Blowing rectifier tubes

Discussion in 'DIY' started by wanders, Jul 11, 2018.

  1. wanders

    wanders Super Member

    Messages:
    1,236
    Location:
    Northern California
    I've built a couple of mono block tube amps - KT-88. Both are functioning and sound excellent. Twice now, I've had issues with the 5AR4 rectifier tubes - in both cases the tube started to flash. The first time I wasn't on the ball and I lost the tube before the 2A-slo fuse went. I could find nothing wrong, so I swapped output transformers, checked all the wiring, and started up again - everything went well on both monos.

    Yesterday, I installed a new, more compact power supply in one of the monoblocks. I checked all voltages without the power tubes and everything was stable. I put the amp into the main system, installed the power tubes and, after thirty or forty seconds, the rectifier started to flash. This time I was quick and got it off before I ruined the tube - the tube seemed very hot. Obviously, the new power supply is the main suspect in this case and I wouldn't bother to post this if I had not had the first issue. I reinstalled the original p/s and it seems stable; But, I've yet to test it with in the system with the KT-88s.

    However, I'm wondering if there is a general condition that will over stress rectifier tubes.like this. I know this is a pie in the sky kind of question and readers would like more info (schematics, etc.) before venturing an opinion. But any general thoughts would be welcome. .

    Thanks
     

     

    Please register to disable this ad.

  2. maxhifi

    maxhifi AK Subscriber Subscriber

    Messages:
    2,590
    Location:
    Edmonton Alberta Canada
    Please provide a schematic, I would guess either too much filter capacitance, too high of voltage, or poor quality rectifiers.
     
  3. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    35,866
    Location:
    Southern NJ
    +1, bad filter cap or too much filter cap. Both will stress the rectifier. Voltage beyond what the rectifier can tolerate is also a problem. The other trouble with these, once you arc the rectifier they seem more prone to arcing going forward.
     
  4. mv213

    mv213 AK Subscriber Subscriber

    When I built my VTA ST-120 a few months ago the brand new rectifier tube lasted thru 2 power-ups. Bob Latino strongly recommended I get a variac and set it to no more than 117VAC. My line voltage was running about 123-124VAC. This apparently pushes the b+ voltages beyond the safe range for many of the newer rectifier tubes. I took his advice and have had no more problems.
     
  5. wanders

    wanders Super Member

    Messages:
    1,236
    Location:
    Northern California
    Thanks for these leads. I have a suspicion that you are correct on over-capacitance in the hi-voltage power supply. I've notified the circuit designer of my concerns and your comments. It's possible he erred, or, more likely, I misinterpreted his spec on a final h/v, p/s cap.
     
  6. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    35,866
    Location:
    Southern NJ
    Its mostly the first cap off the rectifier where you have to watch it. Off the top of my head I want to say a 5AR4 wants 40uf max if its also at max voltage and current.
     

     

    Please register to disable this ad.

Share This Page