Univox U1061 B+ needs lowering

[Jazz P Bass]

Any thoughts on lowering the B+ Vdc of this U1061 beast.
Zener, MosFet?
Last time that it was in it was 710 Vdc.
I tried different brands of 6L6 tubes & the amp invariably eats them one at a time.
At one point it killed the OT so that was replaced with a Hammond OT.

[The Dude]

Just to be sure before making mods: I see there are two primary taps on the PT. Is it wired to the 117V (Orange) tap and not the 100V (Yellow) tap?

[g1]

How is the heater voltage? If it is also high, I'd be more inclined toward a bucking transformer in the PT primary.

[loudthud]

That 1760W (2K primary) is really the wrong OT for this amp. Running high power will smoke the 6L6GCs. With B+ that high, a 5K primary is needed. Something like the Hammond 1650R or a newer version with simplified secondary.

[g1]

Perhaps because they are biased so cold, Musicman with similar voltages (2275-150) uses 2K8 primary (rated for 150W).

[Jazz P Bass]

I will be receiving the amp in a day or 2.
Stay tuned.

[J M Fahey]

Given what you spent you shoukd have changed the PT instead.

[Jazz P Bass]

Well a little back story.
All that I have done (twice) is replace tubes.
A previous repairer installed the OT.
Got the amp in and the tubes are literally blown.
I hooked up the Yellow (100 V) tap and here is what I have.
B+: 423 Vdc
Scree: 328 Vdc
Bias: -63 Vdc
Heaters: 7.7 V ac
The circuit looks stock, component value wise.
The only oddity is there are 47 ohm/ 5 watt screen resistors installed. (not too sure that these should be there)

[g1]

I would expect all those voltages to be higher if running 120V into 100V tap? Is the schematic wrong about the colours of the primary taps?
What are the readings at same points when running into the 120V tap?

[The Dude]

Yes. You should use the 117V (orange) tap. That is what I meant in post #2. I just wanted to be sure it WASN"T wired to the other tap.

[Helmholtz]

B+: 423 Vdc
Scree: 328 Vdc
Bias: -63 Vdc
Heaters: 7.7 V ac

So heater voltage high, but B+ low??

[Jazz P Bass]

Duh.
I was on the incorrect test point.
B+ on the Yellow tap is an insane 850 Vdc.

[Jazz P Bass]

So what would be the best approach for a buck transformer?
Something like this?

[J M Fahey]

Forget buck transformer, those are bandaids for, say, 10% adjustments. .
You have the wrong PT, period (for 2022 tubes that is)

Get a Bassman 100/Twin PT and fit it there.

Keep those filter caps in series, but with a resistive divider (like on Fender/MB/etc. supplies), you will not have a PT center tap any more.

Donīt waste more time and money on it, solve the real problem.

Just to make it clearer:

All that I have done (twice) is replace tubes.
A previous (clueless) repairer installed the OT.
Got the amp in and the tubes are literally blown.
I hooked up the Yellow (100 V) tap and here is what I have.
B+: 423 Vdc
Scree: 328 Vdc
Bias: -63 Vdc
Heaters: 7.7 V ac
Donīt ever reconnect that 100V tap on an already murderous PT

[mozz]

Bucking transformers are not a bandaid, they work. Also much cheaper for the customer than replacing a power transformer. You can't really go by those last voltages with no tubes installed.