Bias adjustment SFSR

[pbabadoo]

Embarrassed to have to ask this, but I have forgotten how to adjust the bias on my '74 SF Super Reverb (AA270 schematic). I got lazy and just kept using similar Groove Tubes sets.

I think that I used to attach some extra resistors (maybe 1 ohm?) to the power tubes and read voltages someplace. Can anyone fill me in on an adequate procedure to set bias? Don't have a scope.

Thanks

[MWJB]

Yes add a 1ohm 2W resistor between pin 8 of the power tube sockets & ground, read idle voltage accross these resistors in <200mV dc.

To actually adjust bias, rather than balance it - either rewire the balance pot as per AB763, or replace the grounded resistor on the balance pot with a small cermet trim pot , wired as a variable resistor (gives you "adjust" & "balance" facilities).

[MWJB]

...oh, and you don't need a scope to bias, set idle current to 33mV per tube.

[pbabadoo]

No wonder it was running hot - bias was reading about 58mV! I added the trim pot as suggested and was able to get it set right at 33mV and balanced nicely.

Many thanks!

[pbabadoo]

Can I leave the 1 ohm 2W test resistors in the amp, or should they be installed for biasing, then removed?

[bnwitt]

Leave them in there. Also, there is a little more to biasing than was mentioned. The milliamps of the power tube cathodes is just a part of proper biasing. Read this article and pay attention to the cathode resistor method:

Bias FAQ

While you were indeed reading millivolts across the 1 ohm resistors, because the resistors are 1 ohms it equates to milliamps through the cathode. You use the milliamps times the plate voltage to determine the dissipation wattage of the tube. Plate dissipation is what you are trying to set and you have to take screen current out of the equation to really get that. If your BFSR is typical, your plate voltage is about 460VDC and a 70% idle current would be around 45 milliamps. 33 milliamps is really cold. See attached chart for 6L6GC power tubes.

[MWJB]

"While you were indeed reading millivolts across the 1 ohm resistors, because the resistors are 1 ohms it equates to milliamps through the cathode. You use the milliamps times the plate voltage to determine the dissipation wattage of the tube. Plate dissipation is what you are trying to set and you have to take screen current out of the equation to really get that. If your BFSR is typical, your plate voltage is about 460VDC and a 70% idle current would be around 45 milliamps. 33 milliamps is really cold. See attached chart for 6L6GC power tubes" This is simply untrue. Plate voltage can be anything from just over 400vdc to 525vdc, I suggest 33mA because that will handle either scenario happily...a typical 2x6L6 fixed bias Fender usually sounds right around 12-15W per tube. If you want to run 45mA per tube you had better make sure that you have some sturdy tubes in there, like JJ, as 45mA will kill a lot of 6L6 style tubes in a high voltage SR. Most currently produced 6L6 tubes should be treated as 25W tubes, idle plate current around 17.5W per tube tops.

[bnwitt]

Originally Posted by MWJB

"This is simply untrue. Plate voltage can be anything from just over 400vdc to 525vdc, I suggest 33mA because that will handle either scenario happily...a typical 2x6L6 fixed bias Fender usually sounds right around 12-15W per tube. If you want to run 45mA per tube you had better make sure that you have some sturdy tubes in there, like JJ, as 45mA will kill a lot of 6L6 style tubes in a high voltage SR. Most currently produced 6L6 tubes should be treated as 25W tubes, idle plate current around 17.5W per tube tops.

Winged C 6L6GC = 30 watts JJ 6L6GC =30 watts and most others on the market as well. I don't know where you are getting the 25 watt thing. I did miss the fact that he has a silverface model, so the voltage is higher as you say. The bottom line and heart of my post is that he needs to check the plate voltage as well as the milliamps through the cathode rather than just setting his idle current to some arbitrary setpoint. I want to teach him to fish not give him a fish.

[MWJB]

"I don't know where you are getting the 25 watt thing." Bitter experience...SED say 30W on the data sheet, but they won't take it. At the end of the day biasing to 21W will be too much for most current production tubes...even if they can take it, the amp won't sound particularly Fendery - though, that doesn't necesarily mean that you won't like it. 33mA is not an arbitrary point...it was considered advice.

