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Russian rod tubes in a battery all-tube mini amplifier prototype
Russian rod tubes in a battery all-tube mini amplifier prototype
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Old 24th June 2018, 07:19 PM   #21
Hearinspace is offline Hearinspace  Canada
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Russian rod tubes in a battery all-tube mini amplifier prototype
Thanks for the drawing. It makes it easier to follow your thinking.
First new question, in the signal circuit drawing, one end of the cathode on each of P1 and P2 (I'm numbering left to right) are grounded and in the battery drawing it's just P1. Whichever is the case you are not using a negative bias on the control grids. Is running P1 at zero bias and P2 at Positive Bias intended?
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Old 24th June 2018, 08:09 PM   #22
Hearinspace is offline Hearinspace  Canada
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Russian rod tubes in a battery all-tube mini amplifier prototype
Forget the question, I skipped past the last sentence of your post above.
Well, hopefully we've kept the ball in the air long enough that somebody more helpful than me has noticed it.
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Old 25th June 2018, 09:54 PM   #23
Thomasha is offline Thomasha
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I managed to reduce a lot of the oscillation by converting the PI to triode mode. Now it even plays clean at lower volume settings without the tremolo.

There is still some motorboating at the last 10% of the volume pot.

I haven't tried the alternative heater setting yet, maybe tomorrow. For now I'm quite happy with the result.
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Old 26th June 2018, 01:06 AM   #24
Hearinspace is offline Hearinspace  Canada
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Russian rod tubes in a battery all-tube mini amplifier prototype
Yeah, it sounded pretty good to my ears in the video.

I still have this notion that the motorboating comes from feedback through the common cathode string. . . . . but that's just a wild guess based on some limited experience with another circuit . . . . . long ago . . . . . . . in a place far, far away . . . . . . . . . : )
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Old 26th June 2018, 04:29 AM   #25
PRR is offline PRR  United States
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Quote:
Originally Posted by Hearinspace View Post
...explain to me how the collective cathode circuitry as shown is not problematic? ...
The heating current is perhaps 100mA. Each cathode current is under 1mA. Since all tubework is +/-20%, it is reasonable to neglect the small cathode current adding to the large heating current.

Or: the heating resistance is about 12 Ohms. The cathode internal impedance (1/Gm) is over 1K Ohms. Only 1% of what happens at the internal cathode interface appears at the heater.

The mutual coupling is small for most situations. In a high-gain chain (such as a guitar amp!!) you may have to deal with it. Capacitors may be wanted. Note that you want to well-bypass the *heating* resistance so this may be a far larger cap than we could get in 1920, even large today (thousands of uFd for full bass forward response).

Small filament tubes were usually designed to have grid returned to the more negative end of the filament. One end of the filament gives Vgk of zero, the other end Vgk is -1.2V, average -0.6V. The grid is negative of most of the filament. Yes, the near-zero end does pass some grid current. In a HOT tube like 12AX7, zero Vgk can be significant grid current. The filament tubes were not so tightly wound, or highly heated, grid current was not a big deal.

The Russian Rod Tubes use side-rods, not a 'fence' IN the electron stream. Grid current is apparently quite small even for Vgk positive.
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Old 26th June 2018, 04:41 AM   #26
Hearinspace is offline Hearinspace  Canada
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Russian rod tubes in a battery all-tube mini amplifier prototype
Thanks for the input PRR, this stuff interests me and I was hoping you might chime in.

Not to argue, but with his stated 31mA heating current, I get 387Ω heating resistance. Does that make it more influential?

Another thing I noticed is that in the signal circuit schematic it shows neither output tube has a cathode connection to ground. Probably just a mis-print but that's all I have to work with . . . . . . . So stuff like that is bothering me.

A full schematic of both heating and signal together would likely have made me pack up and be quiet some time ago.
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Old 27th June 2018, 07:55 PM   #27
PRR is offline PRR  United States
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Quote:
Originally Posted by Hearinspace View Post
...schematic it shows neither output tube has a cathode connection to ground...
Not sure what plan you cite. This one has a full path.
Attached Images
File Type: gif filament-maze.gif (40.8 KB, 82 views)
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Old 27th June 2018, 11:27 PM   #28
Hearinspace is offline Hearinspace  Canada
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Russian rod tubes in a battery all-tube mini amplifier prototype
Well, I don't think it exonerates me, but one thing I do see right away that I missed was that neg terminal of the 10uF cap connected to the left end of V6 filament.
I'll look again. Thanks.
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Old 28th June 2018, 05:25 AM   #29
Thomasha is offline Thomasha
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I think what might be messing up with the signal has something to do with the bias of the output tubes. One is at a higher bias, in the beginning of the series string with the leak resistor connected to another filament level to compensate this. While the second one is at the filament voltage with a grounded grid leak resistor. They have quite different voltage drops at the filament, with the second one presents a higher voltage drop (hotter bias?)
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Old 2nd July 2018, 06:17 PM   #30
Thomasha is offline Thomasha
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Another update!
Click the image to open in full size.

I added a tone control and some minor changes.

There still some noise, but it only is present for the last 10% of the volume pot. It seems that the signal at the PI is been received by the preamp tube. By grounding the output of the first stage all the noise is gone. I blame the layout, which is really bad with all the cables around.

So now I will work on a proper PCB layout and just keep some spare grid stoppers just in case the noise is still there.

Something that kept me busy was the PI. I was measuring the signal at the grids of the power tubes and I can't find a way to balance the PI. If I choose the pair of resistors for a given volume setting it works, but if I change the volume the differene between signals increase.

For example, I adjusted the voltage for a 17V p-p at both grids, that resulted in the 150k and 30k resistors as in the schematic. When I increase the volume say to the max. swing that I could get, 32v p-p, the second side, after the voltage divider stays at 23v p-p. It could always be that I'm hitting the power tubes too hard. But the same occurs when I reduce the volume...

My current bias is -3.5v, so 32v is almos 10 times this value. The problem is that I kind of like the distortion when at full swing. It is really full sounding.

Well I tried the feedback idea presented by Merlin, but it had the same behaviour. I guess I need to try a fixed grid leak resistor instead of the volume, and add a cap between both so that the grid bias stays constant right?

I guess my approach is wrong since the tubes are at fixed bias, and not cathode bias as in other circuits.
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