Help troubleshooting - I'm out of ideas

[Dr Tony Balls]

Hi all.  Working on a new amp and trying to implement a "one-tube reverb" a la the thread provided below and i'm having problems.  Hoping someone can sanity-check me and figure out what i'm missing.

https://el34world.com/Forum/index.php?topic=7957.0

The amp itself works and sounds great with the reverb tube (V2) removed, however with it inserted i'm getting significant 120hZ hum.  Here's my key observations so far along with a schematic:

- The hum goes away by turning the reverb control
- The hum is present even with the reverb tank removed




This would limit the source of the hum to something happening between the reverb tank and the reverb control, i.e. the area around V2B.  Here's what i've tried and noted so far:

- Grounding the grid of V2B does not eliminate the hum
- Replacing the cathode cap and/or resistor do not eliminate the hum (C15 & R31)
- Replacing the plate resistor and or coupling cap do not eliminate the hum (R32 & C16)
- Chopsticking wires does not eliminate or reduce the hum
- Replacing V2 does not eliminate or reduce the hum
- Disconnecting the grid connection to V2B (at the tube socket) DOES eliminated the hum


What am I missing here?  Plate/Cathode voltages of V2B are 144 and 1.03 VDC, respectively.  Below is a shot of the area, though its a tight build so maybe hard to discern whats going on:

[sluckey]

What resistance do you measure from V2 pins 7 and 8 to ground? Where are the ground connections for V2? Can you post a pic that shows all ground connections for V2, ie, tank ground, grid and cathode grounds, reverb pot ground?

[Dr Tony Balls]

What resistance do you measure from V2 pins 7 and 8 to ground? Where are the ground connections for V2? Can you post a pic that shows all ground connections for V2, ie, tank ground, grid and cathode grounds, reverb pot ground?

V2 pin 7 to Ground: 215 kΩ  (makes sense b/c of R30)
V2 pin 8 to Ground: 1.48kΩ  (makes sense b/c of R31)

The ground connections are shown in the following pic.  The cathodes for V2 (pins 8 and 3) are connected to ground via the eyelet board as labeled in the upper part of the pic.  The reverb pot ground is connected to the same spot. All grounding for the board is connected together and links from the board to a PT bolt. 

The grounding for the reverb tank is done via a terminal strip shown in the lower part of the pic.  Both tank grounds as well as the grid ground are connected here.  I thought about the possibility of a grounding issue however 1) I would think that would manifest itself in other parts of the circuit not being isolated to one section, and 2) this is the way Fender did their reverb wiring for ages....cathodes on the board, tank and grid resistor grounded at the rear of the chassis.

[sluckey]

All grounding for the board is connected together and links from the board to a PT bolt.

This may or may not be your issue, but since you have tried all the logical tests, could be worth trying. Separate your power grounds from the preamp/reverb grounds. Fender connected the reservoir cap grounds to the chassis near the PT. All other filter caps and board grounds were connected to the brass plate (call it the preamp ground if you wish). This keeps the nasty power amp grounds contained at the PT end of the chassis, well away from the sensitive, low signal end of the chassis. As you have it, the reverb cathodes are connected to the same ground point as the power amp grounds. That reverb recovery triode is the most sensitive triode in the amp since the signal from the reverb tank is the smallest signal in the amp, even smaller than the preamp inputs.

Sounds like something easy to try when writing about it, but I realize layout could make it more difficult. Still worth a try IMO.

[Dr Tony Balls]

All grounding for the board is connected together and links from the board to a PT bolt.

This may or may not be your issue, but since you have tried all the logical tests, could be worth trying. Separate your power grounds from the preamp/reverb grounds. Fender connected the reservoir cap grounds to the chassis near the PT. All other filter caps and board grounds were connected to the brass plate (call it the preamp ground if you wish). This keeps the nasty power amp grounds contained at the PT end of the chassis, well away from the sensitive, low signal end of the chassis. As you have it, the reverb cathodes are connected to the same ground point as the power amp grounds. That reverb recovery triode is the most sensitive triode in the amp since the signal from the reverb tank is the smallest signal in the amp, even smaller than the preamp inputs.

Sounds like something easy to try when writing about it, but I realize layout could make it more difficult. Still worth a try IMO.

BRAVO, you've done it.  Thank you immensely for your help.  It's a problem i've not run into before, however i'm not usually building circuits with reverb and that must have caught me off guard.  To test it I just liftet the ground connection of V2B and ran it to the chassis elsewhere with an alligator clip and sure enough the hum is gone.  I'll separate my preamp from power amps grounds more officially now and go forth.

Thanks again.

[Latole]

Sluckey is a master for trouble shooting. 

[peterc]

That is really interesting. Not something I have run into before, but noted. Thanks

[Joel]

This: http://valvewizard.co.uk/Grounding.pdf is one of the best things I've ever read about grounding in tube amps.

In the last amp I built (5W single ended thing based on the AX84 "Clean Preamp") I used the improved bus ground idea from Merlins grounding chapter and referenced it to the input jack only.  Quiet!

[dude]

I’ve always separated my power supply and preamp grounds as far as possible, even if l use a buss bar to the back of the pots it’s grounded to the input jack, from there to a terminal screwed close to the chassis with the cathodes of the first preamp tube.
Never had any hum, all my amps are done this way.
Now the Hammond AO44, l’m working on is a rats nest, grounds all over the place but still l’ll do my best to keep HT and preamp grounds as far as possible from each other.

[silverfox]

Perhaps I'm missing something but, I don't see where the filament wires are twisted.

Opps. I see the problem has been resolved.

silverfox.