Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: marshallguy on May 24, 2023, 01:49:30 pm
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Hi, I replaced an open B+ 500v/ 47uf cap with a FET brand on my friends hot rod deluxe combo amp. Everything was good, B+ 430vdc and I checked the bias which went from 60 to 70mv after warm up but stabilized. The Power tubes are Sovtek 6L6WXT+. He said the amps about 15 yrs old and the tube over 10 but he hardly ever uses it. I figured any where between 60 to 70 mv is good with a 30w each pair of tubes though Fender wants cold bias at 60mv a pair. I don’t know what the stock tubes were in this amp or their wattage rating.
Anyway, I’m playing the damn thing for about 10 mins on low volume and explaining the the 2 channels adjustments and such, switching back and forth and suddenly the power starts to drop out and a hum shows up even after I unplugged my guitar!
After turning it off, I recheck the same filter cap and it’s reading about 44uf. Hmmm, ok, not open but not 50uf. I also checked the other B+ rail caps, all good. I turn it back on, hum is gone and I recheck the bias to show him and the bias goes from 60 to 70 in short orderthen to 75, 76, I turn it off before it keeps going and going up. Not good.
So is it now a bad bias filter cap or one maybe just of the the power tubes that’s the probable cause? Unfortunately one can’t easily determine which tube is drifting up or if it’s both together.
Could a bad power tube take out a B+ rail filter cap?
JC
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Posted in the wrong board. .... moved here
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Try the 6L6 tubes one at a time. Don't play, just watch the meter.
I have seen 6L6 in fixed bias just rise and rise the longer they are powered, the hotter they get.
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Try the 6L6 tubes one at a time. Don't play, just watch the meter.
I have seen 6L6 in fixed bias just rise and rise the longer they are powered, the hotter they get.
Worth a try, I assume you know these aren’t fixed biased?
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Try the 6L6 tubes one at a time. Don't play, just watch the meter.
I have seen 6L6 in fixed bias just rise and rise the longer they are powered, the hotter they get.
Worth a try, I assume you know these aren’t fixed biased?
Best to post a link to a schematic so no one has to assume.
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Try the 6L6 tubes one at a time. Don't play, just watch the meter.
I have seen 6L6 in fixed bias just rise and rise the longer they are powered, the hotter they get.
Worth a try, I assume you know these aren’t fixed biased?
Best to post a link to a schematic so no one has to assume.
Trying to download full schematic with bias power section on my iPad not helping. 😆 Halfwave rectifier with a couple of filter caps, voltage divider resistors and 25k pot.
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I think I know what the probable cause is. I just remembered a friend of mine has a similar Fender using the same 60mV bias point and adjustment and he was complaining about the bias not being stable. I recall now he replaced the bias pot and that fixed it. He found a forum that others had similar problems not sure which though. Anyway he’s traveling out of country so I’ll have to wait until he returns to verify but almost certain that was it.
Thanks for all the replies.
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Try the 6L6 tubes one at a time. Don't play, just watch the meter.
I have seen 6L6 in fixed bias just rise and rise the longer they are powered, the hotter they get.
Worth a try, I assume you know these aren’t fixed biased?
The Hot Rod Deluxe certainly is fixed bias.
BTW, Hoffman has a huge schematic library. You can find the link below this post. The complete HRD schematic is in the Fender section. Sometimes it's just easier to copy/paste the link to the library file rather than post an edited schematic. Either way is commonly used on the forum.