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Cathode vs. Fixed Bias: Why?


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1/31/2001 5:52 AM
Mark Lavelle
Cathode vs. Fixed Bias: Why?
It looks to me like cathode biasing should be significantly cheaper, and less hassle, than fixed biasing. I'm planning on building a relatively low power 6V6-based amp (similar to the one in TOT) and thinking there's no advantage to doing it with fixed biasing as I don't intend to use the 6L6s that the design can also handle.  
 
 
 
Am I missing something? Do I need to do a lot more reading? Is there any real tone difference between the two?  
 
 
 
Thanks,  
 
Mark
 
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1/31/2001 6:26 AM
anonymous
There's a significant tone difference! It's all a matter of what you like but for me, cathode bias makes am amp to mushy. I prefer fixed bias. The Vox AC30 and the tweed Deluxe, though they are very different amps in design and tone, they are both cathode biased and it really comes across as one simularity in their tone. It's why Matchless amps never sound right to me, but that's just me. If you like those amps, and a lot of people do, it might just be your thing. I'd recommend you build your amp so it's switchable fixed/cathode bias, the November schematic on AX84 will give you an idea of how to do this, that way you have a choice.  
 
 
 
Kelley
 
1/31/2001 5:29 PM
Mark Lavelle

Thanks for the pointer to the AX84 November project. Switchable sounds like the way to go, since I'm aiming for an experimentation platform, anyway. Just curious: is it safe to switch biasing modes while an amp is on (i.e., not in standby)?  
 
 
 
Care to try and define "mushy"? It's not a term I'd apply to at least one of the cathode-biased amps that I'm intimate with...
 
1/31/2001 6:38 PM
John Kelley Brown

I wish I could define it. To those like me who are not into that kind of tone, adjectives like stale, lifeless, mushy, frumpy, compressed, describe it best, for those that like cathode bias, you will hear adjectives like, sweet, warm, swirl, brown, tweed, intimate, to discribe the exact same tone. It's all subjective really and I think it depends on how you approach playing guitar. Brian May is one of my favorite guitar players, he gets a great tone from a Vox AC30, (though the has to use a treble booster, otherwise he says, "it gets a bit wooly"), but after owning 2 AC30s I realized, hey, this is just not my sound, I can get a great tone from a plexi Marshall or a Fender Super Reverb, but I really struggle with a AC30, Tweed Fender Deluxe or Matchless. I see a pattern there. Some feel that cathode bias is more "touch sensitive". I find this is not true, I demand touch sensitivity out of any amp I play and I get it in spades with a well designed fixed bias amp, but without the mush. I like the notes to "jump" and I've never had that happen for me when I'm playing through an amp with cathode bias. I cant get notes to "jump" with cathode bias where with fixed bias I get plenty of "jump". So use the switch, get back to this thread with some adjectives for what you experience, I'm always interested in how other player percieve tone.  
 
 
 
Kelley
 
1/31/2001 9:26 AM
Carl Z

Mark;  
 
 
 
Cathode biasing really isn't significantly cheaper than fixed bias. At most you'll save maybe two or three bucks. As for hassles, fixed bias is actually a much smaller hassle than cathode bias. With properly selected components and a pot or two you can set the amps bias in less than 10 minutes! With cathode bias you need to do a certain amount of cut-and-try to get the resistor value right. Then every tube change you run the risk of getting a pair of tubes that takes a lot more bias voltage and you're right back to square one. I tend to think that fixed bias came into common usage due to the flexability, speed and accuracy of setting up the amp on a production line basis once they got consistent performance from off the shelf rectifier diodes.  
 
 
 
As for the tone I tend to agree with the other poster here in that it has a somewhat looser feel...all other things being equal. Tone and feel wise, it seems that more people actually prefer fixed bias. Since you're building it yourself, try them both and go with what you prefer.  
 
 
 
Carl Z
 
1/31/2001 5:32 PM
Mark Lavelle

Two or three bucks doesn't sound right - you've got to rig a B- supply in addition to adding trim pots...  
 
 
 
Anyway, I'm going to take a serious look at going for both.  
 
 
 
Thanks.
 
1/31/2001 6:07 PM
Rebel420

Cathode Bias:  
 
1 - 5-10 watt wirewound resistor - $0.50- $1.00  
 
1 -- bypass cap --$0.25- $1.00  
 
so a total of $2max/ $0.75 min.  
 
 
 
Fixed Bias  
 
1 diode $0.05  
 
3 1/2-1 watt resistors $0.05-$0.60  
 
1 trimpot $0.50-$1.00  
 
2 filter caps $0.50-$2.00  
 
so a maximum total of what, $3.65, or $1.10min? a difference of $1.65 if using premium components, or $0.35 if using typical components? Realistically,  
 
Even going from cheap CB compared to premium FB components, you are looking at about $3 difference.  
 
When you are talking about production scale, then things are much closer....  
 
 
 
(prices pulled out of Mouser elec.)
 

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