Plate stop resistors?

[raiken]

BullSht.... When I attended ET "A" in Great Lakes when I was a young ET in the Navy, that was "exactly" how it was explained to me ; at that time. "The entire tube rings !" You need to get your facts straight, sir....


-g

And you need to quit believing everything you're told... They have to dumb it down for new ETs who don't have a clue...

But go ahead...humor us - please explain exactly how the "entire tube rings" and thus causes parasitic oscillation (
not microphonic). This ought to be good...

[Enzo]

And that large hunk of glass would ring at the frequencies of parasitics?

[J M Fahey]

Well, I'll relax in my favorite seat at the ringside, grab my two pound popcorn bag , and get ready to watch this medium weight boxing match.
Here we go again

[alchemy]

Hmmm... While this is mildly entertaining, I suppose I will give up hope of any sort of technical discussion and just assume for the time being that they were put there to stop oscillations. Guess I'll know for sure after I remove them...

[R.G.]

Hmmm... While this is mildly entertaining, I suppose I will give up hope of any sort of technical discussion and just assume for the time being that they were put there to stop oscillations. Guess I'll know for sure after I remove them...

Back on the side of sanity, I'd guess that they're there to protect the OT and PT in case of a tube short.

It is possible that they're parasitic oscillation stoppers, given that there are two output tubes sharing a plate and there are some conditions where a pair of plates might oscillate if their interconnecting wiring had the wrong resonance and Q. Could happen. It could even be that the amp builder found that it oscillated and inserted them for a cure.

But if so, it's likely a bandaid fix. There many amps similar to that which don't use the resistors, and work fine. And it's not because the tube resonates even if it's an oscillation fix. Hmmm. Maybe that OT design has a high leakage inductance and gets an oscillation going... grasping at straws here.

My bet is on shorted-tube disaster prevention, though. Short them out and see. We used to call this game "You Bet Your Tubes". 8-)

[soundmasterg]

The Sunn 2000S uses plate stop resistors also. I remember noticing that as it was different than the 200S which didn't use them.

Greg

[Enzo]

When I see them in something like an SVT, my immediate reaction is they are ballasting the tubes so they share current more easily. That is just an opinion.

[jimboyogi]

According to the schematics I have, Vox used 100 ohm plate stoppers on the AC10, the first (1959) iteration of the AC15, and the first AC30 (with EL34 output valves). I always assumed they were there to help balance current between the output valve pairs, but I was just guessing

Later versions of the AC15 and AC30 moved the 100 ohm resistors to the screens instead.

[Steve Conner]

Parasitics and stoppers is standard stuff for radio hams, you can read it up in the ARRL handbook (or the RSGB one for you Commonwealth citizens)

What happens is that the stray inductance of the plate wiring, together with the plate-to-ground capacitance of the tube, forms a tuned circuit that resonates in the VHF band. The grid and screen circuits do likewise with their wiring and their respective electrode-to-ground capacitances.

If any two of these circuits have similar resonant frequencies, and the tube has enough gain at VHF, you have an oscillator. Depending on the strays, it can end up as a Hartley, a Colpitts, an electron-coupled oscillator, a TATG or whatever. I think this is what Mooreamps means when he says "the whole tube rings". But the basic gist is that you have a radio transmitter hitching a ride along with your audio amplifier. The transmitter has no load, so you can damage tubes and burn out components, just as if you keyed up a real tube transmitter with the antenna unplugged.

The stopper resistors damp these unwanted resonant circuits by being in series with them. That is why they have to be mounted as close to the tube as possible: because the wires are part of the resonator.

Slugging the electrodes to ground with capacitors doesn't necessarily help. You're adding more C to a LC circuit, so it doesn't damp the resonance, it just changes its frequency.

Tubes like the 807 and 6146 are notorious for parasitics: they were designed for radio transmitters, so they have lots of gain at radio frequencies, and they just want to oscillate.

The resistors don't do anything to help current sharing: their values are too low. Indeed, because the screen currents in beam tubes vary so widely due to mechanical tolerances, you can argue that any resistance in the screen circuit makes current sharing worse.

[jimboyogi]

The resistors don't do anything to help current sharing: their values are too low. Indeed, because the screen currents in beam tubes vary so widely due to mechanical tolerances, you can argue that any resistance in the screen circuit makes current sharing worse.

Hi Steve,

If you are keen, can you please elaborate on this?

[Steve Conner]

The screen current in beam tubes is like the base current in transistors. It's nominally zero, any screen current you do get is an error caused by imperfect alignment between the control grid and screen grid wires.

And by their very nature, imperfections can't be the same in every tube. If they were, they would be a systematic error that could easily be corrected.

[Enzo]

learn something every day.

[jmaf]

I've inserted a resistor before the OT center tap to insert an artificial sag, after a tube rectifier was converted to SS. 100 ohm sounds about right to simulate tube rectification. My 2 cents.

I like "ghetto fuse", BTW.

[alchemy]

Back on the side of sanity, I'd guess that they're there to protect the OT and PT in case of a tube short.

It is possible that they're parasitic oscillation stoppers, given that there are two output tubes sharing a plate and there are some conditions where a pair of plates might oscillate if their interconnecting wiring had the wrong resonance and Q. Could happen. It could even be that the amp builder found that it oscillated and inserted them for a cure.

But if so, it's likely a bandaid fix. There many amps similar to that which don't use the resistors, and work fine. And it's not because the tube resonates even if it's an oscillation fix. Hmmm. Maybe that OT design has a high leakage inductance and gets an oscillation going... grasping at straws here.

My bet is on shorted-tube disaster prevention, though. Short them out and see. We used to call this game "You Bet Your Tubes". 8-)

All of these seem possible, as does the current sharing theory. When I got the amp there was a 3 or 4 inch black whole inside the chassis where one of the screen resistors had blown up and taken everything around it with it. That tube had completely shorted, but all of the output tubes tested bad. Never could figure that out....

They are 3 watt resistors which seem kind of small to be in the plate circuit of a KT88. The first pair I put in got way too hot when I had the amp close to full power.

If I am going to remove the plate resistors and see what happens to the current, would it be better to remove them one at a time or all at once? I'm guessing all at once to maintain the balance.

And lets suppose one burned up? I'm assuming since there are two pairs of tubes in parallel that the other three tubes will keep going, albiet not properly. But will the tube who's plate resistor burnt up shut off, or will the screen begin to act like the plate and begin to draw excessive current?

Thanks for the help, as always. Still wrapping my head around all this, a little each day......

[alchemy]

I've inserted a resistor before the OT center tap to insert an artificial sag, after a tube rectifier was converted to SS. 100 ohm sounds about right to simulate tube rectification. My 2 cents.

I like "ghetto fuse", BTW.

That crossed my mind, but since they are separate reistors on each tube socket I don't think that's the idea. That and the fact that it's switchable between SS rectified and a pair of 5v3 tube rectifiers....