CATHODE RESISTOR

[susomora]

HI GUYS,
JUST IN CASE I WOULD LIKE TO CHANGE THE BIAS RESISTOR CIRCUIT FOR A FIXED RESISTOR VALUE , HOW MANY OHMS SHOULD I GO FOR ? .I'M GOING FOR JJ EL84'S.
THANKS IN ADVANCE .
SUSO

[Shannon Parks]

Try 390 or 430 ohms, at least 2W rating.

[paart]

IT DEPENDS UPON HOW MANY TUBES YOU WILL BE RUNNING OFF THE RESISTOR. Oops! darn! I had the Caps lock on! If you are going to be using 1 resistor per output tube, then follow the values suggested above. Should you plan to pair the tubes off one resistor, then you should use half that value--about 200 ohms or slightly more. If you plan to run all four outputs off one cathode resistor, ala Dynaco, then 100 ohms or so should do it. Dynaco ST35 used a 95 ohms resistor, which is just about impossible to find. Remember to double the wattage rating each time you halve the value.

The adjustable setup in the DIY35 is still the best way to go. I can't think of any reasons not to use it. There are some weak arguments for reduced distortion with one common resistor, but one failed output tube with this design can wreak havoc on the remaining tubes!

[erichayes]

During preliminary R&D on the 1773, before I was convinced fixed bias was the way to go, I tried individual, paired and common cathode resistor modes.

Believe me, the argument for a common resistor offering lower distortion is an urban myth, unless you enjoy listening to sine waves of the same frequency out of both channels. As soon as you start throwing different frequencies at the two channels, the distortion with the common resistor arrangement is measurably inferior to that of individual or paired resistors.

[sorenj07]

what about a Blumlein-style garter bias setup?

[susomora]

I just wanted to know how to proceed just in case I wanted to simplify the lay out. Of course I would go for a resistor per cathode since i want to have a bypass cap per tube and of course leave each chanel as it is .
I have to admit that another reason to ask this was to know how to avoid messing inside the amp guts when it's turned on ...

[paart]

The little bit of "messing around" that you have to do after startup is well worth it. If the bias is too high, then the life of the tubes will be shortened. If the current through the tubes is either too low or unequal, higher distortion will result. After setting the bias initially, reset it after a few hours of operation, double check it again in a week or so, and you should be good for a long time.

[EWBrown]

390 to 430 Ohms, 2 Watts, ohms per tube, bypass each with a 100-220 uF 35V cap.

Sometimes J/Js run a tad "hotter" than vintage 6BQ5s and Russian made 6P14Ps, use a higher rK value for lower cathode current, and la ower rK for higher current.

To echo earlier statements, the four-cathodes-into-one-resistor approach was done simply for economic (cost cutting) reasons. It will work with closely-matched tubes. If any of the tubes gets too greedy or anorexic, then the rest of them pick up the slack, and things can quickly get unbalanced. There are no sonic advantages, and the slight cost cutting isn't worth the potential for problems.


In a small SEP UL amp, I've been pushing a pair of 6P14P-EVs hard at 45 mA and a B+ of 365VDC and 300 ohm rK). the heavy-duty 6P14P-EV's or (used but still good) 7189s can take it, but the J/Js would retire to an early grave with that kind of "punishment".

I wouldn't do this to nice vintage NOS tubes, but it's legal, and even moral, to do this with $2 to $3 tubes Yellow_Light_Colorz_PDT_08

/ed B in NH

[erichayes]

I've never experimented with the Garter, having become perfectly content with using fixed bias. It might be interesting to see what it would do in a post-PI driver circuit, though.

That's your next project, Tom: pair of 3-500Za running AB2, driven by a couple of 6550s off a LTPI or balanced differential.

http://www.mykit.com/kor/ele/circuit/3- ... xample.htm

Go down about 2/3 for the operating parameters. With an air core OPT, you'd probably have a really sweet sounding 100 watt amp.

[TomMcNally]

I've got LOTS of 4-400's ... they would be fun to mess with.



Maybe a big subwoofer amp, down in the basement
so the fans wouldn't be too noisy.

[CpuZapper]

Hello all, I wanted to start off with fixed bias first, 400 ohm for each tube but use a bypass cap of 470uf 35v. Is this to large of a bypass cap?

[Shannon Parks]

Hello all, I wanted to start off with fixed bias first, 400 ohm for each tube but use a bypass cap of 470uf 35v. Is this to large of a bypass cap?

Did you mean autobias or cathode bias? You can go as big as you want.

[CpuZapper]

Hello all, I wanted to start off with fixed bias first, 400 ohm for each tube but use a bypass cap of 470uf 35v. Is this to large of a bypass cap?

Did you mean autobias or cathode bias? You can go as big as you want.

I'm a bit of a newbie. I originally was going to setup a adjustable cathode bias like the DIY35 but thought I would use 400 Ohm resistors and a bypass cap 470uf 35v to start. I guess I should call this a fixed cathode bias, is this a much better description or do I still have the terminology wrong?

I think if autobias is a good option I would look at that later after I get this project up and running.