Russian rod pentode circuits

[BusyBee]

Ah yes I do recall seeing pics of Russian Nuvistors with flying leads. The RCA ones that need socketing also have many leads cut off flush on the bottom as they form the structural support for the electrodes internally but not all are needed for connection to external circuitry. The RCA ones can have very low noise and microphonics which suits the audio work I do.

I had a quick google for possible sources the other day and in fact the two most prominent search returns (for rod pentodes and other types) were for Russian suppliers and they were offering them for 72p each which is about what I was expecting having briefly googled them maybe 6 years ago and before I had learned any more about them here of course. There are obvious ethical problems with buying from Russia and then the issues likely over banking sanctions etc so I always intended to try and get them from Ukraine of course. Ukrainian and Bulgarian suppliers seem to be only on ebay, not sure if that's good or bad! Probably puts prices up but gives a little more peace of mind I guess.

It is all a bit confusing though as they have very much gone up in price from ebay, £1.50 - £2.95 each from what I can see and some suppliers seem to have listings for £1.50 ones and for £2.95 ones with the same descriptions and they look the same in the photos.... also there are several potential ebay shops for them and having never imported valves from Eastern Europe, never mind during a war, I don't know if a specific supplier is one of the good ones or not....
I'm probably a bit paranoid but another worry is that if ordering say £60+ worth of various valves to keep postage down then it's not as if I could go and bang on the door of the supplier if I never receive them!

Ideally I want to find a supplier that is both trusted and has all the valves I want. The 6C17K-B (in Cyrillic. I prefer Cyrillic for such valves as there can be a few interpretations of Cyrillic lettering and mistakes in translation can also occur) is interesting for a start. A disk seal planar frame grid triode for UHF/low microwave frequencies, I believe directly heated but not sure from descriptions if they mean indirectly heated but cathode and heater are connected, around 22mA/V Gm and Mu of around 180 with reputedly zero microphonics. Very small. Connecting them to anything could be fun... hoping soldering directly to the ring terminals would work, and avoiding them taking off at say 1GHz may be a problem. Only about £2 each though and promise very low noise for low level audio amplification. I have seen them put to this use in a few places on the internet so not my idea no. I saw some on ebay from a Bulgarian or Romanian supplier that were only something like £8 for 12 (but around £11 delivery!) but although described as new they appear to come without boxes (not sure on that) and appear rather "dirty" but I think that could just be blackening of oxidising silver as all the "terminals" are silver plated. The £2 ones from Ukraine have boxes and look shiny.

If anyone has specific recommendations for suppliers that can be trusted that would be great.

(must re-read Mark's post about quoting!)

I have tended to use ebay as there is some degree of protection. Russian suppliers there were closed off, the last I looked. A pity really as I did have one supplier I ordered several times from. I suspect that those suppliers are not happy bunnies.

I have tested a 6C17K-B. The diagram shows indirectly heated with the cathode connected to one end of the heater. I have seen online that some people have soldered directly to the case and had problems but I did so on the one I tested and it worked without problem. They have a silver plated case, I think, and solder takes very easily. I would maybe beware of distorting the case / internals with excessive heating, although on test with full heater voltage these do get pretty hot so maybe not a problem. On reflection it's probably the case that the correct holder acts as a heat sink as well. I have tried to find these online but had no joy so maybe they are more equipment specific. It would be possible to make one. UHF / microwave valve holders do seem rare and often more expensive than the actual valves, from what I have seen.

This shows the connections well. There are English translations available and I may have one if you want it:

[Jez]

Thanks for the datasheet. I'll manage in Russian.

One idea that occurred to me would be an arrangement of different sized Terry clips as a holder. Obviously not for use at UHF though! Could make a useful test jig for them though.

Soldering to them will be my first approach and like you I've seen reports that this kills them but other people saying it was fine. I suspect that if not quick enough then the very localised heating of the iron distorts the packaging and breaks the seal causing them to go to air.

What did you use them for and how did they perform? I've read that the extremely high Mu is not very consistent from one unit to another so for stereo audio use you need to buy several to stand a good chance of getting a pair that allow good channel balance.

[Murphyv310]

Hi
I use Terry clips but use heat shrink on the valve firstly. In projects that are audio related foam as I've previously stated.

[Kalee20]

Here's the circuit of the self-balancing push-pull amplifier using 1j24b and 2x 1j29b.

The 180pF capacitors across the transformer primary are to suppress residual HF spikes from the AC filament heating. As mentioned, there's  a neutralisation circuit which injects ripple of appropriate amplitude and phase into g2's, but I found that the ripple disappeared (apart from spikes) with the trimpot just about at its central position - which says something for the constructional symmetry of the rod pentodes.

The O/P transformer is sectionally wound (secondary and closely-coupled bifilar feedback tertiary) split in half, each side of the primary.

Frequency response is also shown into a resistive load.

It's no doubt not quite up to Jez standards, but it has drawn favourable comments from a former hi-fi retailer.

[Murphyv310]

Excellent Kalee.
The frequency response is really good.
What is the distortion levels?

[BusyBee]

The amplifier looks good to me too. It's nice to see a good amplifier that is a bit out of the ordinary.

Tracy

[BusyBee]

What did you use them for and how did they perform?  I've read that the extremely high Mu is not very consistent from one unit to another so for stereo audio use you need to buy several to stand a good chance of getting a pair that allow good  channel balance.

I haven't used them as yet. I was doing a bit of experimenting in UHF and got them for that but after successfully building a wideband attenuator further forays were fraught with issues, the main one being (obvious really) I don't have great experience or suitable test equipment for those freqencies. Now that the TV is digital it is far less interesting to me anyway.

