Princeton Reverb Hum

[Fletcher Munson]

I've a just completed Princeton Reverb build that has an annoying hum that I can't figure out.

The amp is modified for 6L6 power tubes and has a couple simple mods noted in the attached schematic.

The hum is relatively minor... it's a background sound and you don't really notice it while playing. But it's there during quiet passages and it's more than I consider normal. The hum is present with the Volume at zero, it's steady until about 4-7 on the dial where it diminishes slightly, then it increases a lot beyond 8.

Turning up the Reverb beyond 7 increases hum. Disconnecting the reverb tank has no effect. The Bass and Treble controls don't have any effect. Increasing tremolo Intensity makes the hum more noticeable, but I don't think it actually increases the hum.

I tried pulling preamp tubes one at a time:

V1 - no effect
V2 - no effect
V3 - hum mostly disappears
v4 - hum mostly disappears

So it seems the hum is located in the V3 area but I need advice on narrowing it down from there.

Princeton Reverb Schematic Markup.pdf

[mikeydee77]

What is your earthing scheme? Photo perhaps?

If the pre amp valve's earth point is sharing the same earth point with higher current earths like the reservoir cap, then this can be a cause of unfathomable hum noise. It's a very satisfying fix to find... I think the most practical scheme is a constellation of earths for each major stage. Wait a minute someone that can actually write coherently has already tackled this, take a look at http://www.valvewizard.co.uk/Grounding.pdf

[Fletcher Munson]

Thanks mikeydee. I'm aware of the Valve Wizard grounding guide. In this case I'm following the vintage fender grounding scheme pretty closely. One problem is the use of the multi-section cap can which groups all the filter cap grounds together. Not much you can do about that. Otherwise, I'm using a brass ground plate and following the Fender setup pretty closely. Will post a picture ASAP.

[Fletcher Munson]

Here's a pic of the layout:

[mikeydee77]

Looks very neatly done and wth top notch parts.

On the cap I think most sensitive area is V1 anyway and that looks to be grounded via he jack socket.
Have you tried gently moving the pre amp wires with a chopstick while powered up, could be a wire placement e..g the V1 grid (pin2) too close to the plate resistor - kind of thing
The only thing I haven't seen before is heater wiring like that? Looks neat but I think better to have them up in the air away from valves and also maybe the rever connections..

I am sure one of the wiser amp builders will chime in with something soon too...

Mike

[mozz]

As nice as it looks, i can't look at it. I would never, ever just push wires through a lug straight then solder them. Maybe because i was taught to make the connection strong first then solder second. Is V1 pin 4 and 5 joined?

[The Dude]

^^^^^^ Agree. You shouldn't depend on solder alone for a connection. Also, it looks like one of your artificial CT resistors for the filament winding isn't soldered. I circled it in this picture. It could just be the picture, but worth a look.

[Fletcher Munson]

Thank you mozz. Yes, V1 pins 4 and 5 are joined together.

Thanks Dude. Yes, the artificial CT resistor is soldered to pin 7.

Regarding using solder alone without wrapping the wire: When you solder to an eyelet, isn't that what you're doing?