Anode to Cathode capacitor?

[Sowhat]

I got an unfinished basket case Triode Electronics JCM800 2203 amp as a gift and figured I would build it up, the previous owner popped for some Mercury Magnetics transformers and some other goodies so I figured it would be worth doing. The first thing I did was open up my Marshall schematic for the amp and check the stuff that has already been built. While doing this I noticed one change Triode did to this amp design that has me a bit puzzled. They put a 100pF cap from the plate to the cathode of the first stage preamp V1A. I'm not real big on tube theory but I think this may be some hack to decrease the plate to cathode capacitance... maybe to prevent some possible oscillation/instability or boost the gain or something like that. If anyone knows of this type of thing I would really appreciate an explanation of what's the point of this.

[tubeswell]

Just another variation on a plate (anode) bypass cap. Used to reduce HF harmonics

[Sowhat]

Wow, thanks tubeswell, I never knew you could accomplish plate bypass like that, I always though that had to be done across the plate load resistor. I wonder if this is some Marshall Tech note that I never saw, the original schematic circuit for this stage only uses a standard cathode bypass.

[tubeswell]

Yes to AC, the B+ (HT) and the ground both look the same, because the filter caps are shunting the AC that appears on the B+ rail to ground anyhow. In the case of a connection to a bypassed cathode resistor, that also looks the same as ground to AC (to the point that the bypass cap is shunting the desired bandwidth to ground). So you can either shunt the HF wiggle to the B+ rail or to ground (or to a bypassed cathode resistor - the more fully that the cathode resistor is bypassed, the more effective the plate AC-shunt will be). Obviously the plate bypass cap in any case needs to be rated for the maximum voltage drop it will see.

[soundguruman]

Plate to cathode is killing high frequencies to stop the stage from oscillating and ringing.
It's a standard mesa boogie tweak.

[mooreamps]

The term is known as Plate Neutralization....The way it works, although I've never seen anywhere ; where they just come out and explain why it works.. The cap has a high impedance at low frequencies with respect to the internal plate impedance of the tube... Thus has a small affect on the gain of the stage....As frequency goes up and the cap impedance is equal to the internal plate impedance, the stage gain equals 1. When tubes tend to break out into internal oscillations, the oscillation frequency is dependent on the internal inter-element capacitance's of the tube..

-g

[Bruce / Mission Amps]

The term is known as Plate Neutralization....The way it works, although I've never seen anywhere ; where they just come out and explain why it works.. The cap has a high impedance at low frequencies with respect to the internal plate impedance of the tube... Thus has a small affect on the gain of the stage....As frequency goes up and the cap impedance is equal to the internal plate impedance, the stage gain equals 1. When tubes tend to break out into internal oscillations, the oscillation frequency is dependent on the internal inter-element capacitance's of the tube..

-g

They only time I've heard of "Plate Neutralization" is when talking about RF applications.
Like others coming out of the hollow sate world of radio, I've tweaked hundreds of RF decks with neutralizing trim caps on their final PA... typically 2pF to 10pF....

[darkbluemurder]

Plate to cathode is killing high frequencies to stop the stage from oscillating and ringing.
It's a standard mesa boogie tweak.

Correct. It is also used in Dumble non-HRM amps in the overdrive section.

Cheers Stephan

[Sowhat]

Thanks everyone for your insight on this... That was probably covered in my tube theory classes back in the 60's when I was most probably gazing out the window daydreaming, wishing I was somewhere else. I took electronics just at the time everyone was transitioning from tubes to transistors and while I seemed to pay attention enough in solid state classes to pass, all the tube stuff (taught by an awesome old school tube engineer) seemed to leak out of my ears, anyway it was more of a cover old obsolete stuff kind of class in those days. Wish I had paid more attention to it back then, it would have saved me a relearning process seeing how popular tube systems have remained despite the impression given at the time that this stuff was simply a history lesson only suited for inside techie jokes about space charges and like that.

[mooreamps]

They only time I've heard of "Plate Neutralization" is when talking about RF applications.
Like others coming out of the hollow sate world of radio, I've tweaked hundreds of RF decks with neutralizing trim caps on their final PA... typically 2pF to 10pF....

Yes, for an RF deck you would want the stage to still amplify at the higher frequencies... Thus, the cap values are used as you stated... For an audio frequency amps, I typically use 100 pf, and I mount it right on the tube socket ; "all of them"...

-g