50 watt problems

[gibboau]

Gentleman,

Some of my "guitar playing" friends found there way into my amp room and decided to crank out a few riffs on my trusted 50watt build a few weeks ago, it wasn't long before i found my way in to the room and discovered that the two dudes had been trying to get sound out of an amp that wasn't even PLUGGED IN to a speaker cab

I switched the thing off right away and proceed to give the a stern lesson in basic amp theory (no load, no power on etc etc)

The amp still makes sound and doesn't blow fuses however its lost a heap of tone and isn't as loud and punchy as it use to be !

so what am i looking at here?
a) Semi Blown OT?
b) Blown Power Tubes?
c) Blown OT and Power Tubes?
d) WORSE

Im not to sure how long the amp was running without load, but i can say it was run for up to a min our two without load

I haven't had a chance to put the amp on the bench and probe it, but have had a few rehearsals and i can tell there is something not quite right about it

I'm interested in your thoughts and theory's

[hourglass]

My experience with EL34 amps is limited, but what I can say for sure is they utterly hate being turned on with no load. I had a pair of GTEL34-M's in the kit I built and after doing some tweaking I turned it on, cranked it up and then.... nothing. Cuz I had neglected to hook up a speaker.

Once I hooked the speaker up it was fine, but the next time I fired it up one of the tubes started to red plate immediately. I checked it with a meter and saw that as soon as I flipped the standby switch that tube was drawing over 200ma.

Prior to this I had flipped on a JCM 800 2204 without a load. It immediately blew the B+ fuse, but once it was changed it kept blowing them. Turns out one of the tubes had a dead short between the plates and the filaments.

You'd think I'd have learned my lesson after the first one, but no. Hopefully after two it's managed to sink in. Of course, I have years and years of doing the same dumb shit with my 6L6 amps with no (knock on wood) issues ever coming from it.

So - my recommendation would be to check the tubes. See what kind of current they're drawing and check your grid resistors while you're in there.

Ryan

[arledgsc]

The police haven't come by to arrest you for murdering your buddies? Most likely your tubes are cooked but the OT primary could be damaged as well. Before you pop in a set of tubes take the original tubes out and drain the caps. Then measure continuity from the each OT primary lead to center tab and verify they read about equal. The primary windings with take a lot of punishment with no load as there is no place for the power to go except back the EL34s and OT primary. Then for peace of mind measure from the OT center tab to the secondary speaker outputs and verify no shorts across the transformer. As suggested above measure the screen resistors while the tubes are out and caps drained. Verify they aren't discolored or blistered. When you feel brave borrow a set of tubes from your buddy's amp and have a go at biasing. Good luck and let us know what you find out. Scott

[gibboau]

Guys thanks for your replies

Here are my findings,

Primary's:
RED 38.5ohms
WHITE 39.5 ohms
I couldn't get a reading on the secondary's but i considered that as a good sign, the multimeter wasn't showing any shorts

one of the screen resistors was fairly burnt, so i was prudent and replaced both of them
I didn't really test the tubes i just put in a spare set i had !
biased it up and presto much better

i'll have a better idea after our next rehearsal, ill see if the amp holds up after hrs of ROCK !!

im hoping this is the end of the dramas, now to go and order another set of SED's

[arledgsc]

Excellent and great job of debugging and repair! If it makes it through practice then I would be confident it is fixed but time will ultimately tell. Be sure and remove the fuse from the amp while not in use to keep your friends honest

[gibboau]

Be sure and remove the fuse from the amp while not in use to keep your friends honest

that's a great idea !!! only problem is ill forget it one day