SET Asylum

RE: 300B - Is this thermal runaway ?

49.37.68.181

Burney

In Reply to: RE: 300B - Is this thermal runaway ? posted by Burney on October 26, 2021 at 06:46:35

Forgot to add, I did change the coupling caps. No relief.

Follow Ups:

RE: 300B - Is this thermal runaway ? - Tre' 08:27:47 10/26/21 (6)
wait, you have two amps and they are both doing this?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
- RE: 300B - Is this thermal runaway ? - Burney 08:47:34 10/26/21 (5)
  I had this 300B ebay PCB to try out. Wired up everything and used a few days. It just wasn't as magical as the EL34 SE I had been using for several years. The EDCOR OPT's were repurposed from the EL34 SE amp and they worked fine with the ebay 300B PCB. No thermal runaway. I also recall 300B cathode had higher voltage, about 72VDC whilst using the ebay PCB. B+ was almost same
  
  Got ambitious and put together the WE91 clone. And here we are in this situation.
  - HiFi Klasse A R�hrenverst�rker 300B Vacuum Tube Power Amplif (Open in New Window)
  - RE: 300B - Is this thermal runaway ? - Tre' 08:51:16 10/26/21 (4)
    Did you build two WE91s and are they both having the same problem?
    
    Tre'
    Have Fun and Enjoy the Music
    "Still Working the Problem"
    - RE: 300B - Is this thermal runaway ? - Burney 09:11:15 10/26/21 (3)
      No. Just the current project in a makeshift chassis.
      - RE: 300B - Is this thermal runaway ? - Tre' 10:53:20 10/26/21 (2)
        
        In Reply to: RE: 300B - Is this thermal runaway ? posted by Burney on October 26, 2021 at 09:11:15
        
        I would look for parasitic oscillation in the driver stage.
        
        Tre'
        Have Fun and Enjoy the Music
        "Still Working the Problem"
        
        RE: 300B - Is this thermal runaway ? - Burney 00:24:10 10/27/21 (1)
        
        In Reply to: RE: 300B - Is this thermal runaway ? posted by Tre' on October 26, 2021 at 10:53:20
        
        Thanks. I installed range of stopper resistors from 220R upto 10k. Both at the driver and the 300B. No improvement
        
        Secondly, as I understand the grid voltage at idle should be much higher than -50vdc that I am measuring right now.
        
        RE: 300B - Is this thermal runaway ? - Tre' 10:39:23 10/28/21 (0)
        
        In Reply to: RE: 300B - Is this thermal runaway ? posted by Burney on October 27, 2021 at 00:24:10
        
        Yes, if the tube is healthy with about 340 volts across the tube and only 50 volts bias there should be some 240ma of plate current flowing. That would melt the plate.
        
        Tre'
        Have Fun and Enjoy the Music
        "Still Working the Problem"

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