Help me understand this amp (1971 Ampeg ST-25)

[glebert]

I have this amp that isn't working, and the power amp design seems pretty dicey to me. I'm concerned that the design (pretty early solid state for Ampeg) may have worked OK when new but may not work well with anything other than the new, exact parts used originally. So far I replaced one of the 1% resistors for being way out of spec and throwing everything off. Currently have all the bias, predriver, driver, and power transistors out (don't even know if I still have the parts I removed) but at the point where I would start putting them back in. Some parts of this amp look like something we would have designed in first year electronics. Want a voltage point at 40V from your 80V supply? Just put two 20w power resistors to make a voltage divider to do it, no problem! (actually my prof wouldn't have accepted that)

Anyway, the part I don't get is Q18. It has 0.7V base-emitter drop so it should be "on" (at least somewhat) but it also has 40.1V from collector-emitter would imply that is is "off." (note: looking at an old datasheet for this part it looks to me like it has much softer Vbe-current curve than what I am used to) The on-ness of Q18 is set by the PNP Q17, which has Ve=8.5V, Vb=8V, and Vc=0.7V. It seems like Q17 is supposed to leak just enough current to get Q18 biased up to turn it on but not so much as to blow it up. That seems like a pretty heavy burden to put on Q17's parameters and the resistors that bias it up, especially if Q18's Vbe-current curve is steeper than the original.

I know that in general we assume that amp designs are robust unless proven otherwise, but in this case there are almost none of these amps alive anymore (there may not have been many to begin with). I wonder if we should assume that premise to be true in this case. I really don't want to redesign the PA section, if it came down to that I might put a class D module in place of the orginal PA. The preamp is actually working fine and sounds great.

Thoughts? Am I overthinking this?

TIA,
Greg

[Helmholtz]

Q18 is on. It's supplied from 80V through R86 and R8. Collector current shoud be around 20mA.

[glebert]

Q18 is on. It's supplied from 80V through R86 and R8. Collector current shoud be around 20mA.

Typo? There isn't an R8 on this part of the circuit.

[Helmholtz]

Typo? There isn't an R8 on this part of the circuit.

Yes, obviously I meant R87.
My laptop keyboard developed some contact problems recently.

[glebert]

I can get that about 20mA would be going through R86 and R87 which gets you the 41.25V at the collector of Q13, and most of that current will go through Q13 (very little going into base of Q14) and then go down through Q18. So Q13 is on and had a Vce of 1V, but Q18 with same current has Vce of 40V.

In looking a little more I realized I have been ignoring Q19, which should have a pretty solid 40.75V at its emitter so the diode drop to its base will try to keep the voltage there at about 40.1V.

[Helmholtz]

Base current of Q14 takes a little while the base current of Q19 will add about the same.

If Q18 were off, it's collector voltage would be considerably higher and Q19 would be reverse biased, probably causing breakdown.

[glebert]

I'm not disputing that Q18 is on, at least somewhat, but if I put some other device in there should I expect it to work the same? I did find some of the original parts and the Q18 transistor is shorted out Presuming that 10000 is an Ampeg part number. Looking at the given substitution from the schematic 40408 they are rare, with a pair of NOS parts being $30 on ebay.

Since I probably have to replace Q17 too my concern is if this can work reliably long term with substitution parts.

[g1]

Keep in mind this is a single supply amp with cap at output to block DC from speaker. Output will sit at half of supply voltage.
It will work with standard generic parts.
There have been other threads where replacements for the 40408 etc. series are discussed. Usually a TO220 style with leads bent over so the case is flush to board.
MJE340/350 type seems to ring a bell. Or similar.

[Helmholtz]

I'm not disputing that Q18 is on, at least somewhat,
.

Well it sounded different in your first post.
Ic = 20mA is healthy conduction. This is not switching but linear operation.

From the circuit parameters you can guess minimum Vce, Icmax and Pmax.
All SS power amps rely on heavy DC NFB to stabilize operating conditions, so chances are that a similar though not identical transistor will work ok.

[Enzo]

I know I have a few 40408 in storage, and I just yesterday cleared a path to the storage drawer thing they would be in. I'll check.

Meanwhile, it is a TO39 90v 700ma part. MJE340 is a 300v 500ma part, which might be OK.

[glebert]

OK, thanks for the inputs. I will get to acquiring some parts and I'm sure there will be more questions later.