Cap and resistor types in guitar tube amp

[voivodata]

Hi
I’ve read quite a few posts with strongly differing opinions on the importance (or not) of the type of passive components used.
I don’t wish to open a can of worms here, just to get some advice and/or suggestions.

I’m up to replacing the resistors and caps in the dirty channel preamp section of my guitar tube amp – it has for stages of ECC83

I plan on using:
- 2W Metal Film resistors for cathode and gridleak in all four stages and for plate and grid stoppers in the first two stages
- 1/2W Carbon composition for plate and grid stoppers in the third and fourth stages and in tone stacks
- Silvered Mica caps for all cap values below 1n

I still am not sure what cap types to use for dc coupling and signal attenuation - Metallyzed Polyester, or Polypropylene

I also have a pile of 630V capacitors of unknown manufacturer or type. They look very much like Xicon Polypropylenes sold at hoffman’s site, but I cut one open and found that on the inside it has stacks or layers of what looks like metal film (at least it is shiny). I’m not sure if these are metal film or metallized paper (don’t know how to tell the difference). Are they any good for use in guitar tube amps?

What about the Evox Rifa paper caps from Farnell - has anybody tried them?

I’d appreciate any thoughts you have on my choice of resistor types as well as any suggestions for the capacitor types to use.

Thanks

PS: I forgot to specify that the amp is a fender tremolux clone and I've already modified (more like rebuilt to a new) the channel in question, so I will not be ruining anything precious.
Currently all resistors are 2W carbon film and caps are Wima polypropylene. By replacing these I'm aiming at less noise and more warmth.
I also wonder if I should replace the phase splitter resistors and caps too?

[voivodata]

Are my questions that "newbie" or is it the topic being worn out.

Somebody help please

G.

[Tweeker]

Use met. polypropylene caps over mylar (polyester), they are cheap enough.

Carbon comps are even noisier than carbon film. For lowest noise, replace the plate loads with wirewounds. For values between 10k-100k "NI" wirewounds are hard to beat in audio apps. Inductance is not an issue at these values, it becomes one as you go lower.

Metal film is a good choice in cathodes, carbon film may be "warmer".

Use carbon film or carbon comp for all grid stoppers.

Noise: carbon comp > carbon film > metal film > wirewound.


Metalized paper caps are for line apps and such, not signal. They are self healing and fail gracefully. Not to be confused with paper in oil (PIO) caps.

[Tweeker]

If by plate stopper you mean a 47 ohm or so resistor paralleled by a 4-6 turn coil for the prevention of oscillations, these should be carbon film or carbon comp.

[Tweeker]

RC stage parts, the cathode bypass cap neednt be electrolytic, or even present.

[Merlinb]

2W metal film resistors are unlikely to sound much quieter than 2W carbon films, and carbon films generally sound "warmer"- so you may get the opposite of what you want! Carbon comp resistors can be quite noisy, but if they're in the 3rd/4th stage you should be ok. Wirewound resistors are great if you can afford it, but hardly necessary for a guitar amp.
As for caps, any poly type will do, although film types are preferred over metallized types. Teflon or polystyrene is the best, although polypropylene is more common. In a guitar amp you probably won't notice the difference though. Some people also like to add a ceramic cap in parallel with coupling caps, to give some high-freq texture.

I agree with Tweeker about cathode bypass caps- avoid electrolytics if you possibly can, and use non-polarized ones if you have to. However, in a guitar amp you can safely reduce the cathode bypass caps to 1uF or 2uF, thus allowing you to use poly-type caps. The lower value will really tighten up and smooth out the bass tone too!

You don't "need" to replace the PI components- that part of the circuit already has excellent signal-noise ratio.

[Tweeker]

Higher wattage resistors are somewhat less noisy than lower, this one one reason you will sometimes see 2 watts where a half watt would do.

[d2134]

I agree with Tweeker about cathode bypass caps- avoid electrolytics if you possibly can, and use non-polarized ones if you have to.

Wath about tantalum electrolytic, are they better for cathode bypass then aluminium electrolytic?

[Tweeker]

No, they are really only suited for non signal roles. Besides, if you can use a tantalum here, you can use a poly.

[Conrad Hoffman]

No comments on the sound of one thing vs another, but regarding resistors in a tube amp, remember that resistors have a maximum voltage rating, regardless of value. Power rating isn't the only consideration. They also need to be derated in high temp applications. For metal films, I prefer the larger RN70 or equivalent types. Note that flameproof metal oxide resistors are very appealing until you find out how non-linear they can be with voltage. Though my favorite cap is polystyrene, remember the maximum temperature rating is very low for a tube amp- I'd stick with polypropylene or polyester.