Power transformer for a Univox U-1000

[jamesedmunda]

Hi
I've got a Univox U-1000 with a bad power transformer.
 Mercury Magnetics will wind me one for $350,
 Is there any alternative.
 Thanks James

[DummyLoad]

ignore what i posted and deleted.


use a fender twin reverb power transformer. rewire the power tubes to use the single 6.3V winding. you'll need to trim the bias supply since the twin part has slightly higher output voltage (60V vs. 50V). you will also need to add an artificial CT for the filament string: typically 2 x 100R 1/2W resistors.


--pete

[sluckey]

This is a Hammond replacement for the twin reverb. Cost $85.30 at AES. Don't know if it will fit your chassis holes.

https://www.hammfg.com/files/parts/pdf/290FX.pdf

https://www.tubesandmore.com/products/transformer-hammond-guitar-amplifier-replacement-fender-120-v

[ckilgore]

Very interesting schematic.  Here are a couple of questions for the amp guru's here:
1. Please explain the signal flow of the second 12AX7 gain stage.  I seem to get lost following the path for CH-2.
2. I don't think I have ever seen a tremolo connected to the signal path using back to back connected triodes (12AU7) before.  Please explain the reason and benefits of that.
Thanks

[sluckey]

CH2 signal path highlighted in yellow.

The 12AU7 is just acting as a variable resistance element. The tremolo signal on the grid causes the resistance to vary at the trem rate and the higher the intensity the wider the resistance swings. This resistance shunts the CH1 guitar signal causing it's amplitude to vary.

[astronomicum]

Hello. Can someone help me with what I am missing? The schematic shows an 800V center tapped power transformer only supplying 470V to the B+. It should provide somewhere in the mid 500s (565V). The 290FX recommended looks like a good choice to provide the 470V specified (640V CT) if we assume the schematic. transformer specification is incorrect, or again am I missing something? Thanks.

[sluckey]

Hello. Can someone help me with what I am missing? The schematic shows an 800V center tapped power transformer only supplying 470V to the B+. It should provide somewhere in the mid 500s (565V).

You are missing the load. Yes, that 400v PT will produce 565VDC into a capacitor without any load current being drawn. But those four 6L6s will put a big load on that 565V and suck it right down. How much depends on the current capacity of the PT. A little PT would sag a lot. A huge PT might not even sag at all. I'd hate to tote that one though.   

I think 470V is reasonable for a reasonable sized PT.

[HotBluePlates]

... schematic shows an 800V center tapped power transformer only supplying 470V to the B+. It should provide somewhere in the mid 500s (565V). ...

... that 400v PT will produce 565VDC into a capacitor without any load current being drawn. But those four 6L6s will put a big load on that 565V and suck it right down. How much depends on the current capacity of the PT. ...

And within that "current capacity of the PT" is the winding resistance of the transformer.  More current drops the output voltage.

We normally don't focus on that, as we don't have specs on the winding resistance of a PT before actually buying it.  However, I've got a vintage Deluxe Reverb and noticed its winding resistance is higher than Hammond's replacement PT (one of the few places that publishes a spec).

I found this was a part of why many home-brew builders wind up with unexpectedly-high B+ voltage (usually blamed on high outlet voltage; this 1964 Deluxe Reverb needs 120vac to land on its expected heater and B+ voltages).

[astronomicum]

Thanks for the replies. Knowing that it would not be unreasonable to expect such a large voltage drop by design is helpful. Univox can not be faulted for overdesigning their PTs.