Tube Amps / Music Electronics
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|11/21/2000 8:45 PM|
||Re: "best" master volume control?|
The "best" MV circuit is the one YOU like. I have heard decent sounding amps with all of these circuits. My least favorite is the Matchless style, the Ken Fisher mod will lose bias to the power tubes if the pot gets dirty and/or opens. A pre p/i MV needs a well designed preamp before it. The post p/i dual pot MV requires the least modification to a relatively stock amp.
An interesting twist to the standard post p/i dual MV is to swap the input & output leads to the MV. This runs the output of the p/i to ground creating more distortion at lower settings. Bedrock used this circuit in their amps. The production model I saw used a dual 500K Lin. pot. I imagine anything from 250K to 1M would work fine, although 250K will load the circuit a bit even when turned all the way up.
|11/22/2000 12:55 PM|
O.K. I'm dense. As to the "Bedrock"-style post PI MV, you're saying (for each side) have the PI output (after the caps) go to the wiper of the pot, one end lug to ground and one to the power tubes? How does running the PI output to ground create "more distortion at lower settings" than the wiper connected to the power tubes? What is it about driving the PI output to ground that causes the PI to distort more?
Any and all explanations gratefully accepted.
|Book Of The Day||
The Ultimate Tone, Volume III by Kevin O'Connor
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|11/22/2000 8:50 PM|
Maybe that's because the signal has to go through the total resistance of the pot when the setting is low?
Just a guess.
|11/22/2000 10:18 PM|
You don't need to be a rocket scientist (or Randall Aiken) to prove this. Just try it. It's really easy to reverse the connections on the pot and hear the
What is it about driving any output into a short circuit that causes distortion? Limited voltage swing! Since the output can't follow the input faithfully it flattens out sooner producing "more distortion". But don't take my limited technical description for it, try it!
|11/23/2000 12:24 AM|
The reason a post MV yeilds more distortion in lower settings is because as you lower the signal going to the power tubes you also lower the feedback level to the PI. Less feedback = more distortion. (Unless you have a no feedback loop.)
Some amps even have a variable feedback control that allows you to dial in more or less feedback hence cleaner or grittier tones.(this is the simple layman's explination)
|11/23/2000 12:45 AM|
If you can hear that distortion I'll buy you a beer, heck I'll buy you a case of beer!
While your assumptions are technically correct and can be measured on test equipment this is not the distortion of MV circuits. You are more likely to hear the effects of less damping but I doubt you will hear a "gritty" distortion from removing the feedback loop, unless your amp has some serious problems.
The Post P/I MV in question distorts more at lower settings because the P/I's plates are being forced to ground, limiting it's voltage swing capability. If you re-read my original posts you will get the gist of what I'm referring to.
Maybe someone more versed in talking about load lines and that kind of stuff can jump in here and either explain this better or tell me why I'm off base. All I know is I saw it with my own eyes and heard it with my own ears, and a chunk of it fell right on my head...
|11/23/2000 4:07 AM|
My answer wasn't meant to be technically satisfying but stated in the simplest terms for the questioner. If you'd like a more technical explination here's a quote from Kevin O'Connor from his book the Ultimate Tone:
" before we look at the details of installing a power amp MV we should look at what it's net effect will be. The power section in most guitar amps has a feedback loop that tries to maintain constant gain despite environmental influences.
The paraphase drive signals are at whatever level the input stage determines to be appropriate to make the feedback signal look like the input signal. If we attenuate these paraphase signals the input stage must work harder to obtain the required feedback. At some point the input stage runs out of gain and output capability and begins to distort.
The output signal is then distorted as well. The greatest distortion occurs when the MV levels are set low. Less signal is transfered from the input stage to the output stage...the poweramp master volume does not allow a truly clean sound to be set up as the conventional MV is able to do. This means the power amp MV is restricted to those situiations where the amp is being set up for one range of tones only: distorted ones."
Got any Samuel Adams?
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