Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by bluetweed, Apr 5, 2008.
Give me some advice!
What does "best" mean? That's a rhetorical question.
Low 20's is the proper bias range for 6V6s in a DR.
(for tone Mike) I know where it should be by the numbers, etc./ for example
Pure 64's 6V6 amp has a high plate voltage like a deluxe reverb, but he likes to run his amp at no lower than 30ma's/ The amp sounds fantastic there!
Then use your ears. There's no consensus on what sounds best.
And don't call your tube dealer to complain about short tube life if you violate the max specs for the tubes
where do people usually set deluxes at?
The people I know use low 20s.
Maybe do a forum poll and find out. I don't know if anyone's done one on that question. But of course, Mike is right about (a) over spec will shorten tube life, and (b) "best" tone is subjective - dependent upon the listener, what you prefer.
The thing that bugs me is, What are the Max Specs for a 6V6?
I figure the majority of 6V6 based amps are running new production tubes. But accurate tube design specs are not easy to come by.
A good number of vintage amps run well beyond original data sheets for decades, But what are the actual limits?
For instance, a PP amp with JJ 6V6S running as follows,
460V Plates, 415 Screens, biased @ 18ma
It sure sounds great, loads of cleans, excellent breakup. But how long?
Now drop in an EH 6V6, or a Tung-sol, or a Sovtek, could they run in this environment?
Now drop in NOS 6V6GTA's
I can get basic specs on JJ's, but the other new production specs appear to not be published?
I like mine at 24ma per tube.
JJ's don't really qualify as 6V6s so use whatever spec is published for them.
For real 6V6GT's, the max power dissipation is widely known (Google "6V6GT data") as 12watts. For 6V6GTAs 14 watts. Using no higher than 70% of idle dissipation for a ~400Vplate amp puts you at about 20mA.
Many original Deluxes were biased pretty hot at 30ma and up.Tone is pretty subjective,so set it where it makes you happy and don't complain if it eats power tubes.
I have mine at 26, I prefer the tone at 28 BUT don't want to eat up the tubes any faster than I already am. It's just a preference, so try it at different levels starting at, say, 19ma.
I have my DRRI at 27 through JJs
IME, the majority of 'original' designs ran their Plate currents very low. This is especially true in the 6L6 amps.....rarely over 50% plate power.
As Mike was indicating, some 6V6's are spec'ed at max 12 watts, some at 14 watts. Then you do the calculations from there (max_power * 70% = bias_current X plate_voltage). Your JJ 6V6 example is suitable for a 12 watt rating (the calculation gives me 11.8 watts). Note there are three variables, so just knowing the bias current setting doesn't tell you whether you're exceeding spec.
I think old amps got away with running above spec because the tubes in the old days were pretty tough.
Most SFDR's I've seen end up with about 450 volts on the plates with new filters and the bias at around 60%. My line voltage here is usually around 120-123. None of the vintage amps were intended to be run at todays line voltage. Somehow they hang in there.
On my SFDR, I'm at 24.something with a plate voltage of about 400 (power tubes are JAN Phillips). I've tried it down to about 18 and up to about 28 but where I'm at sounds best to me.
I tweaked the bias on my '71 SFDR by ear until I found my "sweet spot". I measured the plate current as .022/.023 on the Marconi 6v6gt's that I got from Mike K (thank you, btw). OK, with 440v on the plate I calculate I'm running at around 84%, right? I hope this isn't too hot, because a notch below this & the amp really thins out. And a notch above - the amp just gets dirty too fast. To my ears, this is really the optimum setting.
Mil grade NOS, you're probably OK.
Hotter bias = earlier breakup? If you dont want that then you dont got to think about it? I am looking for less to think about...