Question about Music Man Rd50 Cap mods

[sjturbo]

I would appreciate some inputs on a cap question please. Few years back Jooshtin posted a schematic with BobW's mods as well as some he came up with for the MM RD50. Specifically some changes were made to the ,I believe supply caps, C33,34,35. These are intended to tighten up the bass, correct? Will I notice a big change?
https://tubeamparchive.com/download/file.php?id=2201

[JMFahey]

Short answer: very very little audible difference, if any at all.

Those stating punchier/authoritative Bass are Audiophiles going from, say, 4700uF caps in their SS amps to massive 22000uF or 47000uF ones and driving sub bass cabinets where it's noticed and matters, doubt you can hear a difference with such a small change and with a guitar amp driving guitar speakers.

[sjturbo]

Thanks much JM! I'm no cork sniffer so I thought I would ask! Do you think there was any other purpose? Would it reduce hum at all? While I'm asking, in order to calculate the plate voltage I need to know the PT specs correct? Anyone know where I can find those? I would like to make sure I know what I want to do before tearing into the amp!

[Stevem]

The schematic is showing a massive 585 VDC on the plates of those 6l6gc tubes !

[JMFahey]

Thanks much JM! I'm no cork sniffer so I thought I would

ask! Do you think there was any other purpose? Would it reduce hum at

all? While I'm asking, in order to calculate the plate voltage I need to know

the PT specs correct? Anyone know where I can find those? I would like

to make sure I know what I want to do before tearing into the amp!

Going from 60uF to 100uF will definitely not hurt, plus today 100uF is a

standard value, easier (and cheaper) to find and probably even smaller, go

figure.
It will also have a little less hum, so go for it .

As of voltage, just quoting from memory (search and post the relevant MM

schematic) , it's a peculiar supply:

1) it's relatively high voltage compared to classic values, please recheck but

I seem to remember +V voltages around 700V DC (or 650VDC)

2) it's dual voltage selectable, to provide full and roughly half power.

3) it's a voltage doubler type, so caps, which are in series, will have to

stand 350V DC each ... of course you need some extra rating for safety

and long life, so buying 2 x 100uF x 400V would be fine.

4) VAC rating for transformer secondary will be 0.707 x desired DC

voltage.

So to charge each series cap to 350V DC each, you'd need a transformer

secondary rated 0.707*350=247VAC .

Just from memory, it might have been rated 240/250V AC on "high" and

have a tap at, say, 160 VAC or thereabouts on "Low" , but please check

the actual circuit.

[sjturbo]

Thanks Stevem! I remember that this amp runs a high voltage to the plates.
JM, I don't have the specs for the transformer but a little background. I bought the amp new in the early 80's and it has never been out of my site. It has the original Sylvania 6l6's and likely has never been abused. As amater of fact the amp rarely goes above 3 or 4 in volume. I have been interested in BobW's mods for a long time but have not had the nerve to tear into it. Regarding the caps, you believe I should go to 100uf 400v on al three of the caps? Can you give me your thoughts on BobW's mods and Jooshtins? I have PMed BOBW but he has not responded. Thanks again.

[Reeltarded]

The schematic is showing a massive 585 VDC on the plates of those 6l6gc tubes !

Check out the ones with 6ca7s.. that is tickling a demon.. insanity.

[Stevem]

Yup, I know as I have worked on quite a few of them with there 700 volts on the plate!
I have never really liked the sound of these class B run output stages in these Amps when they where cranked up passed 5 or 6, plus there is just too much SS in there responce for me also.
Can't say why as there are Fully SS amps that I like!

[Reeltarded]

I wonder what one of those little RS $15 meters looks like on 720v.

[ToneMerc]

Yup, I know as I have worked on quite a few of them with there 700 volts on the plate!
I have never really liked the sound of these class B run output stages in these Amps when they where cranked up passed 5 or 6, plus there is just too much SS in there responce for me also.
Can't say why as there are Fully SS amps that I like!

The 4-5 Dumble clones that I built with the Musicman style plate supply sound great cranked.

TM

[ToneMerc]

The schematic is showing a massive 585 VDC on the plates of those 6l6gc tubes !

Current issue JJ 6L6, EL34 or 6CA7 no problem, Sovtek 5881 not a problem as well.

TM

[ToneMerc]

Regarding the caps, you believe I should go to 100uf 400v on al three of the caps?

Use either a 80uf or 100uF/450V F&T at the plate/screen supply and a 22uf /500V and don't look back.

TM

[Reeltarded]

The HDs make great power amps for an MP-1. Stunning in a real sense.

[sjturbo]

Current issue JJ 6L6, EL34 or 6CA7 no problem, Sovtek 5881 not a problem as well.
TM[/quote]

Thanks ToneMerc, for the suggestions on the caps. Regarding the tubes you listed, would rebiasing be needed on these? I currently have the original Sylvania 6l6's in it. By the way I sent you a pm. Please let me know if you received it. I'm having some trouble with the website and I wondered if my pm's are also not getting received. Thanks!

[JMFahey]

FWIW those cathode drive MM squeeze down to the very last drop of juice from tubes, when one starts going weak just *junk* the tubes because there's no one drop left.

As of being somewhat SS sounding (in a good way) , yes, it's true.

A good part of tube sound is the interaction between a low current capability PI driving power tube grids to death, and suddenly having to supply actual current (when power tube grids turn positive), a task they were never meant to achieve, and grid rectification appears.

That grid rectification also shifts bias in a dynamic way, and wildly changes operating parameters, real time.

All that creates a complex interaction which gives standard tube amps a special flavour.

None of that happens in MM, my personal interpretation is that they are plain SS amps, with the power transistors boosted by tubes, which add *some* flavour, no doubt about that, but not all of it.

FWIW it's easy to see, just overdrive a MM and scope it.

The clipping signal is quite squareish, very close to an SS one.