Pros and Cons of Different Types of Master Volume

[mcabe]

Apologies if this has been discussed here before, but I am interested in learning a little more about the pros and cons of the different master volume implementations. Adopting the Ken Fischer ruberic ((link removed)) there are 4 different types of master volume.

1. "Type 1": Post-phase inverter master volume with a dual gang pot shunting signal to ground.

2. "Type 2" or "Fischer/Larmar": Post-phase inverter master volume replacing the load resistors sitting between the phase inverter and the power tubes with a dual gang pot.

3. "Type 3" or "cross-line": Post-phase inverter master volume inserting a 1 meg pot across the grids of the power tubes, so the inverted signals cancel each other out.

4. "Type 4": pre-phase inverter master volume placing a pot before the phase inverter shunting signal to ground.


I don't think that there is a single best master volume, but each has its place depending on the circuit and the desired effect. I am interested in learning more about where these different implementations excel and where they are lacking. For example, if your goal is to generate preamp distortion at low volumes and you find phase inverter distortion to be "fizzy," you should implement a pre-phase inverter master volume. Or if your amp has negative feedback, it may not work well with a cross-line master volume.

So what are the tonal and other advantages and disadvantages of each? In particular what are the differences between the 3 post-phase inverter master volumes? Hopefully, I didn't leave out or mis-characterize anything above.

 

[jonrpick]

Subscribed...

 

[robrob]

If the phase inverter is a 'less than unity' cathodyne (concertina) then a pre-phase inverter master volume is usually preferred because you get the added control of PI distortion level. This is a simple mod involving adding one 1MA pot and some wire.

For gain type PI's like the ubiquitous long tail pair it's usually preferred to use a post phase inverter master volume (PPIMV) so you can control the actual input to the power tubes.

I think the Type 2 MV is best because it doesn't add anything to the circuit, it just makes the power tube grid leak resistors variable. A 'con' is that it is not a simple mod.

I installed a Type 3 cross line MV in my 5F6A Bassman because it is simple and effective. I haven't read about the issue with feedback and the Type 3 MV, could you give us more detail?

Tonally I don't think there's much difference between them and in a blind comparison I'd be surprised if anyone could consistently pick one MV over another.

 

[mcabe]

Good stuff Rob! I'd say that any master volume controls the amount of input to the power tubes. Pre vs post PI just changes how much you drive the phase inverter as well. The cathodyne phase inverter is often associated with unpleasant sounding overdrive tones, so the pre-phase inverter master volume is preferred. Merlin over at valvewizard recommends adding unusually large grid stoppers to the cathodyne PI to fix the problem. I imagine, if you were getting good sounding overdrive from a cathodyne PI, a post phase inverter master volume would be warranted.

I haven't read about the issue with feedback and the Type 3 MV, could you give us more detail?

I don't have any personal experience with the cross-line master volume and negative feedback, but I have heard reports of it not playing well with amps that have a presence control. Perhaps the presence control becomes ineffective when the master volume is turned down? Seems plausible since both the presence control and master volume work by applying negative feedback to the signal.

 

[robrob]

Yea, I could see when negative feedback is applied to a long tail pair phase inverter it is usually injected into the phase inverter's cathode and the grid of the non-inverting phase inverter triode so the inverted and non-inverted signals have different amounts of NFB so at low master volume settings it could generate some unwanted distortion but I haven't experienced that with my Bassman.

 

[mcabe]

I gather one potential downside of a type 2 master volume is that if the wiper of the pot fails, you can lose bias on the power tubes, which is why you see people put 2M2 resistors parallel to the pots. In a fixed biased amp, can the DC on the pots from the bias make the master volume scratchy, or is the bias voltage too small to matter?

Out of curiosity what is the effect of losing bias on a fixed or cathode biased amp? Burn up your power tubes, or something worse?

 

[robrob]

When the bias voltage goes away the blocking action of the control grid goes away so the tube will flow too much current from cathode to plate and will exceed the tube's power and heat dissipation limits and burn up. It can take the output transformer and other components with it.

 

[mcabe]

When the bias voltage goes away the blocking action of the control grid goes away so the tube will flow too much current from cathode to plate and will exceed the tube's power and heat dissipation limits and burn up. It can take the output transformer and other components with it.

Same result for fixed and cathode biased power tubes?

 

[mcabe]

Or if your amp has negative feedback, it may not work well with a cross-line master volume.

BTW, I think post phase inverter master volumes of any kind may not play well with negative feedback, so this is not a specific downside of cross-line master volume.

 

[robrob]

Same result for fixed and cathode biased power tubes?

Yes.

 

[clintj]

One consideration of post PI on an amp with global feedback is that feedback becomes ineffective when the PI is overdriven. The loop goes to open gain, and you get a spike in volume. You usually don't notice it in normal play other than the threshold between clean and dirt becomes pronounced. The other side effect is that once feedback goes away, the presence control also ceases to work.

If I build a PPIMV, I use a dual 1M pot with modestly sized blocking caps before and after the pot on each leg. This keeps DC off the pot, preventing scratchiness (Bassman owners know that sound on the Presence knob). I also dial back the amount of NFB a little bit.

 

[jamesdlow]

I'm seen a variation on the Type-2/Lar-Mar on the Victory V40. It uses 1M dual gang pot and 270K safety resistors.

I think the advantage of this setup is that with the resistors in parallel, the lower resistance makes more difference when calculating the total resistance. The better tolerance of the fixed resistors keeps the total grid leak resistors more consistent between the two power tubes.

 

[GlideOn-Designs]

I'm seen a variation on the Type-2/Lar-Mar on the Victory V40. It uses 1M dual gang pot and 270K safety resistors.

I think the advantage of this setup is that with the resistors in parallel, the lower resistance makes more difference when calculating the total resistance. The better tolerance of the fixed resistors keeps the total grid leak resistors more consistent between the two power tubes.

I had to register and resurrect this thread after I double-taked this casual mention of a new Master Volume variant, coming from a guy in the middle of installing one.

I was about to attempt a Type 2/Lar Mar and paused because I lack two key ingredients to make it work - a reliable dual 250k pot (reads whopping 285k and 220k on other) and lack of 1.8M resistors. Or 2M. Or 2.2M. Can't seem to know which it is that is needed. My pot is also far away from pins 5 of my power tubes and it has to through a rats nest of wiring to make it there.

So I have this spare Alpha Dual 1M pot and it's rather excellent with both sides coming within 7k of each other and I'm generally happy with the Type 3/CrossLine Master Volume for how well it works at all but extreme settings where buzzy Cross distortion occurs from about 0.5 to 1.

I like the simplicity of the Crossline because it's fewer components and less to go wrong in an uncertain application. However, I like the implimentation of safety resistors and caps taking away risk of potentially harmful voltage on questionable components.

And yes, I'm aware of PEC pots but go ahead and try to find one in the real world right now. They are either all bought up or discontinued.



So...tell me more of this Type 2 Hybrid MV you've come up with? Obvious question, but are you using the usual .1uf coming off of the PI?