Preferred power tube voltages?

[mdroberts1243]

EDIT: Less relevant now that I've figured out I can't do anything about the plate & screen voltages short of replacing my transformer or bolting the variac into the cabinet.

-mark



In my BM 100W build I've used a large 1st dropping resistor (3.4k). I haven't seen much discussion about what the preferred operating point is for the plates and screens on the four 6L6s... I notice everybody else using a lower dropping resistor (2.2 to 3k) and getting higher B1 and B2 than me.

Anybody care to offer an opinion on the ideal? Is there a similar sweet spot for the power tubes as there is for the preamp?

My current voltages: (at 116.6VAC)
B1 409
B2 408
B3 381
B4 306
B5 298

V1a 183 / 1.66
V1b 190 / 1.56
V2a 198 / 1.59
V2b 204 / 1.47
V3 high 237
V3 low 231
V3k 39

Power tubes are biased for about 17W static power dissipation right now.
Thanks,

[mdroberts1243]

Well, at the risk of talking to myself... I'm using TAD 6L6WGC-STR (30W) tubes right now and I've seen some mention on the web to keep the voltage under 425 and close to 400. I've also seen mention that the old GE tubes had a sweet spot around 19W static power dissipation. 19W would be 63%.

At 120V (only with a variac, thanks to my local hydro) the plates have 417V on them and the SPD is ~19W... at my nominal 114-116VAC it's closer to 400V and 17W.

[talbany]

Sweet spot for me has been 430-450 plates TAD 6L's For El34's aim for 470v-490v
For the preamp v1a get as close to 200v as possible v1b 205- 210 OD v2a 225-230 v2b 230-235.
If you set the string correctly you shoud start with 200v then increase about 10volts on up the chain. Don't forget to add the 150k dropping string to simulate the FET. Also adds character.

[mdroberts1243]

Sweet spot for me has been 430-450 plates TAD 6L's For El34's aim for 470v-490v
For the preamp v1a get as close to 200v as possible v1b 205- 210 OD v2a 225-230 v2b 230-235.
If you set the string correctly you shoud start with 200v then increase about 10volts on up the chain. Don't forget to add the 150k dropping string to simulate the FET. Also adds character.

Thanks Talbany,

I've given up on changing the plates, but I do have control over the PI voltages... any opinion in that area? Right now I have in the 230s to low 240s and with the 'virtual' ground in the high thirties I guess the PI tube thinks it is running around 200V too.

[talbany]

That sounds a little low to me .My sweet spot for the PI is around 280 on non feedback side and 10 volts higher on the feedback side.

[dogears]

If you have the BM PI, it is really hard to get those voltages, unless you have 480V B+ or so....

The BM I had here had 245-255 on the PI.

That sounds a little low to me .My sweet spot for the PI is around 280 on non feedback side and 10 volts higher on the feedback side.

[mdroberts1243]

If you have the BM PI, it is really hard to get those voltages, unless you have 480V B+ or so....

The BM I had here had 245-255 on the PI.

That sounds a little low to me .My sweet spot for the PI is around 280 on non feedback side and 10 volts higher on the feedback side.

Thanks Guys,
This gets to the heart of the matter... right now I'm at 3.4k for the 1st dropping resistor, I could raise the PI voltages if I lower that... with the 3.4k the PI has 244V, 235V and 40.4V on the cathodes at 120VAC line.
I think the ~200V is probably enough headroom for the PI, but wondered if there are any other benefits lurking with a higher operating voltage traded off against a lower 1st dropping resistor.

[talbany]

Voltages on the BM will be a little lower due to the lower value cathode resistor 511 and lower tail 9k. As well as a differant negative feedback circuit. More gain but less headroom

[kleinm]

I know its heresy, and I'll probably get booed off the forum, but you could increase your FET-simulating resistor to get more voltage to all of your B+ nodes. My PT is rated at 320-0-320V, like your 290FX. I have a 220K FET simulating resistor (dodges beer bottles) and I get 440VDC B+1 on the plates of my TAD 6L6WGCs.

I've tweaked my dropping string to 1K/18K/2K2, and with this power transformer the voltages on V1, V2, V3, and the 6L6s are right on with what's frequently recommended. This is also with a 330R choke replacement resistor.

I hope this helps. I'm going to wipe the blood and beer off my face from all the thrown projectiles....

[groovtubin]

I know its heresy, and I'll probably get booed off the forum, but you could increase your FET-simulating resistor to get more voltage to all of your B+ nodes. My PT is rated at 320-0-320V, like your 290FX. I have a 220K FET simulating resistor (dodges beer bottles) and I get 440VDC B+1 on the plates of my TAD 6L6WGCs.

I've tweaked my dropping string to 1K/18K/2K2, and with this power transformer the voltages on V1, V2, V3, and the 6L6s are right on with what's frequently recommended. This is also with a 330R choke replacement resistor.

I hope this helps. I'm going to wipe the blood and beer off my face from all the thrown projectiles....

I for one would not do that, the forum is for INDIVIDUALITY, for those who think NOT, i`d say there`s a nice COLD room in Siberia wating on you! lol!

jim