Adjust cathode bias pot - both channels move together. Why?

[Hearinspace]

This may be a problem that needs a wiring diagram or schematic and if so I can post one tonight but I'm hoping it's something that at least one or two people here have encountered before and will recognize.

In a nutshell, with pots for bias on the cathodes of single stage DHT's in both channels of a preamp, if I adjust the bias on one side, the bias of the tube on the other side tracks with it. Works both ways.

The B+ supply string to ground is PT > FullWave Bridge > LCLC > split to two channels >LC > Line Out Transformer > DHT > Cap bypassed bias pot > signal ground.

The Ground buss runs from chassis through the bridge > PS caps 1,2, 3 right/left. Each channel's signal ground buss running from first PS cap > bias pot > input volume pot wiper > input selector switch (grounds form inputs are switched with hots) .
(There is also hum and/or buzz that varies in sound and intensity with all the changes that I've tried so far looking for the fault.)

Present filament supply is on a separate board and is P Transformer > split to two separate channels consisting of FW Bridge > LCLC > LT1084 >

I've looked for shared ground faults to chassis but don't find any. Can there be a DC reference through the filament supply bridges referencing both filamentary cathodes to each other?

[Arnulf]

Um, with no regulation of supply voltage any change of bias voltage (and consequently quiescent point / quiscent current) will inevitably change the B+ voltage of both tubes. Change in B+ will consequently lead to change in quiescent current of the other tube which (with resistive bias cathode arrangement such as potentiometer or straight resistor) will cause bias voltage to change.

Physics. Use fixed or didoe (LED etc.) bias if you don't like the way it works now.

[kevinkr]

Originally Posted by Hearinspace

T

Present filament supply is on a separate board and is P Transformer > split to two separate channels consisting of FW Bridge > LCLC > LT1084 >

I've looked for shared ground faults to chassis but don't find any. Can there be a DC reference through the filament supply bridges referencing both filamentary cathodes to each other?

Does the power transformer have independent filament windings for each channel's filament supply?? If not the bias is the sum of the plate currents flowing in both tubes - and adjusting the bias on one automatically changes it on the other as well. Sensitivity to the plate supply voltage should not normally result in more than a few % variation in the plate current of the other channel.

[Hearinspace]

Originally Posted by Arnulf

Um, with no regulation of supply voltage any change of bias voltage . . . . . . . will cause bias voltage to change.

Thanks Arnulf. Yes I know that but in my experience so far if quiescent current draw on one tube is increased , that pulls down the B+ netting a drop on the other side . . . . sort of a See-Saw effect. In this case, when the current through one side is increased the current through the other side increases too.

Originally Posted by kevinkr

Does the power transformer have independent filament windings for each channel's filament supply?? .

Kevin, thanks and sorry. I wasn't sure how to paint the picture with words so it would be clear in one go. The power transformer for the filaments is an 18V Hammond feeding two separate rectifiers and their associated filter strings.

The B+ transformer is an old Lundahl 350VAC LL1650. I thought about using three of its 6 V windings for one of the filaments but asymmetrical loading of the Lundahl secondaries can sometimes lead to problematic hum fields, so I put two rectifiers and their circuits on the one Hammond.

[kevinkr]

Originally Posted by Hearinspace

<snip>

Kevin, thanks and sorry. I wasn't sure how to paint the picture with words so it would be clear in one go. The power transformer for the filaments is an 18V Hammond feeding two separate rectifiers and their associated filter strings.

<snip>

I must be dense but I don't understand from this whether or not you have two totally separate filament windings (2 x 9V?) on your filament transformer - if you don't that is the issue, if you do then you have a wiring/design issue.

Can you post schematics showing clearly what you have done?

[Hearinspace]

Originally Posted by kevinkr

I must be dense but I don't understand from this whether or not you have two totally separate filament windings (2 x 9V?) on your filament transformer - if you don't that is the issue, if you do then you have a wiring/design issue.

Can you post schematics showing clearly what you have done?

Hi Kevin, The Hammond only has one secondary - 18VAC. I'm using whatever I have on hand to explore a bit and so things are a little mis-matched.

I just tried putting one of the rectifiers on three of the Lundahl 6V windings and now the bias works as desired.

Thanks !

PS. Do you use buss grounding? I had the whole B+/Audio circuit on a single buss running from PS through everything with cathode and lastly grid at the farthest end. I also had hum that I thought might be associated with the cause of the bias problem and I tried to cure that by cutting the tube's ground circuit off the end of the buss and rerouting it straight to the first PS Cap. Now there's no hum but lots of 120Hz buzz. I'd have thought it would be a good idea. Perhaps not?

[kevinkr]

I do generally use buss grounding, and have no problems with hum or buzz in any of my components. (And this with 32Hz, 100dB+ efficient speakers)

Generally the first cap in my power supply gets the HT transformer center tap, the main chassis ground and the end of the ground buss, and that's it. Usually the highest current grounds circuits get placed near the last filter cap in the HT supply and everything follows from there.

The ground buss should be grounded at one, and only one point to avoid ground loops and to prevent supply ripple currents from developing a voltage across the IR drop of your ground buss.

[SY]

Originally Posted by kevinkr

Generally the first cap in my power supply gets the HT transformer center tap, the main chassis ground and the end of the ground buss, and that's it.

That's interesting- in my boxes that use bus grounds, I attach the chassis ground at the other end of the bus, at the input.

[Hearinspace]

OK, all is well- switched back to the previous buss scheme where it's essentially all in one row with grid at one end rectifier at the other . Because the way the bridge was physically situated I put chassis ground between rectifier and first PS cap. Now all hum and buzz is inaudible though for the first time I am hearing a lot of what seems to be tube rush but with a distinctly white noiseish shhhhhhhhhhhhh. I think I remember reading somewhere that this kind of noise can be caused by HF oscillation. I'll try something tomorrow but for now this is sounding pretty good. Thanks !

[Cal Weldon]

Hi Iaian.