New 14 watt Express build

[wpaulvogel]

I just finished a lower power Express. I wound the transformers. It’s got a 265-0-265 power transformer that supplies 345 volts at idle. The output transformer is 8k to 4, 8 & 16 ohms with 4 primary 3 secondary interleaving. Runs 6k6gt pentodes at 11 watts clean or 6v6gt at 14 watts clean. Still playing around with screen resistors but has 1k right now. Using Amperex 12ax7 preamp tubes and Sylvania 6v6.

[joeboo88]

Very nice!

[martin manning]

Well done! What’s the story behind your decision to wind your own transformers?

[gktamps]

Wow!
DIY to another level, winding your own!

[aceofbones]

That's a beautiful build.

[romberg]

Very nice! I'd love to hear more about the transformer winding.

Mike

[wpaulvogel]

Well done! What’s the story behind your decision to wind your own transformers?

With my own transformers I can establish the voltage and current for the power transformer and with the output transformer I can put in the interleaving that I want. I started this because getting Marshall transformers ( vintage wound) was taking so long.
I also noticed on the lower power output transformers that there isn’t a complex interleaving. On this transformer the primary has an equal number of turns per side and the DCR is balanced to within less than 1 ohm. It’s more work than just buying transformers but I’m happy with the results.

[martin manning]

Well done! What’s the story behind your decision to wind your own transformers?

With my own transformers I can establish the voltage and current for the power transformer and with the output transformer I can put in the interleaving that I want. I started this because getting Marshall transformers ( vintage wound) was taking so long.
I also noticed on the lower power output transformers that there isn’t a complex interleaving. On this transformer the primary has an equal number of turns per side and the DCR is balanced to within less than 1 ohm. It’s more work than just buying transformers but I’m happy with the results.

Thanks. Did you do a bifilar wind on the OT primary to match the resistances? What about impregnating the windings with varnish? Did you set up a vacuum rig? Are the components (core laminations, bobbins, end bells) readily available?

If I recall correctly, one of the Aussie forum members has done this as well. I can see the appeal of rolling your own as the number of manufacturers declines, but it seems like it would be labor intensive to do it on a small scale without specialized machinery. Personally I'm hoping that continued demand will support a niche market for audio transformers, and that some of the capacity that has been lost will be made up elsewhere. I see Allen has stopped selling transformers, probably because he was using Marvel/Mag Components as a supplier. I'm afraid it's going to be a while before it all shakes out.

[pdf64]

I wonder why smaller OTs have never tended to have much interleaving? Does their smaller physical size mean that their frequency response is ok without it being needed?

[harleytech]

Very Nice !

[Phil_S]

Much admiration for this build.

Quite some time ago, I built an Express like thing with a pair of 6AQ5's, not so different from what you did. The challenge was taming the B+ to an appropriate level for the power tubes while not dropping it too much for the preamp. I thought it came out reasonably well -- a very nice amp -- all sorts of good things to say about it. Unfortunately, it didn't really float my boat and now it just sits there... It needs an upgrade to the plate load resistors to 1W to try to tame the hiss (annoys me, but not bad) and it needs a proper head shell. After I do that, I'll probably offer it for sale here.

I think your "solution" was better. Not everyone can (or is willing) to wind their own. Congratulations.

[katopan]

I've always wondered how an Express with a 6K6-GT output stage would go. I bet it's killer!

Great to see someone else winding their own transformers too. A labour of love (and hate) but you get what you want and in some cases what you can't even buy from elsewhere.

[ChopSauce]

Much admiration for this build.

To say the least...

[wpaulvogel]

Well done! What’s the story behind your decision to wind your own transformers?

With my own transformers I can establish the voltage and current for the power transformer and with the output transformer I can put in the interleaving that I want. I started this because getting Marshall transformers ( vintage wound) was taking so long.
I also noticed on the lower power output transformers that there isn’t a complex interleaving. On this transformer the primary has an equal number of turns per side and the DCR is balanced to within less than 1 ohm. It’s more work than just buying transformers but I’m happy with the results.

Thanks. Did you do a bifilar wind on the OT primary to match the resistances? What about impregnating the windings with varnish? Did you set up a vacuum rig? Are the components (core laminations, bobbins, end bells) readily available?

If I recall correctly, one of the Aussie forum members has done this as well. I can see the appeal of rolling your own as the number of manufacturers declines, but it seems like it would be labor intensive to do it on a small scale without specialized machinery. Personally I'm hoping that continued demand will support a niche market for audio transformers, and that some of the capacity that has been lost will be made up elsewhere. I see Allen has stopped selling transformers, probably because he was using Marvel/Mag Components as a supplier. I'm afraid it's going to be a while before it all shakes out.

I included a picture of the pattern. I don’t use a bifilar wind. It’s a lot of labor but I get the transformers finished more quickly than ordering them custom wound. I have a 5 gallon vacuum chamber that allows me to put my varnish can inside with the coil submerged. I first vacuum the coil dry to remove moisture, then dunk it and vacuum to impregnate the varnish. I leave the coil submerged for 48-72 hours outside the vacuum. I then stack the laminations and go back to vacuum, then 24 hours soak. It takes close to a week to cure out the varnish. Here’s a good balanced pattern.

[wpaulvogel]

I wonder why smaller OTs have never tended to have much interleaving? Does their smaller physical size mean that their frequency response is ok without it being needed?

I think it’s a matter of production cost and possibly the fact that because smaller transformers are for less powerful amps and we may not be able to easily hear the difference in the lower volume stage as in the higher power amplifiers. I really don’t know. Also a complex interleaving of a guitar amp transformer may be overkill because it’s narrow frequency band. It does sound good though