A simple push-pull audio amplifier using russian rod tubes and power transformers

As one sometimes does, I was perusing EvilBay a while back and saw some ex-USSR sub-miniature pentode tubes for sale.  In looking up the part number - 1Ж18Б, which is usually translated to "1J18B" (or perhaps "1Zh18B") I was intrigued as they were not "normal" tubes.

Many years ago I'd read about the type of tube that is now often referred to as a "Gammatron" - a "gridless" amplifier tube of the 1920s, so-designed to get around patents that included what would seem to be fundamental aspects of any tube such as the control grid.  Instead of a grid, the "third" control element was located near the "cathode" and "anode" - or even a pair of anodes.  As you might expect the effective gain of this type of tube was rather low and despite its working, it really didn't catch on.  It was the similarity between the description of the "Gammatron" and these "rod" tubes that interested me.

Figure 1:
A close-up of a 1J18B tube.  Note that the internals are a collection of rods
rather than "conventional" grids and plates.
Click on the image for a larger version.

Some information on the "Gammatron" tube - not to be confused with the later-used "Gammatron" product name - may be found at:

• The Radio Museum - link.
• The N6JV virtual tube museum - link.

In reading about these peculiar "rod" tubes I became intrigued, especially after reading some threads about these tubes on the "radicalvalves" web site (link here) and the "radiomuseum" site (that link here).  Since they were pretty cheap I ordered some from a seller located in the former Soviet Union.

This past holiday week I managed to get a bit of spare time and decided to kludge together a simple circuit with some of these tubes which are pentodes with the suppressor grid internally connected to one side of the filament.  The first circuit was a simple, single-ended amplifier with one of these tubes wired as a triode.  Encouraged that it (kind of) worked I decided to put together a simple push-pull amplifier for more power.

Figure 2:
Diagram of the push-pull amplifier using 1J18B tubes wired as triodes.  On T1, a single 5 volt winding is
the audio input and the series 120 volt primaries, wired as if for a 240 volt connection, is used as a center-tapped winding
for the 180 degree split to feed the two tubes.  The speaker is connected to the "115" and "125" volt taps of T2.
No serious attempts were made to maximize performance.
Click on the image for a larger version.

Figure 1 (above) depicts the electrical diagram of the amplifier that was literally constructed on the workbench using a lot of clip leads and "floating" components as shown in the pictures.  Because this was a quick "lash-up" I used components that I had kicking around with no real attempt whatsoever to obtain maximum performance.

The audio source for this was my old NexBlack MP3/OGG audio player, designed to drive only a standard pair of 32 ohm headphones.  To get some voltage gain and to obtain the 180 degree phase split to provide differential drive to the pair of tubes I fed the audio into one of T1's 5 volt secondaries with the grids connected to the dual 120 volt primaries in series, using the middle as the center-tap to which a "bias cell", a single 1.5 volt AAA cell, was connected to provide some negative voltage.

Even though T1 was a simple split-bobbin dual primary, dual secondary power transformer, it worked reasonably well in the role of audio transformer.  With the 5 volt to 240 volt secondary and primaries, the turns ratio was approximately 1:48 implying a possible impedance transformation of 2304-fold across the entire "secondary".  In this application the actual impedance is not important as it was only the "voltage gain" and the 180 degree phase split that was sought.  In the configuration depicted in the Figure 2 there was more than enough drive available from the audio player to drive the tubes' grids into both cut-off and saturation.

Both V1 and V2 were wired in "triode" configuration with the screen (G2) tied to the plate supply and the audio and operating bias being applied to the first grid.  Because these tubes' filament voltage is specified to be in the range of 0.9 to 1.2 volts, a 4.7 ohm series resistor, R1, was used to drop the filament voltage from NiMH cell B2 to a "safe" value of about a volt.  The plate voltage was provided by five 9-volt batteries in series with a bench supply to yield around 60 volts - the recommended voltage for this particular tube.

Figure 3:
The amplifier, wired up and scattered across the workbench.  The audio
player and T1 are along the left edge, the tubes are in the middle and
the output transformer, speaker and batteries that make up the
plate supply are seen to the right.
Click on the image for a larger version.

