Acoustic G60T head: DC supply for channel switching

[gmoon]

I could use some advice--I picked up a "non-working" Acoustic G60T head and cab for a good price. When I first brought it home, I replaced the fuse, pulled the power tubes and checked all the usual internal voltages. No problems.

I replaced the power tubes with known good ones, and fired up the amp. It worked fine on the gain channel (Vol2) only. However--No channel switching (no LEDS), no reverb. But cool! No issues with the basic tube circuit, or the transformers!

Tapped from the same secondary winding as the bias voltage source, there is half-wave rectified chain of voltages, 65V-20V-10V, each subsequent one regulated down from the single +65VDC source:

--65V for the reverb recovery stage, then passed to a power transistor/zener setup that regulates down to
--20V for the reverb driver (LM383), then passed to a 10V zener that regulates to
--10V for the LEDs and the optoisolator bulbs (channel switching)

It should be noted that there was an add-on board--a small rectifier/filter board screwed to holes drilled in the PT. The previous owner had removed the small transformer that fed that PS.

Looking closely at the 65V->20V regulator, someone replaced the two 10V zener diodes with std. rectifier diodes (not good) and removed the power transistor (2N6474), which had probably fried (before or after the zener changeout, it doesn't matter). The transistor had been mounted on the top of the PCB. The small add-on mod replaced that 20V source.

At this point I took an unregulated 12V DC wallwart (18V DC w/no load) and hooked it up to the PCB at the "20V" marker. The load dropped the wallwart down to 16V, short of the normal 20V but close enough. With that PS connected, the amp is fully functional--channel switching, reverb, etc.

So, I could just retrofit a 15-20V power supply (18V switching looks like an option).

'Course I could just repair the on-board supply. Could the 2N6474 be replaced with a better, stronger power transistor like a TIP41C? Along with the zeners, of course... Or is this a flawed regulator circuit?

[sluckey]

Could the 2N6474 be replaced with a better, stronger power transistor like a TIP41C?

yes

[PRR]

> TIP41C

The "C" is the 100 Volt job, right? That's OK.

> is this a flawed regulator circuit?

In one sense: why did the original blow? Perhaps because when something goes wrong, the trnsistor tries to flow "infinite" current, and 65V times infinite current is infinite Watts. (OK, the 10K in the Base sets some limit, but still perhaps 1/2 Amp and dozens of Watts more than the bare or barely-heatsunk transistor can stand.)

Blow another two bits. Put a resistor between the +65V source and the transistor Collector. I'm thinking there's no need for even 200mA. The transistor can do that with only a few Volts C-E. So 65V-5V-20V= 40 Volts across the resistor at 0.2A flow, 200 ohms.

In a dead-short the full 65V appears across the resistor, 21 Watts. If you were constantly poking screwdrivers in there, a 20W or 25W part would be blowout-proof.

At nominal 0.2A load, 40V across the resistor, 8 Watts.

I suspect normal load is more like 0.1A. 20V across resistor, 2W.

For normal use, a 5W should do, a 10W is sure to take any normal use. Either will burn-up in a dead-short, so you know what happened.

Meanwhile, back at the transistor.... the worst-case heat will happen at 0.113A. 22.5V across resistor and 22.5V across transistor, 2.54 Watts in each. A TIP41 should have several square inches of metal heatsinking to handle 2.5 Watts long-term. That's WORST-case; at any other load the heat in the transistor is lower, it won't cook (the resistor may). Also the worst-case current in the transistor, dead-short, is 65V/200= 0.325 Amps, well below the 6A rating. Witht the resistor in line, the transistor can't feel more than 2.54W 0.325A, far-far below the "infinity" it could get without the resistor.

Sorta like a lamp-limiter for starting sick amps. If something is very-wrong, you don't have infinite power available, stuff doesn't explode.

[gmoon]

Thanks, Steve.

@PRR:

Yeah, the 100V one. Hmmm, the TIP35C is 100V, 25A CC, and only about a buck. Although maybe it's better to fry the transistor than the transformer... Adding a fuse here also wouldn't be uncalled for, I think.

The unregulated 12V supply I've tested with is 500mA rated, and only drops to 16V with the load, so I'm guessing the current draw doesn't exceed 100-150mA (at that voltage) with everything functioning. That's easy to verify.

OK--adding a 200 ohms/5W current limiting resistor should be very do-able.

I've spend a little time reading up on "series voltage regulators" today. Interesting stuff; can't get much simpler (except for a lone R/zener pair).

RE: The little I've learned about these amps in the last week--there are rumors some of the amps had overheating / fire issues early on...

Thanks!

[PRR]

> Adding a fuse here

With the 200 ohm resistor, current can't get over 0.32 Amps. Unlikely to fry the transformer.

[gmoon]

With the 200 ohm resistor, current can't get over 0.32 Amps. Unlikely to fry the transformer.

OK, cool.

Devils advocate, thought--the amp has a second fuse holder which serves as only a holder for a spare. You can see the tail of it below/right of the PT, so it would be simple to wire it and protect the PT from anyone who does poke around with a screwdriver...

[gmoon]

OK, bear with me...

The current draw on the temporary DC supply @16V (15.9) maxes at 110mA on the clean channel (the clean LED indicator is transistor driven so it's about 10-12 mA higher than the gain channel).

For arguments sake, let's make a conjecture/guess that the draw @20V could be about 135mA. I'm thinking the total "normal" current draw for everything including the current limiting resistor then should be approx. 405mA, and the current limiting resistor 160 ohms.

So... ~400mA has got to be close to the max current a separate bias winding can safely provide. There's no way to know for sure for an out-of-production transformer, but it makes me nervous...

[PRR]

> OK, bear with me...

[gmoon]

  ...rimshot...

[Willabe]

OK PRR, just WHO'S bear is in that little RED WAGON and WHO'S little RED WAGON is that? DO YOU or THEY have a PERMIT for either of them to pull them down the street, in public ????


                     Brad