Quickie resistor question.

[balancedtriode]

Hi I am about to build A quickie lit I have had in the house for a while and I have a few resistor bases questions
1. instead of using a 4.02k resistor could I use a 3.9k without negative effects?
2.Is using a carbon film resistor a good Idea in stead of the carbon comp?, or will that have a negative effect aswell.
any info would be appreciated.
thanks,
-coltrane

 

[Paul Joppa]

Changes of 5% or 10% in resistor value will have little effect.

The 220 ohm carbon composition resistors are "grid stoppers" and it's important in theory that they have little inductance; composition is usually considered the best in this regard. Film (carbon or metal) is usually etched in a spiral which creates inductance. There are reports of low-value metal film (under 1000 ohms) working well, so the theory may not be the last word, but it's all the guidance I have.

 

[earwaxxer]

coltrane - I replaced the carbon comp just because I'm neurotic. You can use any good low impedance resistor. The carbon comp is supposed to be more 'analog'. Its and old technology. Is it wrong. No. Like I said, if you are crazy about the stability of what is available, you have options. BTW I think I used a metal film. YMMV. Enjoy!

 

[Paul Joppa]

To augment the discussion, the known problems with carbon composition are unstable values - they do change with age (and sometimes with humidity!) - and noise - they make noise in proportion to the DC current they carry. Neither of these are an issue with grid stoppers, which are not critical as to resistance and carry no DC current.

The 4K plate load resistor does carry current, and I recommend metal film, wirewound, or bulk metal foil (in order of increasing cost!), as these are known to be quiet. Carbon film can make noise, but the best manufacturers are fairly quiet. Ech of these has a different reputation for the sound, though I think the consensus is a bit tenuous...