Tubes Asylum

RE: I never even thought of the line voltage but that could very well be an issue if it varies.

24.114.44.194

Jonesy

A

In Reply to: RE: I never even thought of the line voltage but that could very well be an issue if it varies. posted by Michael Samra on May 13, 2014 at 17:15:40

New output tubes now. But had same symptom using the old tubes. Just in the past couple of months of course. Otherwise they seemed good.

I'll try putting the old ones back and see what happens.

Many good tips to try.

Have a long weekend coming up if I don't get any chances during the week.

Thanks Mike!

"I know just enough to get into trouble. But not enough to get out of it."

Follow Ups:

Yeah, check the plate voltages when the bias reads differently - FenderLover 23:16:41 05/13/14 (7)
Does the amp have the same idle bias readings with the phase inverter pulled?

Anyhow, good luck with your amp!
- RE: Yeah, check the plate voltages when the bias reads differently - Jonesy 10:56:23 05/14/14 (6)
  Oh geeze. I don't even know where to check the plate voltage.
  
  So far all I have been able to do over the years is check resistors, capacitors, and replace things that bulge, leak or look burnt!
  
  Or I just follow the advice someone gives me on the forum.
  
  Jonesy
  
  "I know just enough to get into trouble. But not enough to get out of it."
  - RE: Yeah, check the plate voltages when the bias reads differently - Michael Samra 15:17:07 05/14/14 (5)
    `Check the plate voltage at pin number 3 of each of the 6550 tubes.Also check the negative voltage at pin 5 and see if it changes.
    
    Honest amplification is better than excessive 2nd order distortion anytime.
    - RE: Yeah, check the plate voltages when the bias reads differently - Jonesy 15:53:34 05/14/14 (4)
      Thanks Mike! Will add that to my list of things to try.
      
      "I know just enough to get into trouble. But not enough to get out of it."
      - Take several measurements at different times of the day - FenderLover 19:41:28 05/14/14 (3)
        
        In Reply to: RE: Yeah, check the plate voltages when the bias reads differently posted by Jonesy on May 14, 2014 at 15:53:34
        
        Maybe able to find a pattern.
        
        I'm going to try a different DVM also... - Jonesy 21:50:19 05/14/14 (2)
        
        In Reply to: RE: Take several measurements at different times of the day posted by FenderLover on May 14, 2014 at 19:41:28
        
        Somethings been nagging at me. When I used to check my output bias and DC balance every 6 months or so, I thought I always had to change from DC mA to DC volt settings along with switching one of the test leads. But at some point I recall getting zero reading set at DC mA for output so thought I was mistaken and both tests were to be done on the same DC volt setting, because only then would I get any readings.
        
        But reading through past posts I think it should be DC mA for output and DC volt for balance. I know not to touch A.C. balance as I need a scope.
        
        Either I've gone crazy or my DVM has. Hopefully the latter.
        
        Jonesy
        
        "I know just enough to get into trouble. But not enough to get out of it."
        
        RE: I'm going to try a different DVM also... - FenderLover 01:01:00 05/15/14 (1)
        
        In Reply to: RE: I'm going to try a different DVM also... posted by Jonesy on May 14, 2014 at 21:50:19
        
        When you measure the voltage (milli-DC), you are really measuring the voltage drop across a known set resistor tied from the cathode-to-ground. The idle current passed can be related to this voltage drop, by Ohm's Law.
        To measure the actual current passed, you need to break the connection between ground and the cathode of each tube. Then, insert your DVM probe to complete the circuit.
        
        Edits: 05/15/14
        
        Ok. I'm good then... - Jonesy 20:51:51 05/15/14 (0)
        
        In Reply to: RE: I'm going to try a different DVM also... posted by FenderLover on May 15, 2014 at 01:01:00
        
        The test points are set up such that 65 65mV equates to 65mA.
        
        Going to get down to some testing this weekend.
        
        Jonesy
        
        "I know just enough to get into trouble. But not enough to get out of it."

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