Need some help debugging my Express clone

[Wayne Alexander]

I finally got the Mojo head cab I ordered so I could take the thing to band practice. I'll post pictures of the head cab on the weekend - it's gorgeous. Basically I had them make a stock shaped Trainwreck head cab covered in rough blond tolex.

Back to my amp's issues. First, it's got a huge hiss with the volume control up high. It doesn't need a guitar plugged in to do it, and it did it even with my Les Paul. Weirdly, the hiss would reduce when I did NOT touch the strings, and it hissed more when I did. It wasn't noticeable when playing, but very noticeable when I was not. What should I be looking for?

Second, it's got a high-pitched oscillation/shriek overtone that increases drastically with the presence pot all the way up, and mostly goes away with the presence pot down. When I run the amp through a THD Hotplate, it's much more controllable, but when the amp is run straight through a cab, especially with the master volume up so full signal runs through the circuit it has a very high pitched whistle/shriek/overtone on top of the actual signal. (I used a PPIMV with a dual 1 Meg pot coupled with .047uf caps on each side of it replacing the two .022uf caps that lead to the wires feeding the power tube grids). What should I be looking for to debug this?

Lead dress doesn't seem to affect either of these issues. I do have the extra 260V tap wires from the PT under the filter cap circuit board, and the extra unused 6200 primaries on the OT also rolled up under there. Would those matter? I don't want to cut them short yet because I may try using them to see what the difference is.

I can post updated pictures on the weekend.

I've been quite faithful to the A1A circuit except I used a 1 Meg bass pot to let more bass into the tone stack (I may go back to the 250K, the bass is pretty overpowering) and I've got that master volume in.

Anyway, any guidance would be appreciated.

[LeftyStrat]

Back to my amp's issues. First, it's got a huge hiss with the volume control up high. It doesn't need a guitar plugged in to do it, and it did it even with my Les Paul. Weirdly, the hiss would reduce when I did NOT touch the strings, and it hissed more when I did. It wasn't noticeable when playing, but very noticeable when I was not. What should I be looking for?

Try pulling each preamp tube, one at a time, starting with V1, until the hiss goes away. This will help you locate where it's coming from. It could just be a noisy preamp tube. Trainwrecks are known to have a little hiss, so you may want to consider picking up a NOS tube for V1 (especially a Mullard or Telefunken).

Second, it's got a high-pitched oscillation/shriek overtone that increases drastically with the presence pot all the way up, and mostly goes away with the presence pot down. When I run the amp through a THD Hotplate, it's much more controllable, but when the amp is run straight through a cab, especially with the master volume up so full signal runs through the circuit it has a very high pitched whistle/shriek/overtone on top of the actual signal.

The fact that you say it is less pronounced with the hotplate suggests it is microphonics. Ken recommended placing the head on foam if you're setting it on top of the speaker cab. It could be a microphonic preamp tube.

If that doesn't work you can also try shielding the line running to your presence control.

[Elcabong]

Regarding the hiss, the amp should be pretty much silent with no guitar pluged in and the grid shorted to ground. Check you jack wiring. Also you could try a different tube in the first position.

The squeal could be caused by positive feedback; in this case, reversing the OT leads would stop it. But I have also experienced hi-pitch squeal before that was caused by the OT wiring being too close to the preamp. It does not seem to be your case.

[erigm]

This won't help your squealing problem, but might help bass response. In your PPIMV try using the stock .022uF/600V (or .1uF) caps on the PI side and 1uF (or higher) caps on the bias side (these should be rated for more than your raw bias supply ...100V should do it). Your MV should have a more consistant bass response this way. The idea is to have the bias side caps at least 10x the value of the PI caps so the series effect is diminished.

[Wayne Alexander]

Well, I finally had time for a debugging session. First I tried shielding the wire from the board to the presence pot and the negative feedback wire from the presence pot (through the 100K resistor) to the output jack. That was either no change or only a slight one.

Then I switched to the 260V taps on the PT instead of the 300V ones, that moved my plate voltage on the power tubes to about 334, it had been higher before.

Next I swapped to Mullard preamp tubes, all 3. No big difference in hum/noise.

Then I noticed that when I put my hand close to the bright switch and the volume pot, I got a shriek. After shielding the wires from the bright switch over to the volume pot with little or no effect, I tried a different switch. I had been using a big one I got from Weber, but the switch itself turned out to be somewhat microphonic. I used one of those micro-DPDT on off on ones, a 3A 125V one I got at Radio Shack, it's not microphonic and that helped a fair amount. I can now turn up the presence control all the way without getting an almost ultrasonic shriek on top of the core tone.

I tried running a set of NOS Mullard EL34's from 1965, but I they biased very strangely (are they wired different from modern EL34's?) and kept blowing fuses, so I settled on a set of Hammond/GE 6v6's for now.

I tried swapping the OT leads to the power tubes, I got a huge shriek, so I swapped them back.

I didn't yet get around to trying the 6600 taps on the OT, I'm still using the 5200 ones.

I've gotten most of the high-pitched shriek overtone out of the core tone of the amp, but it still has a fair amount of low hum (filaments?) that seems to feed on itself and get a bit louder when left alone. It's there with no guitar plugged in, and it's there to some degree even with the amp volume turned down, but it's louder with the amp turned up.

I have an oscilloscope and a signal generator and I'll eventually try to look at the waveform in various places throughout the amp (I haven't used the scope yet, just recently got it).

[Elcabong]

Good troubleshooting technique, Wayne. The oscilloscope will be helpfull to determine where this hum is originating.