Blues Jr help...it's burning up power resistors!

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bluesbear
Hey get your own solder
Posts: 357
(3/31/04 12:00 pm)
  Blues Jr help...it's burning up power resistors!
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 Hi.
I'm trying to figure out what's wrong with my harp player's Blues Jr. The amp fades in and out, then R-32 burns out. It's the first resistor just after the 47mF filter cap which is just past the recifier. I'm thinking that one of the diodes is bad. Does this seem like a possibility to you better informed people? I'm still pretty much going on guesswork here but I don't particularly like guessing with someone elses amp.

members.aol.com/blusbear/...rSchem.jpg

Thanks!
Dave
 
OldHouseScott
Posts: 17
(4/1/04 9:33 am)
  Re: Blues Jr help...it's burning up power resistors!
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 In my mind, it has to be a short at point X. If it were at point Y, the second stage filter resistor would go as it tries to dissipate ~7w (R32 would only be dissipating 1.5w).

My guess would be a bad output tube, shorting out the screen grid to the suppressor grid. All of B+ then appears across R32 (plus the screen grid resistor which is only 100 ohm), so it burns up trying to dissipate 46 watts as heat.


 
bluesbear
Hey get your own solder
Posts: 359
(4/3/04 8:23 am)
  Re: Blues Jr help...it's burning up power resistors!
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 That seems to be it. I changed out the power tubes; I'd only tested them before and they tested good. I only had time to give it about 5 minutes try but it only took a few seconds to start smoking with the old tubes. I'll give a little more of a tryout later but I think that's got it.
Thanks, Scott, I owe you one!
Dave
 
 
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