5F6A Bassman voltage check

[Chuck H]

Is this using the 710V tap on both amps? Or are you saying you're getting acceptable tonal results from either voltage tap? Because you can play around with a hotter bias at the lower voltage. Bias doesn't have to be a "one size fits all" adjustment. So maybe 35mA at the higher voltage and 40mA at the lower voltage. Not that it matters a lot if you're getting good results at 35mA at either voltage. But it's a parameter you can experiment with if you like. A hotter bias at the lower voltage can reduce any crossover distortion you may be hearing and sound thicker and looser.

[Jonny toetags]

Is this using the 710V tap on both amps? Or are you saying you're getting acceptable tonal results from either voltage tap? Because you can play around with a hotter bias at the lower voltage. Bias doesn't have to be a "one size fits all" adjustment. So maybe 35mA at the higher voltage and 40mA at the lower voltage. Not that it matters a lot if you're getting good results at 35mA at either voltage. But it's a parameter you can experiment with if you like. A hotter bias at the lower voltage can reduce any crossover distortion you may be hearing and sound thicker and looser.

Both are running the 710V tap.

[Tom Phillips]

Both are running the 710V tap.

What plate voltage did you end up with at the current ~35mA bias setting?

[Jonny toetags]

Tom, I'm reading 433V on one amp and 435V on the other

[g1]

That sounds much better, right in line with the 430V Chuck was talking about with a 5U4, so I think you are good.

[Tom Phillips]

I agree.
Seems that you have a good set up.
Glad it worked out.
Cheers,
Tom

[Chuck H]

Tom, I'm reading 433V on one amp and 435V on the other

How did you go from 475Vp @ 22mA to 434Vp @ 35mA without changing the rectifier?

EDIT: I just ran a sim on PSUD2 and when set up to the circuit you describe I get 476Vp @ 22mA (per tube) and 468Vp @ 35mA.

[Jonny toetags]

Chuck H:
-I have no idea. I didn't change anything else....the only thing I've been doing since last Friday night is playing them hard for hrs at a time.
In between playing, I would check/tweak bias (currently ~35.5mA) and check voltages.
Last night, I only checked voltages before, during and after.
Is that drop that uncommon?

g1:
-I'm still running the TungSol 5AR4 rectifiers. I haven't tried a 5U4.

Should I be worried about that voltage drop now?

[Chuck H]

Chuck H:
-I have no idea. I didn't change anything else....the only thing I've been doing since last Friday night is playing them hard for hrs at a time.
In between playing, I would check/tweak bias (currently ~35.5mA) and check voltages.
Last night, I only checked voltages before, during and after.
Is that drop that uncommon?

g1:
-I'm still running the TungSol 5AR4 rectifiers. I haven't tried a 5U4.

Should I be worried about that voltage drop now?

I'm worried about that voltage drop.

Do me a favor and:

Pull the power tubes from one of the amps and check the unloaded plate pin voltage as they are wired now. Also check your filament voltage.

[Jonny toetags]

I will get those readings and post back.
(I have some NOS 5U4's on the way anyway)

[Chuck H]

I will get those readings and post back.
(I have some NOS 5U4's on the way anyway)

Well, a 5u4 at this point would get you too far below spec. They will be useful if we discover something is amiss and you end up with the voltage I expected.

[Jonny toetags]

I'm guessing components 'breaking in' wouldn't affect the voltage?

[Chuck H]

I'm guessing components 'breaking in' wouldn't affect the voltage?

Not like that. 40V from 22mA to 35mA is a VERY disproportionate change for the circuit as I've tested it. Maybe I'm wrong, or there was some mistake in an earlier measurements circumstances or recording. I hope so because it sounds like you're "there" if we don't find a problem.

[Jonny toetags]

Not like that. 40V from 22mA to 35mA is a VERY disproportionate change.

I figured.
I will get all those readings and post back soon.
I hope there's nothing terribly wrong with these amps....I need to ship them out early next week

[Tom Phillips]

I'm guessing components 'breaking in' wouldn't affect the voltage?

Nope. But since both amps are giving you essentially the same final voltage readings now I suspect that there may have been some condition during your initial voltage readings that caused the 475V reading such as:
1) 475V really is the unloaded value for your amp rather than the reading during low bias setting. Or the bias setting at the time was actually lower than 22mA.
2) High line voltage when you took the 475V reading
3) Test equipment issue
4) Some other mixup