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I have sent several emails over the past year or two regarding problems with blowing el84 power tubes in my Slll 18w. You suggested a Zener to ground which seemed to reduce B+ to an acceptable range but V5 has blown again. I installed a 22v 5 watt Zener and all was well for the past year. Now blowing V5 again

Here are my voltages now:

1. B+ 368vdc (diagram shows 345)
2. Pin 7 V5 363vdc
3. Pin 7 V4 364 Vic
4. 9.54 Vdc at the intersection of the 150R and 220uF that shows 12 Vdc on the diagram
5. 342 Vdc at the next turret of the 100R that shows 340 Vdc

6. 290 vac at pins 1 & 7 of the rectifier tube as shown on the diagram

I'm at a loss at this point as to what I need to do. I see no shorts or arcing and no sign of overheating regarding any components.

Any help or suggestions you could give would be greatly appreciated!

Thanks,

Steve Kirkpatrick









Sent from my iPhone

Start with no tubes, does the fuse blow?
Add the rectifier tube, does the fuse blow?
Add the preamp tubes, does the fuse blow?
Add the output tubes, does the fuse blow?


You might have an intermittent short somewhere. I have found shorts on amps that were a single strand of wire (a single strand in a 50 or so strand stranded wire). Use a magnifying glass to look carefully at joints and wires.

Are you using a 2A slo-blo fuse if your AC input is 120V?

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http://pdfelectronics.com

The fuse isn't blowing with tubes out, tubes in. I've just had to change mainly V5 3-4 times in 2 years. V4 may have gone once. Voltages all check. Will look for any possibility of a short. Wondering if OT transformer could have intermittent voltage spikes or fluctuations......?

Yes to the 2amp slo-blo and house voltage is 120v

I believe you tested the OT resistance and it was fine. Might be fine threads of wire you cant easily see but this is a very strange one. Voltages are a bit high though. I would completely rewire the socket and board to V5, maybe even replace the socket. Increase the screen resistor to 470R for each tube and maybe add a 200 ma slo blo on the HV CT to protect more.
You could add a VRM and turn down the voltage as well.
Maybe you are getting spikes in your mains, so a surge supressor power bar or add one inside the amp.

_________________
Stephen
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