Liverpool build Hum problem..Fixed

[gahult]

My Liverpool build as I have a hum in it.
It is only there with PI tube V3 installed. Pull V3 it's gone. Pull any other tube it's there.
The hum is present with all controls down to zero so I thought it could be filament related but, I am sure it's in the PI.
I have tried 10 or more tubes in V3, even my good Tungrams but, no luck.
Any thoughts?

Thanks,

Gary

[Jana]

From your description, I would suspect that it has something to do with the ground for the speaker jacks, the ground for the phase inverter and the Presence pot/cap ground. They all could have a "good" ground, but if these items are grounded in different places there can be a hum introduced. If this is the problem, what usually cures this is to use isolated speaker jacks and run the ground from the jacks over to the PI/Presence ground area. The secondary of the OT is grounded at the speaker jacks but a smaller wire is run from the jack ground to eventually make chassis ground along with the PI/Presence. Does this make sense?

A way to test this is to remove the speaker jacks from their holes, float them in the air, and connect a jumper cable from the speaker jack ground terminal over to the PI ground.

[gahult]

Jana,

Thanks, will give that a try.
Here are the voltages on V3. Keep in mind my AC runs at 122+v here.

Pin
1 202
2 40
3 57
6 204
7 36
8 57

Gary

[Jana]

I have never built a TW so I can't comment on your voltages other than to say that they look a bit suspect. There is 17 volts and 21 volts difference respectively between the grids and the cathodes of V3a and V3b. That seems high. Usually there is about 1 to 3 volts difference on the long tailed pair PI.

[M Fowler]

Gary,

Did you ground the post above B3 that goes to junction of R17/R18?

I have seen other post showing:
V3
1 238
2 45
3 65
6 240
7 42
8 65

So yours seems to be inline to me. Especially if not running a matched PI tube.

Mark

[gahult]

Mark,
Yes, it is grounded to the grounding buss where R14 and C10 are grounded.
I am going to try some other tubes (5751, 7025, 12au7, 12at7) in V3 position and see what the voltage readings are. Maybe find some closer readings. I can pull the V3 12ax7 out of my Express it reads 22v/35v on pins 2/3, 7/8. Could be a good comparison as the boards are the same.
Will try later, gotta run to the doc for followup on surgery.

Thanks,

Gary

[M Fowler]

Gary,

That is a good plan. Other than carefully going over the wiring and soldering I do not know how to solve the problem. How goes the shoulder?

I got a snow day today a rare event for VA to close.

Mark

[El_Martin]

Hi Gary!

Grounding in TWs can be a real PITA.
Redone it several times on my 2 hole Liverpool.

Is it changing, when you pull V1?
Try grounding the inputs of the gain stages from front towards PI.

Even if you turn all the knobs down, there are some gain stages increasing the noise (hum).

Good luck!
Martin

[gahult]

Thanks to everyone for your input.
This was a problem I created by not going "by the book".
I tried to take the easy way out, or so I thought, duh, by using 1 wire to ground the cap stack wiring all the leads on the back together.
Don't do that. I learned you have to follow instructions. Do they have to draw me a picture? Yup, I had one but I didn't use it.
There is a reason for everything..lead dress, lead dress.
I separated the caps and put the other two ground wires in and....NO HUM..just the high gain hiss.

In closing I would just like to apologize for my bone headed stupidity and say go by the proven, correct way of the builds.

Gary

Just an after thought. How does one get by using can caps and not having an issue with 1 neg. ground lead?

[M Fowler]

Gary,

C20 & C21 form the 80u for B+1 in the schematic. C19 is by itself for B+3 that is why they are separately grounded.

Mark

[RJ Guitars]

Gary,

Thanks for taking the bullet for all us boneheads out here... you've got a lot of company that has shared that special moment when the light came on !#@%@^#$:oops:

rj

[soma_hero]

With the can caps I used in my Liverpool, the two 50uf in one can connect to ground, and the other 40/20/20/20uf in one can connect to the same ground point. Same ground point but different wires. No humming at all!