Restack transformer from PP to SE

[Mush]

Hi:
I've got a pair of Hammond 1650-P PP output transformers with a primary impedance of 6.6k that can handle 200ma dc per side.

I'm am now completey enamored with the 845 tube and want build an SE amp of my own.

Would it be a stupid idea to restack the laminations on the Hammonds and air gap the E-I for use in an 845 or GM70 SE amp maybe running at 80ma?

Thanks very much for your help and have a happy new year.

Ciao -- Minto

[sorenj07]

it would take some pretty severe engineering know-how, or someone with that know-how to help you i'm not even sure that you could use the windings as they are . . .

[Wavebourn]

Know how: don't restack'em, use some MOSFET CCS on an opposite shoulder to compensate DC bias. You'll pay for electricity to greatly save on iron and copper.

[kimjand]

Beware of the isolation rating of the Hammond 1650. It may not be suitable for use at the high voltages I suspect you have in mind.

[tubelab.com]

don't restack'em, use some MOSFET CCS on an opposite shoulder to compensate DC bias.

You can't do this here. The 1650-P is a 6600 ohm transformer. Using half of the secondary (with a CCS on the other half) would present a 1650 ohm load to the tube. The 845 would not be happy with that.

It is possible to take the transformer apart and restack the laminations to create an air gap. I have done this. In addition to the previously mentioned insulation issues there is another problem. When you add an air gap to a transformar that was not designed for one you lose about half of the inductance that was present in the transformer. The core will be more prone to saturation. If all worked out ideally you could expect the transformer to support the power level of an equally sized SE OPT. The 1650-P weighs 8 pounds which would equate to a 10 to 15 watt SE OPT. Since this restacking operation is not ideal, expect it to handle even less power. I have restacked some "80watt" guitar amp P-P OPT's (I have a lot of them to experiment on). These transformers will really handle about 40 watts (6 pounds) in a P-P amp. When restacked for SE they can do about 5 watts before showing saturation on bass notes.

If you are going do do an 845 amp, you are going to need some decent OPT's and you need to be prepared to deal with high voltages. Mine runs at 1100 volts. DEADLY STUFF!!!

[wrenchone]

Restacking is not really a good idea, as any decent transformer is varnished to within an inch of its life, It will take paint stripper or something of equal violence to get the varnish to let go. I'm doing this right now to recover the 6 ml lams from some chokes I have on hand, but I'm doing it solely to get at the lams - the choke windings were rendered toast. Better to buy the proper iron in the first place rather than restack. What adds insult to injury is that the P-P transformer lams will be interleaved to minimize the gap and maximize the primary inductance, and the SE transformers must be butt-stacked so they can be gapped. This means you really would have to tease the lams all the way apart and completely reconfigure.

[Wavebourn]

Originally posted by tubelab.com


You can't do this here. The 1650-P is a 6600 ohm transformer. Using half of the secondary (with a CCS on the other half) would present a 1650 ohm load to the tube. The 845 would not be happy with that.

You are free to use as many tubes as you want in parallel. And you are free to use different types of tubes. Output transformers are most significant and precious parts of tube audio amps. Using biased PP transformers in SE outputs is one of options to save costs (excluding electricity consumption, of course, that is consumed by filaments as well!)

[Mush]

I guess I'm stuck with it as a PP transformer. I have 2 power transformers that will put out around 650vdc at 400ma that might be useful . Does anyone recommend a pair of PP 845s loaded 6.6k plate to plate?

Thanks - Minto

[tubelab.com]

Restacking is not really a good idea, as any decent transformer is varnished to within an inch of its life

I guess that the transformers that I have are not decent because one good whack with a hammer and a block of wood was all it took. They were surplus Schumaker guitar amp transformers that I got for $20 each. They sound pretty decent though. The Hammond 1628SEA's that I have appear to have been held by their leads and dipped in some black varnish. One of them is so thick that you can not make out the individual laminations. No way you are getting those apart.

[Wavebourn]

Originally posted by tubelab.com


I guess that the transformers that I have are not decent because one good whack with a hammer and a block of wood was all it took. They were surplus Schumaker guitar amp transformers that I got for $20 each. They sound pretty decent though. The Hammond 1628SEA's that I have appear to have been held by their leads and dipped in some black varnish. One of them is so thick that you can not make out the individual laminations. No way you are getting those apart.

They are nice transformers, I have 3 stereo amps made with them, they sound excellent even for Hi-Fi (the amps were designed for acoustic guitar + vocal concerts, i.e. from 80 Hz). And it was one of that amps when one 6L6 run away turning into a CCS so it became SE, but people kept saying it sounds decent.

...however, I paid $50 per pair, no shipment: we met in person enjoying some discussion about audio electronics. He paid for them more than he got selling them on ePay.