Keep frying this resistor in the reverb circuit

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mlannoo
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Keep frying this resistor in the reverb circuit

Post by mlannoo »

Reverb quit working one day in my '78 Fender Pro Reverb (the nasty 70w ultra linear model). first i checked all connections. then tried a couple other 12AT7 tubes. then opened it up and saw a fried 680ohm resistor in that part of the circuit. so i replaced it and fired it up ... and it fried again.

any thoughts on what to do next? i'm new to diagnosing problems like this. i've attached a couple images ... one is a layout showing the bad resistor in the reverb circuit on the board and the other is the schematic for this model.

obviously something else is making that resistor fry out ...

thanks.
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morcey2
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Post by morcey2 »

Have you tried other tubes after you replaced the resistor? Was the same tube in for both barbeques? Check the 560pf capacitor that goes between the plate and cathode and make sure it hasn't gone bad. That would put 390V on that resistor, which I'm pretty sure it won't handle. Not unless you have a 500 watt power resistor in there. 8O

I'm pretty much shooting in the dark, but hope this helps.

Matt.
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mlannoo
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Post by mlannoo »

morcey2 wrote:Have you tried other tubes after you replaced the resistor? Was the same tube in for both barbeques? Check the 560pf capacitor that goes between the plate and cathode and make sure it hasn't gone bad. That would put 390V on that resistor, which I'm pretty sure it won't handle. Not unless you have a 500 watt power resistor in there. 8O

I'm pretty much shooting in the dark, but hope this helps.

Matt.
i'll give it a try. actually i think i had the original 12AT7 back in there when i replaced the fried resistor ... dope!
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Jef
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Post by Jef »

In my 70 watt Pro reverb it wasn't the cathode resistor but the 12AT7 tube that fried. The tube is biased way too hot.
I replaced the resistor with a 2k2, bypassed it with a 25/25 cap (which are the BF values) and clipped out the 560pF cap.
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Post by Geek »

Hi,
Jef wrote:The tube is biased way too hot.
Indeed! 6V bias with 400V on the anode is freakin' insane for that little guy. Especially with today's "hot rod" versions.

I agree 2K2 will cool it down. Heck, even a 1K5 would add years onto it.

Cheers!
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Gregg

mlannoo
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Post by mlannoo »

Jef wrote:In my 70 watt Pro reverb it wasn't the cathode resistor but the 12AT7 tube that fried. The tube is biased way too hot.
I replaced the resistor with a 2k2, bypassed it with a 25/25 cap (which are the BF values) and clipped out the 560pF cap.
thanks for the replies ...
i understand the concept you're suggesting ... the cathode resistor isn't large enough - so the tube is biased too hot. sounds like that may have been a design flaw?

But I do have to say i'm a little embarrased to admit i don't fully understand the solution you've offered. is a "2k2" resistor a 2Kohm (2watt) i'm assuming? and i'm not sure about the next step: "bypass it with a 25/25 cap" .... is that 25uf/25volt? bypass what?

Here is my crude attempt to visualize what you're suggesting. I could be waaay off here!

thank you
matt
Last edited by mlannoo on Tue 05/15/07 8:44 am, edited 2 times in total.
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Alexo
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Post by Alexo »

is a "2k2" resistor a 2Kohm (2watt) i'm assuming? and i'm not sure about the next step: "bypass it with a 25/25 cap" .... is that 25uf/25watt? bypass what?
2K2 means 2.2K or 2,200 ohms, I would go with a 1 watt resistor but a half watt would be fine, I bet.

25/25 means a 25uf, 25 volt electrolytic cap, the same type you probably have on your preamp cathodes and such.

By "bypass," they mean to run the cap in parallel with the resistor, like in this picture...

HTH
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Jef
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Post by Jef »

Yep, that's it!

Jef
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mlannoo
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Post by mlannoo »

excellent! thanks guys. i will try that tonight.

what is the reason you decided a 2.2k resistor and 25uf/25v bypass cap would fix the problem (other than that being Blackface values)? I'm trying to understand exactly what that does. the 2.2k resistor is changing the bias of the tube right? Does it cut down the voltage on the plate?

matt
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