Least intrusive master volume for princtone reverb

[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.

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.

It works fine how you 've drawn it. And yes your right about the bias reference.

Sorry about the dis-information. I was not paying close enough attention to the original circuit.
So with the bottom of the pot connected to the R40/R41 junction, the change in PI bias between different pot settings will not be noticeable? (470K vs 1.47M grid leak at opposite ends of pot)

[potatofarmer]

Sorry about the dis-information. I was not paying close enough attention to the original circuit.
So with the bottom of the pot connected to the R40/R41 junction, the change in PI bias between different pot settings will not be noticeable? (470K vs 1.47M grid leak at opposite ends of pot)

If you're concerned with grid-leak bias occurring, you can use a 500k-A pot instead; the input impedance will still be massive. Or exclude the 470k grid stopper; it's not necessary... though it does improve the distortion tone significantly.

[Shane]

Yep Paul's right with both points. This is the best master I've tried on split load inverters.

[elipsey]

If you're concerned with grid-leak bias occurring, you can use a 500k-A pot instead; the input impedance will still be massive. Or exclude the 470k grid stopper; it's not necessary... though it does improve the distortion tone significantly.

OK, I'll try it. Why does the grid stopper improve tone?

[Shane]

Have a look at the bits toward the bottom of the page where it says 'avoiding unpleasant overdrive tones'

http://www.valvewizard.co.uk/cathodyne.html

Cheers
Shane

[Shane]

oops double post sorry!

[soundguruman]

[QUOTE=elipsey;341574]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.

The best master volume is no master volume.
Because the power tubes need to overdrive to get the real sound you are looking for.
And a master volume prevents this from happening.

All other methods (master volumes) are a compromise.

The real sound comes from the power tubes screaming for holy mercy.
And you will always be trying to get that sound, and won't.

The better compromise(s) are:

A. Less efficient tubes
B. Less efficient speaker
C. Change output from pentode to triode, not difficult
(install a pentode triode switch instead of a master volume)
D. Use a good power attenuator, mount it in the cabinet.
E. Speaker emulator

Then you maintain tone, high frequency crispy-ness, sensitivity.
That you otherwise lose with a master volume.
There's quite a big difference in sound quality.

See, musician is always trying to put master volume in amp.
But this is not producing what musician wants, or expects.
Using other methods result is more useable, from musicians standpoint,
of sound quality.

[Shane]

Soundguruman is right in saying driving the power tubes is what you want to get that big 'ol fullsome tone.

However I have to say I think all the methods he suggests to achieve it will change the original amp tone a lot. You will not preserve the original tone.

Now don't get me wrong those 'different' tones wont be 'bad' just different, and his suggestions are all very valid approaches.

Also its true that a master volume also will cause the amps response to be different from its non master sound..but again its just ....different, not bad.

If you want relatively unchanged tones and get that amp running flat out tone, then you're gonna have to get into things like power scaling etc....even thats not perfect, and quite intrusive and relatively complex to install. But thats a whole other discussion................

You, of course may not want power tube overdrive at all!

Cheers
Shane

[soundguruman]

Soundguruman is right in saying driving the power tubes is what you want to get that big 'ol fullsome tone.

However I have to say I think all the methods he suggests to achieve it will change the original amp tone a lot. You will not preserve the original tone.

Now don't get me wrong those 'different' tones wont be 'bad' just different, and his suggestions are all very valid approaches.

Also its true that a master volume also will cause the amps response to be different from its non master sound..but again its just ....different, not bad.

If you want relatively unchanged tones and get that amp running flat out tone, then you're gonna have to get into things like power scaling etc....even thats not perfect, and quite intrusive and relatively complex to install. But thats a whole other discussion................

You, of course may not want power tube overdrive at all!

Cheers
Shane

Used to be that everyone wanted 100-300 watt amp.
Now most players are recognizing how ridiculous that is, they want 6 watt amp.

So we are all trying to turn amp to lower loudness...
instead of just starting with 6 watt amp.

After all, you are not playing outdoor stadiums...
even Princeton is too loud.

It's ALL a compromise.
No matter what you do, there is always some sacrifice somewhere.

There is no perfect master volume, which preserves original tone of amp 100%.
No matter which method is used.

And, I am kind of surprised that nobody has designed one.

[elipsey]

Have a look at the bits toward the bottom of the page where it says 'avoiding unpleasant overdrive tones'

The Valve Wizard

Cheers
Shane

Thanks for the link, that's a very helpful discussion. The last time I built a Princeton Reverb clone with a post-PI master volume, I noticed that when it was heavily overdriven it sounded kind of... funky. It's hard to find good words to talk about this kind of thing, but I guess I would say it sounded sort of sproingy, like a guitar did it with a mouth harp.

That's why I thought it would be cool to try the MV before the phase inverter this time.

[elipsey]

See, musician is always trying to put master volume in amp.
But this is not producing what musician wants, or expects.
Using other methods result is more useable, from musicians standpoint,
of sound quality.

I get what you're saying, but the musician is also sort of the customer, so I talked with him about that stuff and I'm just trying to give him what he wants in the best possible version of what he asked for. If it's done in a way that only changes the circuit by adding a grid stopper to the PI input and the knob is on the back of the amp, then unless he likes weird sounding cathodyne distortion he's better off than where he started even if he never uses the master volume. The mod is easily reversible, cosmetically invisible, and doesn't stop him from using a attenuator or whatever.

Anyway, I'm not telling him it will sound the same as cranked amp. I think it's matter of taste.

[elipsey]

You, of course may not want power tube overdrive at all!

xactly

[elipsey]

I tried replacing the grid leak with a 1MA pot as discussed, and the taper was not usable. It goes to full volume at something like "1" out of "10". I checked the pot and made sure it wasn't wired backwards (with reverse taper) or defective. I'm wondering if I really understood the instructions. Here, again, is a picture of the circuit:


Is this right?

[Shane]

Hmmm.. your schematics right, dont know why it's reacting like that, never had a problem with the ones I've installed.
Maybe you could try a 500KA see if thats any better.

Cheers
Shane

[g1]

If you think it may be taper related, try wiring it backwards. Yes it will work backwards but as a test it will show you if it has to do with the taper.
Or simpler still, measure the pot from wiper to ground when set at "1" on the dial.