FDP Home Page / FDP Forum / FAQ's

The FDP is made possible by the following companies and individual members like you.
Please use the links below to show them we value their sponsorship.

WD Music

Amplified Parts

Musician's Friend

Apex Tube Matching

Antique Electronics Supply

Amazon

Sweetwater

Guitar Center

MOD KITS DIY

Yellowjackets Tube Converters

Jensen Loudspeakers


* God bless America and our men and women in uniform *

* Illegitimi non carborundum! *

If you benefit and learn from the FDP and enjoy our site, please help support us and become a Contributing Member or make a Donation today! The FDP counts on YOU to help keep the site going with an annual contribution. It's quick and easy with PayPal. Please do it TODAY!

Chris Greene, Host & Founder

LOST YOUR PASSWORD?

......................................................................

   
FDP Jam
Calendar
Find musicians
in your area!
  Search the Forums  

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

roadhog96

USA / Sesame St.

Mar 6th, 2017 01:36 PM   Edit   Profile   Print Topic   Search Topic

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.

amphead4

Cincinnati, USA

Mar 6th, 2017 02:53 PM   Edit   Profile   Print Topic   Search Topic

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.

roadhog96

USA / Sesame St.

Mar 7th, 2017 08:14 AM   Edit   Profile   Print Topic   Search Topic

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?

amphead4

Cincinnati, USA

Mar 7th, 2017 08:47 AM   Edit   Profile   Print Topic   Search Topic

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)

roadhog96

USA / Sesame St.

Mar 8th, 2017 12:16 PM   Edit   Profile   Print Topic   Search Topic

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




Reply to this Topic
Display my email address             Lost your password?
Your Message:
Link Address (URL):
Link Title:




Moderators: Chris Greene  Iron Man  reverendrob  

FDP, LLC Privacy Policy: Your real name, username, and email
are held in confidence and not disclosed to any third parties, sold, or
used for anything other than FDP Forum registration unless you specifically authorize disclosure.

Furtkamp.com 
Internet Application Development

Copyright © 1999-2017 Fender Discussion Page, LLC   All Rights Reserved