MC30 voicing

invalidbuffa

Super Member
I discovered yesterday that my MC30's were voiced by my grandfather to have more (or I should say adequate) bass with the A7's by changing the driver capacitors C10 and 11 from .22 to a .47 mfd.

.47mfd 600v k40y's are becoming rare and quite expensive... can I achieve the same voicing by increasing C8 and C9 between the phase splitter and voltage amplifier?
 
Whelp, looks like I'm wrong. I measured the frequency response of one amp with the .47's first, then I swapped in the originally spec'd .22's and measured again. Both response curves were nearly identical! I guess the feedback loop is doing its job.
 
I believe you would need to carefully look at the response curves below 14 hz to see any real difference. Maybe 1 dB at 18. The real issue would be in the sub sonic realm below 10 hz.

I can not imagine any tech with any sort of factory connection fussing with trying to correct what can be best described as a speaker dislike by altering the amps bandwidth.
 
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A lot of people do a lot of different things in the name of sonic "improvement" but as you've discovered, with varying degrees of success. In another thread, a member is trying to reconfigure a vintage tube amplifier from a well known manufacturer in order to accept output tubes it wasn't designed for. My opinion is that it's a fool's mission that so far has resulted in a cascading series of issues as successive parts must be changed in order to meet a condition arbitrarily imposed by the owner. Not only is it likely to cause more harm to performance than good, but the value the unit will be decimated.

Takes a great deal of expertise to beat a manufacturer at its own game and a little knowledge is a dangerous thing. If better performance is desired, it's a lot simpler to buy something that was born better to begin with.
 
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Which leaves the mystery of why the cap values were changed in the first place...

could be the original caps were leakers, so someone replaced. They did not have same value so used a substitute value.

On the MC30, the 100uf feedback cap is key. If that is original would replace it with a bypolar cap, same value then bypass the bypolar cap with a .22uf film cap.
 
I owned a JBL SA600 in the late 60's. Whenever I turned it on, for the first few seconds, I could not touch the volume or tone controls or I would get extremely loud scratchy, scraping sounds in my speakers (like 100db crumpling paper). JBL actually told me to reduce the value of the capacitors going into and leaving the pots. The caps were so big, the RC charge up time was long enough to send DC into the pots. Rotating pots with DC in them does not sound good. Bigger is not always better.
 
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Attempting to force more low frequency energy through a transformer without it going into saturation will take a lot of modeling of the circuit.

My suggestion: return the values called for in the MC30 schematic and be done with it. As long as the operating voltages are in spec, you've got one of the greatest amps ever made.

Why dink with it? (Dinking = a technical term for trying to out-guess the designer)

Cheers,

David
 
Just to be clear I have no intention of making any modifications to these amps, other than adding the power supply bypass caps at the recommendation of Jim McShane and reverting them to the 15329-and-up revision.
 
I suspect a polarizing voyage film probably would fit that space. Would that be preferable to the BP?

The cap is actually a cathode bias resistor bypass for the input stage 12AX7 and technically not part of the feedback network. Although a film cap in that position would be nice, it would also be physically quite large and would have to be mounted somewhere on the chassis because it wouldn’t fit on the tag board. Since there’s a polarizing voltage constantly present, a conventional bipolar cap isn’t necessary and considering typical construction of these things, probably undesirable. IMO, a high quality electrolytic is probably the best choice, possibly bypassed by a decent film cap of 1-10uF...pretty much as Mac designed it.
 
Jim included with the recap kit a Nichicon Muse 100v/100uf as a replacement for the cathode resistor bypass. It works fine, the amp sounds great, and I can't imagine doing much else would yield any improvement. I should note that since it's a radial cap the leads aren't long enough to go through the holes on the board and attach to the terminals, so the old capacitor is clipped out and hook-and-crimp is used to install. This discovery was made after I completely desoldered the capacitors, leads and everything. In the future I would go with an axial cap like a sprague atom.

As 62caddy pointed out, it seems like trying to outthink Corderman and his team is an exercise in futility.
 
Jim included with the recap kit a Nichicon Muse 100v/100uf as a replacement for the cathode resistor bypass. It works fine, the amp sounds great, and I can't imagine doing much else would yield any improvement. I should note that since it's a radial cap the leads aren't long enough to go through the holes on the board and attach to the terminals, so the old capacitor is clipped out and hook-and-crimp is used to install. This discovery was made after I completely desoldered the capacitors, leads and everything. In the future I would go with an axial cap like a sprague atom.

As 62caddy pointed out, it seems like trying to outthink Corderman and his team is an exercise in futility.
Pretty much how I feel about substantial mods and tube type rolling on the Mac UC tube amps. Otoh, upgrading parts during resto and simplifying the front end pot and switch clutter I'd do for my own use
 
I have a pair of 60s and a 225 .
Jim furnished both cap sets.
The 60s I wouldn't change a thing on. The 225 IMO is a notch down (still better than quite good)
I don't think the suggestion was outside the realm of what might have been done had said parts been available in 1960.
OTOH , I'd be skeptical that there was much difference either way (assuming everything was up to spec).
I think we all pretty much agree they (MC) got it right in the first place.
 
Curiously, Stu Hegeman amps used metal film power resistors on higher current areas where Mac used carbon clay. Afaik, metallized polypropylene caps weren't generally used yet for audio, but mylar plastic film types were becoming available. The amps would certainly meet spec in measurements with any new stable types, but there are those types that have more theoretically ideal characteristics and better long term stability.
 
Curiously, Stu Hegeman amps used metal film power resistors on higher current areas where Mac used carbon clay. Afaik, metallized polypropylene caps weren't generally used yet for audio, but mylar plastic film types were becoming available. The amps would certainly meet spec in measurements with any new stable types, but there are those types that have more theoretically ideal characteristics and better long term stability.

in the citation amps, some of those were wire wound resistors, very quiet, low noise parts.
 
The AKdatabase has 3 different schematics for this amp, two factory and one Sam's.

All three have different va.urs for the mentioned cap......I believe it is C3. I did not have time to pull out my factory supplied schematic from back in the day but it should not be surprising to see Mac engineering tweek their designs.

It is also noteworthy that they did make a provision to alter the low frequency response in the factory schematics. Interesting.
 
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