The problem may be coming from the grounded-emitter transistor stages, which make good AM demodulators (like having a radio tuned to all the stations at once).

Might try lifting the emitter leads on the small transistors and tack in some small resistors (perhaps 470 ohm) and see if that helps. An input resistor can also help, in the range of 1k-10k.

Good Luck!

Does it sound like the hiss you get in-between stations on an AM radio?

I would guess a corroded part somewhere that needs to be resoldered. Or maybe a bad pot?

Working on a transplant. Sort of a Recto knock-off, but with an effects loop for each channel, and less features. (Nothing is really mounted yet, just doing a test-fit.) Still working on the channel-switching and fx-loop circuits.

The schematic shows a 1000uF output capacitor, so there shouldn't be any voltage across the speaker, unless that capacitor is bad (which could very well be the problem.) Replace that and see what happens.

Just got one of these, seemed like the perfect thing for wiring a preamp, since it works like a breadboard:
http://www.radioshack.com/product/index.jsp?productId=2102846

I don't know anything about snubbers, so I can't help you there. I'm sure diodes of proper rating are fine.

Edit: sorry I yanked the preamp circuit, needs more work.

Edit: Preamp circuit is back, seems to work pretty well this way:
http://www.diystompboxes.com/analogalchemy/sch/v4preamp.html

Made some minor changes to the balance control and lowered the two 10K emitter resistors to 1K:
http://www.diystompboxes.com/analogalchemy/sch/diodepwr.html

Power supply schematic (using the Radio Shack transformers):
http://www.diystompboxes.com/analogalchemy/sch/rs_pwr.html

I think it does sound a little better to use the diodes, so I went back to that on my build.

I took the diodes/resistor out and couldn't tell the difference, so I guess I'll forget about that idea.

The higher voltage preamp didn't pan out either, went back to a 4-stage vulcan circuit at 9V which sounded much better.

So at this point everything is sounding good and even got some tone controls in there. When I get it all boxed up I'll try to record some samples.

Check the DC voltage across the speaker lead. If it's not pretty close to zero volts, then that can cause some hum. Maybe try this with the speaker disconnected as well.

You're right, the diodes really don't do a whole lot. They round out the waveform a bit more, especially if the 470K is increased (but too high and the transistors won't bias). The hi-fi and headphone versions I've made don't use diodes and sound great.

On the preamp circuits the diode helps a lot, because it keeps the transistors out of saturation, but I doubt it's more than a subtle effect here, except perhaps at full blast.

Still need to do an extended full-power test through a load resistor, but other then that seems to be a nice little amp that gets plenty of volume.

Thanks for the comments and help guys!

One thing that happens a lot is that the leads on some part get corroded, and need to be re-soldered. Sometimes you can pull a capacitor completely out of the board because the corrosion has separated it from the PCB!

I'd get the boards out there if people wanted them, but there are some caveats: Because the output drifts slight with temperature, exact output centering is not guaranteed. I'll explain the biasing procedure to see what you all think:

1. With speaker/input sources removed, adjust dc balance and vbe multipler back-and-forth until the output is zero and voltage between output emitters is around 75mV.

2. Connect speaker (or load resistor) and adjust controls again. Check again after a warming period. In the future, the amplifier will begin with a very slight offset (and accompanying hum) which disappears after warming up.

3. Glue trimmers into place.

So the balancing/biasing is a little tricky. I've made changes to the balancing network, which now limits the possible output deviation to around +/-2V, making adjustment easier and somewhat safer.

The amp also requires choosing positive-feedback values appropriate for the application. (This can also program the level where power amp distortion begins.

I have a nice preamp going, using some leftover MPSA06's running off the +35V supply. Gets about a "Marshall" amount of gain, and seems to be a perfect match. Will try to post a sample.

Checked this out some more.

With the output shorted, and no input signal, one of the output transistors gets warm, and one of the emitter resistors reads .650v across it. It's either one transistor or the other, but never both, depending on how the balance/bias controls are set. I can hold it for 20-30 second, and it's fine except for the warming. Does a VI limiter always throw to one side or the other like that?

The amp is pretty quiet if the balance/bias is tweaked slightly after plugging in the speaker. (plugging in the speaker throws the balance off very slightly). Raising the quiescent from 20mA to 75mA gets rid of most of the hum.

So I guess it's working alright. Sounds nice cranked up, which is what counts. Some build pics:

I ordered the boards. Rebuilt the amp using the new design and it works, except I think the balancing control is too touchy (easy to fix). The rest seems okay, so I'll run it full blast tomorrow and see what happens.