The schematic is on Schematic Heaven.
http://www.schematicheaven.com/boogieam ... _22cal.pdf
Instead of the long tailed PI a negative voltage is applied to the cathode of the PI tube to set its cathode voltage to 2V. The negative feedback is applied to the other grid of the PI tube.
Mesa Boogie used a similar approach in the Subway Blues model.
Has anyone ever seen or tried this? What would the benefits of this PI be compared to the usual long tailed PI?
As always thanks a lot for your input.
Stephan
Mesa Boogie Studio .22 - unusual phase inverter
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Darkbluemurder
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Darkbluemurder
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Re: Mesa Boogie Studio .22 - unusual phase inverter
I added the link to the schematic for convenience.
http://www.schematicheaven.com/boogieam ... _22cal.pdf
http://www.schematicheaven.com/boogieam ... _22cal.pdf
Re: Mesa Boogie Studio .22 - unusual phase inverter
It saves several parts, but demands a negative bias supply.
Fender's "long tail" appeared before general use of fixed-bias with handy negative supply. His trick avoids the negative supply, but puts the grids up around +50V, which means more caps to bring signal to grids.
There's trickier stuff in these Mesas. MB was trying new things; also perhaps getting away from Fender's Intellectual Property.
Fender's "long tail" appeared before general use of fixed-bias with handy negative supply. His trick avoids the negative supply, but puts the grids up around +50V, which means more caps to bring signal to grids.
There's trickier stuff in these Mesas. MB was trying new things; also perhaps getting away from Fender's Intellectual Property.
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David Root
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"Trickier Stuff"
I've not seen this one before.
V1A-B is different. Looks like a tweed mixer with a single input and an elevated grounded grid. Presumably most of the current is drawn up thru the common cathode, some splits to the cathode on the right hand side and the grid gets a little current thru the 470k resistor, sort of like a screen. Is this a pseudo semi-pentode? Minus the suppressor grid, which makes it a pseudo semi-tetrode?
V1A-B is different. Looks like a tweed mixer with a single input and an elevated grounded grid. Presumably most of the current is drawn up thru the common cathode, some splits to the cathode on the right hand side and the grid gets a little current thru the 470k resistor, sort of like a screen. Is this a pseudo semi-pentode? Minus the suppressor grid, which makes it a pseudo semi-tetrode?
Re: Mesa Boogie Studio .22 - unusual phase inverter
> V1A-B is different.
Sure as heck is.
> Looks like a tweed mixer
Hmmmm... could be that's what was on their mind.
> Presumably most of the current is drawn up thru the common cathode, some splits to the cathode on the right hand side and the grid gets a little current thru the 470k resistor...
V1B grid is 1V negative of cathode, so draws no current in the 470K. A and B have equal bias and flow equal current. Only A gets signal on its grid. At a glance, it's just a mixed-up plain-triode amplifier. It would be "better" without the B side. But I assume M-B arrived at this plan "by ear", and liked it this way. I could work out the technical parameters, but that won't tell the "sound". Hmmm... there's more than first-glance here, the second triode does reduce the "need" for a cathode capacitor, and shifts the distortion from more even-order to a lower THD point with some odd-order.
> Is this a pseudo semi-pentode?
I'd almost say it goes the other way. It's a pretty soft triode. It's a bicycle built for two, but one rider is just dead weight.
I can't make out WHAT the 470K does. It may just be leftover from starting with a Tweed double-input and messing around. If they got it sounding good, they may have been afraid to go further and try taking the 470K out. AFAICT, it just adds hiss. This hiss may be just above the self-hiss of an average 12AX7, or it may be reduced by the "poor performance" of this plan. And if it "sounds good", the added hiss and cost is "worth the waste".
I have to assume there is a dot on the grid of V2A. Then when in Lead mode, V1AB amplify the input, but the 0.001uFd coupling cap only passes above ~500Hz. There is still a path through the 3M3 to V2A, and the boost from V1AB arrives out of phase with that. So there may be a cancellation dip below 500Hz. And a lot of phase shift over the 200Hz-800Hz range which includes most lead guitar fundamentals. If these are shifted relative to their harmonics, later clipping has a different effect than normal low phase shift inputs.
Note also the odd value cathode-cap under V2A.
And M-B is kinda notorious for "errors" on their schematics. What I see there is plausible but not sure to be correct.
Sure as heck is.
> Looks like a tweed mixer
Hmmmm... could be that's what was on their mind.
> Presumably most of the current is drawn up thru the common cathode, some splits to the cathode on the right hand side and the grid gets a little current thru the 470k resistor...
V1B grid is 1V negative of cathode, so draws no current in the 470K. A and B have equal bias and flow equal current. Only A gets signal on its grid. At a glance, it's just a mixed-up plain-triode amplifier. It would be "better" without the B side. But I assume M-B arrived at this plan "by ear", and liked it this way. I could work out the technical parameters, but that won't tell the "sound". Hmmm... there's more than first-glance here, the second triode does reduce the "need" for a cathode capacitor, and shifts the distortion from more even-order to a lower THD point with some odd-order.
> Is this a pseudo semi-pentode?
I'd almost say it goes the other way. It's a pretty soft triode. It's a bicycle built for two, but one rider is just dead weight.
I can't make out WHAT the 470K does. It may just be leftover from starting with a Tweed double-input and messing around. If they got it sounding good, they may have been afraid to go further and try taking the 470K out. AFAICT, it just adds hiss. This hiss may be just above the self-hiss of an average 12AX7, or it may be reduced by the "poor performance" of this plan. And if it "sounds good", the added hiss and cost is "worth the waste".
