Least intrusive master volume for princtone reverb

[elipsey]

I want to add a master volume to a princeton reverb clone in a way that makes the least change possible to layout. It would be nice if the pot had a switch so it could switch out of the circuit when not in use. I plan to put the pot on the back of the amp in the hole where the ground switch was traditionally located so as to avoid drilling new holes.

So the question is, what's the least disruptive way to do this? It hasn't been as easy as I hoped to find pots with various kinds of switches. I have a push pull DPST pot, and a pot with an SPST switch the closes when the pot is turned clockwise from the '0' setting (like a champ power switch pot). I haven't found any DPDT amp pots, although that would seem to be ideal. Anyhow, the weber schematic for the 6a14 (PR clone) uses a dual ganged 1M pot between the PI and power tube grids. This seems like the most obvious solution. I'm not sure I liked hearing distortion from the PI in this amp the last time I did it this way, although I can see that it's arguably part of the amp's sound.

Here's a picture of that from their schematic:


The problem with that is that it adds a couple of big caps that are sort of inconvenient to have to crowd in (.1 orange drops), and I can't find a dual ganged pot with a switch.

So I'm thinking about a single pot w/ switch before the PI, configured in rheostat mode, so I can use a pot I already have with an SPST switch. Here's a diagram:


I like this because when it's switched out, the circuit it identical to the non-MV circuit, and I only have to add one smaller cap, and if it breaks the amp will still work, sans master volume. I guess the downsides would be that the taper will be different since it's not a voltage divider, and the tone will be differnt at low settings because it will mess with the impedance seen by the previous stages, right?

Any thoughts?

Anyone have a more elegant or minimal solution?

[jimboyogi]

I would recommend a Lar-Mar style post PI MV. Effectively it is a dual ganged pot replacing the output tube grid leak resistors. No extra caps required. I have used this method for both fixed and cathode bias, and I really like it. This also means that when turned all the way up, it is effectively out of the circuit, and no need for a switch.

The only thing to be careful of is that the grid leak is now via the wiper/track of the pot. This means that it is vital to make sure that the connection doesn't fail, which pots have a tendency to do sometimes

The way to safeguard this is to hardwire a fixed resistor between the pot wiper (power tube grid) and the pot track end that connects to ground/bias.
Some people use a 2.2M fixed resistor in combination with a 250K log pot, this gives ~ 220K when all the way up, so identical to the original grid leak value. However if the wiper/track connection fails, then the grid leak value becomes 2.2M, which is a lot higher than recommended spec, which worries me!
Instead I use a 470K resistor in combination with a 500K log pot. If the pot connection fails, grid leak value is 470K, which seems much safer to me than 2.2M.

Another thing, if you don't like the sound of the PR cathodyne PI when distorting, then I would recommend putting a hefty grid stopper right on the cathodyne grid (pin 7). This is gone into in detail on the Valve Wizard website - The Valve Wizard (and even more detail in Merlin's pre-amp book, money very well spent IMO). The recommendation is 470K to 1M, and is reported to greatly improve the distorted sound.

[soundguruman]

I want to add a master volume to a princeton reverb clone in a way that makes the least change possible to layout. It would be nice if the pot had a switch so it could switch out of the circuit when not in use. I plan to put the pot on the back of the amp in the hole where the ground switch was traditionally located so as to avoid drilling new holes.

Anyone have a more elegant or minimal solution?

[Alan0354]

So I'm thinking about a single pot w/ switch before the PI, configured in rheostat mode, so I can use a pot I already have with an SPST switch. Here's a diagram:
[ATTACH=CONFIG]28236[/ATTACH]

I like this because when it's switched out, the circuit it identical to the non-MV circuit, and I only have to add one smaller cap, and if it breaks the amp will still work, sans master volume. I guess the downsides would be that the taper will be different since it's not a voltage divider, and the tone will be differnt at low settings because it will mess with the impedance seen by the previous stages, right?

Do the pre PI stage, but connect the pot as volume pot, not as the variable resistor. You are changing the frequency response and changing the loading or the previous stage.

[Shane]

Just replace the 1M resistor R39 with a pot wired as a normal volume pot with decent sized resistor( 470k ish) from wiper to grid. works fine.
Cheers.
Shane

[Chuck H]

Build one of these.

[potatofarmer]

Another vote for just replacing the PI's grid leak with a 1M pot. A switch isn't needed since it's effectively the same as the stock circuit with the master all the way up. I used this for a master on a build with a cathodyne phase inverter and it works really well.

[elipsey]

Do the pre PI stage, but connect the pot as volume pot, not as the variable resistor. You are changing the frequency response and changing the loading or the previous stage.

I understand, just wondering if anyone has tried it that way, and if it actually sounds bad, or maybe it's good enough/matter of taste?

[Mick Bailey]

The way you've got the pot configured in the second pic not only gives a variable load to the preceding stage, but is acting as a variable T filter so you'll get a tonal shift too. I'd go with wiring a pot as a volume control to the PI if you want minimal intrusion.

[elipsey]

Just replace the 1M resistor R39 with a pot wired as a normal volume pot with decent sized resistor( 470k ish) from wiper to grid. works fine.
Cheers.
Shane

Is this what you mean?


Interesting. Does it affect the bias of the PI when you turned the knob?

I like how it doesn't change the layout. I'm a little iffy about putting DC on pots though. In the location I'm using the pot would short to the chassis if the nut got loose and it rotated. Also, it seems like having a big DC offset on a pot can get a little noisey when you turn it. What if the control was AC coupled and in paralell with the grid leak?

[g1]

The bottom of the pot should go to ground, not to the top of R40. Then there will be no DC on the pot.

[elipsey]

Sorry, I must not be getting how this works. I though that the junction between R41(1k) and R40(56k) provided the bias reference for the grid, so if I remove that reference, the PI won't be biased properly anymore. What am I missing here?

[Shane]

It works fine how you 've drawn it. And yes your right about the bias reference.
There is DC on the pot but very little DC current thru it so noise is pretty much unnoticeable.
I've wired it like that lots of times with no issues.

Cheers
Shane

[Chuck H]

Or build one of these: It's as unobtrusive as you can possibly get, you won't be hearing the ugly end of the PI that's normally hidden in the cutoff duty of the power tubes and you can use it with other amps too.

[elipsey]

Or build one of these: It's as unobtrusive as you can possibly get, you won't be hearing the ugly end of the PI that's normally hidden in the cutoff duty of the power tubes and you can use it with other amps too.

I know I shouldn't overlook the attenuator option, especially for a small amp like this where the power requirment is lowish, but I'm trying to do what the owner asked for... Also, I'm sort of afriad that if I give people attenuators, they will cook their tubes or break whatever isn't up to snuff in the amp and then get mad at the tech...

EDIT: I'm totally putting an attenuator on my next 5F2A build though -- just a hifi lpad that can handle 10 or 15 watts, good to go! It seems to me like the smaller the amp, the better an idea attenuators are. Anybody here try an AC4TV with the built in one? i think it really kicks ass, and the parts probably cost 40 cents.