5F6A Bassman voltage check

[Jonny toetags]

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

I guess there's a benefit in building 2 of the same amp at the same time.
I'm going to test the wall voltage tonight as well before I power up the amps.
I might even put the bias to the lowest point and test as well.
Could be any of the factors you mentioned i guess.

[Chuck H]

That was my hope too. But when I run PSUD2 with a Bassman like circuit, using the default 31 ohm PT resistance, a 5AR4 rectifier, 710V secondary (355V in the sim) and 22mA per tube I get 472Vp.

Changing to 35mA per tube I get 463Vp. That's a 9V drop. Pretty damned far from 40V.

My concern is that there may have been an error in the tap change wiring and there is excessive current draw or possibly a primary miswire.

[Jonny toetags]

That was my hope too. But when I run PSUD2 with a Bassman like circuit, using the default 31 ohm PT resistance, a 5AR4 rectifier, 710V secondary (355V in the sim) and 22mA per tube I get 472Vp.

Changing to 35mA per tube I get 463Vp. That's a 9V drop. Pretty damned far from 40V.

My concern is that there may have been an error in the tap change wiring and there is excessive current draw or possibly a primary miswire.

Just in case, this is my PT: http://www.classictone.net/40-18029.pdf
Right now I have RED/RED connected to my Rectifier.

If I look at their chart, the RED/RED should have a B+ of 422V?
Maybe I did take a bad reading of 475V?? Although I don't see how.

EDIT: WAIT a second.
I DID do something else when i switched the taps, I added 2 protection diodes as per this post:
http://music-electronics-forum.com/t40027/
I put two 1N4007's wired exactly like that diagram, although i have no other wires on pins 3 or 5... my reds go to 4 and 6.
Would that explain the extra rectification? I thought they were only for protection. Should I remove them?

[Chuck H]

The protection diodes aren't changing anything significantly. But note that the 422V figure on the diagram applies to a current draw of 200mA. You're nowhere neat that. Or at least, you shouldn't be. So we need those voltage readings. Unloaded (go ahead and pull ALL the tubes), filament voltage and also take readings at each node in case that reveals a current draw that shouldn't be there. Also, do you have a 25W resistor under 100R around? It would be useful for a possible next test.

[Jonny toetags]

The protection diodes aren't changing anything significantly. But note that the 422V figure on the diagram applies to a current draw of 200mA. You're nowhere neat that. Or at least, you shouldn't be. So we need those voltage readings. Unloaded (go ahead and pull ALL the tubes), filament voltage and also take readings at each node in case that reveals a current draw that shouldn't be there. Also, do you have a 25W resistor under 100R around? It would be useful for a possible next test.

Ok.
Excuse my ignorance, but where/what is "each node"

[Jonny toetags]

OK.
So I pulled the chassis and brought them home to my workshop (where I do all my building)
With all tubes pulled except rectifier, I'm reading 512V on pin 3 of the powertubes
With powertubes in, I'm reading ~460V (it fluctuated from 455V to 461V)
Filaments show 6.6V


I took the other measurements from my rehearsal space. I guess the power was fluctuating as it's a huge building with about 150 rehearsal rooms

[Chuck H]

Hmmm... Well then, it would seem that you have low voltage at your practice space. How low? Maybe five or six volts (=NBD).

You may want to try it out for tone at the new (correct?) voltage. I expect it'll be fine or better

Two amps in the box!

[Jonny toetags]

Hmmm... Well then, it would seem that you have low voltage at your practice space. How low? Maybe five or six volts (=NBD).

You may want to try it out for tone at the new (correct?) voltage. I expect it'll be fine or better

Two amps in the box!

Yeah...it was the first time I actually I took any readings over there. Won't be doing that again.
I have the NOS 5U4's coming in tomorrow.
I guess I will give them a shot, lower the voltage a bit and then rebias here, where I'm used to the voltage.
I'm still thinking 460v is a little high.

[Chuck H]

Well it's not too high for an amp in general. Perhaps too high for a "Bassman"... Unless that Bassman is a vintage amp running on modern mains voltage. Then it would be just like yours. My advice is to try it at that voltage and see if it doesn't kick your ass. Since you have the 5u4's coming there's no harm in trying them. You could provide the amps with both rectifier types too. With instructions that for a vintage Bassman as it would be today, use the 5ar4 (gz34) and for vintage correct voltages, use the 5u4 It's still two amps in the box.

[Jonny toetags]

The 5U4 brought the voltage down to 444V.
I'm going adjust the bias to about 39mA and compare the two and see which I prefer.

EDIT:
I actually think I prefer the stock 5AR4 rectifier/460V/36mA bias.