Plate Resistors on power tubes?

[spidergawd]

I'm working on a red knob Sunn Model T for a friend. It has been hacked up at some point in it's life, probably more than once. He decided since I have to change so much, I might as well convert it to a first generation Model T. The wiring is pure point to point on terminal strips, Pretty awesome for an amp made in the late 70's and perfect for modding.

That first gen schematic is almost exactly a '59 bassman up through the phase inverter, where there it hits a 150 watt ultra linear poweramp. There I found something rather odd. Each of the four power tubes has a 50 ohm 5 watt resistor between the plates and the output transformer. Ive never seen power tube plate resistors before. The only other amp i can find with this setup is the Sunn 2000s. Both these amps came with 6550s, however this Model T on my bench is sporting a nice set of Genelex KT88s.

The Sunn 200s, which has kt88's on it's schematic does not have the plate resistors. I know most of the sunn tube amps are essentially Dynakit hifi amps with guitar preamps slapped on to the front.

I have a hunch that the 50 ohm resistors were put in there to get the A+ voltage down from 500v to 480v for the 6550s. I think the dynaco kit was probably built for KT88s which, if im reading the datasheet correctly, can handle 820v of plate volatge at idle. I'm looking at a recent datasheet for svetlana 6550s and it says they can handle 685v, but perhaps 1970's 6550s couldn't handle that much.

Is this likely the reason? Or is there a some reason they'd do this to affect the impedance of the output transformer primary?

Since the amp is running KT88s and I know my friend prefers them, should i think about taking these out? Any benefit to the higher plate volatge? What effect in general higher impedance have on power tubes?

[roberto]

Plate stop resistors help prevent pentodes from going into parasitic oscillations.
It was quite common in the past, when multiple big power tubes were paralleled.

[Stevem]

Go and view the schematic for a Fender Bassman model AB165, is this how you mean these resistors are installed?

Ampeg SVT amps and Sunn 300 models use resistors in the plate circuit as blow out protectors so that if one tube shorts out the amp keeps playing for you, but if the amp only has two output tubes this set up is meaningless and I do not think it has anything to do with oscillating,

I have a Sunn Sonaro head and when fitted with good testing set of 6550s or KT88s it outputs the same 60 watts of clean power, however the KT88s add a bit more gain in the mix so the amp fitted that way clips the output stage sooner, but man are they more expensive then 6550s!!

[roberto]

I do not think it has anything to do with oscillating

You Stevem have really a strange approach to others...

For your information, the first two links that come out searching plate stoppers with google:
https://music-electronics-forum.com/for ... -resistors
https://www.diyaudio.com/forums/tubes-v ... tions.html

You can find it on old technical handbooks too.

[FUCHSAUDIO]

I recently experienced this in a few production amps with 4 power tubes, due to output transformer changes. Mild instability on transients. I added 30-OHM 3-watt resistors on the power tube plate leads and it cleaned right up. You do have the added bonus of it acting as a fuse, but not my intent. Lots of amps had this back in the day. I would stay below 50 ohms and at leas 3-5 watts, depending on value. LIkely avoid wire wound resistors.

[Stevem]

If such low resistance helps out with oscillating then it’s done for the same reason such as when it’s done on the long leads that are sometimes needed to feed SS output stage transistors.

This low added resistance evens out the impedance across the 3 wires going to each transistor, so in short it compensates for poor design/ layout, but it does eat up power output to a small degree.

I have fixed many Southwest technical SS power amps in the mid 70s that would blow themselves up when cranked up enough due to this issue, but in terms of a tube output stage it’s used first as a 5 watt fuse,

[Colossal]

I would stay below 50 ohms and at leas 3-5 watts, depending on value. LIkely avoid wire wound resistors.

Avoid wirewounds due to inductance or low voltage handling or both?

[spidergawd]

Go and view the schematic for a Fender Bassman model AB165, is this how you mean these resistors are installed?

Ampeg SVT amps and Sunn 300 models use resistors in the plate circuit as blow out protectors so that if one tube shorts out the amp keeps playing for you, but if the amp only has two output tubes this set up is meaningless and I do not think it has anything to do with oscillating,

I have a Sunn Sonaro head and when fitted with good testing set of 6550s or KT88s it outputs the same 60 watts of clean power, however the KT88s add a bit more gain in the mix so the amp fitted that way clips the output stage sooner, but man are they more expensive then 6550s!!

Not like the AB 165. Those resistors are between the power tube plates and the plates of the PI. Exactly like the SVT where they are between the plates and the primary of the OT. Thanks for those examples, Sometimes Sunn's designs seem a little weird, good to know there's president for this decision.

[roberto]

Not like the AB 165. Those resistors are between the power tube plates and the plates of the PI.

That one is local feedback on the power tubes.

[roberto]

Avoid wirewounds due to inductance or low voltage handling or both?

Generally speaking, "stoppers" should always have lowest possible inductance, so carbon comp or carbon film.

[Dr Tony Balls]

I'm working on a red knob Sunn Model T for a friend. It has been hacked up at some point in it's life, probably more than once. He decided since I have to change so much, I might as well convert it to a first generation Model T. The wiring is pure point to point on terminal strips, Pretty awesome for an amp made in the late 70's and perfect for modding.

I've done this before! A guy brought me a 1st gen Model T that was hacked up to hell and I brought it back to normal. Here's what it looked like when I got it:



It's hard to tell from the photo but the mod wizard added SIX preamp tubes and some pots and switches which included a reverb stage, some cascaded gain stages, etc. Thankfully, the power section (everything post PI) was largely untouched. Didnt even attempt to power the amp on, I just gutted out the crap and rebuilt it proper, including terminal strip wiring. The gutted chassis:



and the rebuild:

[FUCHSAUDIO]

Avoid wirewounds due to inductance or low voltage handling or both?

Generally speaking, "stoppers" should always have lowest possible inductance, so carbon comp or carbon film.

Interestingly enough, the Sunn pics below used wirewound resistors....I'm not sure how potentially bad that might be, but FWIW I use flameproof MF. I don't think WW resistor's publish inductance figures. If the value is low (the Sunn uses 50 ohms) I would think the inductance shouldn't be too bad. Worth testing with and without them.

[pdf64]

I seem to recall seeing that some wirewound resistor types are available ’non inductively wound’, might even have been an upgrade option, ie such that the same range was available with a basic inductive winding, and also, for a small surcharge, with a non inductive winding.

[roberto]

Yes, bifilar or Ayrton–Perry windings can do that.

[spidergawd]

I've done this before! A guy brought me a 1st gen Model T that was hacked up to hell and I brought it back to normal. Here's what it looked like when I got it:

Holy moly! What was going on with those strip boards? Im surprised the 6.3v winding could handle that many extra tubes! You gotta think somebody knew they were doing something wrong when they had to use that many zipties.

Your rebuild is beautiful! Better than stock. Im surprised how quiet the one i'm working on is with the stock wiring. Non-twisted filament lines just mingling anywhere they please. It seems like Sunn just had luck on their side. My concert lead is like a work of madness.