G&K 800RB keeps blowing up

[abbey road d enfer]

There's a circuit that supposed to be adjusting for heat on this right?

That's transistor Q13, which is mounted on the heatsink, with thermal compound to ensure proper heat conduction. Of courese, if either Q5 or Q13 is duff, it doesn't work.

 

[gridcurrent]

short out the forward bias circuitry until you are confident the amplifier is stable.
consider an output inductor (typically 10 uH or less),
often essential in harsh environments such as long speaker cables.

 

[JMFahey]

When a PCB gets so trashed it can´t be relied on.
As CJ mentioned before, lots of tiny traces broke here and there, there are tiny cracks you may never catch.
With the components, $$$ time and effort invested you could have bought another ... at least a working one on Craigslist or even at the used rack at GC.

Worse is you have zero guarantee it will hold.

I would junk the power board and install one of these inside:
https://www.amazon.com/WinnerEco-IRS2092S-Channel-Digital-Amplifier/dp/B0711BC7SX

You went well beyond duty, trying 3 times, now go for the Plan B.

Amp will now become a *working* 500W/4 ohm or whatever the supply can give it mono Bass amp, will sound every bit as good as before, but of course you will lose biamping.

If so important (I guess not), you will certainly find a lower power module which is happy with the secondary supply voltage.

But from my experience, players, and specially Bass players, are very happy just being able to plug a nice cabinet and rumble away.

 

[abbey road d enfer]

I would junk the power board and install one of these inside:
https://www.amazon.com/WinnerEco-IRS2092S-Channel-Digital-Amplifier/dp/B0711BC7SX

It's not that simple; the rails are +/-85V, exceeding by 20% the max rating.
It would be possible to power this module from the +/-60V rails, but the power would not be the expected 300W.
BTW, forget about the 500W rating of this module, marketing inflation.

 

[JMFahey]

Wow, didn´t remember this old amp used such crazy high rails for *just* 300/4  .

For that power/impedance Industry standard  is +/-70V or so, sometimes +/-65V if you have a good power supply , hence my suggestion, which works perfectly in most any Ampeg (SVT3 Pro=+/-65V),  Fender (BXR300=+/-70V) and so on and on and on.

You´ll find similar rails in similar power Harke, Ashdown, Laney, whatever, ... because .... um .... that´s what´s needed.

300/4 means 34.6V RMS , round that up to 35V RMS.

Which means: 49.5V peak.
Add slightly over 4V for transistor drop, + emitter resistor loss, you need +/-54V rails.

Now real World supplies drop under load, plus ripple appears; an average supply drop in a well designed commercial amp (DIYers and Audiophiles don´t count, they don´t have to work within a budget) is around 15%.

So no load rails in a Commercial 300/4 amplifier must be around (54/.85)=+/-63.5V
Call it +/-65V to have a little extra margin and we are talking.

Spec +/- 70V if you want even more margin andd/or use a cheaper/smaller/lighter transformer.
Or if you use MosFet outputs which often need somewhat higher voltage drop.

But +/-85V is *crazy*  , it means dropping (54/85)=36.5% !!!! 

Only explanation I find, because GK are no fools, is that they do it on purpose, to get more "dynamic/musical" power for the same RMS rating.

FWIW more modern GK700 RB still uses +/-85V rails ... but it´s rated 480W RMS into 4 ohms !!!!

Back to the OP problem: pity he got stuck trying to repair such an unusual amplifier, with such a stressful supply (it´s definitely not easy on the amps it feeds) and to boot with a nuked PCB.

Still offer my "Plan B" as a practical solution, but now searching for a power module which can stand +/-85V  ... even if those will drop *sharply*  under load.

Plan C: get a full ClassD power amp + integrated SPMS all-in-one-board, similar to ICE Power modules ... and in fact what G&K uses today.

What also do Fender, Aguilar, T&C Electronics and most others.

 

[samgraysound]

So unfortunately, this band has 3 or 4 of these heads they take on tour. I think they use 2 at once at the show and the others are backups in case they blow. So they all need to be exactly the same sound wise, I don't think they will go for swapping in a different power amp.

Which leaves me in pickle, because I either need to really truly fix it, or refund them what I've charged to repair it. I could look for a parts unit.

Sam

 

[JMFahey]

Oh, previously undisclosed data which changes everything.
Yes, in this case they need exact same.

Personally, I would hunt for a working one (cosmetics don´t matter at all) and do a full PCB transplant, period.

Check the usual suspects , Craigslist, EBay, etc. but I woukld definitely ask GC, they are all over the place and might have a suitable one at *another*  shop, very far away.

It costs them nothing (and won´t charge you extra) for bringing it near you, where in any case you can pre-test it before spending a cent.