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Can I Change the Cathode Bias Cap Value?

4/5/2004 3:41 PM
Can I Change the Cathode Bias Cap Value?
What would happen if I used a 25mfd-25V electrolytic capacitor instead of the existing bad 20mfd-25V cap across the 140-ohm cathode resistor coming off the 7591A power tubes in my '65 Ampeg Rocket 2? Would it affect bias in any way?
4/5/2004 6:24 PM
In my experience, changing the cathode bypass cap in a cathode-baised output stage from 20uf to 25uf will not bring about a noticable change in tone or frequency response.  
That said, in my experience, replacing an old electrolytic cathode bypass cap with a fresh one has positive effects on tone and frequency response. I notice fuller tone, stronger lows and mids, and clearer highs. It's a bit like removing a pillow from in front of a speaker.  
Changing uf values for this cap *can* cause noticable changes in tone and frequency response, but in my experience, the difference in uf value needs to be greater for it to be noticable.  
If you replaced the 20uf with 5 uf, you'd probably notice a lot less low end.  
Re: whether or not it will affect bias in any way - yes it will, but not drastically in the case you describe (20uf > 25uf). Anything greater than 22uf is not going to improve bass response much more anyway, so you're already operating within a very good-for-guitar frequency response range witht eh 20uf.  
But that 20uf cap is probably original and therefore not functioning at optimal performance anymore, so replacing it with a fresh one of similar value will only make the amp sound better, IMO.  
Hope this helps,  
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4/8/2004 5:23 PM

Thanks Craig. That was a lot of great info. My follow up question is this:  
In general terms, how does a change in cap value affect bias? I've changed bias by playing with resistor values, but I don't understand how cap value changes affect bias.
4/8/2004 10:22 PM
Try this: measure cathode voltage with the cap, then without. See if you notice any change in voltage.  
4/8/2004 10:34 PM
I got a little more ambitious and did a google search on "Cathode bias" and came up with some good references for you (and me!)  
Hope these help,  
4/9/2004 5:21 PM

Thanks Craig. That explained it. I really appreciate the links. Don
4/8/2004 11:28 PM

Changing the cap won't affect bias, but it will affect frequency response.
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