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FDP Forum / Fender Amps: Vintage (before 1985) / Screen Grid Resistors


USA / Sesame St.

Mar 6th, 2017 01:36 PM   Edit   Profile  

Fender used a 470 1W Carbon Composite and I've read quit a few articles were it was suggested to change these to a 470 flame proof metal oxide rated at 2W or 3W. I think the metal oxide could be a good thing.

This topic was brought up on this forum At least a few years ago and I remember someone saying it wasn't a good idea to up the wattage, that you should keep the stock rating of 1W and they preceded to say why but I cannot remember for the life of me what that reasons were.

Does anyone remember or know why it's not a good idea to use 2W or 3W. I can think of a good reason why you should but not why you shouldn't.


Cincinnati, USA

Mar 6th, 2017 02:53 PM   Edit   Profile  

If the screen shorts, you might want the resistor at act as a fuse and fail before the PT runs hot. But it won't be a weak link if you increase its power rating.

Flame proof is good. We were on the third row at a concert once and a Marshall head gave off a huge cloud of smoke. Probably a burning carbon comp screen resistor.


USA / Sesame St.

Mar 7th, 2017 08:14 AM   Edit   Profile  

You know that makes a lot of sense I never thought about that. Agree it would be much cheaper to replace a resistor than a power tranny.

What would you say about raising the rating for the 1K and 4.7K power supply resistors to 2W or 3W, good or bad idea?


Cincinnati, USA

Mar 7th, 2017 08:47 AM   Edit   Profile  

The 1K drops 10 volts, the 4.7K drops 40 volts.

P=V*V/R =10*10/1000 = 0.1 watts

P=V*V/R =40*40/4700 = 0.34 watts

If you were bold enough to touch them in use (DON'T DO IT!), you would find that they don't run very hot in normal operation.

Let's play "beyond worse case" scenario. What if the third cap shorted in a TR (SS rectification) and it had minimal voltage drop across the PT and choke and some fool had jumpered a wire around the fuse.

The 1K resistor would now drop about 450 volts and the power dissipated would be P=450*450/1000 = 202 watts. In this imaginary failure mode, the resistor would be the weak link and you wouldn't select component specs to withstand that type of failure. The conventionally rated resistor is going to overheat and fail so flame-proof would be preferred.

Upgrade to flame-proof metal but I see no technical reason to go to 2W or 3W rated parts. Just my opinion...

(edited because I just can't write clearly in one take)

(This message was last edited by amphead4 at 04:56 PM, Mar 7th, 2017)


USA / Sesame St.

Mar 8th, 2017 12:16 PM   Edit   Profile  

Thank you for that explaination, I definitely will be using flame proof metal oxide and sticking to 1W resistors for these. Leo knew what he was doing why mess things up.

FDP Forum / Fender Amps: Vintage (before 1985) / Screen Grid Resistors

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