Cathode resistor value for SE 6L6/EL34/6550

[Steve Conner]

I find the TNT suppositories do a better job than any tablets. Get thee hence, spammers!

[jmaf]

I find the TNT suppositories do a better job than any tablets. Get thee hence, spammers!

Right on. I read a review of the new 3/4" TNT-S WXT+ suppositories on the Spammer Digest, sales may increase 200% using them.

[Steve A.]

I appreciate the humor but I'm thinking of deleting all of the responses to the spammer (and the responses to the responses- including this one!) after maybe a week. The reasoning being that if someone is searching for information in the old posts here maybe 5 years from now do you think that they really want to wade through responses to spammers?

Any thoughts on this?

Thanks!

Steve Ahola

[Gingertube]

Steve,
There was a BIG hint in your last post above.
Max Rg1 for 6V6 and 6L6 in cathode bias is 500K (100K for fixed bias) whereas it is 1M max for EL84 (300K if fixed bias). That is because of the amount of "normal" grid current these tubes exhibit.
I'm not sure which schematic is relevant - note that on the Dark Star schematic you posted above there is no "physical" Rg1 and the actual Rg1 value (resistance from grid1 back to 0V) depends upon the setting of tone stack pots - that is a "dodgy" way to do things.
Cheers,
Ian

[Steve A.]

Steve,
There was a BIG hint in your last post above.
Max Rg1 for 6V6 and 6L6 in cathode bias is 500K (100K for fixed bias) whereas it is 1M max for EL84 (300K if fixed bias). That is because of the amount of "normal" grid current these tubes exhibit.
I'm not sure which schematic is relevant - note that on the Dark Star schematic you posted above there is no "physical" Rg1 and the actual Rg1 value (resistance from grid1 back to 0V) depends upon the setting of tone stack pots - that is a "dodgy" way to do things.
Cheers,
Ian

Crap! There was a 220k resistor to ground on the PicoValve circuit but it seemed to work fine without it so I left it off, thinking that it was like the tone stack was feeding a PI and not a power tube. I will add it back and recheck the voltages.

Thank you, thank you, thank you!

Steve

[Steve A.]

Thanks, Ian! It was the missing grid load.

I soldered back in the 220k resistor right after the cap that was between the MV and the grid stopper. Voila! I got some readings with a 6L6GC that don't look abnormal:

332vdc plate
326vdc screen (3k3) I + 2mA
25vdc on cathode (470R) I = 53mA

Amazing what a big difference that one little grid load resistor made! (With the cap between the 250kA MV and the grid we lost that load- even with the volume turned to 0.

Funny thing is that on the stock PCB the 2nd stage was missing a grid load resistor shown on the schematic which I caught and corrected. But I did not notice that same problem in how I wired up the output section. My bad!

Thanks again!

Steve Ahola

P.S. It was too late to crank up the amp at all but it did sound okay at lower volumes.

[kleuck]

Cool, but your 6L6 is cold-biased now
Try a 250 ohms cathode resistor.

[Steve A.]

Cool, but your 6L6 is cold-biased now

So you don't want the bias cold in the wintertime?

Try a 250 ohms cathode resistor.

What sort of current should I look for across the cathode resistor with a B+ of 332vdc? (Like a minimum and a maximum value)

Thanks

Steve

[jmaf]

So you don't want the bias cold in the wintertime?


What sort of current should I look for across the cathode resistor with a B+ of 332vdc? (Like a minimum and a maximum value)

Thanks

Steve

You'll want the tube biased at halfway between 0 and cutoff. You should see cutoff at around -50 V give or take, so you want it at around -25 VDC.

25/250 = 100mA

Pumping out 2.5 Watts idle just on that resistor....class A eats trees doesn't it...

[kleuck]

Looks too hot imo, i would be looking for a total cathode current around 60 mA, hence a 300 ohms resistor (according to the GE 6L6GC datasheet)

Edit : well their examples are conservative actually (they are just the same as for the 6L6GB), jmaf is right, though 100% of dissipation is still hot, 90% would be safer.

[tubeswell]

I appreciate the humor but I'm thinking of deleting all of the responses to the spammer (and the responses to the responses- including this one!) after maybe a week. The reasoning being that if someone is searching for information in the old posts here maybe 5 years from now do you think that they really want to wade through responses to spammers?

Any thoughts on this?

Thanks!

Steve Ahola

nuke'em

[Steve A.]

Sounding great- here are the latest measurements (the current current?)

10pF cap from plate to grid of V1a/b (paralleled but with split cathode)
1k/5W screen resistor
270R cathode resistor (330uF bypass cap)

313vdc plate
310vdc screen 3mA
20.1vdc cathode 74mA 22.223

So would that be 20.8 watts?
W=E*I
= (313-20.1vdc) x (74-3mA)
= 293vdc x .071A

V1 B+ = 220vdc --> 56k --> 140vdc

V2 B+ = 244 --> 100k --> 143vdc (V2a plate/V2b grid and cathode)

Damn I forgot to measure the voltage at the cathodes!

A big thanks to everybody for all of their help on this!

Steve

[Gingertube]

Yup - that would be 20.8 Watts Anode dissipation - a conservative setting for a 6L6

You could drop the cathode bias resistor a little more BUT if you want to swap a 6V6 in, then I would check that before changing anything. For a 6V6 you want to keep anode dissipation to around 12 Watts. The 270 Ohms may be to low for a 6V6 already. One sure way to tell, plug one in a see what happens.

I'm messing about with cathode bias resistors on the current restoration project. An old PA Amp running Push Pull, 807 with 600V on the anodes and 300V on the screens. I "acquired" a box of about 30 off loose 807 from a local "audio nutter". Put them all through my AVO MK3 - more than half of them were NBG due to excess grid current or low cathode to heater resistance.

Happy that it is comming together for you.

Cheers,
Ian

[kleuck]

Yup - that would be 20.8 Watts Anode dissipation - a conservative setting for a 6L6

For a 6L6GC only, that's already too hot for a GT.
Steve, you can use a swith to choose between two resistors (or use them in //) like i did in the SP6, to be able to use 6V6 and EL84 too.
You can go to 14 watts with 6V6, probably with a 390 ohms resistor here.