Valve amp volume control implementation.

[Diabolical Artificer]

Attached is a circuit for a common cathode triode gain stage, which is pretty bog standard in most valve amplifiers, apart from the volume control that is. The problem is with the volume pot set at low/zero, , the grid resistor is "nulled" out by the bottom half of the pot set to around zero ohms,with the control grid sitting pretty much at ground, which AFAIK, is bad practice.

I've seen some guitar valve amps with this sort of setup, apart from the addition of a grid leak resistor or two, but better amp's and some pre-amps use one triode as a fixed low gain
"buffer" prior to the volume control.

How would one implement a volume control without the use of another triode "buffer"? Is there a configuration/circuit that allows the use of a pot before the gain stage but doesn't have the problem as above?

Cheers, TFL, Andy,

[G6Tanuki]

The control-grid won't be at 0V *with respect to the cathode* because of the resistor/capacitor in the cathode circuit.

In practice you can delete the fixed resistor from grid to ground and allow the grid to take up its necessary potential simply by way of the potentiometer.

In some circuits I've seen, the potentiometer is wired differently: one end to ground, input to slider, other end to grid. Whatever, the grid needs to be at ground-potential if the R/C network in the cathode circuit is to do its stuff.

[turretslug]

I think that one reason for the (generally several megohms, i.e. rather higher than the value of the potentiometer to avoid excess law deviation) grid resistor is to reduce the brutality of the audio crashing of a control with iffy track/wiper interface.

[kalee20]

Originally Posted by Diabolical Artificer

The problem is with the volume pot set at low/zero, , the grid resistor is "nulled" out by the bottom half of the pot set to around zero ohms,with the control grid sitting pretty much at ground, which AFAIK, is bad practice.

Why would it be bad practice?

[Radio Wrangler]

The fixed grid leak is a worthwhile precaution because pots sometimes develop worn sections where the slider goes open circuit and without the fixed leak that would lead to a floating grid and excess anode current.

With a decoupled cathode and the anode straight to the HT rail, the circuit as drawn is a bit lacking in outputs The analogue equivalent of the famous write-only memory.

The pot backwards is a trick to avoid the open slider problem and save a whole resistor! The stage before has to be OK with working into a dead short at min volume and the 'law' is dependent on the source impedance to the pot.

David

[Diabolical Artificer]

Apologies, answering your posts a bit out of order

"With a decoupled cathode and the anode straight to the HT rail, the circuit as drawn is a bit lacking in outputs" Doh!!! Forgot to draw an anode resistor, but hopefully you all got the gist.

"Why would it be bad practice?" This is what I read in an article,and can't remember the exact details but the gist was it was bad practice, this chap also says the same thing @ around 33:00 in this video - https://www.youtube.com/watch?v=fD83Oi6gCUs

Thinking about it now after reading your comments, as you say G6Tanuki " The control-grid won't be at 0V *with respect to the cathode*" so, where the grid is with respect to the cathode is the important DC parameter.

That said wouldn't the efficacy of the grid leak resistor go down as the bit of the pot in parallel with it goes down in value causing a possible positive drift of the grid?

Andy.

[Herald1360]

That last statement sounds backwards to me.

It might be relevant if the valve was contact potential biassed where the cathode is grounded and a high enough grid leak (usually 10 megs or so) allows the slight grid current to develop a negative voltage across the resistor, but no-one would connect a volume control directly to such a grid, there'd be a dc blocking capacitor involved.

Adding a CR coupling network between a pot wiper and valve grid can be quite effective at reducing crackle as a worn pot is rotated.

[kalee20]

Definitely to me too - the only case where you want to maintain a constant (and high) resistance from grid to 0V is if the valve is contact potential biased.

Adding a fixed, high-value resistor from grid to 0V as per Andy's original post is not a bad idea for the reasons Herald1360 and Radio Wrangler say - it stops the grid floating if the volume control gets worn out and can help reduce crackles when it gets 'iffy.' But it certainly won't matter if the resistor is progressively reduced to zero by a normal, fully-functional volume control.

[G8HQP Dave]

Originally Posted by Herald1360

Adding a CR coupling network between a pot wiper and valve grid can be quite effective at reducing crackle as a worn pot is rotated.

Yes, the omission of a coupling capacitor is the only bad practice in this circuit. Having the capacitor present not only reduces noise from a worn pot, but also slows the rate of pot wear as it keeps grid current away from the wiper-track interface.

[Aub]

When i've done this, i've put a capacitor between the pot's slider and the grid/grid resistor junction.

Cheers

Aub

[Diabolical Artificer]

Thanks all.

Andy.