Time will tell as my interests vary. One way or another I do spend some time playing with oscillators and still have some interest in VHF so there is scope. Their construction would lend itself to a mechanically stable design even if there are potential issues with temperature variation in the circuitry. It is, however, very unlikely I would use them for audio. I do have a circuit I found somewhere in which someone has but I think more a curiosity than a game changing design.

Tracy

[Kalee20]

Excellent Kalee.
The frequency response is really good.
What is the distortion levels?

Thanks! Though I don't know - distortion has not been measured.

I do have scope traces of the output, just below the clipping point, with both sinewave and square wave drive. The sinewave looks pretty good and sinuous, so distortion is at least low.

You can see residual HF noise from the HF AC filament heating, as mentioned before, some optimisation here would no doubt clean it up. But it's not a problem.

[Mike Watterson]

Small choke in series with grid and small cap on anode to 0V. The electrode capacitances are extremely low compared to D9x valves.
Years ago I used quad high profile DIL transformers for isolated CPU I/O cards on an industrial controller. One connection was 1MHz which was used as both PSU (multiple + and - voltages using 1N4148s and regulators for cpu and op-amps, ADC or DAC.) and CPU 1MHz clock. The other three used as shift clock,serial data in and serial data out. The CPU hadn't a true UART, just a parallel read/write shift register.
I'm sure the 11mA is possible for a 1j24b. Not sure if they could do the 1j29b. Possibly an idea with them is to suitably feed the filaments in antiphase as the top end is common. Then HF ripple on the two parallel beams reaching anodes would cancel. One plate of g1 is shared and other two g1 plates are in parallel. Similarly for the g2 rods. But the g3 rods and anodes treat it as a single beam.
   
There are four sheet beams from the filaments. A pair in each horizontal direction from the red dots. There is some attempt at shielding using 3 shield rods at either side of beams symmetrical with the two rods used for the separate ends of filaments which are spring loaded. So the common f+ ends of the two filaments are at the base. All the rod pentodes have spring loaded filaments and you can see the filament end of the spring move with filament power on /off

The top common connection of the two filamentary cathodes is regarded as LT+ and the separate bottoms are LT-, so series operation is slightly nonsymmetrical and not as good. They always recommend decoupling the centre tap on series operation.
Note 1j29b-r and 1j29b-v have different bases.
   
Note the labels are wrong on the photo for the 1j29b I examined. That's Joe Sousa's photo.
Note in Pentode OR Triode mode the g3 must go to a low voltage, either f-, f+ or within + or - a few volts. So with isolated filaments the g3 ought to go to common of filaments. F3->f1, F1->f2 and F2 and F2 in the photo are shield rods.

It's possible the shield rods on most are for additional support. The 1j37b has only one dedicated shield rod. The diagonally opposite rod is feeding top end of filament via the usual spring.
   


   
Triode mode allows a bit more power or much lower HT.

Here is a failed attempt at Push Pull at 45V HT. Small/Miniature mains transformers work far better. None of the available sub-miniature transformers are suitable. Most are for old Germanium transistor designs.
   
A 600 Ohm to 10K step-up transformer for microphones might work for a very low power o/p on a 1j24b to drive a 64 ohm speaker or 2 x 32 Ohms earphones in series, but it might easily saturate and pretty poor.

A ceramic "beeper" disc fed from a 10K Ohm anode load works at 18V to 22V on a 1j24b, and can be rigged as an earpiece. Having it in a short 6mm to 8mm long tube closed at one end and mounted a few mm from a 6mm to 8mm hole in the other end works well.

[Kalee20]

For the 1j29b:

The top common connection of the two filamentary cathodes is regarded as LT+ and the separate bottoms are LT-

Though, this seems like a convenient convention only. Internally, the production difficulties for having tapered rods, or non-parallel spacings, would be significant! To prove it, I tried swapping over the filament leads with a DPDT switch, and the anode current changed by at most 2%.

I also tried it with the (single filament) 1j24b, same result.

...series operation is slightly nonsymmetrical and not as good. They always recommend decoupling the centre tap on series operation.

Yes I agree - I used mine in parallel.

For a directly-heated filament, there must of necessity be a potential gradient along the filament to get it hot. So no way can it be a unipotential cathode. Ideally, the reference point would be midpoint, which of course isn't accessible, but the equivalent is the mid-point of the filament heating supply.

If this is a transformer, it's easy.

The other point is that the filament voltage 'eats up' some of the grid voltage swing - as the grid goes more negative, the positive end of the filament cuts off first while the rest of it still passes current. Unless there is compensation, such as grid rods tapered along their length, or assembled on a slant (which is already determined not to be the case), the electron beam gets progressively shut off and it takes a further 1.2V to cut off the negative end of the 1.2V filament. So even if the characteristic is completely linear up to commencement of cutoff, there'll be curvature over the last 1.2V and the cut-off can't be any sharper than this.

It's a distortion mechanism.

With filaments in series this happens over 2.4V, it's a good reason to not use this option!

I used 0.6V - 0 - 0.6V filament supply, referencing everything to the centre tap, and there's practically zero change in current as the filament voltage swaps polarity. It's square wave, which has the best possible peak:RMS ratio, so there's never any instant when the total filament voltage is greater than 1.2V.

Note in Pentode OR Triode mode the g3 must go to a low voltage, either f-, f+ or within + or - a few volts. So with isolated filaments the g3 ought to go to common of filaments. F3->f1, F1->f2 and F2 and F2 in the photo are shield rods.

Why? I've used g3 as a control voltage for AVC, also g2 (offset in level), and I've used g3 in a frequency changer for oscillator injection, going between 0 to -20V, and it works well. Each case, there's more than 'a few volts' on g3!