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Replacing power transformer...need cross reference help.
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Author:  Bluesman [ 26 Jan 2009, 18:27 ]
Post subject:  Replacing power transformer...need cross reference help.

Hello all, I need some help. I have gotten myself sidetracked away from other projects and I'm going through a 1960 Silvertone Model 29.I'm replacing caps, resistors, etc., that need to be, and the power transformer is caput. I have a Sams Photofact Folder for the amp, which includes the schematic and every part that is in the amp. The parts list simply shows a Silvertone power transformer #46070-3. It is 117V@ .85A on the primary, and 125V@ .570A and 6.3V@ 4.1A on the secondaries. No CTs. Can someone steer me to a good replacement? No luck finding any kind of cross reference and I'm not that experienced selecting transformers. A Google search for that specific Silvertone number got nothing.

Thanks, Greg

Author:  Gio [ 26 Jan 2009, 22:00 ]
Post subject:  Re: Replacing power transformer...need cross reference help.

Hi Greg, I looked at the Hammond catalog and no luck for a HV + Filament transformer with those ratings in one unit. That is a good sized transformer.

Author:  Bluesman [ 27 Jan 2009, 05:06 ]
Post subject:  Re: Replacing power transformer...need cross reference help.

Thanks, Gio.

I have several 6.3V filament transformers I can use if need be. I also have an old Stancor with the correct output, but at .25A instead of .570A. I have room to use separate if it comes to that.

Does it seem like overkill on that power transformer? This amp is a push pull using 2 pair of EL84s, a pair of 7199s, and a 12AX7. It's rated at 10 watts per channel.

Thanks, Greg

Author:  Gio [ 27 Jan 2009, 13:33 ]
Post subject:  Re: Replacing power transformer...need cross reference help.

That does seem high.

  • Under 100mA for each push-pull pair
  • perhaps 5 mA for the 12AX7
  • and max 20mA for each 7199

Add some for losses and you would think that 300 mA would be plenty. Are you sure it is 125VAC on the secondary? That seems low for use with those tubes.

Author:  Bluesman [ 27 Jan 2009, 18:10 ]
Post subject:  Re: Replacing power transformer...need cross reference help.

Gio,

According to the schematic, 125V in and three outputs from the PSU. 195V, 320V, and 330V. There is actually 325V on the plate of the EL84s, 160V on the pentode plate and 30V on the triode plate of left channel 7199, 150V on the pentode plate and 40V on the triode plate of right channel 7199, and 90V on each plate of the 12AX7.

All values measured from socket pin to common negative with all controls at minimum and proper output load connected.

I could come up with an totally different PSU, I suppose, but don't really want to go to that much trouble if I don't have to.

Thanks, Greg

Author:  Gio [ 27 Jan 2009, 18:41 ]
Post subject:  Re: Replacing power transformer...need cross reference help.

Ok, initially you had mentioned 125V secondary.

I doubt you will be able to find something that will have those secondaries so you will have to make some changes to the power supply. Best best may be a single 250VAC secondary to feed everything and resistors as required to get the right voltages.

Author:  Bluesman [ 27 Jan 2009, 19:04 ]
Post subject:  Re: Replacing power transformer...need cross reference help.

I worded my reply incorrectly.

There is a 125VAC secondary from the transformer into the power supply circuit that results in the three outputs I mentioned before.

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