Replacement OT for Univox U-1101

[Gaz]

Got a Univox here and the OT is arc'ing. subbed in this transformer I had around and all is well.

http://www.classictone.net/40-18072.html

I would use it, but it's a bit too heavy for the flimsy steel chassis, which is already slightly concave.

Would a regular Twin OT be up for the job or too puny? The amp puts out about 120W with 700vdc on the plates and 350vdc on the screens. I upped the screen resistors to 1K and am running 6550s.

Thanks!

[Stevem]

You have a 1101 model and not a 1011?
Can you post a schematic ?
The amps that I have seen use 6l6 tubes and as such can only handle at most some 500 volts on the plate!
Has the PT been changed on that amp?
Also 6550s are good to 660 volts so you would need to go with EL34 types, KT88 or the real big boys of the KT90 or KT120 leage!
If the amp did use 6l6 types than the 6550s and others will need more heater current and you will need to confirm that the heater voltage does not drop below 6.1 volts with the bigger bottles in the output stage.
If this takes place you can add another small filament PT to power two of the output tubes.
A twin OT would work impedance wise, but current wise it would burn up if not arc over first from the 700 volts maybe, and if so a OT like a Hammond 1650R is what I would be looking at with a plate of street metal or aluminum under it to support I t on the chassis better.
Also note that you may need more bias voltage on pin 5 to get the bigger bottles happy, and my have to mod that circuit a tad, or pull ac off of the primary of the PT and convert and knock that down to a dc voltage the tubes need.

[Gaz]

Sorry, meant to link to the schem, and you're right about the model # mixup.

First off, thanks for the concern, but everything's fine with the power tubes and PT. It has enough filament current, and the 6550s are perfectly happy at that voltage. Remember, the screens are only at >350vdc.

I am really just concerned with finding an off the shelf OT. I was wondering if you could explain what you meant when you said:

...current wise it would burn up if not arc over first from the 700 volts maybe...

Do you think the insulation could not take 700vdc? Also, how do you determine the current rating? I thought the lesser power OT would maybe just lessen the bandwidth, but I really on't know which is why I asked in the first place.

Thanks.

[Stevem]

Well, looking at the schematic at idle the plate voltage with 6l6 tubes is 650 volts, and then flat out wide open the plate voltage drops to 625 which sounds wishful to me as I bet that with all fresh power supply filters and good tubes that the plate voltage nose dives down to some 580!
A replacement twin OT should be HI Pot tested I would think and that's done at some 2000 volts I think, but that with the OT at room temp and not sitting at 140 degrees or more!
The twin OT will likely handle the 650 volts and the current I guess since that amp is making its wattage more by the means of higher plate voltage where as the twins OT is passing more current, but less voltage!
Of course if you never run the amp at more than 3/4s of its max power I would say all will be well, but I would add a 4 inch 220 volt fan and just power it with the 110 as it will be noiseless and more enough air to keep the output stage up and running!

two changes I would make would be to up grade that 50 volt 30uf bias filter to a 100uf at 75 to 100 volts, and any power supply and or preamp plate load resistor that is getting hit with near 300 volts or better should be 1 watt in size and not 1/2 watt.