Can Cathode Biased ever sound like Fixed?

[tubeswell]

Yes - do let us know please.

[MarshallPlexi]

When I get it built I'll wire up a switch to the cathodes that selects between a 350R resistor and the 30V Reversed Zener. That way I can tell how it sounds in each setting and which one sounds better. I want the amp to have the quick attack of the Fixed bias.

Thanks again!

[cbarrow7625]

Cool. Can't wait to hear the report!

[Dave H]

So if I read the spec sheet right, that would require a Reverse Polarity unit since the standard configuration is with the Anode to the Zener case. Cathode to case is listed as reverse polarity on the spec sheet.

This one right?
http://mouser.com/Search/ProductDeta...%252bstVK44%3d

30V / 50W / 420mA

You want the standard one - Anode to case. The cathode of the diode goes to the cathode of the tubes.

Dave H.

[tubeswell]

Hmmm... having though about this a couple of times this morning. I'm not sure about that. A zener diode has a precise, low, reverse-bias breakdown voltage. So would you not put it in the opposite way to a conventional diode? (i.e. with the anode end pointing toward the tube cathode) In the attached Pic (courtesy of the valve wizard) the zeners are used to clamp the screen in this way.

[imaradiostar]

I've read that some people prefer the tone of cathode bias with a zener to clamp the voltage drop as you increase the current- in other words with a zener (or string of them) in parallel with the cathode bias resistor. This way it sags and compresses like a cathode biased amp at the edge of PA distortion but when saturated it tightens up like a fixed bias amp.

Of course the end result will vary depending on the primary filter cap size, output tube, reflected load impedance, plate voltage sagging and so on...

I've never built one this way myself but I have a design coming up that may lend itself to this.

jamie

[tubeswell]

Another e.g. of reverse-biased zener voltage clamp

THis one was on an electronic tutorial page

[Chuck H]

FWIW I am currently using a cathode bias arrangement with a zener ACROSS the resistor.

Using my scope, I determined the where amp starts to clip relative to the voltage rise on the cathode resistor. Then I chose a zener value that would "fix" the bias just above that voltage. The result is that the amp has a softer attack and compression when clean but tightens up when pushed into overdrive. I did this to reduce crossover distortion inherant in cathode biased el84 amps and it worked very well.

I am using a 100uf cathode bypass and I hear no switching hash from the diode. You cant even tell it's there except that the amp is tight and fast when overdriven.

Chuck

[Dave H]

Hmmm... having though about this a couple of times this morning. I'm not sure about that. A zener diode has a precise, low, reverse-bias breakdown voltage. So would you not put it in the opposite way to a conventional diode? (i.e. with the anode end pointing toward the tube cathode) In the attached Pic (courtesy of the valve wizard) the zeners are used to clamp the screen in this way.

We are saying the same thing but I think you have the anode and cathode naming reversed. I have it as in the attached file.

Dave H.