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Ty_Bower wrote:

hilldweller wrote:Screen resistor to protect the $$$ KT-88s
Eliminates screen instabilities. I found this somewhere on this forum. Yellow_Light_Colorz_PDT_12

I think this might be the one. What size did you use? 100 ohm?
http://www.diytube.com/phpBB2/viewtopic.php?t=1455

Yes I used 100 Ohm. I figured that if I used a higher rating, I would sacrifice some output power. The 100 ohm resistors work out real nice, I have had no problems.

I wanted to find some 100 ohm, 1 watt, carbon composition resistors. Mouser wants $2 each for the things, and they're not even in stock.

I wonder if I would be better off substituting for a 1/2 watt carbon comp, or a 2 watt carbon film instead?

Where'd you get your Panasonic X-Y caps?

Ty_Bower wrote:I wanted to find some 100 ohm, 1 watt, carbon composition resistors. Mouser wants $2 each for the things, and they're not even in stock.

I wonder if I would be better off substituting for a 1/2 watt carbon comp, or a 2 watt carbon film instead?

Where'd you get your Panasonic X-Y caps?

The caps came from mouser P/N 75-125LS10-R Vishay/Sprague .01 mfd @ 250v
The resistors: Radio Shack P/N 271-152 100 ohm @ 1w Metal Oxide 5%

Here are some 2 watt CC resistors that will work nicely. 150 ohms is a sufficient value for this position. I find it interesting that anybody would be concerned about losing power when they're triode strapping an amplifier, cause that's going to happen bigtime regardless of which value you use for the resistor.

http://www.tubedepot.com/r-cc-2-nos.html

-Paul

Paul,
I don't think we're talking about triode strapping the amp. The resistors in question are in series between the screen grids and the UL taps on the output transformers. The idea came from an article at AudioXpress by David C. Gillespie. He writes that upgraded power supplies might create a situation where "screen instability" can occur, resulting in arcing and damage to the screen grids.

As far as the caps on the line cord are concerned, I thought the Roger W. Stevens article prescribed 2KV rated caps. I ordered some from Mouser, but I didn't pay close attention to what I ordered. I ended up getting #594-S103M69Z5U283L0R. They've got these funny hats on top with a little notch cut in them. I think they're supposed to arc over if you massively overvolt the things. I'm guessing they will still be OK for my purposes.

The official name is "spark gap", and you nailed it.

Screen grid-stopper resistors are used to
tame 'parasitic oscillations' due to the
flow of electrons.

Do not use carbon film resistors. They
can catch on fire.

Some recommend paralleling 2W CC
resistors to achieve at least a 10W
rating. But, these applications use
resistences in 5-10k range, not 100ohm.

I think I've touched on this in previous threads, but . . .

"Screen grid stopper" is a misnomer. The screen grid is either working in phase with the plate of an output tube (UL or Williamson operation), or it's an independently powered accelerator having no signal imposed on it (pentode operation).

A control grid stopper, on the other hand, is intended to eliminate parasitic oscillation, and is usually valued between 1K and 10KΩ. ½ watt carbon comp is the construction of choice (although I want to challenge that sometime in the future), and the resistor needs to be located as close to the control grid as possible.

The screen grid resistor would be more accurately called a ballast resistor, in that its intention is to equalize G2 current between tubes. It also lowers the potential on the screens to be below the plate voltage in UL operation. Anyone who's witnessed brightly glowing screens can appreciate this addition. The usual value for a ballast resistor is between 33 and 220Ω, but YMMV.

Here are some 2 watt CC resistors that will work nicely. 150 ohms is a sufficient value for this position. I find it interesting that anybody would be concerned about losing power when they're triode strapping an amplifier, cause that's going to happen bigtime regardless of which value you use for the resistor

I have been triode strapping my amps with pentodes or tetrodes for years using a uf4007 diode and a 100 to 250 ohm 1/2 watt resistor in series per tube and I can attest that you don't lose power and it sounds significantly better IMHO to Pentode or UL operation.

http://www.audioasylum.com/forums/DIYHiFi/messages/6928.html