DIY Audio Projects Forum

I inherited what looks like a 1970’s kit-amp and I really need help finding out what it is. There is no writing on it of any kind except for "Radio Filter" imprinted on a small box connected to the power cord. It was my father’s (who was an electrical engineer), and it was “bundled up” with a Southwest Technical Products Corporation (SWTPC) “198/A Preamp” (found here: http://www.swtpc.com/mholley/Catalog6/Page01.jpg). I searched the internet for pictures of SWTPC kit-amps of that era, but had no luck. I know my dad built both of them, so I would love to put them to good use, but I don’t want to hook them up without knowing more (especially about the power amp).
Thanks in advance!

The following pictures are:
(1) Amp front-exterior (2) interior (3) Filter? connected to cord (4) rear-exterior

From some help from another forum I discovered that the "box" resistors are 40409 and 40410 and the outputs are RCA 40411's. Other than that and that it's maybe 70 to 100 watts per channel, I don't know anything else. Here's another picture:

I found out some great stuff about this amp!
I was informed that it's a class AB amp (two "Brute 70's" put together for 70 watts r.m.s. per channel stereo - from Popular Electronics, Feb 1967) with less than 1% harmonic distortion at any power level, and with a built-in 35 dB negative feedback system. According to the magazine article, also included is a "thermal feedback loop to enhance stability," and the "Brute 70" was called the "Rolls Royce" of power amps. I guess my dad built it to specs, except that he mounted the PCB outside the chassis and next to the heat sink. it's a superb amp that's comparable to 280 watt 'modern' amps that have very little distortion.

Did you find this site? http://www.swtpc.com/mholley/PopularEle ... eb1967.htm

Gold, bone!

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design

I'm not sure it's quite "gold" like Geek wrote, but I really appreciate your help anyways.
It's a page from the same site (http://www.swtpc.com) that I listed on my original post (http://www.swtpc.com/mholley/Catalog6/Page01.jpg),

I had one of those built for me back in 1968 and used it as a PA slave amp for several years. It sat in storage for about 40 years and I have recently resurrected it as a guitar amplifier. The amp is monstorus! Very clean, a bit hummy but extremely powerful. My amp was made using mil spec parts and the 40406/07/08/09 were used as is but the driver stage was done with 2N3767/2N3741 instead of the 40409/40410 and the output transistors were 2N3055's instead of the 40411's. It also runs on +-56 volt rails rather than the published +-35 volt.

I have blown the thing up and when it goes it goes with fire and smoke!!! It blew up because of old cracked brittle wiring and not anything to do with load or demand. In repairing it I found I had cascade failure back down to the 2nd stage. In replacing the transistors I found that you need particularly robust replacements, the regular replacements just won't haul the freight. This is most probably due to the custom beefing up done by the original builder because +-56 volt rails are technically too high for the parts, but mil spec stuff is always beefier than standard commercial parts. I switched the outputs to MJ15004's because they are 25 amp ones versus the 15 amp 2N3055's. The driver stage seemed happy with plain old NTE replacements as did the pre-stages.

It's a touchy SOB but once you get it dialed in, you got a monster on your hands.