Princeton Reverb Toubleshooting Help

[Stan Malinowski]

Here's the problem:

After about 15 minutes after turning the amp on I get a loud popping and crackling sound when playing the bass strings. No such noise when playing the upper strings. When I shut the amp off there is a noticeable pop.

I have tried two complete different sets of tubes but the problem persists. Could I be looking at a bad socket (or two) for the power tubes?

Any input on troubleshooting would be appreciated.

[John Phillips]

From the fault, it sounds like a combination of something that's heating up, and also being disturbed by bass frequencies...

It could be all sorts of things - a cracked resistor, a cold solder joint, loose tube socket, bad cap - difficult to advise where to look first.

If you feel happy running the amp opened-up, wait until the fault appears, then poke around inside with an insulated tool (chopsticks are good!) and see if you can identify something which makes the popping noise when you touch it or move it.

[Stan Malinowski]

UPDATE: - I thought I found the problem. Last night when the amp was cold I turned it on and started "wiggling" the tubes a bit (the chassis is out of the cabinet). I found that V1 (1st stage preamp 7025) tube would crackle and pop a bit when wiggled, all other tubes were fine. I had a spare Groove Tubes 12AX7 tube which I inserted and noticed it fit tighter than the 7025. I played at a low volume (about 2-3) for about 30 minutes and the problem seemed to go away. This morning I let the amp warm up for about 30 minutes before playing. I played for over 20 minutes with the volume set at about 2-3 and had no problems. I kicked the volume up a little bit over 4 and after about 5+ minutes of aggessive playing the problem returned. I noticed that when the problem occurred the chassis became very microphonic in the areas of the first two preamp stages and the input jacks. The tapping of the chassis with my finger produced large thumps thru the speaker along with the problematic pop and crackle. In addition, this tapping could also get the tubes to squeal.

So...seems like my problem is most likely in the preamp section rather than the power tube section like I originally assummed. Seems like the best course of action might be to probe around with my trusty chopstick in the preamp section.

Again, any input is appreciated.

[Stan Malinowski]

PROBLEM SOLVED!

I cleaned all the tube sockets and retensioned the pin clamps - problem did not go away. I started probing with the chop stick, tapping the various preamp components. Many of the components produced the microphonics when hit. Tapped on the wire which ran from a common point on the board to ground and PRESTO - a loud pop! Looks like the wire from the eyelet on the board to ground was loose. Soldered in a new wire, all microphonics and pops are now gone! The ground point in question was the ground where most of the components associated with the reverb driver were grounded, along with a couple of compoenets from the second preamp stage.