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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
bdgregory
1,710 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.
tommyboy60 OP
3 posts
 

Yes, I mean 6L6 cathode bypass cap and from what I read going from 50uf to 100uf will have very little change in response of the 6L6 but going lower than 50uf will.  Someone actually pointed me to a bypass circuit calculator and it figures it all for you.  This calculator does not have the 6L6 as choice (mainly pre-amps like 12AX7) but going from 50uf to 100uf does not change the gain and very little effect on the response.  You go under 50uf and you start to lose the low frequency response and lowers your gain.    At this point I think I am leaning toward the 10/10/10/20 and 80/40/30 can caps.  People tell me and I am in agreement that the individual caps are superior to the cans but call me anal/thorough, I would like to keep the look without the extra clutter.   Now my dilemma is replacing the current .1 Stromberg paper caps on the primary side and secondary filament side with safety caps.  Been reading about the X1/X2 and Y1/Y2 safety caps and this is my new dilemma.

tommyboy60 OP
3 posts
 

Well, I completed the restore and it looks and sounds great. All of the coupling caps I replaced were really leaking and way out of spec, one was leaking 2amps at 400v and not going down, most increased current leakage during testing which should not be the case.  A couple of the bypass caps would not even register as caps with my Sencore and my cheap tester.  I also replaced a few resistors that were out of spec.   I did use cans, sorry, I like to keep original as I can and they fit really nice and tested excellent.   I mostly used orange drops for everything else.   Yes, I even took out the time to unsolder pins to add the new components, I was going to do the old wrap around but I kinda knew I wouldn't before I started.

I am not changing the nonpolar plug and I am currently waiting on the safety caps.  If the .1 line caps that are installed in there now are anything like the other caps I have replaced, then they are coming out either way. 

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tablejockey
3,861 posts
 
Cool looking amp.

Are those chicken head knobs bakelite?
Big fan of the 6l6G sound. To my ears a great compromise of the El34 mid and KT88 bass.
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