Pedal note distortion

[Matt Green]

Everytime I play pedals or octaves in the left hand, the high notes I'm also playing get distorted but not in a good way. The voice coil on the 15 seems to be ok. No noise as I move the cone back and forth. I think i hear the same problem thru the speakers of the a100. Problem might be in preamp but don't know where to start?

[tpappano]

Is this something the organ did not do before at the same volume levels? It might be normal behavior of the amps. Amplifiers have a “power bandwidth”, for example you might be able to play a chord at the top end at max swell pedal or at the bottom and might not have distortion, but do both at once and you might have distortion. You would have to back off the swell a little for them to sound clean. Is your Leslie driven by the A100's speaker amp, or directly from the preamp?

[Matt Green]

The Leslie is driven from the preamp to the six pin connector box. The sound in the low end gets worse as the organ and Leslie warm up. It sounds like the fifth harmonic starts farting out of the 15 even though only the 16ft pedal is pulled. I'll try to get audio clip up.

[Matt Green]

BTW, are the p 12ax7 and 12au7 completely interchangable on the ao28 because I can't tell the p difference between them

[tpappano]

I haven't had to go deep into my preamp, (yet) it is all original at the moment. I wouldn't think the two tubes would behave quite the same when swapped, but I haven't tried it. I suppose I could, I have an A100. One thing I have noticed is that the ao28 seems to have plenty of headroom and its output at GG seems to stay pretty clean no matter what I do. You could actually have a failing tube, and there are probably a couple capacitors that would cause problems, too. I'll look at the schematic tomorrow, other more experienced ao28 users might have an immediate diagnosis 8)

[Matt Green]

Here's a pic of the preamp. You think maybe that 25mf 50v should be replaced?!!!! Jeez

[Matt Green]

Here it is

[tpappano]

I think that one was being asked to perform outside its ‘comfort zone' 8) I think that might be C59, but I don't see C59 on the schematic. I does show what looks like two C56s, a misprint maybe. Probably needs to be rated more like 400v instead of 50 8)

Can you check what's on the other end of that red wire?

[David Anderson]

tpappano's suspicions are correct. That red wire is a 280V DC supply node. Someone paralleled a 25uF 50V capacitor with the stock 10uF 350V section, probably intending to add extra capacitance to one of the 30uF 25V cathode bypass sections also in that same multi-section can capacitor. But he hooked it to the wrong terminal; thus, the 50V capacitor blew up.

The bright yellow and purple wires are the cathode bypass capacitor sections of that can.

Also, no the 12AU7 and 12AX7 in the preamp are not interchangeable. Each needs to be in its proper socket.

[Matt Green]

Should that cap just be taken out and not replaced? And do any of the other connections seem to be wrong?

[Matt Green]

Is there any way to tell the difference between the 12au7 and ax7 when the writing is no longer on the tube?

[David Anderson]

Is there any way to tell the difference between the 12au7 and ax7 when the writing is no longer on the tube?

Not visually, unless you have a lot of experience with tubes and happen to be aware of particular details of internal construction. I have a tube tester which I could use to figure it out, but I'm guessing you probably don't.

Sometimes if you chill a tube and then breathe on it, you can see enough traces of silkscreening to figure out the type.