[bnwitt]

I've been biasing Winged C and JJ's to 70% of 30 watts for years and never had a problem with tube life or tone. I'm on the west coast and about 60% of my service work is on Fenders. I have also built over a hundred Fender clones over the last 13 years and nary a problem with this. By arbitrary I meant that without the plate voltage taken and the math done, you're not hitting the mark. The only 6L6 amps I will bias as cold as you suggest are the ones with bias vary tremolo and that is only for improved tremolo effect. Obviously your experiences have been different but I don't know many techs who would argue the point that doing the math isn't better than selecting one milliamp setting for all silverface amps which you seem to be advocating. To each his own I guess.

[MWJB]

You're right that it is best to do the math & cross reference plate voltage against current, but in reality many guys will use probes on an amp with an external bias adjust pot, many probes don't allow you to read loaded plate voltage (a variant of the Weber bias rite does)...hence I aimed to provide a real world scenario, more for others looking for bias advice, as much as for the OP.

I have had several winged Cs let go at rather less than 30W dissipation in cathode biased amps, never felt inclined to push their limits in fixed bias amps...if the tone is great at reasonable current (28mA+) then that might do, in certain circumstances I might push nearer to 40 depending on plate voltage/client requirements.

FWIW a tube vendor I spoke to who sells & bench tests thousands of Winged Cs suggests that even 25W max dissipation may be a little on the optimistic side & that the classic 5881 rating of 23W may even apply.

[bnwitt]

That's really interesting. I wonder why your mileage on winged C's is so vastly different than mine. I have found them to even hold up well to 80% biasing (of 30 watts) in metal head amplifiers. Not something I like to do but have done with the player present getting his tone while I bias. In the last 10 years I have only had one Winged C 6L6GC fail prematurely. They are always my first choice for current production with JJ's second.

Of course I understand the probe thing, but the original poster had the amp open and added 1 ohm resistors so I figured he was already in there.

What cathode biased amps with 6L6's are you talking about? What kind of screen voltages did you have? Just curious, and I don't mean to hijack the thread.

[MWJB]

"What cathode biased amps with 6L6's are you talking about? What kind of screen voltages did you have? Just curious, and I don't mean to hijack the thread." Off the top of my head...5B5 tweed Pro w/330ohm cathode resistor, mid 50's mA at 400vdc(corrected), Electar Tube 10 70mA @ a shade over 400vdc (corrected)...

Didn't record screen voltage, but the Pro had a 10K dropping resistor between plates & screens, so must have been around 30-40v under the plates.

Conversely, I have run them at 40mA @ 470vdc in one of my own amps (not a tolex Fender) & they seem to hold up fine.

[pbabadoo]

Very educational argument, guys. Thanks for all the info - it is much appreciated. What I do know is that the amp ran way too hot and was noisy when biased at 58mV across the resistors (no kiddin', right). That's probably due to drift over the years, the current set of tubes, and the solid state plug-in rectifier. I'll get back in there, check the cathode volts and maybe adjust things on that basis. By the way, I'm using Electro-Harmonics 6L6's.

[bnwitt]

Well 58ma is way too hot regardless of whether you target 25 watts or 30 watts for the tubes if the plate voltage is close to 500VDC. Just so you know, the plate voltage is measured on pin 3 of the octal socket. I've attached a jpg of the numbering of an octal socket looking at it from tube side and wire side. It doesn't hurt to put electrical tape on your meter leads all the way just short of their tips to prevent shorting of socket pins when you are checking voltages. It's pretty tight in there and shorting pin 3 to pin 2 is a bad thing. Also, I've attached a couple of bias charts with extended voltage ranges. One for 25 watts and one for 30 watts on 6L6's. Go with what you want. I will say that once you get into the 70% zone, you should go a little up and a little down from there while plugged in and playing, to see what setting floats your tonal boat. Then see where that leaves you wattage wise. If your voltage was 520VDC you'd have had 30 watts (0.058*520) or 100% at idle. That's too much for sure.