Plexi blowing HT fuses

[Riffmonster]

Hi all,

I built a 50 Watt JMP clone with 1969 specs (SS rectifier) a few weeks ago and it worked great from the start without any problems. I have played the amp for about 10 hours in total. Yesterday I made a stupid mistake and did not realize I had set the amp on 4 ohms, while my attenuator and cab were on 16 ohms. Anyways, I only realized this mismatch after maybe 3 hours of playing when I switched the amp off with the standby switch (not the power switch yet). The fuse blew before I could turn off the power switch. Turned out the Standby switch was shortened afterwards. I replaced it but the amp is now blowing fuses immediately when I try to turn it on (turning on the power switch before the standby switch). I tried it already with and without tubes. I did not have the time to take a close look at the amp yet, but would anybody have an idea what happened and where to start troubleshooting? I suppose checking the OT, PT and filter caps first makes sense?

Many thanks for the help. I will try to upload some build pictures later. Nothing smelled or looked burned inside when I quickly checked it.

Regards,

Simon

[martin manning]

Since you ran with 4x normal primary impedance I would suspect burned/shorted screens or OT primary insulation damage. Look at the screen supply and screen resistors, but unfortunately it's likely to be the OT.

[Riffmonster]

Thanks for the quick reply Martin, this is very helpful. I will check the screen resistors and supplies and hope it’s just them and not the OT. I will keep you posted. Still can’t believe I made this mistake

[martin manning]

You can try disconnecting the OT CT from the power supply with tubes out, and see if the fuse holds.

[Riffmonster]

Sorry, I mixed up the terms for the fuses (English is not my first language). The mains fuse blows but not the HT fuse. Is your test to disconnect the OT center tap still valid?
Thanks.

[martin manning]

Yes.

[Riffmonster]

Still blows the fuse with the OT CT disconnected. I checked the resistance from the OT leads to ground as advised here http://www.geofex.com/ampdbug/outtrans.htm and it all looked good. Maybe it is not the OT after all?

[martin manning]

What happens if you pull out the HT fuse? With the tubes out, that should leave just the HV rectifiers, the reservoir cap, and the bias supply connected to the PT. Do you have a bulb limiter? No sense in wasting fuses.

[Riffmonster]

Thanks for the help! For some reason my diodes were shortened. I replaced them and the amp seems to work normal now but I am a bit concerned that I am overlooking something. What could have caused the diodes to break in the first place when flicking the standby switch? Two be honest, I only used two UF5408 diodes and not 4 like in the original schematic.

[martin manning]

The diodes have to block 2x the B+ voltage, so standard practice is to place two 1kV diodes in series on each leg of the PT. Sounds like you may be ok, but definitely double up on the rectifiers.

[Riffmonster]

Yep, I put four diodes in and played the amp for some hours today without any problems. Thanks for the help again!