Music Man Power Supply [was Re: #102 died yesterday :( how,why]

[Bombacaototal]

4x6550 + 7 preamp tubes seems like to much for a 7A heater winding. 4x1.6 + 7x0.3 = 8.5A. Much more that the winding provides. Problem remains with 4xEL34. You’d need a separate tranny for the preamp tubes.

Just coming back to this, I realised it's actually 8 preamp tubes. Definately will need an additional preamp tube transformer.

[rootz]

Or a custom wound transformer. No idea if that's an option for you.

[Bombacaototal]

To me they look like those old Schumacher transformers. I am thinking:
PT (also due to the size based on the pic of the rear of the amp) is https://www.tubeampdoctor.com/en/mains- ... /130w?c=73
OT is https://www.tubeampdoctor.com/en/output ... -150?c=292

But the power tubes do look fatter than the 6CA7, so surely 6550. Also the guy who sold the amp at some point seems to confirm 6550 (attached).

But yes looking at the 6550 spec sheet I am not sure the spec would be up to the PT, and the impedance on the OT seems far from ideal...and HAD didn’t seem to mismatch as he kept 4ohms/8ohms

Looks like one of those rabbit holes I got into with the 001 and Borderline tremolo....Only HAD knows what he did on both...

[talbany]

Yes those tubes look to be the old-er GE's 1972 (these are the ones Marshall used in their later amps when they converted over to the 6550's for reliability issues with the 34's in the 70's) Very rugged stout tube. 700V on the plates driven with a cathode follower is going to be loud as hell!! I wouldn't be too concerned about getting the primary impedances perfectly!! matched for optimal power output purposes. Those GE's are were a one of a kind sound the metal guy's love em for their crunch and tight low end. You may be better off spec-ing out for a KT-88 if your going with a newer type tube especially if you are concerned about reliability and longer tube life.
I tried both KT-100's and 6550's and prefer the KT's
Just an opinion here. Do what you will.

Tony

[rootz]

Tony, the silver alligator, was that a one off with MM transformers and 700V on the plates of a quad of 6L6 tubes?

So than it boils down to the question: what power stage do you want to use?

[Bombacaototal]

Tony, the silver alligator, was that a one off with MM transformers and 700V on the plates of a quad of 6L6 tubes?

So than it boils down to the question: what power stage do you want to use?

The silver alligator had the MM transformers, 700V on plates and quad of 6550.

I have a 2x6L6 and a 4x6L6 amps already. Would love to do a 2x6550 (but with the 150W transformers and mismatching the impedance of the OT for the half power). I think the Alligator was a blues amp and the crunch and tight low end you described of those GE might be why HAD chose them. I never tried a KT-100, would be interesting to put them on an amp. Are the PT/OT specs good enough for those? Also thanks Tony for the info that the tubes on the amp look like the GE 1972 6550. They were indeed more rigged back in the days. But working out of spec like HAD did gives me anxiety, specially on the PT side.

Still undecided on the power tubes but should aim for a 2x6L6 with these transformers, more within spec...
PT https://www.tubeampdoctor.com/en/mains- ... /130w?c=73
OT https://www.tubeampdoctor.com/en/output ... -150?c=292

I can unmatch the OT for 8ohms/16ohms and get within spec

For choke this should be enough, right!?
https://www.tubeampdoctor.com/en/choke- ... -etc.?c=57

Just need to find an external mounting 6.3V transformer which can take 10A or more...Or would adding a fender style 6.3V (4A) in series with the PT heaters but at the first Phase Inverter 9Pin tube work?
https://www.mojotone.com/amp-parts/Powe ... ransformer

Also found this handy chart with MM amps, models and their power tubes
http://www.pacair.com/mmamps/Misc__Info ... chart.html

I still need to check in more detail Martin's bias too..

[martin manning]

I think HAD may have taken some inspiration from the 2x 6550 160W Fender PS160, which uses a doubler PS and runs 700V plates/350V screens. Some nice photos here: https://reverb.com/item/162619-1970s-fe ... be-pa-head

[rootz]

Haha 160W from a pair of 6550a! At 20% THD?? An SVT will do close to 120W /pair at 660v/330v, but I can’t remember if that was at 2 or 5% THD. So a quad 6550 @700v should get you at least 240W. Now if you lower the primary impedance of the OT and use a hefty CF driver tube you could go class AB2 and ooze some extra watts out of it. Not that you’d need any more lol...

[martin manning]

These are not Hi-Fi amps, and even vocals sound better when the PA adds some color ;^)

[Bombacaototal]

I think HAD may have taken some inspiration from the 2x 6550 160W Fender PS160, which uses a doubler PS and runs 700V plates/350V screens. Some nice photos here: https://reverb.com/item/162619-1970s-fe ... be-pa-head

Thanks Martin. I am adding the schematic for future referece. Will go through it in detail and see what I can come up with..

A quick question about Pentode operation. The VOX AC30 (with EF86) has the 100nF (C12) cap to ground, but when I was studying Pentode operation at valve wizard http://www.valvewizard.co.uk/pentode.html that cap (cg2)seems to go to the cathode of the tube...Which is the correct?

[donvan]

Just a very tiny derail here...
The SVT schematic a page or two back shows two diodes in parallel, reversed polarity, at the input of the first stage. What is the purpose of that? Clipping?

[sluckey]

Derails are welcomed. Heck, this whole thread is a derail!

The diodes will clip at .7 volts peak, thus insuring that nothing bigger than .7v peak (or1.4v peak to peak) is applied to the power amp. Hopefully, the normal signal level is less than .7v peak. The diodes have no effect on a normal level signal.

[donvan]

Thanks sluckey!

[rootz]

These are not Hi-Fi amps, and even vocals sound better when the PA adds some color ;^)

That is true Martin. Max power often get specified at different THD levels, or no THD level is mentioned at all. That makes it hard to compare. I also wonder if It wouldn't be better to work with more conservative voltages and double the output tubes to get more reliability. I must admit though that I'd like the challenge to make an amp like this work reliably at high voltages. Vg2 at half Va seems to me like a good starting point for reliable operation.

The driver stage of that Fender PA amp is interesting too. Transformer used as phase inverter and a 6V6 driving it. One benefit I could think of is class AB2 possibilities without a CF or Source follower MOSFET driver for the output tubes. This observation might be slightly off topic though, again

[Bombacaototal]

Like any conversation, it may start sat a topic and deviates to many other things, landing on yet another subject! Keep the derails coming

I am back looking at the bias. I am fairly keen on trying the doubler arrangement. Thanks Rootz, Tony and Martin for the suggestions.

Or you could use the 32-0-32 VAC @100mA winding with a doubler. Use a dropping resistor from the screen node and a cap to lower the voltage at the plates of the PI. To recap: several solutions possible.

Again the 200-130 has a 35-0-35 V tap for the solid state preamp. You could run a voltage doubler off that would give you around 70*2 -140*1.41= 197.4 V DC regulate it and set up a bias range pot to dial it in might be another option.Otherwise buy a dedicated transformer

The TAD MM200-130-250 PT's 64VAC (32-0-32) would work fine using the quadrupler circuit I posted a while ago to use the 60V bias tap on a Twin PT.

I put together a schematic. As per Tony suggestion, given the voltage will give around 197.4 VDC I thought about using a dropping resistor (probably between 5K and 10K) and a supply cap to feed C-

I am grounding yellow (the central tap of the 2x 32V).

Would you please advise if there are any mistakes on the attachment.