Noise in Dumble circuits....

[ic-racer]

Here is another way to do it (in context). Check out the "Hum Balance" control in the lower right.

[markr14850]

Here is another way to do it (in context). Check out the "Hum Balance" control in the lower right.

Just hum balance is better than no hum balance, but balance plus elevation really helped in my case.

[fabiomayo]

So sorry to ask this but I don't think I understand. The B+ is the 6.3v
winding on the PT? What about the other side of the winding?

I'd like to try that on my somewhat noisy build. Shielded preamp
heater wires didn't help.

Is it anything like this?


Edited: Changed the picture to reflect heisthl corrections below.
Calculate the resistors value according to how much lift you want
and your B+ voltage.

[heisthl]

No - it's like this, I hacked your drawing:

[fabiomayo]

No - it's like this, I hacked your drawing:

Terrific. Thank you so much. I'll try it as soon as I can get the trimpot.

[heisthl]

Here is another way to do it (in context). Check out the "Hum Balance" control in the lower right.

Here's an easier to follow Fender method:

[mlp-mx6]

But I do not think that the Fender method raises the ground reference very much, if at all. The pot is functioning as the 100ohm resistors would. It is a *slightly* variable pseudo-center tap.

To elevate, you would need to connect the hum pot wiper to a point ABOVE ground voltage, usually 10V-20V. This effectively reduces the willingness of the cathode circuit to pick up induced current fluctuations from the AC heater. The big cap across the lower resistor shunts the AC noise to ground. Without that cap it is VERY noisy.

[heisthl]

I completely agree - it is amazing though that the 100 ohm pot actually did allow you to tune out some hum.

[drz400]

Has anyone tried DC heaters? that may take care of some of this, I personally don't worry about it- tube amps are noisy

I used a switching DC power supply that Bogner uses on the Shiva. You can get it from Digikey for $25, easy , no transformers.
0 and I mean absolutely no hum at any volume you can run the power tubes as well (series) just make sure you ground the DC, or you will have noise.
Raising the ground ref also helps on some amps but unless the grounding is perfect (not using chassis) in some cases it makes thing worse. DC heaters on the other hand seem to always work.

[giorgiosecco]

[quote="markr14850"]For me, a 10-turn 200ohm precision wirewound pot for balance, with about ~20v of elevation on the wiper does the trick. It's a *huge* difference. For people who have never tried this before, add this to an amp tonight!

Even with the 10 turn pot, I'm amazed at how small a window of movement it is for optimum hum silencing. And the best position is nowhere near centered.

The 20 volts just requires 2 resistors (for me, 470k and 24.7k) as a voltage divider and a cap (for me 47uf), fed from the plate supply. This is all easily mounted on a center-grounded 3 terminal strip.

I'm not sure if I got it right,would you be so kind to post a schem for me?it would be of great help..

[heisthl]

Go up this page 5 posts to the schematic in my post.

[Mark]

So who has tried it and does it work as described?

I have often wondered why items such as this haven't been a standard part of the amp if they work so well. Of course the dollar value is a big part of it.

[giorgiosecco]

Go up this page 5 posts to the schematic in my post.

thank you so much..what if you have 300v secondary?which resistor value do you suggest?if you had the time to show me the calculation method,would be reaallly appreciated!must be something that involves ohm law...

[fabiomayo]

thank you so much..what if you have 300v secondary?which resistor value do you suggest?if you had the time to show me the calculation method,would be reaallly appreciated!must be something that involves ohm law...

Ohm's law alright

Here's how I do it (other hopefully will do better):
(Vout/Vin) = R2/(R1+R2)

Vin is the B+, let's say 300v
Vout is the elevation you want, let's say 20v
R1 is the first resistor in th VD, let's say 470k (arbitrary)
R2 is taken from the equation and is roughly 33k. Just choose the closest standard value you can find...

Hope it helps.

FWIW in my build (50w HRM with different layout) no matter where I set the multi-turn trimpot the noise is the same. Of course, my build hums a little bit

[giorgiosecco]

thank you fabio,I think its worth trying,before trying dc filaments,my express clone sounds great but hums!