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RevMen 10th March 2011 12:19 AM

Removing Tone Controls from VoM Preamp/Amp
3 Attachment(s)
Hello, Tube Friends.

My current project is turning a Voice of Music console pull into a stand-alone integrated amp (of sorts). I hope you can help me plan what I recognize is a common modification, but one I still am not sure about.

I successfully restored it to its original operating condition by replacing caps and a few resistors, as well as a good DeOxit session on the pots. It works great, but I really hated the way the "Tone-O-Matic" loudness control sounded, so I removed it completely, which was pretty easy.

Removing the loudness circuit improved the sound drastically, but it's still not quite there. It has a nice, full, powerful sound, but still is lacking that clarity and presence that I get from my K12G kit amp. I think my next move, then, is to just remove the tone controls completely.

So I'm kinda fuzzy about how to do this because the preamp has, as I understand it, 2 gain stages, the second of which incorporates negative feedback from the output side of the OT. I've been comparing my schematics to schematics of other preamps, but not quite seeing enough similarity to determine my next move. I feel like it might be a very simple operation, but just don't have the experience to recognize it. I've looked at all of the similar threads here I can find, but still don't see the answer. I guess I need some hand-holding.

Attached are the amp and preamp schematics, as well as a diagram showing the components I removed when I disabled Tone-O-Matic.

Eli Duttman 10th March 2011 01:22 AM

1 Attachment(s)
I read the power amp as having 1 side of the PP pair driven directly from the preamp and the 2nd side being driven by an inverting 12AX7 section. IMO, that's not at all impressive.

You said integrated amp. From that, I gather you are using a digital signal source and are not using the optional (?) tuner. If I read things correctly, that unit came with a piezoelectric cart. in the TT.

You have the makings of an "El Cheapo" variant. Get rid of the preamp and the unused 5 VAC filament winding can be voltage multiplied to yield the B- rail.

BTW, as was all too common in that era, the 5U4 is being abused. The schematic shows a 60 μF. 1st filter capacitor. The 5U4 data sheet gives 40 μF. as the max. value.

RevMen 10th March 2011 01:57 AM

Sources are a DAC and a stand alone phono section, and whatever other line level input devices I may have. I'm not using any of the original sources. I'll be simplifying the input selector to just be direct between input jacks and a new, simpler rotary switch.

I am considering just tearing it all apart and building something simpler and probably better with the tubes and iron. Maybe that's the way to go. I would like to try bypassing the tone controls first, though, to at least hear how it sounds.

Eli Duttman 10th March 2011 03:34 AM

Mouser stock # 105-14572 will nicely take care of the replacement source selector requirement, at modest expense.

While you may have issues with too much gain, eliminating the tone controls is easy enough. Discard everything, other than the NFB connection, to the left of the final triode of each preamp channel. Ground pins 1, 2, and 3 of both V1 and V2. Buy a pair of inexpensive 100 KOhm log. taper Alpha Taiwan pots. from Mouser. Route the wipers of the new selector switch to the "tops" of the new controls. Connect the new controls' wipers to pin 7 of V1 and V2. Ground the "bottoms" of the new controls.

RevMen 10th March 2011 02:39 PM

Excellent. Thank you.

I suspect I'll just be rebuilding everything as an El Cheapo or Musical Machine variant in the future, but for my own edification I would like to experiment with the current arrangement.

danneskjold 19th May 2013 04:49 PM

Hi RevMen!

Did you go throgh with this mod? If so, any advice for a newbie who is trying to do the same thing on an identical unit?

Full thread here

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