Screen Resistors in 5150 III EL34 50W

  • Thread starter Thread starter 7704A
  • Start date Start date
7704A

7704A

Well-known member
Got a friend's amp on the bench, looks like the screen resistors need replacing after a possible power tube failure. Can anyone confirm the value and type of the screen resistors? I intend to call fender/evh support monday morning, but the sooner I can get this wrapped up the better. Assuming no bands have been discolored or burnt off (only three bands?), it looks like the resistors are 900 Ohms, which strikes me as a hair low. Probably flameproof or flame-retardant based on the 6L6 and EL84 5150 schematics I've found so far, probably a few watts in rating. My initial thought is that 1k 5W wirewounds should do the job fine, but my friend would like the stock fender values if possible.

Also, if anyone has a schematic for the 50W EL34 5150 III, it would be very much appreciated. The 6L6 version gets the job done for the most part, but there are still differences (e.g., screen resistors).

Picture is of the one resistor that hasn't gone open-circuit. Measures around 1.2k if I recall, though given the board looks a little toasted around it, I don't wholly trust it and intend to replace it since it's <$1.
1699758955886.png


I found this video that shows the output section board, but the resistor values look pretty wacky... Either 89R or 900M. :confused:


Thank you for any input.
 
Update: Contacted EVH Support. Rep was as helpful as he could be without giving out the schematic, which he is not allowed to do. In the end, he directed me to an authorized service center to make sure he didn't misinform me. The service center was very helpful (GPS Electronics), and told me what the official schematic spec'd for the screen resistors. 1k 1W 5%, no mention of them being flameproof. The amp now has new resistors installed, new tubes, has been biased, and appears to be functioning as intended. We'll put it to the test tomorrow at a gig. Fingers crossed.
 
1 Watt. With my variac set to 120VAC, I measured 506V on the plates, if I recall correctly.
 
You must log in or register to reply here.
Back
Top