Recapping Fender Bassman 10 for a friend

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Recapping Fender Bassman 10 for a friend

Postby bleeder_boy on Mon Sep 05, 2016 2:26 pm

Good morning - New to the forum and guitar amps generally. But I've played around a bit on tube amps for audio purposes. A buddy plays gigs around town and has a fender bassman 100 which he says he's stopped using. According to him, it burns through tubes (expensive ones, too) at an alarming rate. He asked me to take a look and after wrestling the thing out of its cabinet, I said okay.

My initial plan was simply to replace the filter caps, clean the connections and do a basic spot check. However, in doing said spot check, I noticed two resistors looked to have carried a bit too much current.

From reading the bands, these resistors appear to be 410 ohms. The schematic that I found online indicates they should be 470 ohm 1 watt. My sense is that I should replace them pronto. But has anyone ever seen this sort of thing before with the fender bassman amp and have any idea what could cause it? Assuming these are 1 watt resistors (I don't know) I didn't think a lessened resistance could do that to a resistor, only that it would affect the bias voltage on the amp.

Any thoughts y'all can provide would be greatly appreciated and thanks in advance!


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Re: Recapping Fender Bassman 10 for a friend

Postby Dean Jr. on Sat Sep 10, 2016 5:24 pm

Those are 470ohm screen grid resistors...by the schematic and by the bands on them. Have you measured those?
They are 2 watt resistors, iirc. I use 5 watt sandcast resistors. You can do some searching and reading as to the function of these resistors...the valve wizard is a good place.
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Re: Recapping Fender Bassman 10 for a friend

Postby Unit_1 on Sat Sep 10, 2016 7:51 pm

+1 what he said
yellow = 4
purple = 7
brown = *10

so 470ohm, and FRIED!

Don't use carbon comp, they die as they age, eventually. I use carbon film for almost everything... 2W would be fine, but of course 5w couldn't hurt...
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Re: Recapping Fender Bassman 10 for a friend

Postby bleeder_boy on Sun Sep 11, 2016 11:39 pm

Purple?!?

Okay, that makes sense. There are four of these in the circuit. The two pictured in the earlier post are identical to the other two except they are larger in diameter and fried. The two fat fried ones read 520 and 554 ohms. The skinny non fried ones read 765 and 620. I'm liking the idea of swapping all out for something in the 5w family. Does brand matter for the grid voltage?

Thanks guys. Any other advice for tuning up this old amp would be grately appreciated.
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Re: Recapping Fender Bassman 10 for a friend

Postby Unit_1 on Mon Sep 12, 2016 4:02 am

These would do nicely, although I like the ones in blue better :mrgreen:

5w 470ohm 700v metal oxide film -- 50 for only 10.90 free shipping. Such a deal!

https://www.amazon.com/uxcell%C2%AE-Wat ... B0087YKTKM

Or 10 for $2.24 free shipping
https://www.amazon.com/Metal-Film-Resis ... B01FSDM7ZO
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The ability to play/make music is a gift that not everyone gets. Those of us who have it should use it.

Asimov:Individual science fiction stories may seem as trivial as ever to the blinder critics and philosophers of today — but the core of science fiction, its essence, the concept around which it revolves, has become crucial to our salvation if we are to be saved at all.
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