Help with Old Triad Power Transformer

[sunnydaze]

Hello everyone,

I'm thinking of building an amp using parts I have laying around. One of those parts is an old grey lay down style Triad PT. Has "Leslie 100-11" stamped on it, and the triad triangle in a box with the number 12 below it.

I've figured out the primary and secondary relationships - First, I checked the continuity between the wires and mapped it out, then I put 10 volts on the primary side and measure the results on the secondary. see the attached PDF.

Primary
Black -- Black

Secondary
Red - Red
Green - Green
Dark Blue - Light Blue - Dark Blue

The problem is secondary HT voltage is too low for what I have in mind. So trying to rig it in series to get closer to what I need. I've never tried this, except when using a new transformer and following the manufacturers recommended hookups.

If I hook the dark blue and red together in series, with my wall voltage of around 112 VAC (taiwan) on the primaries, get about 400 VAC. That's a bit high for what I have in mind. However, if I go with the reds and the blue in series but use the a red and the Blue/White as the output, get around 320 VAC, which is close to what I need.

Is there a problem with doing this, and should I just keep the blue and red which are tied together and unused isolated with shrink wrap and treat the PT as one with no CT . I'm not really sure how this should be handled.


Thanks in advance.

Mike

[martin manning]

http://www.ax84.com/bbs/dm.php?thread=26006 ID's this PT as being from Leslie 102 power amp, the schematic for which http://www.captain-foldback.com/Leslie_ ... cs/102.gif matches the configuration you have. Voltages for two of the three secondaries seem to line up, but the third (HV CT secondary) doesn't. Are you sure this PT is good?

[sunnydaze]

Thank for the quick reply Martin.

No, I'm not sure if it's good. I wasn't able to read the numbers on the schematic well enough to compare. When I used the "+" to upsize the schematic, the numbers blurry on my laptop.

I did recheck the AC voltage on the red HT with full 112 wall power on the primary - about 238 VAC. What should I be seeing?

Any comments on hooking the blue and reds in series to get higher voltage? I couldn't see that schem very well - are they using a voltage doubler?

Thanks
Mike

[sunnydaze]

I'm not sure how it will do under load, but I can say there was no continuity between primary and the secondary or between the various secondary windings. Only the blue had the CT, which was the light blue/white wire.

Mike

[martin manning]

The non-CT secondary shows 240 VDC after a FWB and two stages of filtering, so something north of 170 VAC, assuming 1960's USA 117VAC input.

Looking again I see the CT'd winding is shows 100 VDC from a FW rectifier, so that should be >70 VAC from each leg to the CT... looks ok, and the voltages are symmetrical too.

[sunnydaze]

Sorry Martin, I just realized your note said the HT with the CT looked off. That's the blue wires with the light blue/white CT.

I re-checked all the secondaries with full 112 VAC wall power on the primary:

Red HT's - 238 VAC
Greens - 13 VAC
Blue (measured at the ends) 175 VAC.

When I get to work on Monday I can print the schematic you provided and should be able read on paper.

Thanks
Mike

[sunnydaze]

Martin - thanks for finding the links and helping out on this. I took me a little while to see the 100 DC from the CT you mentioned. I guess you are taking the difference between the 400 VDC on the top versus the 300 VDC from the CT.

That power supply schematic is a little different than what I'm used to seeing, will need to study it a bit.

If I wire the red non CT HT with the blue HT in series and but use a Red and the blue/white as the "ends", I get about 320 VAC, which will work for what I have in mind. That leaves me with a blue and red connected together and an unused blue. The attached file shows what I have in mind.

I think the safest route would be to use bridge rectifier on the red & blue white connections and leave the blue and red series connection isolated rather than treat as CT. This leaves me with an unused blue, not sure I can use as a bias supply since the its partners (the other blue and blue/white) are now mixed up with the red windings. Any comments or guidance would be appreciated.

Thanks again.

Mike

[martin manning]

You'll have to use a FWB since there is no "centered" center tap. 320 VAC will yield something like 450 VDC. The current capability of the two windings is unknown, I guess, so that is an open question.

I believe you can use the other lead from the blue winding for a bias supply. Just treat it like a bias tap on a FW rectified power supply.

[sunnydaze]

Thanks for the help, Martin.


Mike

[tictac]

I've got one of those PT's too.... as the earlier schematic shows the Leslie power supply stacks the two B+ windings to get 400VDC and 300VDC. I believe this

believe this PT powered four pairs of 7??? (the 6BQ5's with the higher plate rating cant remember the exact number) so it should be pretty beefy for a variety of applications

I'm using mine to build an amp that uses 1625's (807 w/ 12v fillaments) which is why I like the 300v winding for the screen supply...

Got mine for $10.00 on Ebay; couldnt pass up such a bargan and it's a Triad!

TT

[sunnydaze]

Interesting Tictac. I got mine a couple of years ago on Ebay bundled with three smaller Triad OT's. Not as good a deal as you found, but good.

I was able to print that schematic provided by Martin and can read it a little better now. Still can't read many of the values.

It looks like the non-CT HT winding (red in my case)is going through a FWB rectifier. The other HT with the CT (blue in my case) is FW rectified, but the CT looks to be "stacked" as you say on a couple of caps, the bottom of which is connected to other HT (red winding post FWB rectified). If I'm looking at that correct, it's the first time I've seen something like this

I'm planning to use mine to power a couple of 6l6's and a bunch of 12XX7 type tubes. Looks heavy enough to cover that. As you probably know, this is a lay down style PT, and luckily, it fits exactly in a chassis I picked up several years ago. It's a big chassis, and this will end up being my experimenter amp.

That "stacked" power supply is interesting, especially the dedicated screen supply. May give that a shot.

Thanks
Mike

[tictac]

Yes the stacked PS windings are not used much in guitar amps but I guess they're pretty common in old Tektronix O-scopes.....

I like that you can use a lower voltage filter cap; you could really increase the filtering to the plates without affecting the rest of the power supply to the screens and preamp....

Another option you may consider is a building a true class A amp since you've got a lower voltage high current PT. The 300V winding can be the B+ and the 100V center-tapped winding could be a high voltage, high current bias supply if you wanted to direct couple the power tube grids....

just a thought...

TT

[sunnydaze]

I like that you can use a lower voltage filter cap; you could really increase the filtering to the plates without affecting the rest of the power supply to the screens and preamp....TT

Which cap are you referring? I couldn't read the values on the stacked caps, kind of curious as to what those values should be.

Another option you may consider is a building a true class A amp since you've got a lower voltage high current PT. The 300V winding can be the B+ and the 100V center-tapped winding could be a high voltage, high current bias supply if you wanted to direct couple the power tube grids....

just a thought...TT

This time I'm trying to use up some of the excess supply in my parts bin. Don't have any single ended OT's. With the blue & red winding's in series, I should be able to get B+ in the neighborhood of 425Vvdc to 450 vdc or so. A lot of terrific guitar amps in that range.

I travel quite a bit and when I'm home, I try to spend time with my three year old daughter - so probably take a while before anything materializes.

Mike

[tictac]

I was thinking Class-A push-pull; I believe Alesendro is one of the builders who make an amp like this.....

TT

[sunnydaze]

Well, it's going to be an experimenter amp, could very well end up being class A PP one day.

Thanks
Mike