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Post subject: Cathode Bias
Posted: Thu Sep 17, 2015 4:59 pm
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I wanted to get a general understanding of cathode bias and how it affects an amp. I'm not planning on building an amp myself, so I don't need to get too technical.
The reason I ask, is a knowledgeable person made the statement that one of the best amps he'd ever played through had cathode bias EL34's. It made me wonder how cathode bias would affect the sound of an amp.
If I understand it right, cathode bias means you can change tubes and not have to adjust the bias, the amp does it itself. If this is correct, it seems like a nice feature, but would it change the character sound of an amp over one that had manual bias? Or was the cathode bias part just thrown in and not meant to say that that was why it sounded so good?
Inquiring minds want to know. Thanks.

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Post subject: Re: Cathode Bias
Posted: Thu Sep 17, 2015 6:50 pm
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Cathode bias is a simpler way of setting the bias, than fixed bias. Usually sets the amp into Class "A" mode. Can be used in either single-ended or push-pull output stages. A cathode-tied resistor on the output stage tube versus the fixed bias --- with its separate bias power stage circuit.

You lose a set amount of power through the cathode-tied resistor. So, the total audio power from a push-pull, cathode biased amp is less than a similar fixed bias amp. Furthermore, the fixed bias Fender amp is set in a Class AB1 mode. Which tends to have greater output power than a similar output stage set with a cathode-bias.

Cathode bias amps have a range in which they can work. So, you can place similar power range of tubes into the amp and the cathode bias setup can accommodate the output range. Eg: A KT-77 or 6CA7 output tube will work fine in a EL34 amp. Without needing to change the cathode-tied resistor(s). A brand new EL34 can be fitted into an amp that has 10 years old EL34 in it, without worrying about bias.

The tone differences tend to be more of a Class and specific tube issue. Class "A" setups sound different, prolly mostly due to the over 70% "on" of the tube, at idle. Whereas, the Fender Class AB1 setup usually has the tubes idling 60-65%. I have not heard a cathode-bias amp set in Class AB1 or Class B.


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Post subject: Re: Cathode Bias
Posted: Thu Sep 17, 2015 7:17 pm
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I see, so the amp doesn't adjust the bias itself, you just have a range of tubes you can work with, without having to rebias. Thanks. The amp being discussed was only 20 watts, so power wasn't a selling point.

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Post subject: Re: Cathode Bias
Posted: Thu Sep 17, 2015 7:54 pm
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Cathode bias amps also compress the signal near clipping.
As the voltage rises across the cathode resistor, it changes the voltage relationship to the grid.
This changes the linearity of gain as it approaches clipping and causes a compressive effect. Nice for tone...


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Post subject: Re: Cathode Bias
Posted: Fri Sep 18, 2015 6:48 am
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Good info, thanks. Looks like I have to add to my "objects of lust" list, one expensive amp.

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Post subject: Re: Cathode Bias
Posted: Fri Sep 18, 2015 7:07 am
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One more question. My Egnator Renegade has two 6L6's and two EL34's and a blend knob. I don't think it's cathode bias or class A. Does it have two separate power circuits?

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Post subject: Re: Cathode Bias
Posted: Fri Sep 18, 2015 7:33 am
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I believe one power supply feeding an output that has two sets of push-pull EL34 and 6L6GC tubes. Not sure if the output stage is cathode biased or fixed bias. Does the amp have arrangement to adjust the bias?


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Post subject: Re: Cathode Bias
Posted: Fri Sep 18, 2015 8:05 am
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More than likely, it does have two bias circuits.
I worked on a stereo Carvin amp with 6L6 in one channel, EL34 in the other channel, and a single bias supply.
Neither side would bias properly.


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Post subject: Re: Cathode Bias
Posted: Fri Sep 18, 2015 8:11 am
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Yes, it has a spot on the rear panel to adjust bias without opening the amp. It's not stereo output.

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Post subject: Re: Cathode Bias
Posted: Fri Sep 18, 2015 1:57 pm
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I just checked my Egnator Renegade. There are two bias adjustment points, one for the EL34's, one for the 6L6's. But, it does not have stereo output. I'm going to check the manual to see if it gives any indication on how it is set up.

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Post subject: Re: Cathode Bias
Posted: Fri Sep 18, 2015 2:49 pm
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According to the manual, I can use: 6L6, 5881, EL34, 6CA7, E34L, 6550, 6V6 (JJ only), KT66, or KT77 tubes, as long as they are installed in pairs. Bias needs to be adjusted for any change, so a fixed bias amp.

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Post subject: Re: Cathode Bias
Posted: Fri Sep 18, 2015 2:54 pm
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Drew365 wrote:
According to the manual, I can use: 6L6, 5881, EL34, 6CA7, E34L, 6550, 6V6 (JJ only), KT66, or KT77 tubes, as long as they are installed in pairs. Bias needs to be adjusted for any change, so a fixed bias amp.

Easily adjustable, however. No soldering on resistors for you.


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Post subject: Re: Cathode Bias
Posted: Fri Sep 18, 2015 4:04 pm
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TimsAudio wrote:
Cathode bias amps also compress the signal near clipping.
As the voltage rises across the cathode resistor, it changes the voltage relationship to the grid.
This changes the linearity of gain as it approaches clipping and causes a compressive effect. Nice for tone...


That's the charm of early Vox and newer Matchless amps. Their model lines had cathode-biased, Class "A" topology.

I've owned a Matchless Hurricane '15 since the 1990s. Very good amp. Consistent tone and reliable.


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