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".