Ideal bias current for Twin Reverb with 6L6GC's?

[animal69]

I just got 4 JJ 6L6GCs for my Twin Reverb. I've done biasing before. What bias current do I want for my 6L6GC power tubes with the Twin Rever? The Weber bias calculator*, at 445 measured plate volts, suggested 47 mA but my amp wont let me go above 45 mA. The Weber site assumes the power is 30W for the tube. Since the Twin reverb is rated at 85W, do I want to divide 85 watts / 4 = 21.25 W for the wattage for each tube, divide 21.25 number by 445 and then multiply by .7? This gives me 33mA.

 

http://www.webervst.com/tubes/calcbias.htm

[Mr. Awesome]

That seems high for me, try this approach.

 

1. Lower the valuue to about 28ma, then from there raise it a little, maybe 2ma play it for a while and see how you like it. Raise it another 2ma to 32ma and play it again for a bit. Try to keep it below 36ma.

 

For me when biasing amps every amp I have ever used gets to a point where it starts to thicken up sound wise, I almost with out fail like the amp set slightly below this point. This is usually middle to slightly cool for the tubes, biasing tubes to the point where they almost red plating is a dumb ass approach IMHO and does nothing to help your tone.

[Casey4s]

I just got 4 JJ 6L6GCs for my Twin Reverb. I've done biasing before. What bias current do I want for my 6L6GC power tubes with the Twin Rever? The Weber bias calculator*, at 445 measured plate volts, suggested 47 mA but my amp wont let me go above 45 mA. The Weber site assumes the power is 30W for the tube. Since the Twin reverb is rated at 85W, do I want to divide 85 watts / 4 = 21.25 W for the wattage for each tube, divide 21.25 number by 445 and then multiply by .7? This gives me 33mA. http://www.webervst.com/tubes/calcbias.htm

 

If you bias at 65% I get 43ma (43.8) per tube with a PV of 445. You could go a little cooler but I think you'll like it better at 65%.

 

[Divide the 30W for the 6L6GC by the 445 PV, then multiply by .65 to get the 43.8.]

[guitarcapo]

30 watts per tube x4=120watts. The twin reverb is NOT a 120 watt amp and there's your problem.

 

445V times .047 amps is 21 watts times four tubes is 85 watts. If you have the current at 47 mA then you have the bias set at 100% dissipation at idle. Way too high.. You want to set the bias at 65% to 70% of that figure....Maybe 31mA or there about.

[Casey4s]

30 watts per tube x4=120watts. The twin reverb is NOT a 120 watt amp and there's your problem. 445V times .047 amps is 21 watts times four tubes is 85 watts. If you have the current at 47 mA then you have the bias set at 100% dissipation at idle. Way too high.. You want to set the bias at 65% to 70% of that figure....Maybe 31mA or there about.

 

120 X .70 = 84, which is about what that amp is rated at.

 

OR

 

30 X .70 = 21, for one tube.

 

31 MA in this case is only about 47%

 

Nobody has claimed that this amp is a 120 watt amp, you must have misread something. No AB PP amp is rated at the actual addition of it's power tube maximum disipation, a 2xEL34 amp at 70% is actually a 35 watt amp not the 50 watts of the two 25 watt EL34 tubes.

[guitarcapo]

I think your math is off. The amp's maximum output is 85 watts. You calculate the 70% off of that. You set the idle dissipation at 70% of 85 watts...not 70% of 120 watts.

[Casey4s]

I think your math is off. The amp's maximum output is 85 watts. You calculate the 70% off of that.

 

Your math is off Sir because 70% of the total of the four 6L6GC's IS 84 (85) watts. You are getting 70% of 70%. The amp is rated at at 85 watts because that IS what 70% of four 6L6GCs comes to.

[Casey4s]

35-40 ma per tube. have a nice day.

 

40 ma is only about 60% of the max in this instance which is fine but a little cool for my taste. If this were my am I would probably bias at 65% or maybe 70%, but it's all a matter of personal taste as to how the amp sounds to the owner and how much longevity they want to get from the tubes.

[Casey4s]

I don't bias with a calulator. I use a scope and I measure idle current after that. Published tube ratings are a "safe" zone that the manufactures used to prevent premature failures. It's not real world. The calculator method sorry to say is for the DYI nOObs, you must make sure you figure in screen dissipation too if you're going to try this:idea: Also, one more tid bit; Make sure you're measuring unloaded voltage if you use a calculator, that means pull your tubes before you do anything. Any easier way, and if you really want to set it up as the manufacturer/fender did, look at a schematic, it shows the loaded voltages for the output stage. Bias it until the voltage matches the drawing specs. Then go back and see how much current you're drawing. You might be surprized.

 

 

 

Don't get defensive Doug, I did not say you were wrong or misinformed. I have a Tektronix scope here and most of the time it has been my personal expierience when biasing with a scope to the crossover, the actual measured ma is pretty close to the math. I was under the impression that the OP was not using a scope so I used the recommended math from an RCA manual to recommend a ball park figure for him. Most of the DIY n00bs don't have a scope or even know how to use one so the matamatical calculation works well enough most of the time and was the standard method before there were scopes in common use. Don't be too hard on the DIY n00bs Doug, they are trying to learn somethihng.

[animal69]

That seems high for me, try this approach. 1. Lower the valuue to about 28ma, then from there raise it a little, maybe 2ma play it for a while and see how you like it. Raise it another 2ma to 32ma and play it again for a bit. Try to keep it below 36ma. For me when biasing amps every amp I have ever used gets to a point where it starts to thicken up sound wise, I almost with out fail like the amp set slightly below this point. This is usually middle to slightly cool for the tubes, biasing tubes to the point where they almost red plating is a dumb ass approach IMHO and does nothing to help your tone.

 

Yes this is what I did. I get my dirt from a great overdrive pedal so I don't like the idea of my Twin breaking up at higher volumes, I like the chimey clean sound. I set it at about 29mA. This gives me a nice clean sound up till about 5. When the volume is past 5 it doesn't make a difference anyways, the current jumps all over while playing (up to 80mA). So I think biasing cool really gives you more options since the current will run off and do its own thing when the volume is cranked.

[guitarcapo]

So you suggest 43.8 ma when the amp tops out (won't let the guy bias any higher) than 45? And you think this is 65%?

 

 

Your advice is "off". Have a nice day.

[Camaromike1984]

I know this is an old thread but, i just had to say. what the amps bias tops out at has nothing to do how high the bias is actually set. what i mean is you could have the bias pot maxed out and still be running too cool. it depends on how the tubes are running and if anyone has messed around with resistor values.

[Bob Kerwin]

I've had my TRRI since 2008. (Have had nothing but Fender tube amps since 1965). I change all tubes annually, hell or high water, even though they do not need it. At about 450 volts I am able to easily bias at 38. That's fine and dandy for me. Leo designed the TR to stay clean at all volume levels. The output transformer would blow grits if one were to bias the amp up to a level where the tubes would be dissipating 30 watts each. It can't handle (OP tranny) the full wattage of 6L6's. But it also keeps the Twin the cleanest amp money can buy, which is why someone buys a Twin. Loud and clean. It still holds its own with a 100 watt Marshall. You know, local bars and night clubs are way too small these days for a Twin or similar volume amp that is way too loud by the time it hits its sweet spot.

[Mikeo]

I have one of the last Fender Twin's that are rated at 135 watts.

I bought it new for $125.00 and the the trade of my Musicman HD 65-210 combo.

I think the year  was 1982 or 1983.

 

 

Edited February 6, 2022 by Mikeo