Univox U1061 B+ needs lowering

[g1]

Bandaids work ... where bandaids are enough.

All measurements shown above tell +B is still too high,and filament voltage is already way too low and bucking will lower both at once,since they are wound on the same core..

What part of "wrong PT" is hard to understand?

I don't understand. B+ is spec'd at 650V on schematic and PT is correct stock unit.
Musicman amps are running 6L6's at higher B+ than this, we don't generally consider replacing their PT's ?

[mozz]

What is wrong with this which you posted? It's 100 watts and i can bet the B+ is dropped when it puts out that much. Maybe the replacement OT is the entire problem. Still don't know why your filament is low.

"Here are the voltages loaded up with 4 6L6 tubes.
Mains Voltage:118 Vac
B+: 580 Vdc
Screen: 287 Vdc
Heaters: 5.5 Vac
Bias: Set to -29 V per schematic
Unclipped output power: 28 Vac (RMS) @ 8 ohms
Current draw at idle from mains: 98 watts"

[Jazz P Bass]

How would that give you 10V at the secondary?

Power tubes are not known to die from low heater voltage and power output looks good.

120 primary- 20-0-20 secondary.
I will just use one secondary for a 10 volt buck.

Then again, I am on the fence as to whether or not the 12 volt buck may be better in the long run.
The amp performs well with that one.

[Helmholtz]

120 primary- 20-0-20 secondary.
I will just use one secondary for a 10 volt buck.
.

Don't understand.
A 20-0-20 secondary has no 10V tap - rather has two (connected) 20V windings.

[Jazz P Bass]

I mislabeled that incorrect, didn't I?
10-0-10 is CT with 2 10 V taps.

[Helmholtz]

Yes, that's a 10-0-10 secondary. Could also be seen as a 20V secondary with a 10V CT.

[Pedro Vecino]

A question: seeing that the high voltage secondary uses the center tap to be able to reduce the voltage by half (screen grids and preamplifier) I understand that I could not use a conventional transformer with a single secondary with the phase inverted there to subtract high voltage . But I think a transformer with two separate secondaries could be used.
I have found this. Could it work in this case? With the idea of being able to apply it in similar cases on 100 watt amplifiers.
It marks 55+55V in separate secondaries and 450mA.

https://es.farnell.com/multicomp/mct...a=MXxZfDB8WXww

https://www.farnell.com/cad/1906781.pdf

[Helmholtz]

But I think a transformer with two separate secondaries could be used.
I have found this. Could it work in this case? With the idea of being able to apply it in similar cases on 100 watt amplifiers.
It marks 55+55V in separate secondaries and 450mA.

Depending on transformer construction (insulation, creepage and clearance distances), it might be a problem that this wiring puts another e.g. 500V between the 55V secondaries.

[loudthud]

There was a long thread on the subject of split power supplies here: https://music-electronics-forum.com/...al-rail-thread

There are additional threads that touch on the subject if you search for "split rail" or "dual rail". Many designs reach 100W with a single pair of KT88, 6550 or 8417 tubes.

[Helmholtz]

I always wondered how lower screen voltage could be an excuse for considerably exceeding plate volt limits in some designs.

The EL34 datasheet shows a design example with dual rails (800V/400V) for a pair. Raa = 11k.
They call it class B, but plate current is 25mA meaning 80% plate dissipation.

I actually own a '50s Siemens PA amp that is built to these exact specifications.
Puts out in excess of 90W before clipping with only one pair of EL34s.

But I wouldn't dare to try this with 6L6s having a plate limit of only 500V.

[g1]

I always wondered how lower screen voltage could be an excuse for considerably exceeding plate volt limits in some designs.

I have also wondered about this and whether we may be misinterpreting the datasheets. pdf64 has raised the issue before about 'absolute' maximums vs 'design-center' maximums in the datasheets.
I know the amp manufacturers will have pushed the limits as far as possible during the 'loudness wars' but I think they will have also wanted warranties from the tube manufacturers. The tube manufacturers would have just showed them on their schematics where they were 'abusing' the tubes and voiding warranty, I would think?

[Helmholtz]

I have also wondered about this and whether we may be misinterpreting the datasheets. pdf64 has raised the issue before about 'absolute' maximums vs 'design-center' maximums in the datasheets.
I know the amp manufacturers will have pushed the limits as far as possible during the 'loudness wars' but I think they will have also wanted warranties from the tube manufacturers. The tube manufacturers would have just showed them on their schematics where they were 'abusing' the tubes and voiding warranty, I would think?

From my job I know that when you buy directly from the manufacturer in large quantities, you can make them screen parts for special applications and sign extended specs.
We often did that with caps, transistors, inductors etc.

So maybe MM and others exceeding Vp max had such agreements with their tube suppliers.
Wonderig if this was the origin of the STR labels.
If so, the exception cannot simply be transferred to standard, non-screened tubes.

Apart from that I learned the hard way to never exceed official component limits unless approved by the manufacturer.

[g1]

So maybe MM and others exceeding Vp max had such agreements with their tube suppliers.
Wonderig if this was the origin of the STR labels.
If so, the exception cannot simply be transferred to standard, non-screened tubes.

Apart from that I learned the hard way to never exceed official component limits unless approved by the manufacturer.

I think if this were the case, the owners manuals would have stated that warranty is void if not using manufacturers own 'special' tubes. I haven't seen such statements in warranties. A bad tube can cause other damage to the amp so I think they would have been particular about this.
As to your "unless approved by the manufacturer", maybe they said the plate limit could be exceeded if screen voltage was kept below 'x' and total dissipation below 'y' ?
The low screen voltage when very high plate voltage seems a fairly common theme.

[Helmholtz]

I think if this were the case, the owners manuals would have stated that warranty is void if not using manufacturers own 'special' tubes. I haven't seen such statements in warranties. A bad tube can cause other damage to the amp so I think they would have been particular about this.

If the OEM tubes typically survived the warranty period (2 years?), that risk might have been tolerable.
Also I think I have seen statements like "replace with original (brand) part only" in user manuals.

As to your "unless approved by the manufacturer", maybe they said the plate limit could be exceeded if screen voltage was kept below 'x' and total dissipation below 'y' ?
The low screen voltage when very high plate voltage seems a fairly common theme.

Maybe, but I couldn't find confirmation in tube literature.

OTOH, lower screen voltage might also be necessitated by high plate impedance in order to avoid screen over-dissipation.

[The Dude]

Why not just mod the amp to use a tube with higher plate voltage specs (EL34, 6550, etc.)? It seems simpler and cheaper than other options considering the original tubes are toast anyway.