Tube Amps / Music Electronics
For current discussions, please visit Music Electronics Forum. New: view Recent Searches.
New: visit Schematic Hell!
The sunn still shines online!

Listen to great tunes streaming live right now!

ampage archive

Vintage threads from the first ten years

Search for:  Mode:  

Cathode bias Bypass Cap

8/26/1998 7:57 PM
Cathode bias Bypass Cap
I built a Spitfire clone awhile back. I have gain coming out of my ears, whereas the others I have talked to have a nice distortion. In my construction, D.Hoffman suggested splitting up the El-84 by giving them thier own Cathode resistor and Bypass caps. Compared to the ones I have heard about would be the only real difference. Other than components. I went with a 250/5w resistor instead of the 120/5w and a 25uF/50v cap on each tube.  
Today, I removed these caps to see if I could notice a change, or blow up the amp. I noticed my gain has changed for the better and my bias dropped a little. Which brings me to the question which I couldn't find in books I had.  
What is the purpose of the Bypass caps on the cathode bias resistor in this amp(others).  
How does that value affect the sound. Say if I split up the value(25uf) for each tube.  
Or am I just imaganing a change.  
8/26/1998 8:27 PM

A cap with a larger value will typically gibe you more bottom end, to a point.  
You might use values from 25uf to about 100uf or so. The voltage needs to exceed the voltage drop across your cathode resistor.  
You could try keeping them split, but they might sound better together. You won't know unless you try.  
The real question is where is the gain coming from. You could try to decrease the size of your grid load resistors to limit the gain somewhat.
Book Of The Day The Ultimate Tone, Volume III by Kevin O'Connor
Have you ever wondered if there is a better way to build a Bassman, Champ, Plexi, an 800, AC-30, Bulldog or Portaflex? Or you wanted to build an SVT with off-the-shelf parts? How about a master-volume amp that doesn’t change tone with the master setting? Everything you need to know is right here, including: proper grounding techniques, wiring methods, and mechanical considerations. Eighteen chapters cover the “iconic” amps everyone knows and loves, with schematics and layouts for each, along with the technical history of the product. Eyelet-board and chassis-mounted tube socket construction is used throughout, for easy servicing and modding. TUT3 is very accessible even if you cannot fully read a schematic and is a "must have" if you are going to build an amp for your self.

Note: The Ampage Archive is an Amazon Associate site. A small commission is paid to the site owner on any qualified purchase made after clicking an associate link such as the one above.
8/26/1998 10:11 PM
Benjamin Fargen

Brent, On the same note.....I have noticed that some preamp designs drop the bypass cap going to the cathode of the first 12AX7, while having a bypass cap on the second 12AX7 tube. I was guessing it was a tone thing..... but I never thought you could drop the bypass cap(s) across the bias resistor(s) for a tone change and have it work..........  
Anymore input on this subject out there?  
See Ya, Benjamin
8/27/1998 2:26 AM
Steve A.
Question on BF Fender preamp Ck/Rk
    BF Fender amps often shared the same cathode resistor and bypass cap between two stages: typically an 820 ohm resistor and a 25uF cap. It is generally recommended to split these up by installing a seperate 1k5 resistor on each cathode, along with a 25uF cap. My question is this: why isn't the value of the cathode resistor bypass cap either doubled or divided by two?  
    I'm not sure if this applies to Class A cathode bias but for Class A/B cathode bias D. Torres says that the bypass cap is to "reduce regeneration, poor phase relations and predominant odd harmonics". He says you might get away without the cap if you have new tubes that are well-matched, but its better to just put the cap in (generally around 50uF). I put in a switch to add a second cap for a total of 100uF, which increases the bass response and gain a little bit, but I usually leave it at 50uF (actually 47uF).  
    Once again, this may only apply to Class A/B cathode bias, but I think that you really want the tubes to share the same cathode resistor (common cathode and all that). I've never seen a schematic showing separate cathode resistors for a pair of output tubes, but maybe D. Hoffman is on to something...  
Steve Ahola
8/27/1998 2:35 AM

When I talked to him about building this amp, and he suggested splitting them up was for the fact that the amp would be running very hot with the common cathode resistor. Seperating them, would make the tubes and the resistors run cooler. Might have been something else also but it escapes me at the moment.  
8/27/1998 2:58 AM
Dave H

"I've never seen a schematic showing separate cathode resistors for a pair of output tubes"
Check out the Matchless Clubman in Ampage schematics.  
8/28/1998 8:15 PM
Steve A.

    With the Matchless Clubman and Chieftain, are the separate cathode resistors used because they sound better, etc., or because one 30 watt resistor can be more expensive than two 15 watters? (Class A will really crank them milliamps through those cathode resistors 100% of the time...)  
    In terms of reliability, in the event that a cathode resistor opened up, a single resistor would effectively shut off the output of the amp and not cause any damage to the transformers (is that correct?) What would be the worst case scenario if one of the two cathode resistors in a Matchless Clubman were to open up? (To make it even worse, let's imagine that you had to answer the phone in the other room and it was your mother and she kept talking and talking and you didn't bother to set the amp to Standby...) Just wondering if there is a potential down-side to using a pair of cathode resistors.  
Steve Ahola
   Page 1 of 2 Next> Last Page>>