Trouble with bad reverb hum in Princeton Reverb build (Solved!)
I finished my build last week and immediately ran into reverb hum. At first, I thought it might be because the reverb connection wires were not routed properly or sheilded. However, this turned out to be a small part of the problem. After a few days I'm thinking that either I have made a serious wiring mistake or there is a grounding issue within the PC board. Here I will add that I have checked the wiring 10X over and can find no errors. Unfortunately it seems that no one else has built this board/Princeton so there are no other finished amps to compare it to. :(
My final experiment was to remove the reverb tank return connection @ PIN 2 on V3. Doing this isolates the 'clean' side of the channel and lets you hear the entire build (preamp/tone stack/tremolo/power supply MINUS the reverb. I then reattached the PIN 2 to V3 and the hum appears. IN FACT, there seems to be some odd voltage on the chassis when the reverb return connection is made, which is odd.
I made a quick recording of the sound of the amp (playing guitar) first with the (PIN 2/V3) reverb return disconnected. The reverb tank is connected the entire time. This is using a Strat and starting off in the 'inbetween'/humbucking position and then going to the straight single coil bridge pickup for a single string 'solo', and here you can hear some typical single coil hum, but nothing odd here. Next in the (2nd half) recording, I have reconnected the reverb return (PIN 2/V3) and just listen to the amp WITHOUT a guitar plugged in. The hum should be obvious, and then, towards the end of this recorded section, I demo what happens when I touch the chassis and then remove my hand. This hum reduction (but not all of it) with the hand touch concerns me because it seems like there is a voltage present that reaches the guitar strings (when I am playing the amp with the reverb connected). Something is wrong here but I can't sort it. Hopefully someone has an idea. Oh, don't mind the playing, it's 45 degrees in my garage and I'm just making noise.
I also changed the 2nd tremolo cap to .02 (stock, it was .01) and this makes the tremolo Speed Control behave as I'd hoped. The very fastest speed is now slower (but still gets fast), which gives the Speed Control the kind of range that makes it easier to find a usable speed. It works for me.
FWIW, I also think that some may want an even slower overall top speed so here I would go with a .03 1st cap and a .02 2nd cap. (And be aware, because the traces (front and back) are connected through the board holes, unsoldering a part to replace it is not an easy task, believe me. My board can be 'flipped up' to have the board sit at a 90 degree angle to unsolder a part if need be. (I use painter's masking tape to tape the board in the 90 degree position, btw. because the attached tube wires (#22) tend to resist the 90 degree band). To accomplish this without unsoldering the various connections, a ran a #14 copper bus wire across the back of my pots and made sure that the wires on the end of the board (the power supply end) are long enough to accommodate this manuever. This way, I can unbolt the pots and input jacks and keep things connected as a complete unit. Yeah, it's a bit of work, but things go back together quickly. All of which is to say, if you can, it's best to make a decision before the solder flows. :)
Last thought, I am using a 3Meg reverse taper (Speed) potentiometer (exactly 3Meg) and there are 3M pots out there that only measure 2M. Measure your parts or you may be building something you aren't expecting.