I have to assume there is a dot on the grid of V2A. Then when in Lead mode, V1AB amplify the input, but the 0.001uFd coupling cap only passes above ~500Hz. There is still a path through the 3M3 to V2A, and the boost from V1AB arrives out of phase with that. So there may be a cancellation dip below 500Hz. And a lot of phase shift over the 200Hz-800Hz range which includes most lead guitar fundamentals. If these are shifted relative to their harmonics, later clipping has a different effect than normal low phase shift inputs.
Note also the odd value cathode-cap under V2A.
And M-B is kinda notorious for "errors" on their schematics. What I see there is plausible but not sure to be correct.
Re: Mesa Boogie Studio .22 - unusual phase inverter
http://www.schematicheaven.com/mesaboogie.htm
The Studio .22 Caliber Plus is 99% the same but differently fuzzy, some parts easier to make out.
The Studio .50 Caliber is the same general idea but some different part values (note V1 gets a cathode cap), so they did fool around some more yet kept the 470K.
M-B has also used plain parallel triodes. Also single triodes with resistor-, diode-, and current-source biasing.
D-180 has the Master Volume in a very gonzo place.
Those guys must just snort too much resistor smoke. It gives them weird dreams about new ideas to try. That's refreshing, since 95% of amp "design" has been copies of Golden Oldies, nothing new. OTOH, the fact that M-B's novel designs have not revolutionized the racket suggests that these strange tricks are not better or worse than any other. (M-B's pioneering ideas on complex switching and robust construction have caught-on, more or less depending how much cheaper than M-B another company wants to be.)
The Studio .22 Caliber Plus is 99% the same but differently fuzzy, some parts easier to make out.
The Studio .50 Caliber is the same general idea but some different part values (note V1 gets a cathode cap), so they did fool around some more yet kept the 470K.
M-B has also used plain parallel triodes. Also single triodes with resistor-, diode-, and current-source biasing.
D-180 has the Master Volume in a very gonzo place.
Those guys must just snort too much resistor smoke. It gives them weird dreams about new ideas to try. That's refreshing, since 95% of amp "design" has been copies of Golden Oldies, nothing new. OTOH, the fact that M-B's novel designs have not revolutionized the racket suggests that these strange tricks are not better or worse than any other. (M-B's pioneering ideas on complex switching and robust construction have caught-on, more or less depending how much cheaper than M-B another company wants to be.)
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David Root
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V1A-B
Thanx PRR, that IS more plausible than a "pseudo-tetrode".
The 470k remains a puzzle though. Could it be leaking a little upcoming cathode current back to ground, possibly adding a teeny-tiny bit of positive feedback into the cathode thru the 680R? With the 470k being big enough to prevent it running away to oscillation? If so, how would that interact with the phase shifts into V2A?
Or am I snorting resistor smoke too?
The 470k remains a puzzle though. Could it be leaking a little upcoming cathode current back to ground, possibly adding a teeny-tiny bit of positive feedback into the cathode thru the 680R? With the 470k being big enough to prevent it running away to oscillation? If so, how would that interact with the phase shifts into V2A?
Or am I snorting resistor smoke too?
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bluefireamps
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Re: Mesa Boogie Studio .22 - unusual phase inverter
It's actually way more simple than all that. I've worked on a .50 Caliber and the schematic is not drawn very well. V1 A+B are just in parallel, grid included. The 470K resistor is just the ground reference for both halves of V1. Before I saw the inside of one, I was also confused as to what was going on. The other weird thing is that the input signal goes into each triode, V1 and V2, at the same time making V1's signal to V2 out of phase with the straight guitar input to V2. It works probably because the straight input is much more attenuated by the 3.3M/1M resistor network compared to the higher gain of V1. Any cancellation is small and probably in the very high frequencies.. It's a strange little circuit!
Dave
Dave
Last edited by bluefireamps on Sun Jul 22, 2007 9:05 pm, edited 1 time in total.
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David Root
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The Case is Solvedd!
Well, you'd never know from that schematic! Maybe it was deliberate, to throw people off.
Re: Mesa Boogie Studio .22 - unusual phase inverter
> V1 A+B are just in parallel, grid included.
Ah!
(duh. That's how the 22+ is drawn, but I thot it was bad mimeographing.)
> the input signal goes into each triode, V1 and V2, at the same time making V1's signal to V2 out of phase with the straight guitar input to V2. It works probably because the straight input is much more attenuated by the 3.3M/1M resistor network compared to the higher gain of V1. Any cancellation is small and probably in the very high frequencies..
Yeah, that's odd but decipherable. My gut says the null is more mid-bass, but still too lazy to work it out. Midbass makes some sense: many "Lead" channels are bright, ie bass-shy. There's a time to vamp mellow behind the singer, and a time to jump out front and scream.
> Maybe it was deliberate, to throw people off.
I've seen just enough "sneaky errors" on older M-B drawings to wonder. Can't blame them if it is to divert mindless copy-cats.
Ah!
(duh. That's how the 22+ is drawn, but I thot it was bad mimeographing.)
> the input signal goes into each triode, V1 and V2, at the same time making V1's signal to V2 out of phase with the straight guitar input to V2. It works probably because the straight input is much more attenuated by the 3.3M/1M resistor network compared to the higher gain of V1. Any cancellation is small and probably in the very high frequencies..
Yeah, that's odd but decipherable. My gut says the null is more mid-bass, but still too lazy to work it out. Midbass makes some sense: many "Lead" channels are bright, ie bass-shy. There's a time to vamp mellow behind the singer, and a time to jump out front and scream.
> Maybe it was deliberate, to throw people off.
I've seen just enough "sneaky errors" on older M-B drawings to wonder. Can't blame them if it is to divert mindless copy-cats.