'65 Princeton Reverb reissue with intermittent low end crud

[eddie25]

Anyone ever experienced this? I tested everything I could without flipping the board. Not the tubes or speaker. All voltages check out, and no obvious changes while the amp is being played. No obvious bad connections. It's actually not really intermittent it basically takes a little while to start then it's just there. Cruddy sounding distorted low end.

Problem is... It's a touring band and I only have a day. I was thinking about shot-gunning the e-caps and bias cap, but I only have 450v in stock for the filters and it's right there at 450v B+. Sound like a possible bad coupling cap? Any common problems with these?

I know this is a shot in the dark, but just trying to do what I can for some friends.

[Leo_Gnardo]

Clean controls & preamp tube pins & sockets, outputs & rectifier too why not. Connexions may be OK but resistors not, especially that first 1K in the power surprise. OK when cold but not so much when it heats up a bit. Sometimes they look OK but on close exam you see a crack down the middle. Speaker connexion, have a look in the plug & make sure it's soldered well. Tighten nuts that hold down input & output jacks. Examine connex at the speaker terminals, even remove the speaker & see whether there's any debris, clots of lint, cracks or rips, spider coming unstuck from the magnet or VC loosening up from the spider. If none of that does it, try replacing tubes one at a time. Whew, I've run out of ideas. Hope one of 'em fixes it for ya.

[eddie25]

Thanks! I feel pretty strongly it is a bad component as opposed to a dirty or loose connection. Only because I ran the gamut on stuff like that, but who knows. I think I will do the ole capacitor audio scope trick and see if I can pick up where it starts, but I'm afraid I may run out of time here.

[xtian]

I hear you say this happens when it warms up. But we still want to figure out if it's vibration induced. Can you repeat the problem when playing thru an external speaker cabinet? If not, then it's either the speaker, or it's vibration induced, right? Also, happens only at louder volumes or all volume levels?

If it is vibration induced, and if you can repeat the problem at will, my advice is to input a sine wave and adjust so the problem is happening and you have free hands. Then use a chopstick to press on the circuit board, the chassis, the ground connections, etc., until you find out where you can silence the issue.

[eddie25]

Thanks xtian. I did try an external cab. I also used a sine wave and poked around and it told me nothing obvious. I even poked around while my friend played and it didn't seem to affect it.

The problem seems to be almost solely low end oriented.

I did get a chance to "audio scope" it, and I noticed that at the grid (pin 7) of V3b the sound was pretty clear and nice. Then I tried pin 7 of V4b, the input of the phase inverter and it sounded nasty (I did adjust my DI preamp levels to compensate for the extra gain).

So I tried the plate of V3b, and it was nasty there as well. I think I will change out C14 (the input cap for the PI) and see. I have had my attempt at "audio scoping" fool me before so I am not super confident.

Wish me luck.

[Leo_Gnardo]

Just by coincidence I got into a heavily modified (you wouldn't believe the krazy krazy stuff somebody did . . .) PR this afternoon. And got reminded of one more thing you should look for, because one was starting to loosen up in mine - the leads from circuit board to brass ground bus.