In the same spirit as T1 the output transformer was also one designed for AC mains use rather than an audio transformer.  In trying a number of different transformers that could be wired with a center-tap on the highest-voltage winding - including the same type as used for T1- I observed that the highest audio output power was obtained when I used the plate voltage transformer that I'd wound for a (yet to be described) audio amplifier that I'm constructing.  (For an article about the construction of this transformer follow this link).

For T2 this transformer was used "backwards" with the 982 (unloaded) volt center-tapped secondary being connected to the tubes' plates in push-pull configuration.  With a tone generator being used as the audio source I experimented with the various taps and winding combinations and found that the best speaker drive was obtained across the "ten volts" of the 115 and 125 volt taps of the primary.  Based on this configuration the calculated turns ratio is therefore around (982/10) = 98:1 implying an impedance transformation of 9604:1.  With the 8 ohm speaker, the total impedance across the entire winding is therefore calculated to be approximately 77k, or around 19k between the center-tap and each end.  In rummaging around I noted that this particular transformer appeared to have the largest turns ratio of any that I had on-hand!

Perhaps due to the "open" construction and flying leads and/or the lack of any swamping/terminating resistance on the grid side of T1 I noted on the oscilloscope some high-frequency oscillation on the audio output which was easily quashed with the addition of 100pF capacitors C1 and C2 on the grids of the tubes.  The addition of C3 as a power supply bypass had a very minor affect, slightly improving the amplifier performance as well - such as it was!

Figure 4:
A close up of the two tubes, flying leads, C1 and C2 and filament
battery B2 in the background.
Click on the image for a larger version.

In initial testing bias cell B1 was omitted resulting in a quiescent current of around 6 milliamps with 60 volts on the plates.  Adding this cell  to provide a bit of negative bias lowered this current to around 2.5 milliamps while also improving the output power capability somewhat.  Increasing this bias to about -3 volts (two cells in series) resulted in lower audio output and a noticeable amount of crossover distortion indicating that too much of each audio cycle was occurring where the tube's linearity suffered and/or it was in cut-off.

The audio output power was a whopping 250 milliwatts or so at 1 kHz and approximately 10% distortion while the saturated (clipping) output power was around 550 milliwatts.  Referenced to 1 kHz, the -3dB end-to-end frequency response was approximately 90Hz to 12kHz with a broad 3 dB peak around 6 kHz.  On the "full-range" 6"(15cm) speaker that was used for testing this amount of power was more than loud enough to be heard everywhere in the room and sounded quite good with both speech and music.  If I had used a higher-power "rod" tube like the 1J37B or 1P24B and adjusted the impedance accordingly I could have gotten significantly more output power from this circuit.

While the overall frequency response performance could have been improved somewhat with more appropriate termination of transformer T1, one cannot reasonably expect the use of transformers intended for 50/60 Hz mains frequencies to provide the the best frequency response and flatness - particularly with the high plate impedances of the output.  Having said this, it is worth noting that power transformers such as that used for T1 may not only be used as a driving transformer but it could have also been used as an output transformer in a push-pull configuration, albeit with a lower impedance and audio output power for these particular tubes.  While the performance may not be ideal, these power transformers worked surprisingly well and their price, variety and availability make them suitable candidates for a wide variety of applications!

After satisfying my immediate curiosity about these tubes for the moment I un-clipped the flying leads, unsoldered the capacitors and resistors and put the parts away.  Some time in the future I'll put together a few more "fun" projects using these interesting tubes.

[End]

This page stolen from "ka7oei.blogspot.com".

On the winding of power chokes and transformers: Part 3 - The plate (high voltage) transformer

This is a follow-up of two previous posts in this series:

• On the winding of power chokes and transformers: Part 1 - Chokes - link
• On the winding of power chokes and transformers: Part 2 - A filament transformer- link

Using what we already know:

Figure 1:
Plate transformer with attached wires and end bells installed.
The windings and laminations are yet to be varnished or the end bells painted.
Click on the image for a larger version.

