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

[rootz]

Maybe something like this? Based on the TAD transformer posted above.

+/-250VDC (bipolar) for the CF, -45V for the bias (easily adjusted to fit the right range) and 720VDC B+

[martin manning]

Maybe something like this? Based on the TAD transformer posted above.
+/-250VDC (bipolar) for the CF, -45V for the bias (easily adjusted to fit the right range) and 720VDC B+

It would be much better to use a small auxiliary transformer. I posted a schematic here: https://ampgarage.com/forum/viewtopic.p ... 89#p361489
Note there is no need for a traditional negative bias voltage when using the CF driver.

This discussion really should be moved to its own thread!

[rootz]

Are you saying you do not need to bias the CF grids negatively?? Because that is exactly how I did it in a power amp I made. See the schematic.

Agreed on the aux psi for the CF. A simple isolation transformer would do.

Also agreed on moving this subject to its own thread.

[martin manning]

Are you saying you do not need to bias the CF grids negatively?? Because that is exactly how I did it in a power amp I made. See the schematic.

No, once you have +/- 160V (or so) supplying the CF driver stages, the voltage at their cathodes can be adjusted to bias the power tubes. See in the schematic at the link I posted. What I meant to say is there is no need for the adjustable negative voltage supply you show hanging off of the bipolar supply.

[rootz]

Saw your schematic. We do the same thing in different ways Martin. There are dozens of solutions to bias the CF grids. On a side note. You could take the positive voltage for the CF plates from the screens node. Might be a bit too high though.

Anyway, back to Marcus

[martin manning]

Ah yes, you are right, I've included that elsewhere. Dozens of ways to bias? I don't know about that... The SSS schematics posted here take the positive side from the HT rail, and that works, but it puts a very high potential across the drivers.

[rootz]

Yes, always a negative voltage off course, but look at many Williamson derived power amps from the 40's onwards (like some Heathkit or McIntosh designs for example). All doing the same in slightly different ways. The SVT is a nice example too. I find studying the schematics on sites like http://lilienthalengineering.com quite helpful, so many clever ideas and many from high end amps when CF drivers were more common. You probably already know this site.

I think you are better off with a separate iso transformer. Smaller caps (lower voltage), less caps, pretty cheap transformers too.

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.

[martin manning]

I was not aware of that site, rootz, thanks for the link!

[talbany]

I agree plenty of ways to go about it. Mine is a parasitic coming off the B+ supply. The problem with these are this tends to give you a much slower (spongy) recovery time for the bias supply as well as a wider operating range fluxuations, so this may affect performance especially if you play at high volumes. I've never had any problems with it but if I were to do it again would use a seperate X-former or as SVT does it w/ dedicated tap. 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

Tony

[martin manning]

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. Several SSS have been built by TAG members using that design. rootz shows that same rectifier arrangement above.

Layout is here: https://ampgarage.com/forum/viewtopic.p ... 05#p381805 Just connect the HT rectifiers as for a doubler and that's it.

[Bombacaototal]

Martin, Rootz and Tony, many thanks for the suggestions and discussions.

The bias arrangement for this circuit is more akin to the other SSS amps and not similar to the 002. I will add some details just to get a full picture and include some additional questions.

Based on this post, there is a list of values used for the shunt bias adjustments
https://ampgarage.com/forum/viewtopic.p ... art=30#top

the bias pots were 10k each. shunt resistors bias adjust: 2x10k and 2x 47k, 2x30k, 2x330k

Taking the schematic kinldy prepared by Aaron here (attached) on the inspired build he did using Twin transformers, there seems to be missing 2x10K from the bias network. Would you guess they are towards the ground in series with the 33K or towards the Cathodes of the CF tube in series with the 47K? Or somewhere else?
https://ampgarage.com/forum/viewtopic.p ... r&start=15

As far as mains transformers TAD has two options:
130W (P# 2-130-E ): https://www.tubeampdoctor.com/en/mains- ... 5-130?c=73
100W/130W (P# MM200-130-250): https://www.tubeampdoctor.com/en/mains- ... /130w?c=73

The latter, TAD MM200-130-250 PT's 64VAC (32-0-32) is the one suggested. Martin, is the pdf attached ("Rectifier_&_Bipolar_Supply_for_CFDriver.pdf") the circuit you are referring to? The 60V tap-powered bipolar bias supply and HV rectifier on the same footprint as the CE chassis/J Boarders PCB? Let me check how to integrate the two orange wires from the PT are (32V) and the yellow (0V) to this design.

The green wire from the PT has 7A, and I don't think a second 6.3V transformer will be needed. It probably can cope with the power tubes and 7 preamp tubes.

Now will the power rating handle the 4x 6550? I am even considering a 2x6550 instead..

As far as OT TAD has two options (both 4ohms / 8ohms):
135W (Raa ca. 4.7k Ohm): https://www.tubeampdoctor.com/en/output ... 135w?c=292
150W (Raa ca. 2.8k Ohm): https://www.tubeampdoctor.com/en/output ... -150?c=292

What would be better, unmatching the impedance on the 135W to get 2.3K or the 2.8K from the 150W? I think 6550 should be closer to 2K, correct.

[rootz]

4x6550a at high voltage I only know from the datasheets and SVT. 600v plates, 300v screens respectively 660v plates, 345 screens. Either way, it isn’t in line with the MM amps and the transformers offeredAFAIK. EL34’s or 6L6 are. So, what power tubes do you want to use?

Not too sure about the primary impedance with EL34 on 720v plates and 360 screens. With 6550 that is easy: it’s in the datasheets.

Alligator schematic shows a kind of SVT biasing system. Even the big 150k stoppers are there.

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.

[rootz]

From the 1969 Philips EL34 datasheet: two tubes push pull, 800v plates, 400v screens —> 11k load impedance. Oh and 100W at 6% THD. Thus 4k7 for 4 tubes at 700V seems quite in line with the datasheet.

The 2k8 transformer looks to be for 4x6L6 at 700v plates, 350v screens. Can find the schematic of the 2275-150, shows this. Can’t yet find this operating point in datasheets though.

[Bombacaototal]

Thanks rootz. Attached a pic of an HD130 and the back of the Alligator. I think the transformers look fairly similar (and both are 4ohms / 8ohms taps). I wonder if HAD went with the 6CA7 power tubes, used on the original HD130 design?

Yes indeed it uses the SVT biasing system, but seems to have 2 resistors too many compared, taking into account that Aaron schematic is missing 2x 10K...I am attaching the SVT schematic as well

[rootz]

Transformers might be similar because of rated power? Not an expert on this though. Alligator is 4x6550 though. I’d expect the primary impedance to be around 1.8 to 2k. Hopefully someone with a definitive answer will step forward.