Can Cathode Biased ever sound like Fixed?

[MarshallPlexi]

If I run the B+ in the amp at 425VDC with a supply of 450VDC (25V volt drop across the Cathode Resistor) and use a 250uF bypass cap, will this setup result in a tone that approximates fixed bias as far as tone and feel?

Thanks!

[MWJB]

No. I suggest that your voltage drop accross the cathode resistor will be in excess of 25v too, assuming that you are using 6L6/5881/EL34 (suggest 330ohms min)?

Fixed/cathode switch is easy enough to rig up, check out the FAQ at London Power website, there's a rough schem.

[MarshallPlexi]

31.5V actually. 350R at 90mA. The reason I am going with Cathode bias is because I've simply run out of room in this design and would like to keep it real simple as best as I can.

[cbarrow7625]

The closest you will ever get to having a cathode bias sound like fixed bias is if you use a zener in place of the cathode resistor. I've never actually seen this done in a power section. It is realtively common to do it in preamp tubes. You will just have to get a zener that is capable fo handling the current (and the heat).

The zener will have a very low resistance when conducting and, like fixed bias, will keep the cathode voltage relatively stable (far more stable than a resistor). The zener will also negate the need for a cap, therefore no time constant and full reproduction down to the lowest frequencies (again like fixed bias).

I'd love to know if anybody has tried this and how it worked. It's probably not very popular just because making a fixed bias supply is likely going to be far cheaper than finding appropriately rated zeners. But for you it should be perfect!

[MarshallPlexi]

Man that's cool! So I would find a zener whose breakdown voltage is roughly equal to the amount of negative voltage that I would normally see in a fixed bias setup?

[jrfrond]

You need a (physically) large resistor and a capacitor to employ cathode-biasing. You need a single diode, 1/2W resistor, 5W resistor and a cap to steal a bias supply off of the main B+ winding (that is, if you do not have a bias winding). No matter how densely-packed a tube amp design is, you can find room for these two components somewhere, possibly on a terminal strip.

Does cathode-bias sound like fixed? No, but for me, that's a GOOD thing, as I prefer the cathode-bias sound.

[cbarrow7625]

Well, you'd find a zener that drops the correct voltage so that the power tubes work at the "correct" design current and power dissapation.

If you already know the voltage than it should be relatively easy. If you don't try to find the sound you like best with resistors / caps first and then pick a zener that duplicates that voltage drop.

Make sure you visit your friendly electronics supply book first to find out what value zeners are available in the power ratings you need. You can't get a 31.234 volt zener! Just get as close as you can.

[tubeswell]

You still may possibly want a (100nF?) cap in parallel with a zener because of the diode switching noise

[MarshallPlexi]

Man this is cool stuff! If I run the amp at 70% at 450V that will require a 370R Rk with a plate current of 86mA. (0.086*370=31.82) So I need a 33V zener. (Closest value I could find)

Will this one work?
http://mouser.com/Search/ProductDeta...BXdsE6YlQYA%3d

Or does it need to be reversed?

[tubeswell]

Man this is cool stuff! If I run the amp at 70% at 450V that will require a 370R Rk with a plate current of 86mA. (0.086*370=31.82) So I need a 33V zener. (Closest value I could find)

Will this one work?
http://mouser.com/Search/ProductDeta...BXdsE6YlQYA%3d

Or does it need to be reversed?

You run the zener with the anode end pointing towards the cathode because you want the cathode to be at a positive voltage w.r.t. the ground return path. The zener in your link appears to only be good for 75mA.

Another thing you need to be aware of is a zener will drop a constant voltage, whereas the voltage dropped across a cathode resistor will vary depending on the voltage swing at the plate. So a zener biased cathode is not self-adjusting like a resistor biased cathode - so you will have to set up the tubes to idle at 70% if you don't want to harm the tube(s) (And you will need a zener that can handle the likely max current through the tube(s))

[MarshallPlexi]

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

[tubeswell]

Except that you need to take account of my editing in my last post (where I talked about the biasing of the tubes at 70% - will 30V be sufficient to bias the tube so that it still works when when the plate voltage is peaking?)

[MarshallPlexi]

30V will idle the tubes at about 75% max.

[tubeswell]

Oops, yes I see now where you talked about it earlier. Well I reckon 75% should be fine

[MarshallPlexi]

Sweet! I can't wait to try this out. thanks for the help!