In the previous post of this series I described the design and construction of a filament transformer with dual 11 volt, 11 amp windings and a multi-tapped primary.  Building on the experience gained I felt confident to take it to the next step:  The design and building of the high voltage "plate" transformer for the (yet to be described) tube amplifier.

Based on the characteristics of the tubes to be used, the plate voltage needed to be "around 1 kilovolt" with each amplifier section requiring "about 100 milliamps" of average current, or around 200 milliamps, for the pair.  Because of the experience gained in the winding of the filament transformer, we could use the design of the primary winding as a starting point.  For example, we know that to achieve a target magnetic flux of 1.4 Tesla and have the transformer be capable of at least 253 volt-amps and attain a rather conservative cross-sectional amperage of 0.4 amps/mm² for the primary winding's we could use:

• 17 AWG wire
• Taps at 220, 229 and 239 turns for 115, 120 and 125 volts respectively, at 60 Hz

During the winding of the filament transformer's primary I observed that I could easily fit 41 turns of 17 AWG per layer.  This meant that the 239 turns only partially filled the final (fifth) layer, so we could afford to add a few more turns to the primary if necessary.

Two secondaries needed:

While the main secondary will be for high voltage, we will also need a 6.3 volt secondary to power the filaments of some of the driver tubes.  Because such a secondary will have relatively few turns we will need to calculate it first, for reasons that will become clear.

Using the "5% rule" we calculate that our 6.3 volt secondary will actually need to produce 105% of the desired voltage (6.3 * 1.05) = 6.6 volts to account for the drop under load.  Taking our 229 turn, 120 volt primary as a starting point we determine that the turns ratio to achieve this voltage would be (120 / 6.6) = 18.182:1 turns ratio.  With our 229 turn, 120 volt tap we would need (229 / 18.182) =  12.59 turns to obtain 6.6 volts.  

What this means is that for our secondary we should round the number of turns up (I'll explain why shortly) rather than down and with exactly 13 turns we end up with a primary-secondary turns ratio of (229 / 13) = 17.62:1.  From this we can calculate the actual, unloaded secondary voltage will be (120 / 17.62) = 6.81 volts - a bit higher than we'd like.

How do we fix this?  We should increase the number of turns on the primary to be able to more accurately obtain the desired voltage, but why increase the number of turns when we could also establish an accurate result by rounding down the secondary to, say, 12 turns and decreasing the number of turns on the primary to compensate?

You may recall that when winding a primary, the magnetic flux is has an inverse relationship with the number of turns.  Because the number of turns on the primary of the filament transformer was calculated to achieve the maximum target flux, we would not want to decrease the number of primary winding turns as that would increase that flux.  In other words, the main down side of adding a few turns to the primary is that each winding will need a proportional number of extra turns as well, taking up additional room on the bobbin:  If things are already tight, adding those turns could result in more wire than will fit.

Crunching the numbers:

• Our voltage ratio:  120 / 6.6 = 18.182:1.  We already saw this number.
• Since our 6.6 volt secondary should have exactly 13 turns, our 120 volt primary should have (18.182 * 13) =  236.4 turns, rounded down to 236.  This increase in turns reduces the magnetic flux from 1.4 to about 1.3 Tesla.

Clearly, a half a turn on the 120 volt winding has a fraction of the effect (18.182^th, to be more precise) as a half turn on the low-voltage primary so we will round this down to 236 turns.  Let us now calculate the 115 and 125 volt taps:

• 115 volts / 6.6 volts =  17.42:1 ratio.  13 turns * 17.42 = 226.46 turns.  I rounded this down to 226 turns.
• 125 volts / 6.6 volts = 18.94:1 ratio.  13 turns * 18.94 = 246.22 turns.  This was rounded down to 246 turns.

Since we already know from when we wound the filament transformer that we can safely put 41 turns on a layer, we can see that for 246 turns we would need (246 / 41) = 6.0 layers - so we will go with that!

