univox u1511 restoration

[jazbo8]

Clearly something is defective or mis-wired, without the power tube installed, disconnecting the B+ node to the OPT should not result in a change of voltage.

[Stevem]

Confirm that what you think is the OT center tap is in deed that center tap!

[martin manning]

The schematic shows 250-0-250 on the HV secondary, which would ideally yield 707 VDC. Line voltage is shown as 117VAC. Check your AC inlet- you may have 120-125VAC today, which would account for the high unloaded DC voltage. Measure plate screen and bias voltages to see if they are all consistent.

[ToneMerc]

The schematic shows 250-0-250 on the HV secondary, which would ideally yield 707 VDC. Line voltage is shown as 117VAC. Check your AC inlet- you may have 120-125VAC today, which would account for the high unloaded DC voltage. Measure plate screen and bias voltages to see if they are all consistent.

Martin, that would be true if it was 250-0 in a doubler rectifier config which this appears to be upon a further look. Thus, at 125v mains that PT would be 267 or so x 2.82 , which would be around 753v unloaded.

It a split rail screen supply so screen ressitors are not even needed.

TM

[martin manning]

I've heard people call this circuit a doubler, but it is a FWB. It does produce double the voltage that you would get if you connect the secondary as a FW, of course. The CT is sitting at half of B+, so running it to the center of the stacked reservoir is a very convenient way to balance the voltages and get a B+/2 screen supply.

Screen resistors are always a good idea to prevent oscillations, so I suggested adding small ones for that purpose.

[ToneMerc]

I've heard people call this circuit a doubler, but it is a FWB. It does produce double the voltage that you would get if you connect the secondary as a FW, of course. The CT is sitting at half of B+, so running it to the center of the stacked reservoir is a very convenient way to balance the voltages.

Screen resistors are always a good idea to prevent oscillations, so I suggested adding small ones for that purpose.

I had to look at it twice because I don't normally see "doublers" wired like this. I think it's a slick way of only needing one PT in inventory to maybe work across several different models.

TM

[martin manning]

The Marshall Major is wired like that too.

[zelenko]

hello. you were right. i have step down trnasformer which gives 125v and i think that is main reason for everything.
when tubes loaded it plate is about 690.

[Stevem]

That's still on the high side for a 6550!
I would place a 1 ohm 2watt 1% resistor from each tubes cathode to ground so as to check how much idle current your pulling as I think you are idling them cold!

[galtjunk]

I have a schematic for the 1501.
It uses 6550's.
I hope it helps.

[martin manning]

Nice! There is a real doubler supply (600V) with the screen at half-B+. Note the stoppers on all of the power tube grids, and the plates too for good measure! Also note the 33k bias feed resistors... might be a good idea to copy when going from 6L6 to 6550.

[SilverFox]

I picked up a 1221 that had the same problem- The output had arc'd and left a burnt spot on the chassis and the tube socket. Had to replace the sockets, rewire the OT and it sounded okay. Jumped the inputs and that gave it some gain-bite. As I recall I ran into the same issue regarding resistors not shown on the schematic but someone else was able to determine, as in above, the values.

I was never really that enthused with it and stripped it out. Tried to use the PT in a conventional FWB circuit and couldn't get any voltage out of it. A total mystery. Perhaps it didn't have enough current to drive a conventional circuit. But it definitely worked in the original design. I don't think it was all the power they claimed though. (2) 6L6) putting out 120 watts. Nah.

This is the chassis I used to build the Sunn Clone recently completed. Does yours have the square hole in the chassis? The OT's are great.

Be advised: I bought 6550's to put in mine for better HV handling but they would not fit! Mine ran at 660. 750, Wow.

silverfox.