Need help finding replacement output transformer

[Gaz]

Hi decided to start a new thread now that my problem is solved in an old Univox U-1011

It had a bad OT (arc'ing at high volume). I tacked in this transformer I had around for another project, and all was well, putting out 120Wrms. Problem is is that this one is a bit too heavy for the flimsy chassis and I don't know if it's really up to the task anyway long term.

Would this one be okay, or too small? Fender Output Transformer, Showman, Dual Showman, Twin Reverb, Bassman 100, 4/8/16 Ohm

Thanks.

[Alan0354]

I took a quick look, I just join in, don't take everything I said that serious.

From the schematic, looks like there is a voltage doubling in the rectifier section that from 250V winding, you get 2 X 250 X 1.4=700VDC of no load voltage. Also the schematic spec. 625V at 280mA. Also from the schematic, the grid bias is about -29V, it is low even when Screen grid is only 325V. The 6L6GC is running REALLY REALLY hot at 70mA and 625V each!!!! The power dissipation of each tube is 625V X 0.07A=44W!!! That is way way too high!!!

Showman/Twin is running with +B=450V and grid bias about -50V. It is running a lot lower current and a lot lower voltage. I don't know whether the OT is capable to handle this. The OT is not big for a 100W amp to start with. Yours seems to push to the extreme plus one!!!

Bassman 100 has a much bigger transformer, but still, the voltage really borders me. The maximum voltage swing almost double, so the peak is almost 2 X 625V - 100V= 1150V.

[g1]

Alan, where did you get that 70mA idle current number? Any data that shows idle current as a function of grid voltage is not valid, it is just very rough "ballpark" figure. The only way to find idle current is by actual measurement.

[loudthud]

If the B+ is sky high like mentioned above, it might make sense to look at replacements for the old Music Man transformers. Mojo sells some but you'll have to hunt for the specs.

[nickb]

Low screen voltage means -29V is about right.

High plate voltage means higher primary impedance. You'll want 4K-5K rather than 2K. If you connect your 8 ohm speaker to the 4 ohm tap to double the primary impedance at the expense of a little low end.

You can probably get away with a 100W instead of 120W since you're trying to save weight but I think it might get a bit hot if you you play full out for any length of time.

[Alan0354]

Alan, where did you get that 70mA idle current number? Any data that shows idle current as a function of grid voltage is not valid, it is just very rough "ballpark" figure. The only way to find idle current is by actual measurement.

It's on the schematic. It said 625V and 280mA for the 4 tubes. I just read it out.

It's believable because the normal idle current of 6L6 is about 40 to 55mA for +B=450V and grid at between -45 to -51V. The schematic specified grid at -29V and +B at 625V. It is believable 4 tubes draws 280mA even screen grid at 325V written on the schematic. But this is just rough guess, the schematic gave the number.

[Alan0354]

Why do they want to run such a high +B? Any advantage with high voltage?

It's one thing it run very high power, the voltage is dangerous as the peak plate voltage can jump over 1KV. My rough estimate is when the other plate swing low down to about 100V, the other plate will swing up the same amount which is 625V-100V=525V above the idle voltage. That is 626+525V=1150V.

I never open an OT and I am not expert in transformers. I know in switching power supply, we need to keep the waveform symmetry by winding the two secondary winding in parallel together and use the beginning as CT. With this, at the end of the winding, the two wire are always at the equal and opposite voltage at the peak. That is if one side is at +450V for normal OT, the other side is at -450V. The difference is 900V. Normal magnetic wires are spec for about 400V insulation, so the two wires side by side can only stand off 800VDC. You still have to derate for AC.

Then even for scattered winding, you can still have possibility of a wire with very high +ve voltage over lay on top of the wire have high -ve voltage.

I don't know, as I said, I am no expert in transformer, just join in, more asking question than giving opinion.

[g1]

It's on the schematic. It said 625V and 280mA for the 4 tubes. I just read it out.