Designing the high voltage secondary:

If you are familiar with tube-type amplifiers you may have already have guessed from the voltage and current requirements that the plate impedance of the amplifier would be quite high:  10k ohms, to be precise.  The output transformers themselves are designed for single-ended triode operation with 8 ohm secondaries, rated for 25 watts (maximum) output.  Going through the math one can see that the turns ratio of this transformer is approximately √(10000/8) = 35.36:1.  If 25 watts RMS were being produced into 8 ohms, this implies that the RMS output voltage is around 14.14 volts, or almost exactly 500 volts RMS on the 10k primary which translates to 707 volts peak.

According to the specifications gleaned from the Edcor support forum (a link to the message thread may be found here) the maximum "safe" voltage across the primary and secondary windings would be 1000 volts.  Clearly, assuming a 10k primary impedance, 25 watts RMS of power and any reasonable plate voltage to achieve anywhere near this output power one will have to exceed this maximum voltage rating - unless a bipolar power supply is used where the high voltage is split - that is, the standing DC voltage between the primary and secondary is reduced to half.  To do this a full wave "bridge" rectifier is used with our choke-input filter network with the centertap of the transformer being grounded.

A final (loaded) DC voltage of around 970 volts for the plate voltage was (somewhat arbitrarily) decided as the target for the tubes that will be used - a reasonable compromise between the constraints of the output audio transformer voltage rating and the efficiency of the tube.  With this in mind let us calculate the actual, unloaded voltage for the secondary.

We know from when we designed our choke that at 200 mA there will be a 60 volt drop, so we will need to increase the output of 970 volts by this amount, which means that we will need (970 + 60) =  1030 volts.  Because the power supply will use a choke input we know that the loaded voltage of such a power supply is typically around 110% of the RMS voltage which means that for 1030 volts DC we will need approximately (1030 / 1.1) =  936 volts RMS.

Using the "5%" rule of thumb to take into account resistive loading of the primary itself we can calculate the actual, loaded voltage for the secondary, as in (936 * 1.05) =  982 volts, unloaded.  Using the 120 volt tap from the reference design we can now calculate our turns ratio and the number of turns, as in:

• 982 volts / 120 volts = An 8.183:1 turns ratio.
• 236 turns (at 120 volts) * 8.183 = 1931 turns which will be rounded down to an even 1930 turns so that the center-tap will be made at the 965th turn.

Based on the recommendations from the Turner Audio and Homo-Ludens web pages (see previous articles for the links) we can use a general rule of thumb of 0.33-0.35mm²/amp and since our current is to be 0.2 amps, we need a wire with the size of at least (0.2 amps * 0.33 mm²/amp) = 0.066 mm².  Consulting our wire chart we see that 29 AWG has a cross-sectional area of 0.0642 mm² resulting in a density of 0.321 mm²/amp - pretty close to our design goal.  As noted in the previous installment, Edcor seems to use a value of around 0.253 mm²/amp for their transformers and if this is applied our primary would be capable of (0.0642 mm² / 0.253 mm²/amp) = 0.25 amps.

As it happens I had 29 AWG wire available when the choke was wound (it, too, was designed for 200mA) so this is the wire that I used.

Will it fit? 

At this point the question must be asked:  Will all of these windings fit on the bobbin?

We know from when we wound the choke that approximately 161 turns of 29 AWG wire will fit per layer, and with 1930 turns total, we'll need 12 layers.  With 29 AWG wire having an outside diameter (with insulation) of 0.33mm and the tape from each layer adding 0.05mm of thickness, each layer will occupy 0.38mm or, with 12 layers, 4.56mm of of bobbin "height". 

We also know from our winding of the filament transformer that one layer of 17 AWG wire plus 0.05mm of insulating tape has a total height of 1.274mm and with 6 layers that comes to 7.644mm.  Put together, the combined height of both sets of windings is 12.204mm - approximately 73% of the 16.5mm available bobbin height.

Figure 2:
Center tap of high voltage plate winding located in the middle of the winding
before Nomex insulation was added.
Click on the image for a larger version.

This figure does not include the low voltage secondary winding (one layer of 17 AWG, adding another 1.274mm) or the extra insulation that must be added between windings (approximately 0.5mm for each of the three) all of which adds another 2.774mm, taking us up to 13.704mm - about 83% of the available space.

While this will be kind of a tight fit, we ended up with the same sort of numbers when we designed and successfully built the filament transformer so we can have good confidence that this, too, will work.

