65 Super Reverb Reissue Plate Voltage/biasing

[old guy]

Greetings my old friends, I've come to pick your brains again ('cause I lost mine playing clubs for too long).
I just picked up my last Super Reverb - a brand new 2017. It's the fourth one in 4 decades ... kinda like 'em
Taking it out of the box, I stuck my bias probes in 'er and set about kickin' 'er up to where I need her to be ... 40 - 42 mA. My starting factory plate voltage was 457dcv and bias came out at 35-36.

My question is; "what is of most importance when biasing this amp - the plate voltage or the bias #???? As you know, dropping the PV raises the bias setting. To achieve my 'perfect PV of 450, it cranks the bias to 46 - 47 which, is too distorted for my musical tastes - great for ACDC covers but not my Larry Carlton stuff.

What, do you think is a good, happy medium setting that won't hinder performance or hurt anything?

[Chuck H]

Well, If you have a very specific plate voltage AND a very specific bias current in mind, you might be in trouble. Because unless this new amp has exactly the same PT specs and power tubes as your last one one or the other (bias current or plate voltage) will be different.

My advice is to dial in the bias by ear and then check to be sure it's within safe parameters. Start by setting up for the same current your last amp had and play for awhile at all settings you normally use. Then move the bias a little hotter and repeat. Then move the bias a little cooler and repeat. Get a feel for which direction floats your boat. When you think it sounds/feels right check current to make sure it's within a reasonable range for the tubes. Done.

EDIT: As to which one is more important, voltage or current.?. Both The correct current is dependent on the specific voltage. Voltage should not be "adjusted" with current draw, though, as you have observed, it IS affected by it. So I'll have to qualify current as the deciding factor. You can figure the idle watt dissipation by multiplying the plate voltage by the current. So... 450 volts times .047A = 21.15 watts! Pretty damned hot if that's tested on a single tube. The factory setting of .036A @ 457V would be about 17 watts.

If I can assume your targets as having been derived from you previous amp then .041A @ 450V would be 18.4 watts. So I would say target about 40mA of current for starters and see how it sounds. If your plate voltage is higher than your old amp by 5V it's honestly NBD. A much larger tonal difference could be heard in the different tubes, newer speakers, circuit component tolerance, etc.

[old guy]

Well, If you have a very specific plate voltage AND a very specific bias current in mind, you might be in trouble. Because unless this new amp has exactly the same PT specs and power tubes as your last one one or the other (bias current or plate voltage) will be different.

My advice is to dial in the bias by ear and then check to be sure it's within safe parameters. Start by setting up for the same current your last amp had and play for awhile at all settings you normally use. Then move the bias a little hotter and repeat. Then move the bias a little cooler and repeat. Get a feel for which direction floats your boat. When you think it sounds/feels right check current to make sure it's within a reasonable range for the tubes. Done.

Thanks Chuck,

Waddaya think ... +/- 5% is acceptable? All info points to plate voltage being 450 for 6L6 tubes. Yupp, I'll do the 'ears' thing all day. Thanks!

[Chuck H]

You may have responded about the time I was adding the edit. I think you'll be happy with about 40mA. Shoot for a current*voltage of 18.4 watts +/- and see how it sounds. This would be the same tube dissipation as your last amp.

[Enzo]

We seem to have two threads going on this...

Remember, these are guitar amps, not NASA laboratory equipment. There is nothing precision here. %5? Old Fender schematics had a note that readings should be taken with +/-20%.

[Enzo]

We seem to have two threads going on this...

Remember, these are guitar amps, not NASA laboratory equipment. There is nothing precision here. %5? Old Fender schematics had a note that readings should be taken with +/-20%.

[pdf64]

...All info points to plate voltage being 450 for 6L6 tubes...

It's a good idea to cite other sources of info that you are referencing; it may be that you've mis-understood them, or that they are nonsense.

[Chuck H]

We seem to have two threads going on this...

Remember, these are guitar amps, not NASA laboratory equipment. There is nothing precision here. %5? Old Fender schematics had a note that readings should be taken with +/-20%.

We seem to have two threads going on this...

Remember, these are guitar amps, not NASA laboratory equipment. There is nothing precision here. %5? Old Fender schematics had a note that readings should be taken with +/-20%.

And it seems we have two posts bringing that to our attention

[nevetslab]

And, of course, there's the tubes themselves. I've never sonically checked this out myself, but, with the same mfgr of tubes, two sets that differ a lot in their transconductance, and hence different bias voltage to obtain the same plate current.....will they sound the same?

I too have noticed recently that Enzo's replys have been in identical pairs. Thought it was my eyes at first.

[bob p]

I too have noticed recently that Enzo's replys have been in identical pairs. Thought it was my eyes at first.

No problem, Enzo is just operating in push-pull mode.

[g1]

Bias is never used to set the plate voltage. The effect the bias has on the voltage is a by-product and not an intended consequence.
So you are kind of putting the cart before the horse there. The voltage is only really considered when you want to calculate the idle dissipation in watts.
According to the schematic, the factory bias is 33mA.

[Enzo]

It isn't me. I see the doubling on some other posters as well.

[Chuck H]

It isn't me. I see the doubling on some other posters as well.

I know.

[Chuck H]

It isn't me. I see the doubling on some other posters as well.

I know.

[bob p]