Tube Amps / Music Electronics
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|1/31/2005 4:35 AM|
|BK||That 47pf Cap across Marshall style PI plates|
Can anyone explain the purpose of the 47pF cap across the two plates in the Marshall style PI network? Would omitting this cause some possible 'ringing' or high frequency oscillation? What would say a 100pF cap do in its place?
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|1/31/2005 5:18 AM|
it AC couples the out-of-phase phase inverter outputs to phase cancel some of the highs. I suppose it's not absolutely necessary if the amp is fine w/out them. Probably more of an insurance thing. If you go higher, you kill off more of the highs. 47pF to 100pF, you may not hear it, possibly. You can go higher to tailor the sound if you want. The vox "cut" control is a big cap (2200pF?) there plus a pot wired as a variable resistor to control how much high become cancelled. Oh yeah, in the Vox, the cap is after the coupling caps, but seems like the same idea. In my '88 2204, Marshall moved it after the coupling caps (same 47pF) value, as another example.
|1/31/2005 7:14 AM|
To expand on Dai's comments, the value is so small that the audible high cut is minimal. There can be a bit of "sparkle" on high notes that many folks can notice.
The big effect is on any ultrasonic oscillations. At those frequencies 47 pf looks like a much smaller resistance (actually reactance) and the cancellation is much higher. This kills parasitic oscillations in the PI.
Fender usually uses a 12AT7 for the PI but with lead dress issues parasitics can occur so they often used such a cap. Marshall's use a 12AX7 with higher gain so the 47 pf is much more necessary. If everything is perfect you can sometimes do without it but Marshall's have such pronounced highs anyway that losing the 47 pf doesn't seem to make much difference to the tone, IMHO.
Looking at my handy-dandy reactance chart a 47 pf cap looks like a 100k resistor to a frequency of 30 khz. At 300 khz, which an oscillating 12AX7 can easily do it looks like a 10k resistor. But in the upper range of human hearing the value is way off the chart - millions of ohms. Thinking of that cap as a resistor of lower value to higher frequencies gives you the idea of how it shorts out those high frequency oscillations.
|1/31/2005 11:02 AM|
|Trace - Voodoo Amps
As others have stated, the cap is there to prevent HFO. It's not a must at lower volumes (in vintage amps designs that is). It becomes more of a must when you crank the amp up, some amps will stay stable and others will oscillate.
You can use a 100pF but you will roll off top end and it will be noticeable. You may like it and you may not but the only way to know is to give it a try.
Hope it helps
|2/1/2005 6:41 AM|
---"What would say a 100pF cap do in its place?}
You can use a 100pF but you will roll off top end and it will be noticeable. You may like it and you may not but the only way to know is to give it a try"
Trace is right - as you increase from 47 pf you're heading into territory where the difference can be noticed by human ears.
My feeling is that 47 pf should be good enough. If you've got still got problems then increasing this cap is not the way to fix it! By the time the oscillation is gone so likely will most of the tone!
Like most of these tricks, the 47pf cap is not a universal cure-all for big problems in other areas.
|2/2/2005 4:38 PM|
|Trace - Voodoo Amps
I didn't mean to give the wrong impression, I meant that if you wanted to roll off some top end you could increase that cap. If the amp is going through HFO and the 47pF is in the circuit then it's best to track down the problem and cure it where ever it exists.
Hope it helps
|2/2/2005 9:45 PM|
why'd marshall do it?
because fender did it. check out the 5f6a bassman circuits (or most any tweed fender circuit using a 12ax7 in a LTPI config).
why'd fender do it? there are lots of great responses below that... I just wanted to point out that it was around even in a "bass amp" that predates marshall, when "clean" was the goal. (ha!)
you'll notice that when fender went to the 12at7 in the PI spot, this cap went away. I suspect this was for three reasons 1) lower gain, less likely to oscillate, 2) one less pot, 3) not just for gain purposes, but compare a 12at7 to a 12ax7 when distorting... listen to the top end- the ax7 is a bit "fizzier", this cap helps smooth out that top end fizz a little bit.
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