Explaining how to repurpose an amp to a tech

[Sandusky]

A local guy does some amp maintenance and repair, but has never done a repurpose (from an old PA amp to guitar amp). Unit is a McGowan MG20, and I have used it as is, but it needs recapping, clean-up, etc. I would like to be able to tell this guy a little about removing the RIAA eq, and whatever else he would need to know to switch this around for guitar use. I could take it to Atlanta for high dollars and have it done by a reputable and experienced tech, but this guy is closer, and I assume would be cheaper. Also, out of my skill set, so it is a job for someone else.
Thanks-
Mike

[km6xz]

I do not know the model or the brand but a redesign will require you to specify your requirements very precisely because the tech will not know how to proceed until a goal is decided on. Phono preamps are intended for 50k or so loads on an magnetic cartridge, and the preamp has a multi-inflection point eq curve, which will likely be easiest to accomplish by designing a new preamp. Do you have the current specs of the amp? How does it sound? Guitar amps are expected to have a lot more distortion and to be operated in overload conditions much of the time so pa amps frequently, even after creating a tone stack suitable for guitar work, and raising the input impedance, have overload characteristics which are not musical. What compelling reason is there to mod a pa/hi-fi amp to guitar amp? Surely it is not cost, a proper redesign and analysis of what is currently there and fine tuning it to accomplish the performance goals you agree on, will cost more than buying a nice tube Marshal or Fender re-issue. If you only want him to cut out the RIAA feedback path, and be done with it, it might be cheap but it will not be the golden amp you imagine. Someone who can make it sound right will be a design engineer, not a tube amp tinker, and will have a lab full of gear for analysis and will price accordingly.

[Sandusky]

I have been led to believe, by talking to my tech in Atlanta, another in Asheville, and some poking around on the web, that it is quite common to take old amps (Dukane, McGowan, Bogen, Knight, Layfayette, Rauland and many others) that no longer are used for whatever PA/Industrial/other original purpose and redesign them for electric guitar use. I believe this has to do with a relatively healthy supply of these items, and the "good iron" used in the 1950s-1960s; and as these units would need a re-capping at least, it makes some sense to do a bit of other clean-up and rewiring while one is at it.
Atlanta prices 3 years ago would've run about $300, last year's Asheville price would've been about $200, so I guess you are right about being able to buy something new for that money. However, I see no reason to support China with a purchase, I wouldn't get to support my local tech, and the styling on the older units is exemplary.
I may be wrong about this being a relatively common procedure, but I don't think so.
Mike

[Prairie Dawg]

As a practical matter, the best way to do this is to do the absolute minimum required to assess the condition of the amp. You might like the way your guitar or harp sounds just as it is. At the same time youi can get an idea of what kind of internal voltages you are going to be working with. If you don't like the sound, then the best way of approaching this is to build a simple, tried and proven circuit in the space provided, such as a tweed Pro, Masco MAP15 or similar primitive guitar amp.

The internal voltages are likely to be pretty modest so there's no point in building for mucho headroom. I just sent off a Bogen amp to a guy and it was producing 350v at the plates of a pair of 6L6GCs with a GZ34 in the rectifier position. That's pretty typical.

You are right about one thing, and that is that tube PA amps were built to last, and the best of them (the older ones) were very durable. When I moved out to California in 1977 my spouse got a job in a nonprofit agency. Halfway up the wall in the office sat a shelf and on that shelf was a Rauland PA amp-one of the green and gold ones, that had been installed in 1955 when the place was built. It was used for an intercom and announcement system: "Shirley, line 3 please." When we moved to Iowa in 1993 it was still there and had been in more or less continuous service since 1955 and was never shut off. The only downtime was for power failures.

I've got a McGohan MG20B like yours that is going to get repurposed one day. I shall dig out the schematic today and see what else I can add to the discussion.

[olddawg]

In my experience, the only way to really repurpose any of these old amps (PA, Phono, Tape Recorder, etc) is to gut them and start over. Use the "old iron" and the chassis, maybe some of the tubes. Build and install a new turret board (with fresh/new/accurate caps and components), ceramic tube sockets, pots and jacks. Build it based on a known good guitar amp design, use the guitar amp layout if possible, and tweek it from there. The design you use will be determined by the B+ and the filament current you have available. It really isn't that expensive to do it this way (especially if time is a consideration) and will save you a lot of headache and disappointment.