Cathode Biased Power Tube Cap Value?

[John_P_WI]

Tom,

Let's use ohms law:

V=I x R

When you separated the 6v6's with individual cathode R's, you halved the current through the resistor. Right?

So to maintain the same "V", we need to double the R to make up for half of the I as seen in the individual R's.

John

[Structo]

Tom,

Let's use ohms law:

V=I x R

When you separated the 6v6's with individual cathode R's, you halved the current through the resistor. Right?

So to maintain the same "V", we need to double the R to make up for half of the I as seen in the individual R's.

John

Thanks John, that was kind of the way I was looking at it too, but then I saw other schematics that head either two or four power tubes and some of them had as low as 50R for the cathode resistor so it didn't make much sense.
Of course it depends on the plate voltage and what kind of voltage there is at the cathode.
Thanks for verifying it.

[iknowjohnny]

You might want to try this while your at it. I did this the other day and it really improved my cthode biased EL34 amp quite a bit in many ways. Use different bypass cap values on each tube. If you have a shared cathode resistor of course you'll have to change over to seperate R/C for each tube. But when you do that and use different value caps on each side, the imbalance really sweetens things up. at least on my amp it did. One side has a 22uf and the other 33uf. i tried higher values on the 33uf side but it seems as tho it only takes a small imbalance to create the goodness. I then also changed my PI plate resistors to throw the balance off there too and that was even better. Give it a shot, you may be pleasantly surprised.

[Structo]

You might want to try this while your at it. I did this the other day and it really improved my cathode biased EL34 amp quite a bit in many ways. Use different bypass cap values on each tube. If you have a shared cathode resistor of course you'll have to change over to separate R/C for each tube. But when you do that and use different value caps on each side, the imbalance really sweetens things up. at least on my amp it did. One side has a 22uf and the other 33uf. i tried higher values on the 33uf side but it seems as tho it only takes a small imbalance to create the goodness. I then also changed my PI plate resistors to throw the balance off there too and that was even better. Give it a shot, you may be pleasantly surprised.

Do you happen to know how well balanced your EL34's are?

I installed that full/half/1/4 power switch the other day, so I did install separate cathode resistors and caps.

After doing that I noticed I have a mismatch of about 4ma between the 6V6's.

Right now I have 50uf on each cathode and was going to try 500uf just to see what it does.

Interesting enough, I am working on a friends 18 watt Marshall amp and when I got it I found a 1000uf cathode bias cap. I was a bit surprised and asked him about that since I had never seen one that high.

He directed me to the 18 watt forum and I found out that the guys over there like it in the 18.
So I left that in there.
I did install individual 1K screen resistors though, he had the stock 100R that affords little protection and doesn't lower the screens enough anyway.

On your PI, did you have the standard 82K/100K?

I have seen mention of guys using 100K for both plates saying it adds some character.

[tubeswell]

So to maintain the same "V", we need to double the R to make up for half of the I as seen in the individual R's.

Hi John

Its actually the other way around

When you use 2 x tubes with the same shared cathode resistor, you use an rk value that is 1/2 of what it would be, if you had a separate resistor for each cathode (in order to get the same cathode voltage that an individual tube would get with a separate resistor) - because the shared resistor is seeing 2 x the current that a separate resistor would see on each cathode.

i.e. if I x R = V, then 2(I) x 0.5(R) = V

So for example, where you would use a 500R cathode resistor for one 6V6 drawing 36mA of tube current to get 18V the cathode, you would use 250R for a shared resistor for 2 x 6V6 drawing 72mA tube current through that resistor to get each 6V6 cathode at 18V.

[iknowjohnny]

Do you happen to know how well balanced your EL34's are?

No, i never checked each seperately. But then you're talking bias current while i'm talking about mismatching the AC signal thru the caps. i did however wonder about whether i might get different/better/worse tonal results by mismatching the bias. in fact i was gonna try that today but didn't have a resistor close enough.

On your PI, did you have the standard 82K/100K?

yeah. And i changed the 100k to 120k. each mismatch helped to soften the tone, or what most people consider removing harshness. At this point my tone is ridiculously spongy and springy. I love it tho. But i want to try different scenarios....maybe even higher on the 120k...say 180k. crazy, but what the heck. I find following rules isn't the best way to find the magic bullet. Or maybe messing with balance of the PI grid leaks or the el34 grid stoppers. At this point i've found so many things that you don't see anyone doing that have done good things to the tone that sans anything that would compromise the amp or my safety, i say go for it ! In fact, i've had much more success with these types of things than trying all the common things that everyone talks about doing. I'm no tech, but i'm finding sometimes ignorance can be a good thing !