Thanks Alan, I see there are 2 sets of numbers shown, one in parentheses. I think the first number is at idle, the second number probably at full power. So it looks like idle would be 40mA per tube at 650V (27Watts) and full power 70mA at 625V (44Watts).
Either way seems very high, I doubt modern tubes would handle that. Perhaps I am misunderstanding what those numbers mean or maybe there is a typo on the schematic?

[Alan0354]

Thanks Alan, I see there are 2 sets of numbers shown, one in parentheses. I think the first number is at idle, the second number probably at full power. So it looks like idle would be 40mA per tube at 650V (27Watts) and full power 70mA at 625V (44Watts).
Either way seems very high, I doubt modern tubes would handle that. Perhaps I am misunderstanding what those numbers mean or maybe there is a typo on the schematic?

I don't know which set of number to use, so I took the worst number. It's the 625V that really bugs me. Why that high a voltage?

I did power scaling by adjusting the screen grid rather than the plate. I did calculation of the grid voltage and screen voltage relation. From the curves I studied before on 6L6, there seems to be a linear relation between Screen and control grid voltage. Say for normal Fender type +450V screen and -51V grid, I got about 40 to 50mA idle current. So I back calculate using 325V screen and -29V grid. At 540V screen, the equivalent grid voltage is -29V X( 450V/325V)=-40V. In my experience, at 450V plate and screen with -40V grid for 6L6, the idle current is way over 40mA. I don't remember the exact number and it depends on individual tube. But I could swear it is over 50mA in this condition with every single 6L6 I tested. That's the reason I took the 280mA number from the schematic.

But again, this is based on my assumption the screen and control grid has linear proportion. BTW, the power scaling with screen voltage did not work that well at all. Yes, I succeeded in power scaling, but no, it did not sound good.

[Gaz]

Thanks for the replies. Haven't had time to read them all just yet, but wanted to clarify the real-world voltage of this thing is 700vdc on the plates, and 350vdc screens. I have 6550s in there now and the PT is fine with that. They are also happy with the high voltage. Promise

This is an old amp when louder and cleaner was desirable - I wouldn't try to hard to get in the minds of the designers. Really just trying to find an OT for this that won't die on me, and get the amp out of my shop

Thanks again for the suggestions.

[Gaz]

If the B+ is sky high like mentioned above, it might make sense to look at replacements for the old Music Man transformers. Mojo sells some but you'll have to hunt for the specs.

That is a really good suggestion! Those have identical plate and screen voltages.

[Gaz]

That is a really good suggestion! Those have identical plate and screen voltages.

Ah, scratch that, they have 3 different MM-style OTs, but only for the two output tube models.

[Alan0354]

Thanks for the replies. Haven't had time to read them all just yet, but wanted to clarify the real-world voltage of this thing is 700vdc on the plates, and 350vdc screens. I have 6550s in there now and the PT is fine with that. They are also happy with the high voltage. Promise

This is an old amp when louder and cleaner was desirable - I wouldn't try to hard to get in the minds of the designers. Really just trying to find an OT for this that won't die on me, and get the amp out of my shop

Thanks again for the suggestions.

If that's not your concern, just use anyone and try.

[Gaz]

If that's not your concern, just use anyone and try.

Sorry, I think you misunderstood. I am absolutely concerned with finding a proper replacement OT, just not why the designers used such a high plate voltage since there's not much I can do about that.

[J M Fahey]

Agree that you need a much higher output impedance transformer.

Why do they want to run such a high +B? Any advantage with high voltage?

Tubes are current limited.

You want more power? ... you add more tubes in parallel and *then* you can lower load impedance.

Nice but ... expensive.

Thereīs 2 cheaper options:

a) you raise plate voltage somewhat , say from datasheet example 360/400V to 430/460V *and* raise screen voltage too, which forces some more current out of that cathodes.
This is what everybody and his brother did, from Fender to you-name-it.

b) you keep classic screen voltage (so you donīt increase available current) but *double* plate voltage.

Double voltage and same current means double impedance.
The Hiwatt/MusicMan/Univox solution.

EDIT:
hereīs

That transformer you got is the wrong spec, itīs meant for solution (a) , not (b).

Hereīs one:
http://www.beyondeleven.com/Mercury-...sicm-130-o.htm