The winding:

While it may seem customary to wind the primary first, that may be just because most transformers that are seen these days are step-down, with the secondary winding handling more current than the primary and thus using larger wire.  It usual to place the smallest wire on the inner-most winding since it is more flexible and  easier to handle on the smaller-diameter "inner" layers of a bobbin, going around the square-ish corners and leaving the larger wire for later when the bobbin diameter is larger and the corners more rounded.

Following this convention a hole was "drilled" in the side of the nylon bobbin with a hot soldering iron and a piece of Teflon™ insulated wire was pulled through, attached to the start of the winding and then insulated with several layers of polyimide tape and a layer of Nomex™ paper insulation.  With that task completed the winding proceeded with care being taken on the first layer to assure both neatness and tight packing - the latter being done by pausing every few turns to slide the wire over to minimize the gap between adjacent conductors.

Figure 3:
End of the high voltage secondary winding, insulated with both
polyimide tape and Nomex ™ paper.  A loop was made in the wire which
brought out at a right angle from the other turns so that the tap would
not interrupt the continued neat, side-by-side windings.  Taps are
always made on the two sides of the bobbin that face the end bells
rather than the sides inside the core where the height is
is more limited.
Click on the image for a larger version.

The first layer done, a single layer of 0.05mm polyimide tape was placed over the top.  When I wound the choke I had only a single width of this tape available, but this time I had a selection of widths so as I proceeded with the layers, the location of the overlap and widths of this tape was changed with each layer to minimize "piling" of the turns where the tape overlaps which would later make it difficult to keep the layers even.

After a few hours of intermittent winding over several days - with each layer individually insulated with 0.05mm polyimide tape - the center tap was reached and for this a loop of wire was made in the conductor at right angles to the lay to which another piece of Teflon wire was soldered which was brought through the side via a hole made in the side of the bobbin with a hot soldering iron.  This joint was carefully placed in the middle of the flat side of the bobbin that would face outward from the core and insulated with a few layers of polyimide insulation and Nomex paper to prevent it from damaging or being damaged by the pressure of turns in the layers above and below.

Figure 4:
Overlay of Nomex ™ insulating paper atop the finished high
voltage secondary winding before the top layer of polyimide
tape and its "creepage" insulation along
the sides of the bobin was added.
Click on the image for a larger version.

After a few more days of occasional winding the last turn was laid down, nearly filling the 13^th and final layer.  I soldered to this a piece of Teflon wire and insulated it and the wire was brought out through the side of the bobbin and the entire secondary was covered with several layers of polyimide tape and 0.05mm Nomex paper.  As a final covering over the Nomex, another layer of polyimide tape was laid down, this time with the tape slightly going up the sides to increase the "creepage" distance between the primary and secondary - a sensible safety precaution, particularly with a high-voltage transformer!

Now, the primary...

The conductors of the primary were now laid down atop the insulated secondary.  As with the filament transformer the 17 AWG wire was brought directly out through the side of the bobbin and tucked out of the way:  The connection to flexible wire would be done later.

Figure 5:
The three "end" taps of the primary winding:  Top-left is the 115 volt tap,
below it is the 120 volt tap with the 125 volt finish on the left.  After
this picture was taken small pieces of Nomex paper and additional
tape were placed below and above the taps.
Click on the image for a larger version.

As with the start of any new winding the first layer of the 17 AWG primary was done with special care to make it neat and tight and each layer was individually insulated with 0.05mm polyimide tape.  When the 220^th and 229^th turns (for the 115 and 120 volt taps, respctively) were reached, loops of wire were put in the conductor, which was brought out through marked holes in the bobbin at right angles to the conductor.

With each tap being insulated with polyimide tape and Nomex paper where they crossed over other windings, the entire primary was then covered with several layers of polyimide tape and Nomex paper.  Again, a bit of insulation was brought up along the sides of the bobbin to provide extra "creepage" distance to provide good insulation for the 6.3 volt secondary to maximize both safety and reliability.

