Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jecars on May 23, 2018, 12:37:51 pm
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[Edited with more accurate schematic] Sorry for the confusion. I forgot how detail-focused this forum can be. Same question below applies....
I have been curious about the function and sizing for LTP capacitor between Rt and ground. Please see attached schematic.
I am referring to "Cx", not "Cg2" (I know about this capacitor). Please educate me or provide a good link. For some reason, my internet search came up empty.
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There should not be a capacitor there. It would not allow current to flow in the triodes.
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Are referring to the design/creation of a Presence control? If so, it is also accompanied by using a feedback network from the OT secondary.
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Someone buggered that Valve Wizard drawing. There is no such thing as Cx. That circuit cannot possibly work. Where did you find that drawing? Did you do it?
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Edited schematic, see Original Post.
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What is it supposed to do? What is it connected to? As it is, I see some sort of high pass filter with no useful purpose. If you're trying to implement a negative feedback or presence control circuit, that's not the way to do it.
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[Edited with more accurate schematic] Sorry for the confusion. I forgot how detail-focused this forum can be. Same question below applies....
That's more like it! :wink:
Yes, we are detail focused, but we can only work with the details you provide.
Cx is part of the typical later style Marshall NFB loop and presence control circuit (The resistor in series with the cap should be a pot if you want to have variable presence). Cx makes the NFB effect frequency sensitive. Just like a simple series cap and pot make a simple tone control.
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The negative feedback signal needs to be applied to G2 of the LTP to complete the circuit.
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Thanks, sluckey. Your insight about simple tone cicuit analogy makes sense.
For fixed resistors (ie. no pot...per detail-focused schematic :laugh:), it seems like simple circuit analysis would allow selection of serial and parallel resistor as well as cap size to attain desired cutoff frequency. If so, one thing I struggle: Seems like I must set resistance (impedance) of circuit in question and set the feedback serial resistor (not shown) so that the voltages match at feedback insertion point. Am I right?
Finally, how effective is all of this in the end. Does the tone shift really matter that much in the feedback loop? Any experience?
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Old and new style NFB. The new style removes DC voltage from the presence pot, eliminating scratchiness.
Leave the parallel resistor alone. That helps set up the bias for the PI tubes. The cap and series resistor (pot) will determine the amount of feedback and the frequency response. Works very well, although the presence pot is one of those "set it and forget it" adjustments (at least for me).
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Thanks, sluckey