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My boss bought a C24 preamp to go with the quasi MC275 clone I built for him 6 years ago. A C24 is cheap compared to other McIntosh gear, he got it for $404 with shipping. He has a golden ear and decided it sounded not all that good, but it could be made to sound good for a song, but the next song would have to be adjusted. He decided he would like it to be tubed. The circuit is very close to the MX110 of the same era, but it seems that McIntosh just replaced the tubes with transistors; like they used the same 1M bass and 500K treble pots. The C24 also has a hiss to it, even though it uses a double volume control, with a volume control just before the tone controls, and a volume control just before the output connector.
He wants the tone controls, and I would like to leave as many controls intact as possible; like the HF, LF filters and loudness.
So I'm wondering, I could just tube the tone controls with a 12AX7 on each channel with one side for gain and the other as a buffer, leave the phono preamp transistorized, and then put in a 12AX7 on each channel with one side for gain through the filters and tone control leaving me about 10db overall gain, and the other for a buffer out.
The C24 has concentric tone, balance, and loudness controls. I found some Fender either bass amps or bass guitars that use the same controls almost. http://www.guitarpartsresource.com/imag ... 36-000.jpg
It's 100K so I could put in Morgan Jone's tone control, or Sy's tilt control, since my boss owns a Quad 34.
What should I do?

Call in sick ?

I personally would not try re-invent the wheel. I personally would do a minor refresh
on the C24 and sell it to buy a tube preamplifier. Get a Conrad-Johnson and refresh it.
A C20 while it DOES sound good, not good enough to justify the price . If you really want to do
the downgrade(?) on the C24 why not add a Baxandall tube tone circuit. There are also
a couple of Vendors that sell McIntosh C22 clone pcbs.......

Just a couple thoughts.......I was thinking this through a few months back.

He was looking at a C22, but couldn't afford it. Then a MX110, they are much cheaper, only a little over $1000. Then found the C24.

Please tell us about the "quasi-clone" MC275 you built. Did it use the Plitron/Amplimo toroid Unity Coupled transformers?

tomlang wrote:Please tell us about the "quasi-clone" MC275 you built. Did it use the Plitron/Amplimo toroid Unity Coupled transformers?

Nope, it just looks like a MC275. He couldn't afford them. I Think I posted a picture of it, but I used Edcor 60 watt transformers. I think it is a Posiden with KT77s.

I would build an outboard phono and line stage and only use the switching of the mac , and make everything reversible .Build the tone controls into the outboard line stage , where they can be set once and forgotten . Use the volume control of the mac also .

There's either something wrong with it, or it is just crap. It was supposed to have been professionally serviced with a 60 day warranty. I'm getting, with the volume half way up, 100mv of hiss and a even louder hum that goes away when I put my oscope ground on. The only electrolytic that I found that was replaced, is installed backwards. The hiss is probably coming from the zener that's regulating the 10 volt supply. There are lots of carbon comp resistors, but they shouldn't make that much noise.

Mc enthusiasts, won't be happy, but I am extremely pleased with the outcome. The owner likes it too.

This is Sy's tilt control, with a added bass boost, to better match the Quad 34.

I also put in a NE5532AP phono preamp, and a CMoy headphone amp, because I ran out of room for toobs.

The C24 appears to be one of the last pieces of gear from McIntosh that wasn't backlit.

Pictures

Is that the original 4-section volume pot, or where did you find one?

All the controls are original, I thought I would leave them incase someone wants to turn it back to a C24.
I'm not using the rear 60K pots.
The bass pot was left unchanged, the treble has 2 X 30K in parallel to make the pot about 50K, and the loudness contour was bypassed. I guess it acted as a loudness, but it mainly lowered the volume to do so.
Another strange thing, you had to install your speaker wires from your amp, back into the rear of the C24 for speaker switching, phase changing and headphones.
The 2 can capacitors have been restuffed.

Can you post a pic of the MC275 look-alike to go with this?

It's at his house, and it's a bit heavy to tote around. I'll see if he'll send me a picture.