What resistors to use on KT88s tubes?

[MGH]

LOL...didn't pay close attention to the pic.

Mundorf (M-Resist Supreme resistors) makes wire wound resistors that many feel are better than Mills. However, the values are limited and they are expensive.

 

[Miles Prower]

Carbon composition stray with age and absorb moisture. If you like that sound then use them for control grids only. For the cathodes use metal film. Personally I prefer metal film throughout.

However, C-comps have much less internal inductance, and are pretty much ideal resistances up to ~100MHz or so. Since grid/screen stoppers are RF components, you want the C-comps here (and for RF projects, if you do them as well). Otherwise, metal film for sensitive signal paths (less noise).

 

[MGH]

So which c-comp resistor do you recommend? Thanks

 

[Wavebourn]

Hi Miles;

please answer my question honestly.

For example you have an amp that has metal film resistors inside.

You have in a drawer some C-comp resistors.

How big is the chance that you open your amp and replace grid stoppers?

Anatoliy

 

[trobbins]

Are you saying that say a 5% different impedance at 1Mhz of a grid/screen stopper will impact the operation of the output stage for an audio amp?

 

[kevinkr]

Are you saying that say a 5% different impedance at 1Mhz of a grid/screen stopper will impact the operation of the output stage for an audio amp?

No, he's saying that the lower inductance construction of a carbon comp makes it more suitable as a grid stopper. I stopped using CC resistors altogether at least 20yrs ago and have found MF resistors to be generally effective, but there may be some instances where they are useful, and in VT RF applications they are the logical choice..

 

[trobbins]

Oh, I thought the inference was that because they were great for RF then they were ipso facto a requirement for stoppers in KT88 audio amps.

 

[v4lve lover]

try ferrite Beads

v4lve

 

[Miles Prower]

Hi Miles;

please answer my question honestly.

OK

For example you have an amp that has metal film resistors inside.

Yes

You have in a drawer some C-comp resistors.

I have a stock of C-comps since I also do RF projects.

How big is the chance that you open your amp and replace grid stoppers?

No chance. I retired the last commercial amp I had years ago. All my designs use C-comps for grid stoppers, metal film for passive plate loads, feedback resistors, and everywhere else in the signal chain.

 

[ChrisA]

My KT88 power amp requires 2 and 5 watt resistors on the grid, screen (?), and cathodes. I was thinking Kiwami's here, just because someone told me he had good experience with these in this application because of their slightly warm balance. I was told the Mills noninductive resistors are also good, but how do these compare to the Kiwami? Any other recommendations? Thanks.

The brand does not matter. The type does, slightly

To all those saying that carbon comp resisters change value with age and so on. Why worry they are only 20% tolerance devices. So even when they are new the value is kind of random, by 20% And this is good enough

The big thing about CCs is the resistance varies based on the voltage across them. In other words they depart from Ohm's Law. But only by a small amount. The effect is a slight compression of the dynamic range.

CCs also create some noise "hiss". The metal type are cleaner sounding and better for a HiFi amp. But hiss and compression as good in a musical instrument amp.

 

[disco]

To some extend magnetic shoulders are prone to pick up a LF magnetic field. All CC are non-magnetic. Exceptionally some carbon film and rarely metal film have this property.

 

[Wavebourn]

Disco, MLT resistors are non-magnetic.

 

[gfederys]

Just to jump in and raise hell. I built a 2A3 SE amp years ago with all Kiwamie resistors, 2+5 watt. I haven't had a lick of trouble with any in the circuit, and anyone in the audio club that's borrowed the amp, thinks it sounds great. I guess I don't understand why some people don't like them.