• RE: Stereo 35 R10 resistor question - llwhtt 18:28:47 07/06/13 (0)
  In Reply to: RE: Stereo 35 R10 resistor question posted by jeffczar on July 02, 2013 at 22:53:13

  If you look at the schematic you'll see that R10 is in parallel with R3. Both R3 and R10 are tied to ground, R10 thru the secondary of the OPT. So the reading of approx. 1.3K Ohms is normal.
  
  Craig
  

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• RE: "all bias resistors" - 1973shovel 05:11:09 07/05/13 (0)
  In Reply to: RE: Stereo 35 R10 resistor question posted by jeffczar on July 02, 2013 at 22:53:13

  You don't go into details, but you do say "all bias resistors", so I'm going to assume your ST-35 has been modified to employ individual fixed bias to the output tubes. The stock ST35 uses one resistor, in a cathode bias scheme.
  
  You've rechecked your connections, which is good. Have you cleaned your tube sockets, and checked the contacts for mechanical security? Since it's taking five to ten minutes to red plate, the contacts could be expanding as the amp heats up.
  
  I'd follow Xaudiomanx's advice regarding checking those resistor values. Lift one leg out of the circuit prior to measuring them.
  
  Lastly, if the assumption of your amp having individual fixed bias is correct, strongly consider converting it to Dave Gillespie's EFB circuit. For a few dollars in parts, you will get better measured performance, and subjectively better sound than standard fixed bias, and longer tube life than standard cathode bias.
  
  Good luck with your great amp!

  • Enhanced Fixed Bias   (Open in New Window)

  
  

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• RE: Stereo 35 R10 resistor question - xaudiomanx 04:57:07 07/03/13 (1)
  In Reply to: RE: Stereo 35 R10 resistor question posted by jeffczar on July 02, 2013 at 22:53:13

  Before changing the R10 resistor because it is measuring improperly I would take it out of the circuit on one end and then measure it. Sometimes resistors measure improperly if they are in the circuit totally. Do that before trying to replace something that might not need to be replaced.
  
  Now the red plate issue would be the output tubes are drawing too much current. The current draw is determined by the voltage on the plate and the resistor on the cathode as this is a cathode biased amp.
  
  Are you using individual cathode resistors or combined? I think the combined of the output tube value of the resistor is 390R/5W and the individual is 180R/5w. 10W is better if you can fit them. Use a good quality noninductive type(Mills). Make sure you have a good ground connection to one side of the cathode resistor. If you are using a bypass cap on the resistor make sure they are oriented properly.
  
  I know this may sound stupid but make sure your tubes are wired correctly. It seems suspicious that both channels are doing it.
  

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  • Sorry! I have it reversed - xaudiomanx 14:35:16 07/03/13 (0)
    In Reply to: RE: Stereo 35 R10 resistor question posted by xaudiomanx on July 03, 2013 at 04:57:07

    So sorry.
    
    Two tubes combined use a 180R resistor and individual they us a 390R. Usually when you double the resistor you used half the cap value on that resistor.
    

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