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Fixed and cathode bias in one amp


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8/18/2000 5:07 AM
PeterF
Fixed and cathode bias in one amp
I posted this thread before on a newsgroup and although I got a few replies, they mostly just listed amps that had dual biasing and didn't really comment on them.  
"My amp has both fixed AND cathode biasing. A couple people have told me that this wasn't possible until they saw it and said "I'll be damned". What happened was this: a few years back, before I started to try to do some of my own repairs, I had someone convert my Ampeg Gemini VI to use 6L6 family tubes instead of the rare 7591's. Whoever did the work (I never meet them) put a 200 0hm resistor and cap from the cathodes to ground without disabling the negative grid bias.  
What are the consequences of this setup? The only thing I've seen on this is that Fender did this very briefly in the late 60's or early 70's but quit because it sounded awful. Why would it sound awful? I think my amp sounds pretty damn good right now.  
It should be noted that I never heard the amp the way it was supposed to be before it was changed. At the time, I was using shit speakers and the tubes were ancient. I just want to know what other folks think of this."  
I did find this in "A Desktop Reference of Hip Vintage Guitar Amps" by Gerald Weber. He is talking about silverface Super Reverbs here.  
 
"Some of them had a combination cathode-bias fixed-bias output stage. This kind of circuit sounds absolutely horrible and is the curcuit that gave silverface amps their bad reputation."  
 
... and in "The Trainwreck Pages" by Ken Fischer, Included in "A Desktop Reference..."  
 
"On January 5, 1965 CBS became the official owners of (Fender)...eventually they started changing circuits in all of their amplifiers. These legendary changes were not for the better. There was an eight month period of trying to combine cathode and fixed bias technologies in one product! These units were a dismal failure and among the worst sounding of the CBS Fender amplifiers."  
 
Anyways, I'm still trying to figure out why dual bias would sound bad. Wouldn't it just sound somewhere in between fixed an cathode bias?  
 
Peter
 
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8/18/2000 5:26 AM
Richie{~}==:::

Very interesting question.. some amps you just couldn't do it,ones useing bias trem type circuts.Unless you figure a way to switch between the two .  
I wondered too about a pot for the resistor in a cathode bias amp to have a more fine tuned setting for measureing.. wonder if anyone has done this and why we don't hear much about it or see it used in any amps. Seems like it would work.  
Richie{~}==:::
 
8/18/2000 9:09 AM
Steve A.
Example: SR AB568
Richie{~}==:::  
 
    I think that Peter is talking about an amp that uses fixed and cathode bias at the same time!  
 
    An example of that would be the SR AB568, with the bias balance pot and the 150R/7 watt resistors on the cathodes of each 6L6 (plus a 5/25 non-polarized cap between the two cathodes). This amp also had the 2000pF caps from the 6L6 grids to ground and 47k plate resistors for the PI (which otherwise looks identical to the AB763).  
 
    Jeez, what was CBS thinking??? No wonder SF amps acquired a bad reputation! The AB763 SR was one of the "Holy Grail" amps so why would they want to screw it up like that?   ;)   (Now that was an example of some very demented hack butchery!)  
 
Steve Ahola  
 
P.S. So what would be the effect of 47k plate resistors for the PI? Seems like you would get a lot of clean headroom before the PI would start to distort...
 
8/18/2000 4:34 PM
dutch

quote:
"An example of that would be the SR AB568, with the bias balance pot and the 150R/7 watt resistors on the cathodes of each 6L6 (plus a 5/25 non-polarized cap between the two cathodes). This amp also had the 2000pF caps from the 6L6 grids to ground and 47k plate resistors for the PI (which otherwise looks identical to the AB763).  
 
Jeez, what was CBS thinking??? No wonder SF amps acquired a bad reputation!
"
 
 
Steve--  
It was the EE CBS hired to replace Leo trying to "get rid of all that distortion". IIRC, he came from the aeronautics industry, so of course he'd have a handle on what musicians want, I mean, audio is way simpler than radar, right? :D  
 
This was the difference between Leo's Fender and the post-CBS version--Leo would actually listen to (like, oh-my-gawd!) musicians to get their opinions on the circuitry and to get ideas of how they wanted the amp to sound.  
 
quote:
"P.S. So what would be the effect of 47k plate resistors for the PI? Seems like you would get a lot of clean headroom before the PI would start to distort..."
 
 
Yes, that would be the effect. This is, IMO, the big difference in sound, as well as the UL-mandated changeover to plastic-insulated wire that required the change in lead dress that caused the parasitic oscillations that they put the dreaded caps all over the place in the circuit to "fix".  
 
I wish I had a BF Super or Twin to try the AB568 output-tube cathode circuit and plate resistor mods on (I promise I'd do it in a 100% reversible manner) to see whether it was the combo-bias, the PI changes, or the parasitic oscillation caps that people really hated....  
 
C ya,  
Dutch
 
8/18/2000 8:57 PM
Steve A.

Dutch:  
 
I wish I had a BF Super or Twin to try the AB568 output-tube cathode circuit and plate resistor mods on (I promise I'd do it in a 100% reversible manner) to see whether it was the combo-bias, the PI changes, or the parasitic oscillation caps that people really hated....  
 
 
    Probably all of the above, but let us know what you find! BTW we just heard from someone who had a similar SF amp that had already been partially BF'd; his amp tech swore that he would not hear the difference with the 2000pF caps on the grids removed... but he said it was like day and night! (That might depend on the musical style being played...)  
 
Steve Ahola  
 
P.S. I had worked at a stereo chain [Pacific Stereo] in the 70's that was bought out by CBS... we were in the CBS Retail Stores division and they just up and closed our "sibling", Discount Records. Rumor has it that they were just trying to juggle figures around on their books since Discount Records seemed to be making plenty of money. They may have had to show a loss in our division to offset the profits in other divisions, or maybe the government didn't like the idea of CBS producing records in one division and selling them in another... Who knows? In any case I don't think much of CBS since they got rid of all of the "hippies" running Pacific Stereo and moved in their own people wearing suits! That took all of the fun out of working there...
 
8/19/2000 11:09 PM
Peter S
Re: Fixed and cathode bias in one amp
Richie,  
FWIW, I have used a rheostat in series with a resistor in a cathode biased amp so I could adjust it. It works fine.  
 
Peter S
 
8/18/2000 7:14 PM
Aaron V.

I say go for it. If it sounds great to you, that's all that counts.  
 
A guitar amp is a composite thing - you really can't consider one small block of the amp without considering the rest of circuit (as Dutch alludes to in his post). Take the ultra-linear amps Fender put out. Many people say they sound like hell (I haven't played one so I can't comment myself). Some people then conclude that a 6L6 can't sound good in a UL circuit. We've all heard some 6L6-based amps using conventional screen hook-ups that sound like shit, but don't conclude that a 6L6 will always sound like crap in that topology.  
 
The bottom line is your ears. Switch it to straight fixed bias and if it sounds better, leave it. If it doesn't, switch it back to the way it was before. Try it with a straight cathode bias. Try varying the ratio of fixed to cathode bias. If you're comfortable doing this yourself or your tech doesn't fleece you, you've got nothing to lose.  
 
My $0.02  
 
Aaron
 

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