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I am restoring a Stromberg AU-42 - 6L6G question

I am restoring a Stromberg AU-42 and I want to keep the original can cap look. I cannot find a 50/40/30 in order to keep the 50uf cap at the cathode side of the 6L6 outputs. The circuit is push/pull (2 6L6’s) and the cathodes are tied together to 50uf/50v parallel to 200 ohm resistor to ground. I believe this is the current source for the 6L6’s. I can find a 70/40/40 and a 80/40/30 can cap. I am wondering if I can increase this 50uf on the 6L6 cathode to 80uf or 70uf. My gut says this will be okay although in the back of my head, more capacitance means more current on demand which my gut says should still be okay. I don’t want to heat up the 6L6’s. Any advice would be helpful. I really do not want to use individual caps but if I have to I will.

Thank, Tom
Epsom Repair Depot

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tommyboy60
tommyboy60 OP
3 posts
 

I have looked at some of my design books with notes. I believe that going higher with the bypass cap from 50uf to even 100uf will not make much of a difference in the way the tube will function. If I go lower than 50uf, from what I can tell, this will start to effect the functioning of the 6L6 responses. I am still doing a little research on this before I actually purchase the parts but I agree that the 70uf would be okay but then again the 80uf would not make much of a difference. Going with the 80/40/30 will give me the exact capacitors I need for the 5Y3 power stage. Your thoughts. My guts was always that it would be okay but it has been a while since my courses in tube theory, like 1982.

Gut the current cans will not work, I tried the first one because I wanted to see if the gutting would be clean but it was not so if I went that way I would use some cans that I already have gutted.

I realize I can keep the caps for original look and install individual caps but I do not like the extra clutter. I am doing a total recap, using orange drops for most of the caps when I can and sliver mica for pf caps. From my vendor it will cost me $75 for the can caps I need and $42 for individual caps or a savings of $33. I know I could pocket the extra $33 since customer has already paid me for the restore but, when possible, I like to keep the original look with less clutter. My wife says I am thorough which is really being anal but we call it thorough being a better term for me. I really like your comments, you can never stop learning.

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oldhvymec
5,101 posts
 
You mean a by pass on a filter cap? They are usually pretty small right?
like .47 .22, you know .XXX  not 50 or are you talking filter caps? I agree you could go up a bit.  I'm no designer, lol, but I'm one tinkering fool sometimes.. It's the nosey mechanic in me... Can't help but think about it...  Through the years I've had to do a lot of repair work on heavy equipment, a tube amp it pretty simple in comparison. Getting it to sound the way YOU want is the real trick... You sound picky.. I'm very much so.
Never walked away without a fix, never gave up...I'm one of those for types for sure..

Orange drops, man those thing last 100 years..Silver micas, what are you going with, the same, orange drops and silver micas? MC225 use orange metalized drops, very special sound...I hate messing with them, I just usually test, them. I've found a couple in 35 years... I hate to lose that sound..

Let's see if rodman99999 will chime in..  Do you have a schemo?
elevick
2,636 posts
 
Can't you leave the caps but have all of the single once mounted underneath the chassis?  It can look neat and professional if someone ever opens it up and from the top it will all look mint.
roberjerman
4,319 posts
 
Go for the 80/40/30 cans!
bdgregory
1,708 posts
 
You should post this on The audioasylum tube diy forum. You’ll get multiple authoritative responses quickly there.

that said, if this is the cathode bias bypass cap, different values will materially affect tone. If it were me, I would not worry about using the multi section cap and mount a single cap across the bias resistor. I would buy the 70/40/40, test the 70, and compare it to a single 50uf cap (maybe other values too) mounted across the resistor.  I suspect you’ll lose bass with the higher values.
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