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Thread: out of spec cathode resistor

  1. #1
    Supporting Member Randall's Avatar
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    out of spec cathode resistor

    I have an all original 66 Vibrochamp in for a re-cap. I notice the 470R 1 watt cathode resistor measures 543R, which is almost 16% off. Given that the schematic calls for 10% tolerance, is this off enough that it matters, or would you keep it in and keep it as stock as possible?

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    It's weird, because it WAS working fine.....

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    Supporting Member mozz's Avatar
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    What is the output tube currently dissipating?

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    Lifetime Member Enzo's Avatar
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    Yeah, don't ask us, ask the circuit. How is the circuit/tube performing at this resistance.

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    Education is what you're left with after you have forgotten what you have learned.

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    Old Timer Leo_Gnardo's Avatar
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    Judgment call. Some folks install a larger value cathode resistor on purpose, to reduce bias current, and we assume wear & tear on the output tube.

    Also, if the value of this venerable carbon comp has already drifted up, might it not continue? There's an uncomfortable thought.

    Last week I had a VChamp in, same drifted resistor, plus a shorted cap in parallel, yet another shorted cap across a preamp cathode R, and if that wasn't enough, the 1K dropping resistor in the power supply roasted solid black from overheating.

    I replaced the shorted caps, replaced the resistors with 3W wire wound, replaced the output tube with JJ, 50 mA bias current, 20W calculated plate power whether idle or running at clip. Made an astonishing 7 watts (!) at 500 Hz to 4 ohms. No red plating visible. Customer's happy. It will run as long as it does - who knows where the clock will stop again. I've decided there are bigger things to worry about. You may do the same, or something else. The results as always, unpredictable. FWIW many times I find old Champs roasting their output tube just like this, and they survive just fine.

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    Enjoy. Every. Sandwich.

  5. #5
    Supporting Member Randall's Avatar
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    This one is measuring 15 watts as it sits with a Sylvania branded grey glass 6V6GT.

    Or 18 watts with a RCA 6V6GTA.

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    It's weird, because it WAS working fine.....

  6. #6
    Old Timer Leo_Gnardo's Avatar
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    And - everything's working fine? Might want to lock in the current operating condition, with the Sylvania 6V6, with a 560 ohm resistor - closest EIA value to your drifted one - and I'd replace its bypass cap too. 25 uF, 50V. I'm sure you'll bend the leads a bit, to keep the cap from being jam up to the roasty hot resistor. Another Champ for the ages!

    If you have those white Mallory bypass caps in there, swap 'em out. They were dodgy enough last 50 years. Don't leave behind any little time bombs. Cheap insurance.

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    Enjoy. Every. Sandwich.

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    I'd definitely up the wattage, too - 5W might be overkill, but better safe...

    Justin

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    "... If an older Boogie and classic Marshall had a (clearly illegitimate) child and you baked it in an oven set to clown shit crazy." - Chuck H. -
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    Supporting Member Randall's Avatar
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    I had already replaced the board lytics, always do when doing the filters. I replaced the drifted cathode resistor with a 560R as suggested, I had a big vintage one on hand, it's more than 1 watt like original, but I don't know how much. And I added a 120R resistor between the unused 20/20/20/20 cap lug to another, and fed supply to the former, and took B+ from the latter. This dropped it by 10V, since it was a full 20% high. This put it at a little over 14 watts with the Sylvania. The RCA runs a lot hotter at just under 17watts, but it sounds better, so I'm going to leave it.

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    It's weird, because it WAS working fine.....

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Randall View Post
    And I added a 120R resistor between the unused 20/20/20/20 cap lug to another, and fed supply to the former, and took B+ from the latter.
    I popped a vein in my head trying to figure out what this means.

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Chuck H View Post
    I popped a vein in my head trying to figure out what this means.
    Looks like Randall arranged a "pre filter" to knock down the hum inherent to Champs and most other SE amps.

    I hope you'll be all right now Chuck. After all tomorrow's Samhain, oops I mean Halloween, and you don't wanna miss that. Witches may be flying, including right here on MEF!

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    Enjoy. Every. Sandwich.

  11. #11
    Bent Member Chuck H's Avatar
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    We'll see about the witches. If there's any trouble I have a broomstick for her.

    Samhain? Maybe in times when Monsanto didn't manufacture produce with alien technology. We don't need to concern ourselves with solstices (or the exact spot in between) anymore. All out food comes from a store. Hermetically sealed and government approved

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  12. #12
    Supporting Member Randall's Avatar
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    "I popped a vein in my head trying to figure out what this means."

    The 20/20/20/20@450 can has one 20 section unused. I saw a thread elsewhere on this where the feed wire from the rectifier is moved to the unused tab. Then a 100-250R resistor jumps to the tab where that feed wire used to be. B+ supply stays on the tab where the rectifier wire used to be. This adds another level of filtering, while leaving the 5Y3 rectifier still seeing a 20uF cap, instead of paralleling the extra unused 20uF section with the first section to make it 40uF. There seems to be debate as to whether the 5Y3 is up to that or not.

    I got the benefit of dropping the B+ 10v, and the amp is quite hum-free for a Champ.

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    It's weird, because it WAS working fine.....

  13. #13
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    I know a sample size of one doesn't make a statistic, but my 79 Champ (now Vibro-Blaster) had the original 40/20/20/20@450 Mallory in it along with the same Sylvabia 5Y3GTA for at least 10 years, and I was hard-pressed to tell if it was on if I wasn't playing it. I never changed the heaters from stock, either. B+ was ~430V.

    I yhink the whole max value filter cap thibgy depends ob the rectumfryer tube used - use a goodun & you'll be okay (within reason).

    Justin

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    "... If an older Boogie and classic Marshall had a (clearly illegitimate) child and you baked it in an oven set to clown shit crazy." - Chuck H. -
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    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  14. #14
    Bent Member Chuck H's Avatar
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    Thanks Mike. I gotcha now. In fact I did the same thing on the last Champ I did, sort of. I actually remote caps to replace the can, but the same idea. I wen't 22uf off the rectifier - 100r - 47uf to the power tube plate.

    Justin, I've used 47uf right in front of the 5y3 on two other Champs and never had a problem. Since those vintage caps were spec's -20/+80 and newer caps are always very close to rating I figured what the hell, 36uf vs. 47uf. As to not rewiring the filaments... I always did as a matter of course but if you are using the stock circuit it really isn't necessary because the preamp cathodes are fully bypassed. If those bypass caps are doing their job there shouldn't be significant filament hum. But if you change things up, like using unbypassed or partial bypass on the cathodes there may be an advantage to wiring up a twisted pair.

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

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