Univox U1061 B+ needs lowering

[loudthud]

By the time you bandaid the B+ down to 500V or so where it needs to be for that OT, the heater Voltage will be about 4V. Try it with your Variac.

[Jazz P Bass]

Customer wants to try the buck route.
I tried it with a 110/ 12 and unloaded B+ is 630 Vdc & the heaters are at 5.9 Vac.
What VA should I shoot for? The one I tried is 15 VA.


I understand the OT may be the incorrect one, but I did not install it, and I don't think the owner wants to shell out $175.00 for a different one.
If it helps, the amp is used in a church at lowish volume.

[Helmholtz]

What VA should I shoot for?

Mains fuse is 3A. So for adequate protection the bucking transformer must be able to safely handle a secondary current of 3A.
Power rating should be at least secondary voltage times 3A.

Make sure to safety- ground the transformer core.

[mozz]

5.9v is a bit low unloaded.

[Jazz P Bass]

Having a problem sourcing a single 12 v secondary.
What I find available is 12-0-12.
Can the secondaries be paralled?
What does that do to the VA?
(sorry if I seem out of my league. I am!)

[Helmholtz]

What I find available is 12-0-12.
Can the secondaries be paralled?

Sorry, no.

Connecting the two 12V ends would result in a short, as the voltages are out-of-phase wrt CT and the voltage across the outer ends is 24V.
You would need two separate 12V windings.

[Jazz P Bass]

Ok.
So I need a 36 VA 12 secondary.
Next.
Loudthud seems to think that no matter what, this OT is the wrong choice.
Any other thoughts on that?
Maybe go with a KT 88 (if the heaters can handle it)

[Jazz P Bass]

I located this at DigiKey.
120V/ 12 V 40VA.

[g1]

Agree with Mozz, 5.9VAC heater might sag when loaded. At least load down that winding to simulate tubes installed.
Schematic shows 650V B+, I don't think you really need to be responsible for getting it lower than that. Wrong OT is on him.
Maybe 12V bucking is too much.

[Jazz P Bass]

Yeah, the 12 V buck is a tad too much.
Unfortunately, I cannot find a 10 v at 30 VA.

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
Current draw from mains at full unclipped output: 195 watts

Something that I noticed the last time that this was in: the OT 4 ohm tap provides the best output into an 8 ohm load (that is how it will be ran, 8 ohms)

[loudthud]

Whatever the operating conditions of the amp, it's burning up 6L6GCs. Would it be possible to run an 8 Ohm load on the 4 Ohm OT tap, or a 16 Ohm load on the 8 Ohm OT tap ?

Edit: That should extend the life of the tubes.

[Helmholtz]

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
Current draw from mains at full unclipped output: 195 watts

Looks good enough.

Something that I noticed the last time that this was in: the OT 4 ohm tap provides the best output into an 8 ohm load (that is how it will be ran, 8 ohms)

Now THAT would be a good solution (why didn't I think of this??).
Means an OT primary impedance of 4k, considerably lowering OT currents and tube plate dissipation at medium output.

[Jazz P Bass]

I did manage to find a 20-0-20 rated at 5 amps. (To get one 10 v secondary.)
I think that I will go with that as the heaters seem a tad low at 5.5 Vac.

Any thoughts on that?

[Helmholtz]

I did manage to find a 20-0-20 rated at 5 amps. (To get one 10 v secondary.)
I think that I will go with that as the heaters seem a tad low at 5.5 Vac.

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.

[J M Fahey]

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?

Of course customer wants to spends peanuts, all do.
I want to lose weight without diet or exercising, go figure.

I suspect heīll still burn through tubes, but hey, itīs his money, he can spend it all at once by doing the right thing or in comfortable installments and solve nothing.
How much do 4x 6L6 cost?

Do the Math, he already went through 2 sets.