Question About Capacitor Upgrade in Tube Amp

The Vishay Z-foils are very transparent and low-noise. These resistors can be ordered in arbitrary custom values from Texas Components. The latest version is the TX2575. These are only available in sizes from 10 ohm to 100Kohm. The older TX2352 is available from  1 ohm to 250K ohms. 

For higher power requirements or resistors outside these ranges, I like the Audio Note non-magnetic tantalums and silver-tantalums, but these can get pretty pricey.

I'm also a strong believer in using high quality film caps. I've had good luck with V-cap ODAMs (very neutral and clear) and Miflex KPCU (a bit warmer with a beautiful midrange tone). 

A couple things to keep in mind. These caps all take a few hundred hours of break-in to sound their best. And some will actually sound worse during the break-in period. 

And most important, these amps have LETHAL voltages inside. Capacitors can hold a lethal charge for a long time. Make sure you are thoroughly familiar with the safety procedures before playing around inside a tube amp. The OP mentioned doing DIY guitar amps so is probably already familiar, but anyone else contemplating doing this kind of upgrade should use extreme caution. 

NOS Allen Bradley Carbon comp resistors are all the rage for tube guitar amps. For their warmth and organic tone. I wonder how they fair in hifi? They do tend to drift in value a fair bit. So maybe not so good in hifi setting.

Carbon comp resistors are very noisy and add distortion. This may be (is likely) desirable in a guitar amp, but not what I want in my hifi amp. 

@calieng - the primary purpose of the bypass cap on the power supply electrolytic is to shunt high frequency noise on the B+ rail. I doubt it will have any affect on the high frequency response of the amp. The value of the cap is really not that significant as long as it is a good quality film cap. 

The biggest influence on the high frequency response will be from the output transformer and possibly from the feedback network. I wouldn't recommend changing these parts without a solid understanding of how the amp works as well as having adequate test equipment to make sure the amp is stable after the changes.

Using a higher quality coupling cap also might give you a small improvement in high frequency response if the one that is in there now is not of very good quality.  

@ieales - I'd be real interested to understand how adding some film decoupling caps cross the B+ supply could cause a tube circuit to oscillate. 

I can see how adding too much capacitance to a regulator could cause the regulator to oscillate, but oscillation in tube amplifiers is rarely due to the power supply, and when it is, it's because the power supply has inadequate regulation (too high an output impedance), not the other way around. 

Yes, obviously changing capacitor values in the amp itself, and particularly in the feedback circuit, can cause an amplifier to oscillate. I spend a fair amount of time reading DIY audio sites (and building my own amps and preamps), and I don't think I have seen a case where adding a small amount of additional capacitance to a coupling cap or power supply cap ever caused an amp to oscillate.