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

[Bombacaototal]

Thanks for adding that info to the pile, Tony!

Raphael, if you are posting schematics, please sign and date them, so people know the source, and I'm sure there will be revisions.

Is the "Bill" schematic posted somewhere for comparison?

Maybe the title of this thread should be changed, now that it has morphed into documenting a specific model?

And, why would you want to use old rectifiers?

Martin, “Bill”’s schematic was posted by Tony and its called “Pentode_SSS_Preamp”. It is not complete. I will amend mine with his buffer circuit and date, add version and sign as suggested.

As far as the diodes I found some on eBay and thought it would be cool to get something close to original, although I know has no sound impact. Is your concern regarding reliability of the old part?

Agree with the title change

[martin manning]

Do you have a link to that Pentode_SSS_Preamp? I get nothing using a search.

Yes, for something like rectifiers I would opt for new parts from a reputable source, especially where high voltage is involved.

[Bombacaototal]

Do you have a link to that Pentode_SSS_Preamp? I get nothing using a search.

Yes, for something like rectifiers I would opt for new parts from a reputable source, especially where high voltage is involved.

There you go. It was on one of Tony’s post of last page

Noted with thanks regarding the NOS diode. Maybe I should source something new too

[martin manning]

Ah, yes. I had not looked at that attachment closely enough to see the name on it.

Silicon diodes are very unlikely to deteriorate over time, but manufacturing processes may have improved since then. Those parts look pretty good, but eBay sellers are sometimes questionable, and new ones are cheap and easy to get.

[sluckey]

There's nothing magical about the RCA "SK" series of semiconductors. The SK series is just a generic line of replacement semiconductors. The series would replace most any common consumer semiconductor. The series had a 1/2" thick replacement cross reference book. Motorola offered the same generic replacement series that was called the HEP series. Sylvania's replacement line was called ECG. Every electronics parts house would carry one of these replacement lines. It was common to find a wall covered with the entire replacement line as well as a cross reference manual hanging on a string.

Those 60 year old SK9010s are probably just as reliable as their modern 10 cent 1N5408 equivalents.

[rootz]

How far do you want to go? Diodes have different switching characteristics. Some switch fast and clean, others not so much. In Hifi you’d try to minimise high frequency spikes by adding snubbers over diodes or over the secondary of the PT. Dumble made a choice to do so in later amps it seems. #124 no snubbers, #183 snubbers on the diodes. Which begs the question: do diodes have any impact on the final sound or could you use e.g. a readily available UF5408?

[martin manning]

Maybe he decided to put the caps there to help equalize the voltages and improve reliability?

[talbany]

IMO Rectifier diodes can affect the feel of the amp (fast/slow)

Tony

[Bombacaototal]

IMO Rectifier diodes can affect the feel of the amp (fast/slow)

Tony

Very interesting, I used a BY448 on an amp, but never really thought much about it except that I was going over spec 1650V, 4A. Maybe its making the amp faster then?

Also thanks for chiming in Slukey. I am glad I got those old parts anyway, as I am from the NOS camp

[norburybrook]

Oh Shit....here we go ..do I have to build another amp soon?


well done fellas



M

[martin manning]

I’m not buying it. I can see some HF noise being generated, but “feel?” How does that work?

[talbany]

I’m not buying it. I can see some HF noise being generated, but “feel?” How does that work?

Sure That "HF" noise can generate "spikes" in the top end (AC cycle is going through the zero-line)Riding on the supply voltage can affect how the amp sounds and in some cases how the top end feels.. Some well respected builders even prefer to go with a Uf5804 (Fuchs, Two Rock)to help smooth out those spikes and help prevent a buzzy-ness riding on the top end.

On the bias supply as well. The switching noise reaches the preamp via the power transformer into the heater supply into the cathodes,into the signal voltage of the preamp tubes.
Noise has a strange way of permeating out through the entire amp and it's effects are felt through both sound and feel. However most people only pay attention to the sound unless it's really obvious. IMO
Some builders and players might consider the newer ones (UF's) too fast and to smooth (compressed) and have a different character to the top end than the older diodes. Especially the Marshall players.
We experimented around with a bunch of different rectifiers years ago including FRED's in most cases the results were rather obvious.

BTW. I am well aware of the snubber caps across the diodes and bypass caps across the filters and voltage equalizing resistors



Tony

[talbany]

Oh Shit....here we go ..do I have to build another amp soon?


well done fellas



M

NO!

[Aaron]

Thanks for the info Tony. I wasn’t too far off with my guesstimates considering I only had the fuzzy photo copies and the preamp schematic.
The only missing piece of the puzzle was the mixing circuit. Might have to dig my build out and see if it makes much of a difference.
I really like the preamp in this amp. Crank that preamp volume to 10! I bought some 6U8A valves ages ago with the intention of doing a similar preamp with one valve.

You wouldn’t happen to have the resistor value sheet for the preamp?

Thanks,
Aaron

[Bombacaototal]

Thanks for the info Tony. I wasn’t too far off with my guesstimates considering I only had the fuzzy photo copies and the preamp schematic.
The only missing piece of the puzzle was the mixing circuit. Might have to dig my build out and see if it makes much of a difference.
I really like the preamp in this amp. Crank that preamp volume to 10! I bought some 6U8A valves ages ago with the intention of doing a similar preamp with one valve.

You wouldn’t happen to have the resistor value sheet for the preamp?

Thanks,
Aaron

Aaron, I just went through my files and compared to Tony's and there are these additional 3 missing from his posts, one of which is the preamp for resistors.

I spent a fair bit of time with those pdfs and maybe my schematic will save you some time. I have done amendments as per Bill's schematic.

Questions pending:
1) bias network on the PI, are my 2x10K, 2x30k correct? Where should the 47K go?
2) where should the extra 1M go on the tremolo
3) on the pentode is the .1uF going to cathode or ground on the original
4) on the supply there are 6x470k, so the 2x47uF (350V) are indeed stacked. Does my current schematic look good? Any ideas on the 2x 40K (5W) and how to go about the PI driver (which doesn’t have a dropping resistor listed and also is missing a cap supply)
5) the lnfb on the reverb, is the 10k inside or outside? I’ve seen amps taking both approaches
6) The reverb entrance, Bill's schematic has it as per my green line, I used the standard approachm which for me makes more sense. Which is correct?
7) I think the 220K on the reverb return is as per AB763 (mixing resistor), and not after the decoupling as per Bill's schematic.

EDIT: Updated schematic further down the thread