More about the 6.3 volt secondary winding:

Because it was on-hand, 17 AWG wire was used for the "6.3 volt" additional secondary.  With a cross-sectional area of 1.04mm², we can calculate its current-handling ability:

• Using the 0.33 amps/mm² recommendation from the Turner Audio site, a safe current is:  (1.04mm² / 0.33 amps/mm²) = 3.15 amps
• Using the 0.253 amps/mm² design Edcor guidelines a safe current is:  (1.04mm² / 0.253 amps/mm²) =  4.11 amps.

Figure 6:
The completed winding - including the 13 turn, low-voltage secondary -
with the just-started core stacking.
Click on the image for a larger version.

Even in the worst-case scenario the addition of a 4.11 amp secondary would add only another 28 volt-amps of load to the transformer - well within its capacity.  Because this winding is on the outside of the bobbin and "exposed", it has good opportunity for cooling by convection and thus the Edcor rating would seem to be applicable - and 3-4 amps is plenty of current for several 6.3 volt tubes.

Comment:  If more current is needed it would be easy to add another parallel 17 AWG conductor to double its capacity.

As with the primary winding - which also used the same 17 AWG conductor - the ends of this 13 turn secondary were brought straight out the sides of the Nylon bobbin for later connection to flexible conductors and this additional secondary was overcoated with polyimide and polyester tape.

Finishing and initial testing:


With the addition of the low voltage secondary, all layers were over-wrapped with another layer of polyester tape to both secure and insulate the windings.  The transformer was almost ready to be tested!

Figure 7:
The stacked transformer undergoing initial testing with a
a variable transformer.
Click on the image for a larger version.

Although there are approximately 111 pieces of iron to be inserted into the core, the process is pretty easy:  Simply lay the bobbin on the table on one of the "outer" faces (where the taps are made and wires are attached) and alternately place the "E" sections atop each other.  With the "E" sections done, the transformer is then set on end to provide access to the vacant slots between every other "E" section into which the "I" sections were dropped.  Once these sections were added to one side, the bolts were slid through the laminations with the "I" sections to prevent them from falling out as I turned the transformer over and the "I" pieces were added to the other side.

With all E and I sections installed, a block of wood and a small hammer were used to abut the pieces of laminations against each other, a process that required several passes on all four sides.  With this done some nylon shoulder washers were installed (visible under the screw heads in Figure 7) to prevent the effect of eddy currents that might be caused by the "shorted turn" effect of the screw, and the bolts tightened.

Using a variable transformer the unit was then tested, first noting that the unloaded (magnetization) current was comparable to that of the previously-tested filament transformer indicating that nothing seemed to be amiss.  Very carefully, the high voltage secondary's voltage was then tested on each side of center tap and I noted that they were within a fraction of a volt of each other, and exactly at the calculated value with 120.0 volts applied:  491 volts on average.  I could not directly measure the 982 (unloaded) volts across the entire secondary since I have no voltmeter that is "officially" rated above 750 VAC.

After a test of the low voltage secondary, which was also measured to be at its designed voltage, I attached permanent wires and the end bells as seen in Figure 1 at the top of this page.  At this point the transformer  only awaits being dipped in insulating varnish - something that will happen after inital testing of the (yet to be described) amplifier prototype.


A future post in this series will describe the final steps in finishing these transformers:  Impregnation in "insulating varnish" and the final painting of the end bells.

[End]


This page stolen from "ka7oei.blogspot.com".

Repairing the power switch on the Kenwood KA-8011 (a.k.a. KA-801) amplifier

Back around 1990 my brother mentioned to me that there was an amplifier, in a box, in pieces, in the back room of the TV/electronics store where he worked at the time and that if I made an offer I could probably get it for cheap.  Dropping by one day I saw that it was a Kenwood KA-8011 Integrated DC amplifier (apparently the same as the KA-801, except with a dark, gray front panel) laying in a box from which the covers were removed with a bunch of screws and knobs laying in the bottom.  I also noticed with some surprise that it had a world-wide voltage selector switch on the back and that the power cord had a Japanese 2-pin wall plug and U.S. adapter - and still does!  All of the parts seemed to be there so I offered some cash ($50, I seem to recall) and walked out with it and a receipt.

