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PostPosted: 12 Oct 2017, 15:46 
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If the insulation breaks down between the heater and cathode the heater AC will modulate the voltage on the cathode.
There is always some leakage between cathode and heater because its not perfect.
Tube heater leakage tends to vary with different tubes, good bad or indifferent.

So rather than scrap tubes its easier to try to create a situation that enables the heater modulation to be within exceptable limits.
In other words compensate for slight leakage. Or lift the heaters potential to stop leakage.
In a perfect scenario Equal potential equals no transfer of current. But its not a perfect world.
Due to the fact that the cathode potentials are different you lift the heaters so you get a ball park working difference between heater and cathode ie not exceed the insulation value of all tubes in the circuit.

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M. Gregg

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PostPosted: 12 Oct 2017, 15:54 
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The ground connection goes between the two triodes, so each "sees" 135V between plate and cathode, in my case. As for the heater, I understand one of the triodes sees it as +50V and the other one sees it at -50V from the cathode. However, I now understand that the sign doesn't really matter here... but how big the voltage is. Thanks!

Back to the practical matter, I disconnected one of the heater AC leads... but it's still buzzing.

I'll try one last thing (although I remember trying this first). In the second link you provided, Broskie showed his own build (quite similar to mine actually) and he mentioned that connecting circuit ground to house ground introduced hum and he disconnected it, leaving house ground connected to the case. I won't hurt trying this again.


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PostPosted: 12 Oct 2017, 16:11 
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If you step back from the problem for a moment,

Then think I have:
DC heaters
Bi polar supply
noise cancellation via the circuit design.
I have two tubes
An input selector

What can be making the noise. Because its either an input to the circuit, or injected via voltage from supplies.

The noise cancellation of the circuit could inject noise via the bottom tube grid, if it was out of balance.
(assuming there was ripple in the supply) Are the rails balanced compared to circuit ground? B+ and B-

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M. Gregg

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PostPosted: 12 Oct 2017, 16:53 
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Yes, they’re within 0,5V of one another, compared to ground, on one tube and 0,1V on the other tube.

Also, disconnecting ground from house ground resulted in noise with the pot mid way.


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PostPosted: 12 Oct 2017, 17:07 
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I have to sign off,

Try shorting the input out with a wire directly on the board.
That will eliminate the input wiring and the volume pot.
If it still hums its not the input wiring.

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M. Gregg

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PostPosted: 13 Oct 2017, 02:30 
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If all else fails,

You need to start looking at the circuit.

Remove all connections from the Circuit board except the AC for the supplies.
And the output so you can monitor it.
Connect wire across the input connections so they short out the inputs.
NB you must physically disconnect all other wiring. You cannot rely on the short to eliminate the input wiring because the grounds may be causing a loop.

The idea is to test the circuit board with no other connections.
No ground loops, no input wiring no grounding.

See if the circuit board on its own hums.

If it does then you need to look very carefully at the circuit diagram and trace it on the PCB to see if the two are the same.
If it doesn't then you need to keep the input shorted out and connect one ground wire at a time and test.

There are other things you can try:
Get a large ferrite bead and wind the circuit ground wire (the one from circuit ground to earth on the chassis)
Wind the ground wire 5 times around the ferrite (through the core).
So you have an inductor between circuit ground and chassis ground.

Regards
M. Gregg

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PostPosted: 13 Oct 2017, 02:39 
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This may sound silly, but... can the bypass caps on the B supply hum? I’m thinkin about C4 in particular, as this is a big one and it’s kinda loose (the big red ones).

I assume that if they vibrate, they can induce hum on the B supply.


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PostPosted: 13 Oct 2017, 02:48 
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MrJackson wrote:
This may sound silly, but... can the bypass caps on the B supply hum? I’m thinkin about C4 in particular, as this is a big one and it’s kinda loose (the big red ones).

I assume that if they vibrate, they can induce hum on the B supply.


The caps shouldn't hum.
If they are then there is an instability.
Are the electrolytic caps humming or getting hot? <<be careful its B supply!
I haven't read the function of C4 under normal circumstances I would try removing them.
But you need to see if they are just bypass or if they are some kind of feedback function.

So you need to read the information. If they are just to reduce the ESR of the supply then try removing them.

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M. Gregg

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Last edited by M. Gregg on 13 Oct 2017, 03:14, edited 1 time in total.

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PostPosted: 13 Oct 2017, 02:53 
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C4 is not electrolytic, but polypropilene, if I’m not mistaken. I’ll check tonight.


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PostPosted: 13 Oct 2017, 02:58 
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I have just had a read and the function isn't mentioned for C4,

You need to connect some old speakers etc when testing.
You can use a scope and dummy load but not everyone has one.

Unless you can see anything about C4. try removing them.

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M. Gregg

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