Plate stop resistors?

[alchemy]

Working on a 4xkt88 bass amp that was a prototype of a boutique builder that is no longer around, I noticed that there are 100 ohm (3 watt) resistors in series with the output tube plates. I have never seen this before, and after digging through a ton of schematics, the only place I noticed it used was on some old Sunn amps.

What is their purpose? To help with oscillations maybe, kinda like a grid stop?

Any light that could be shed on this subject would be most appreciated to help satisfy my curiosity...

Cheers...

[Wilder Amplification]

The Ampeg SVT used a 10 ohm 5 watt resistor between the plates and OT primary. More than likely a sort of protection device.

[Gaz]

More than likely a sort of protection device.

Care to elaborate?

[alchemy]

X2

[mooreamps]

What is their purpose? To help with oscillations maybe, kinda like a grid stop?

Any light that could be shed on this subject would be most appreciated to help satisfy my curiosity...

Cheers...

Yes it does. Remember about the reduced inter electrode capacitance and higher frequency response of a Pentode vacuum tube ; which I have seen used in HF and VHF US Navy transceivers... They are not only capable of amplifying signals in these frequency ranges, but also used as oscillators in these bands.. The plate stop resistor helps prevent these pentodes from breaking out into parasitic oscillations, particularly when running them with the higher plates voltages as I do; and working in conjunction with the grid stop resistor... I also use the 100 ohm 5 watt plate stop resistor in my builds ; but it is only most effective when one leg of the resistor is soldered directly to the tube socket itself....

-g

[redelephant]

Do u use plate stoppers on outputs with only a single pair of tubes as well? I use them when several tubes are paralleled since they also help balancing out the load sharing, but have decided not to use them when there are no tubes in parallel. I can see the point though, and will probably start including them, tho wouldn't 20-50ohms be plenty?

[Steve Conner]

These kinds of things usually get added when the designer finishes his amp and discovers that it oscillates. First he tries grid stoppers if it doesn't have them already (it should)

Then screen stoppers, and if those don't work, the plate stoppers go in. The SVT was the most famous amp to have them.

Amps with paralleled pairs of output tubes tend to need them more. There are more possible resonant modes in a mesh of paralleled tubes with long wires going around. Usually it takes more than just grid stopper resistors to keep one stable. Plate, screen and grid stoppers right at every socket are not a bad idea.

And yes, the plate stopper resistors can also act as ghetto fuses.

[Robert M. Martinelli]

Hi all,
if it was "only" to keep the thing from oscillating, a simple plate shunt-to-ground capacitor would have done the job of sending "down the drain" all the frequencies above the range of interest (say, 16kHz) nicely (though, having to safely withstand the full HT, it would have been necessary to use a 1000V one....quite costly....); those resistors also act as limiters and help somewhat with mismatched valves when paralleled, as already pointed out.

JM2CW

Best regards

Bob

[mooreamps]

Do u use plate stoppers on outputs with only a single pair of tubes as well? I use them when several tubes are paralleled since they also help balancing out the load sharing, but have decided not to use them when there are no tubes in parallel. I can see the point though, and will probably start including them, tho wouldn't 20-50ohms be plenty?

I use them for all pentodes ; as well as "other" components in both SE and P/P power stages not mentioned in my last reply. I have used other values higher than 100 ohms, but again that seems to be the common value I've seen in older military transceiver equipment, Not Only Used In Just Some Guitar Amplifier Gear...... Besides, 100 ohms is a good value to work with when I am measuring and balancing the plate current from the power tubes.


-g

[Wilder Amplification]

And yes, the plate stopper resistors can also act as ghetto fuses.

Precisely what I meant when I mentioned using them as a protection device.

Ghetto fuses...I like that!

One thing I never knew was that plates could "ring". I figured with them having a much higher mass than the grids that it would take an awful lot to get one to go into oscillation.

With the shunt cap to ground off the plate, the cap would have to see a large voltage drop when the valve is in cutoff. Whereas a 10R resistor would only see not even 1/4 that at full output.

[mooreamps]

One thing I never knew was that plates could "ring". I figured with them having a much higher mass than the grids that it would take an awful lot to get one to go into oscillation.

The plate does not ring. The entire tube rings... Perhaps you should google "parasitic oscillations"....

-g

[raiken]

The plate does not ring. The entire tube rings"....

-g

Actually, you're both wrong. Nothing "rings" mechanically with a parasitic oscillation, not the grid, not the plate, and certainly not the entire tube, so the mass of the plate or grid or tube itself has nothing to do with it (unless you are talking microphonic oscillation, which is a completely different animal than parasitic oscillation).

Parasitic oscillation is an electronic oscillation, not a mechanical oscillation, caused by an unintended feedback path that results in an in-phase signal when the loop gain is greater than unity. The entire thing could be cast in an immovable block of something and it would still oscillate, because it doesn't depend on mechanical factors as a condition of oscillation.

Google "Barkhausen criterion" for an explanation.

RA

[mooreamps]

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

[R.G.]

I use them for all pentodes ... Besides, 100 ohms is a good value to work with when I am measuring and balancing the plate current from the power tubes.

Howzat working for you when your tubes drift?

8-)

[R.G.]

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....

You believe the way the military explains things to new ETs?
8-)