Standel S60G power amp

[dbeasley]

Hello everyone!

I've been working on a real oddball lately - it's a Standel S60G solid state guitar amp.

Complaint is wierd crackly low headroom. I inject a signal into the input and at the output of the preamp it looks great. Very clean. But measuring signal output from a dummy load plugged into the speaker output shows a weak, clipped signal.

So I take a closer look at the power amp - it is all pcb, and not the easiest thing to troubleshoot. So for the sake of my personal edification I pull the pcb out and trace out the schematic for it which I'll include in this thread. A couple of things:

There appeared to be 2 components snipped off the pcb - I have those labeled with a "?". They are in series. I have no clue as to what they were, or if they are the culprit.

I'm not as familiar with SS as with tube circuits, so I'm having a hard time following what's happening here in the schematic. I have tried to be as accurate as I could in labeling the correct EBC pinouts on the various transistors and such.

My ultimate question is, could someone explain how this circuit is actually working? I see there is a bipolar supply, and the TIP31/32 are complimentary pair transistors - which to my knowledge means they work somewhat like a class AB amp? The main power amp transistors are the 2N3773s, which are in the TO3 package. I'm not 100% how those are driving the signal as well.

Thanks in advance!

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[pompeiisneaks]

I've seen some simple tests you can use for the transistors themselves. They act like dual diodes and diode test mode should show no connection one way and a typical type diode drop the other way. If you get continuity and/or diode drop both ways it may not be very happy. I don't know about the missing components, where does that C go to at the end of them? is that power supply? If they're gone it may have been some kind of resistor and/or resistor cap network that allowed the power to come in and bias the specific transistor, and if that's missing it may be near cutoff and thus very crappy output. Without the original schematic, though, it may be hard to do more that what I just did, guess

Just found this, it 'looks' similar at first blush and shows a cap and a resistor there, if I'm looking right:

http://www.standelamps.com/support/sche ... /ic005.pdf

~Phil

[dbeasley]

So funny you post that schemo, I just stumbled across that very schematic only a few minutes before!

Clearly my circuit drawing skills need some work.

After doing more research I see that the pairs of drivers in the output stage are called Sziklai pairs, similar to Darlington pair transistors.

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[Stevem]

Check all resistors around the TO3 outputs that are of values of 100 ohms or less as they bias the outputs and it's critical that they be dead on!!
When I fix amps like this in regards to the resistors of 1 ohm or less I make them up from some times ten 5% 1/2 watt resistors in parallel and or series to get dead on the value needed.

I 1966 Kustom that I fixed 2 months ago would only hit 60 watts of RMS output instead of the 90 of unclipped power that it should due to the bias being off!

The last Sandel 2-12 amp I fixed had the circuits somewhat modular style in that the power amp section was all layed out as you would expect for that era on terminal strips, but each preamp section and the reverb section was encased in small fishing weight boxes and sealed up in fiberglass resin, what a chuckle I had over that!!!
I think I have pictures I will try to post up on Monday so you folks can have a good chuckle also!

[dbeasley]

Steve,

Thanks for that. I really should know more about these early SS amps but I get so few of them in.

Luckily, this amp lacks the epoxy preamp modules. I guess they held themselves in very high regard to "epoxy patent" their designs haha!

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