Why do master volumes thin out the tone?

[clintonb]

Why do master volumes thin out the tone?

I'm talking about the traditional pre-phase inverter master volume.
I've never tried the post-phase inverter type.

[black_labb]

do you mean when the master volume is low, and the stages before it are distorting?

thats because distortion turns a signal into the original frequency and harmonics of that frequecy. for example lets use a pure 80hz signal to keep it simple. before distortion it is just 80hz. after distortion there is less 80hz, but 160hz, 240hz, 320hz and on. as you can see, the distorted signal has more high end content now. the same thing happens with the guitar signal, but it has harmonics already there, which also distort.

[clintonb]

Black_labb, thanks for the response.

do you mean when the master volume is low, and the stages before it are distorting?

I guess I'm talking about both situations: when the preamp is clean and when it is distorted.

My silverface twin reverb has a master volume, but I don't turn it down so I can crank the channel volume to get more distortion. Instead I turn it down just to get a little less bass and then I turn up the channel volume to compensate for the volume loss.

With my Crate Vintage Club amp, I'll turn the master down just because when I turn the gain level up too high, the amp gets too loud. But when I do that, I lose bottom end.

thats because distortion turns a signal into the original frequency and harmonics of that frequecy. for example lets use a pure 80hz signal to keep it simple. before distortion it is just 80hz. after distortion there is less 80hz, but 160hz, 240hz, 320hz and on. as you can see, the distorted signal has more high end content now. the same thing happens with the guitar signal, but it has harmonics already there, which also distort.

Did you mean to say "less 80hz" or did you mean to say "no less 80hz"? Are you suggesting that it only sounds like there is less bass because of all the additional high frequency content?

[GibsonLover]

That's a classic MV problem. Regardless where it's wired in, pre or post MV, look at the time constant formed by the nearest coupling cap and the MV control to ground. As you adjust the MV resistance, the time constant changes, and so does your corner frequency, thus rolling off the lows and leaving you with a thinner sound. The same thing happens at a normal volume pot position, and is compensated by the treble cap connected to its wiper.
Without the cap, the treble loss is less noticable in the preamp than a MV position because some of those lower frequencies can be filled back in with distortion products (noted earlier) in the preamp stage.

[GibsonLover]

...and another thing ( I meant pre or post "PI" above)... anyway - The MV control I like the the "cross coupled" version that some people dislike for various reasons. Shunting the out of phase signals together, without changing the "R" values to ground sounds like it keeps the low end in tact. At least it sounds good on my Matchless Chieftain head. Shorting the AC signal like that loads the PI differently, and contributes it's own problems, but I likes what I likes...

[black_labb]

Did you mean to say "less 80hz" or did you mean to say "no less 80hz"? Are you suggesting that it only sounds like there is less bass because of all the additional high frequency content?

i said less, but i should have said less in relation to the harmonics. you always have more 80hz, as distortion in a tube is amplification, and distortion. you get to a point where the amp cant put out the peaks of the waves, and then you get distortion.

[clintonb]

That's a classic MV problem. Regardless where it's wired in, pre or post MV, look at the time constant formed by the nearest coupling cap and the MV control to ground. As you adjust the MV resistance, the time constant changes, and so does your corner frequency, thus rolling off the lows and leaving you with a thinner sound.

OK, that is what I was suspecting. I was wondering if some kind of unintended filtering was happening. I need to study up on RC time constants and filters, etc.

I'll have to check out that "cross coupled" master volume circuit.

[clintonb]

The MV control I like the the "cross coupled" version that some people dislike for various reasons. Shunting the out of phase signals together, without changing the "R" values to ground sounds like it keeps the low end in tact. At least it sounds good on my Matchless Chieftain head.

Is this also referred to as a "cross-line" master volume?

[GibsonLover]

Yup - that's it. The beauty is that it can be easily tried out with clip leads if you want. Notice that on the Matchless, it's cathode biased, so if you have a fixed bias amp make sure you know what you're doing.

[Enzo]

Also your ears are involved, google up "Fletcher-Munson curves."

[d95err]

The MV control I like the the "cross coupled" version that some people dislike for various reasons. Shunting the out of phase signals together, without changing the "R" values to ground sounds like it keeps the low end in tact. At least it sounds good on my Matchless Chieftain head. Shorting the AC signal like that loads the PI differently, and contributes it's own problems, but I likes what I likes...

In my experience, the cross line MV affects the tone much more than a dual-pot post PI MV. The cross-line version forms a highpass filter with the coupling caps from the PI, thus resulting in loss of bass as you turn it down. I also experienced that it acted more or less like a switch. Very little change for most of the sweep and then everything way down low when the tone had already lost lots of bass (I did use a log pot).