Deep, searching Adcom GFA1 questions...

[Stuart Easson]

Hi All,

Looking for some insite into a possible latent failure mode in a pair of amps I am restoring...a sanity check on my reasoning etc...

The GFA1 amp is spec'ed as a stereo, 200w/8ohms, 350w/4, it has a big toroid, fan cooled heatsinks, 10A fuses on primary side, ergo approx. 1kw consumption. Not unreasonable for a high output, high curent amp...even as parts they would be worth the $110 I paid for the pair...but...

The amp as originally designed is bridged, and I don't mean that it's a monoable amp, I mean each of the 2 channels is comprised of a pair of 50-60w amps that are bridged. Nothing intrinsically wrong with that, passlabs, jeff rowland etc make some great bridged amps.

The issue is that each of the 50w amps has a single 2sd424/2sb554 (150w/15a) pair for the outputs, which again as a 50w amp would be fine. But if my understanding of transistor ratings and SOA curves is correct this output stage doesnt have seem likely to have a long and healthy life driving full power into 4 ohms.

The output rails seem to be approx. +/-50v so the absolute max output voltage swing (assuming 10% regulation on PSU, transistor losses etc) will be approx. +/-80v. This already implies peak currents in excess of the transistors ratings, and if one considers the peak power rating of the output transistors they seem to be well past the point of destruction...or am I missing something...I planned on using these as sub amps, but I don't want to spend time, effort and money fixing the amps only to fry them if someone gets enthusastic with the volume...I'd rather take care of the issue, upgrade 'em now while they are in pieces all over the table.

Assuming this analysis is OK, anyone know of more substantial TO3 outputs, of course they need to be similar to the toshiba's characteristics to...

Thanks for any help

Stuart

[Original Burnedfingers]

I assume they are TO-3P (plastic) and not TO-3 metal. The GFA's
I used to own had the TO-3P outputs. If by chance they are metal
I would change them to MJ15024's and 25's. If they are P's I have had good luck with 2SA 1302's and 2SC3281's


Personally I have driven these amps on a 4 ohm load and ended up with smoke. It is my opinion that a better designed amp be used for the task. I share the same feeling for the Crown designs that also contain two 60 watt amps in bridge mode per side.

Joe

[Stuart Easson]

Thanks for your reply.

These are 'real' TO3 metal cases, based on my web searchesnot many people are aware of these particular Adcom amps, as far as I know these are their first products and precede all the multi digit GFA amps by some years.

The schematic was drawn in 1979...updated in 1981, apparently for the GFA1a, which has different protection circuitry, probably needed to defend the output stage...

I'll look into the motorola transistors, the different package doesn't bother me too much, in fact in the plastic form I may be able to use more transistors, paralleling them...

Stuart

[mlloyd1]

i had one years ago. it used to3 metal can output devices. in my opinion, this amp was built before adcom made the step up to the robust designs of the later products. in general, my opinion is one should not used a bridged design to drive 4 ohm loads unless it was specifically designed to do so.

mlloyd1

[Gordie Steinhof]

Greetings,
I am also in the process of repairing a smoked GFA-1. One side burned driving 6 ohm speakers. Inside I found the drivers on the good side had previously been replaced with 2SB600 and 2SD555. Since they survived, I'm going to replace the burned 2SD424 and 2SB554 outputs with them. They are available in matched sets from ampslab.com.
I've been looking for the schematic for the GFA-1, where did you find yours?
Gord

[Stuart Easson]

Hi,

Thats an easy one, Adcom. They have been very generous, I've asked them several times for service manuals and they have always eiher emailed them to me or sent them snail mail. The GFA1 came on paper or I'd just pass it on.

I used mje15030/31 drivers and mj15003/4 output pairs, having fixed it I tested the amp into 4 ohms (resistive) to maximum power with no problems.

Stuart

[Gordie Steinhof]

Thank you Stuart,
I got the schematic and know what parts to buy. A very good start.
Gord

[K-amps]

Originally posted by Stuart Easson
Hi,

Thats an easy one, Adcom. They have been very generous, I've asked them several times for service manuals and they have always eiher emailed them to me or sent them snail mail. The GFA1 came on paper or I'd just pass it on.

I used mje15030/31 drivers and mj15003/4 output pairs, having fixed it I tested the amp into 4 ohms (resistive) to maximum power with no problems.

Stuart

THe Mj15003/4 would be close to the perfect upgrade for 554/424 used in lower rail designs. The other choice would be the Mj21195/96's which are manufactured under TOS licensed processed tht created the 424/554/600/555 in the first place.

[mpmarino]

Hey Stuart,

You got those on Ebay didn't you! I didn't know much about 'em but I was actively bidding. You beat me at the end of the auction.... Thank You ...really!

Marc

[wsu99999]

I know this is a super old thread.... But I am too thinking about using one of these as a sub amp. I have a Dayton DVC310-88 12" Subwoofer in a small sealed enclosure and planned on using this amplifier as the amp.

After hearing about the issues in this thread, I'm having second thoughts. The voice coils are supposedly 8ohm each and 4 ohm if ran in parallel. Any thoughts guys?

First post - thanks for your help!