Figure 1:
 Spoiler alert:  This is the KA-8011 with the repaired power switch.
As noted in the text, the original, blue-painted panel meter lights were
replaced long ago with blue LEDs.

When I got home with the amplifier I knew that I had my work cut out for me particularly since, in those days before the widespread internet, I had no schematic for it and no-one that I contacted seemed to be able to find one.  Powering it up I noted that the speaker protection relay would never engage, indicating that there was a fault somewhere in the amplifier.

A visual inspection of the awkward-to-reach back panel's circuit board revealed several burned-looking leads sticking up from the circuit board where transistors had exploded near several burned resistors.  After a few hours of reverse-engineering a portion of the circuit I realized that the majority of the circuit at fault was one of four identical phono preamp input circuits (there are two separate stereo phono inputs) and associated low-level power supplies.  Between the intact amplifier sections and being able to divine the color bands on the smoked resistors - along with some educated guesses - I was able to determine the various components' values and effect a repair.

Figure 2:
The power switch, with a broken bat.
Click on the image for a larger version.

The amplifier now worked... sort of.  I then had to sort out a problem with the rear-panel input selector switch, operated by a flat, thin ribbon of stainless steel in a plastic jacket that was engaged from a front-panel selector.  I managed to cut off the portion at the front that had been damaged where it was pulled-on from the front panel having been loose in the box, punch some new holes in the ribbon, align the two (front and rear) portions of the switch mechanism and restore its operation.

Snap!

Having done the above, the amplifier was again operational and I have used it almost every day in the 25+ years since, needing only to replace the blue-colored incandescent meter lights with LEDs, powered from a simple DC filtered supply.  In the intervening years I also had to replace some of the smaller electrolytic capacitors on the main board that had gone bad, causing the speaker protection circuit to randomly trip on bassy audio content and with slight AC mains voltage fluctuations.

Figure 3:
Comparing the old (top) and new (bottom) switch components.  In order
to prevent it from interfering with the body of the switch some of the
metal on the new bat would have to be removed.
Click on the image for a larger version.

I was annoyed when one day, a few months ago, the power switch handle - which had been bent before I got the amplifier - and then "un-bent" during the repair - broke off in my hand when I turned it on.

Using the "bloody stump" of the power switch for a few months  I finally did a search on EvilBay to look for a new switch.  While I didn't find a power switch I did see a "tone control" switch for the same series of amplifier - so I got that, instead.  When it arrived I noted, as expected, that most of it did not mechanically resemble the power switch or look as though it would easily mount in the same location, but it did have essentially the same metal bat on the end as the original that I figured I could fit onto missing portion that had broken off the power switch.

Comment:

Even though the "new" switch was much too small - of insufficient current rating - to have been used to switch the mains (AC input) power, it would have sufficed to operate a relay.  To have done this would have required that new holes be drilled in the front sub-panel to match those of this new, smaller switch. While this would not be "original" circuitry, it would have looked the same from the front panel.  This is a possible option should the power switch itself become unreliable some time in the future due to the heavy capacitor-charging currents at turn-on.

Removing the original power switch I laid the two side by side and made notes of the differences between the metal bat of the new and original - which was narrower in some places to clear parts of the switch body - and taking a file to the new one I took off some metal to clear the possible obstructions.  I then noted on a crude drawing the length and orientation of the new bat based on the axis of the switch's pivot point.  Because the bat of the original switch was embedded in a block of molded Bakelite I knew that I would have to somehow attach a portion of the new switches' bat to the old, so I carefully disassembled with old power switch, cutting off and saving the original rivet on which the switch pivoted, noting where everything had gone and saving the small springs, contacts and some small Bakelite pins.

Figure 4:
The new bat, butt-soldered on the old switch.  Note that the bat from the
"new" switch has been filed to better-resemble the shape of the
original bat to clear the switch body.
Had I not been able to find a "similar" switch on EvilBay I could have
probably measured the original switch, found some scrap
steel of similar thickness and made a suitable replacement entirely
by hand with careful filing using the bat of another switch as a template.
Click on the image for a larger version.

