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Restack transformer from PP to SE

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
 
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!!!
 
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.
 
tubelab.com said:


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!)
 
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.
 
tubelab.com said:


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.
 
They ARE nice sounding transformers. I have a pair of them in my 300B P-P amp. I have tried several different OPT's in that amp, but I keep puting the Schumakers back in.

I have dissected these and they are not well constructed. There is no interleaving, just one half primary, then the secondary, then the other half primary. The outer primary section is wound with thicker wire to equalize the DCR. There is very minimal varnish, so dissassembly is easy.

The ones that I have came from the liquidation of the old ADA company. I bought 200 of them a while back and have been using them in guitar and HiFi amps ever since. I have cranked 140 watts through one of them using 2 6LW6 tubes in P-P on almost 700 volts. No issues, and none have failed so far.
 
tubelab.com said:
They ARE nice sounding transformers. I have a pair of them in my 300B P-P amp. I have tried several different OPT's in that amp, but I keep puting the Schumakers back in.

I have dissected these and they are not well constructed. There is no interleaving, just one half primary, then the secondary, then the other half primary. The outer primary section is wound with thicker wire to equalize the DCR. There is very minimal varnish, so dissassembly is easy.

The ones that I have came from the liquidation of the old ADA company. I bought 200 of them a while back and have been using them in guitar and HiFi amps ever since. I have cranked 140 watts through one of them using 2 6LW6 tubes in P-P on almost 700 volts. No issues, and none have failed so far.

do you mean this transformers? I love them.

bardvox_3.gif
 
George - It sounds like your transformers were probably just dip impregnated rather than subjected to a complete vacuum impregnation cycle, otherwise, the varnish would be between all the lams. This would probably be sufficient to keep the lams from rusting and to nominally nail the windings to the core and prevent undue vibration.

However, when kraft paper is used in transformer construction (a common practice), it does not have a proper safety/temperature rating unless impregnated with an appropriate varnish. The paper will also be hygroscopic until this is done. A transformer with a plastic bobbin and polyester tape interlayer insulation is another matter. In that case, vacuum impregnation can enhance the insulation between turns and layers, so that it has a higher voltage withstand. Parasitic capacitance is also increased, but that's the breaks...
 
This would probably be sufficient to keep the lams from rusting

Well you can see from the picture of Wavebourne's transformers that they do indeed rust. My transformers look just like his. I have been told that this was a "spec" transformer used by several guitar amp makers, and made by several different vendors. I have a couple of samples made by other vendors. I have also seen 4400 ohm versions for sale on Ebay.

Parafeed is an option for using P-P transformers in a SE application, and yes I have used these Schumaker OPT's in a parafeed circuit. The issue here is finding a suitable choke.

It is even possible to use a CCS circuit in place of the choke. This requires an even greater HV supply and is enormously inefficient. I started doen the road toward an 845 fed by an 813 CCS, but never finished it due to other priorities. Someday. It is definitely a project for an advanced builder.
 
I wouldn't worry too much about rust on the laminations and suspect cleaning it up will do more harm than good. Essentially rust is an oxide formed on top of the existing oxide used to reduce interlam eddy currents. In cleaning up the unwanted oxide, you will surely remove the wanted oxide which i suspect will leave you far worse off than simply having a rusty looking piece of iron. There are plenty of paints that will cover and inhibit further accumulation and if the rust is so bad that it compromises the overall integrity of the iron you have a lost cause anyways.

dave
 
Ty_Bower said:


Off topic, but...

Just how bad is rust? Can it be cleaned up, or will I just make things worse?

It does not bother at all.

Transformers are decent, though very simple. I regret to say I could buy more like Tubelab did, but did not buy... Unfortunately ADA warehouses were burn in fire... David being a brilliant designer was upset about that. His guitar amps are still in demand because he found the way to control tube amps through MIDI interface.