A couple of issues with my newly built plexi 100

[HandWired]

Hey guys,

after firing the amp up for the first time things seemed to be working pretty well. Then after a few hours of operation the HT fuse blew. I replaced the fuse and everything was good again for a day or two until i turned it on and noticed distortion even at low volumes. It actually sounds kind of nice yet i am aware that the amp is obviously sick in some way.

any ideas?

thanks.

[HandWired]

Ok so i opened her back up and eek, all the 1ohm resistors attached to the ground lug and pin 1 of v4-7 have been fried.

[Roe]

.6w 1ohm resistors works fuses. perhaps you have blown tubes. any problems with PO or redplating?

[HandWired]

.6w 1ohm resistors works fuses. perhaps you have blown tubes. any problems with PO or redplating?

do you mean the resistors work like fuses?

I didn't really notice any obvious issues with the tubes, just distortion at low volume and when turned up the amp would just distort even more and sound thinner and worse than it did before, no volume troubles though, still loud as hell.

I guess i will replace the resistors and perhaps get a new set of power tubes and see how that goes.

[neikeel]

.6w 1ohm resistors works fuses. perhaps you have blown tubes. any problems with PO or redplating?

do you mean the resistors work like fuses?

I didn't really notice any obvious issues with the tubes, just distortion at low volume and when turned up the amp would just distort even more and sound thinner and worse than it did before, no volume troubles though, still loud as hell.

I guess i will replace the resistors and perhaps get a new set of power tubes and see how that goes.

Post some decent hi res pics first - just in case something is amiss - fuses are cheaper than valves and transformers

[HandWired]

This is the max size it would let me post here

This has slightly bigger versions.
http://img37.imageshack.us/g/69206713.jpg/

[flemingmras]

When you get those resistors replaced, BEFORE you install new tubes take some voltage measurements with the amp on in play mode and post them up. Specifically, we need readings on pins 3, 4 and 5 of your output tube sockets.

[HandWired]

OK, Resistors replaced, same tubes, voltages are

[axeman]

Were did you have the burned resistor at? Also whats up with pin 1 and 8 of the power tubes?

[HandWired]

Were did you have the burned resistor at? Also whats up with pin 1 and 8 of the power tubes?

the resistors were the 1ohm resistors between the ground lug and pins 1 and 8 of the power tube sockets.

after the repair, the voltage on pins 1 and 8 ranges from about 30 - 35 mV depending on the tube socket

[HandWired]

So i don't know what to do. The voltages seem alright (right?) just like they did before the failure. There didn't seem to be any obvious power tube trouble either.
Has anyone else managed to burn these resistors?

[Roe]

check for redplating when playing, esp. when cranked. stop if redplating occurs

[neikeel]

So i don't know what to do. The voltages seem alright (right?) just like they did before the failure. There didn't seem to be any obvious power tube trouble either.
Has anyone else managed to burn these resistors?

Yes - it was a shorted out valve that took it out.

Also, make sure you have your amp plugged into a load when you switch it on with output valves in

[Dunkmop]

Its always the simple stuff. Sounded like something was loose or a problem with the valves considering those small resistors went.

Was it just 2 of the 1 0hm resitors that blew or all of them? Valves work in a push/pull combination so may only affect half the power valves.

Always check for redplating and heater filaments when first starting up new valves. Fit and remove a few times to make sure the socket and valve contact well.