Clamping the old part in a vise I cut off most of the original bat, leaving about 5mm of metal remaining.   Carefully comparing the old and new piece I then marked where, on the new bat, that I would have to cut to allow the repaired piece - consisting of the new and old butted and laid end-to-end - have the same length as the intact original.  Doing so, purposely cutting the "new" bat slightly long, I did some fine tuning with a file until the two pieces laid down precisely lined up as they should.

Attaching the new piece

Using some silver solder intended for stainless steel I applied some of its liquid flux - apparently a mixture of chloric and hydrochloric acid - and using a very hot soldering iron I "butt-soldered" the two pieces together in careful alignment and then filed the surfaces flat to remove excess.  While the bakelite switch body can handle a brief application of a soldering iron, I knew that it would not tolerate the heat from a proper, brazed joint.

This (weak!) solder joint was intended to be temporary, needing only to be good enough to allow a sleeve to be made by wrapping an appropriately cut piece of thin, tin-plated steel (from my junkbox) around the joint.  Once this sleeve was checked for proper fit and folded tightly, additional flux was applied and the entire joint - sleeve and all - was soldered, the result being a very strong repair with the restored bat being of the same length and at the same angle as the original.

Reassembly:

The trick was now to get every thing back together.

Figure 5:
The steel sleeve being installed over the butt solder joint,
before final folding and soldering.
Click on the image for a larger version.

Reinstalling the pivot and making a few clearance adjustments to the original switch's frame with a small needle file, the original rivet was then soldered into place and the entire assembly washed in an ultrasonic cleaner to remove the remnants of the corrosive flux from the bat and switch body.

In the base of the switch, the contacts, which were the same as those had it been an SPDT switch, were reinstalled - this time, rotated 180 degrees so that the previously unused contact portions would now be subject to electrical wear.  These contact were then "stuck" into place with a dab of dielectric grease so that they would not fall out when the switch body was inverted.

Figure 6:
The repaired switch, reassembled,  with the new bat spliced on.
Click on the image for a larger version.

After reinstalling the springs and pins, the rear part of the switch with the contacts was placed over the top of the moveable portion, held in the mechanical center, and the base was carefully pushed into place, compressing the internal springs and pins.  Holding everything together with one hand the proper operation of the switch was mechanically and electrically verified before bending the tabs to hold everything into place.

In reality the reassembly didn't go quite as smoothly as the above.  During one of the multiple attempts to get everything back together the smaller-diameter rear portion of the small, spring-loaded Bakelite pins used to push on the contacts snapped off.  To repair these pins the front, larger-diameter portions - that which pushed against the metal contacts - were placed in the collet of a rotary tool and a shallow hole was drilled into the rear portion where the broken pieces had attached to fit short pieces of 18 AWG wire:  By rotating the piece into which the hole was to be drilled, the exact center is automatically located.

The pieces of wire were then secured using a small amount of epoxy - a process accelerated by placing the pins in a 180F (80C) oven for an hour.  After the epoxy had set the wires were then trimmed to the length of the original sections that had broken off and the ends smoothed over with a small needle file to prevent their snagging on the spring.  The result was a repair that was stronger than the original pins that easily survived switch reassembly.

The results:

Figure 7:
After reassembly it was noted that the gray "skirt"
was hitting the front sub-panel frame, preventing it from
being set to the "off" position.  A bit of heat was applied to
set a permanent bend so that it would clear this panel.
Click on the image for a larger version.

The amplifier was then put back together - very carefully.  The only real issue that I noted was that the gray plastic skirt/escutcheon on the bat ended up about half a millimeter farther away from the switch body and closer to the sub panel than before, causing it to snag on the front sub-panel's cut-out when I attempted to move it to the "off" position.  Careful softening of the plastic with the rising heat of a soldering iron and bending it very slightly allowed it to clear.

Putting all of the knobs back on, tightening the bushing nuts and screws as necessary before doing so, I then tested the amplifier on the bench and was pleased to find that I'd not managed to break anything.

Finding that everything was working fine I put it back on the shelf where it belongs where I continue to use it often.

[End]

This page stolen from "ka7oei.blogspot.com".