Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: rdrgtr on September 25, 2010, 12:05:24 am
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I noticed something the other day when I was looking at a preamp circuit (dumble I think). The circuit called for a 100pf cap from the plate resistor to the cathod. Basically the cap was between pins 1 & 3 on the first tube stage. High freq. roll off? Just curious. :huh:
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Yeah, it rolls off some of the highs to "smooth" out the tone.
With respect,Tubenit
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Any benefit of that application over a small cap across the plate resistor? (sometimes called a "snubber" IIRC)
Chip
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Snubbers are usually used to control high frequency oscillations. The down side is it may snub into the audio band. Often high gain amps need a snubber and can fortunately afford some hi frequency attenuation.
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Well, I'll tell you what I've done. Such as on the TOS build, I tried BOTH ways on different gain stages. Sometimes I like the plate to cathode ......... sometimes I preferred across the plate resistor.
I have a cap substitution box that makes quick work of experimenting that way.
I could not find a particular guideline for me other than simply listening to it.
With respect, Tubenit
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In principle it makes little difference. Plate is "hot", either B+ or cathode is "pretty near AC ground", so it is a simple plate-to-ground high-roll-off.
You might hear a difference one way or the other.
I suspect in many cases you can just go wherever is easiest.
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Plate-to-cathode (or plate to ground) is better from the point of view of PSRR, but in practice it makes little difference.
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Or tie a cap across the plate resistor as in some Fender silver-face amps.