Dumb Question

[toneman]

I've been building and repairing amps for 15 yrs and have read alot and have books up the #$^%@ but have no definitve answer(can't remember 51 yrs young) The .1 cap in the Pi used to be 200V in old Fenders as well as others. What the hell does it exactly do???? Blocks DC, Can I use a cheaper cap. Does it need to be more than 200V. Do I need to wast a premium cap here? Does it effect tone? Used a OD 716 once and only once--The cat has not been the same ..

[Structo]

I 51 years young myself.
Are you talking about the coupling caps?

I think at least on the Dumble style knockoffs we use the 600v PS orange drops.
These amps usually have around 300v on the plates.

[jaysg]

On Fenders, the one closer to ground is 200V and the one coming off the preamp is 400V or more. One could argue that they should be matched construction [both high voltage], to achieve close to identical response.

[toneman]

Looking from the back to the front of the amp the 3rd cap from the left- first 2 are the driver caps then the cap in the PI to ground.

[drz400]

I've been building and repairing amps for 15 yrs and have read alot and have books up the #$^%@ but have no definitve answer(can't remember 51 yrs young) The .1 cap in the Pi used to be 200V in old Fenders as well as others. What the hell does it exactly do???? Blocks DC, Can I use a cheaper cap. Does it need to be more than 200V. Do I need to wast a premium cap here? Does it effect tone? Used a OD 716 once and only once--The cat has not been the same ..

200V there is fine, you wont be hearing that cap it is taking the grid to ground

[heisthl]

200V there is fine, you wont be hearing that cap it is taking the grid to ground

Actually in a D style clone it has about 1 volt DC on it and whatever AC the GNF divider (presence) allows. The cap should be of good quality because there is fidelity involved with the NF. Not that I've ever done it before but listen to your amp without that cap sometime. Distortion so unmusical you never want to hear it again.
There is no reason it has to match the other grid's cap(PI input) brandage or size except that it looks cool.

[Guitarman18]

Others have used lower voltage caps as can be seen in this Funk build in the files section. It's a Sprague 225p @ 400v


Cheers, Paul.

[Fischerman]

I'm with drz400...the main 'input' for that second triode is at the cathode...the .1uF cap there at the grid is blocking dc and allowing all signal to shunt to ground (in an amp with no NFB that cap is directly grounded). However what heisthl said makes sense regarding the NFB so maybe it affects tone by some non-negligible amount. I always just use what I'm already using elsewhere in the amp. It looks better IMO and I would only save a few cents by using a lower voltage/cheaper cap there.

Considering the total cost of the amp and the fact that I consider that cost to be 'pro-rated' over the life of the amp...I'm spending what...an extra .01 cents per year by using a regular OD there?

Not that I've ever done it before but listen to your amp without that cap sometime. Distortion so unmusical you never want to hear it again.

Or you can just graze it with your soldering iron...not that I've ever done that before.

[drz400]

200V there is fine, you wont be hearing that cap it is taking the grid to ground

Actually in a D style clone it has about 1 volt DC on it and whatever AC the GNF divider (presence) allows. The cap should be of good quality because there is fidelity involved with the NF. Not that I've ever done it before but listen to your amp without that cap sometime. Distortion so unmusical you never want to hear it again.
There is no reason it has to match the other grid's cap(PI input) brandage or size except that it looks cool.

A good quality yes, is it going to make much of a difference even with NF ?, I have not personally heard it, naturally removing it would cripple the function it needs to go to ground. The effect of changing it from one style / brand to another isnt going to make much of a difference IMO and I have certainly tried that as well as played with the value and using that as a mix input (didn't care for that). No reason to skimp though, if you have a 200V part don't be afraid to use it is all and I would sweat Polyester, Polypro etc

[toneman]

Thanks for the input,the 225P looks like the ticket. Some times realestate in some builds are an issue.

[Guitarman18]

toneman wrote:

Some times realestate in some builds are an issue.

Those little flameproof 1% MF Dale resistors used for NFB are great realestate savers. It can get pretty tight with a .1uf 6PS @600V and a 3W metal oxide resistor in there.