Lee Jackson Metaltronix Amplifiers

[rockopusahol]

My guitar player dug this broken amp out of his basement after it sitting there for 4 years. The amp popped a B+ fuse every time you turned it on and it had a bunch of crappy chinese preamp tubes and like 3 different brands of EL-34s in it. I tossed all of'em. I threw some JJ's in the preamp and Svetlanas in the power section and WALAH! The amp biased like it should and worked fine for about a month.

Eventually our guitar player wanted to replace the Svetlanas with JJ EL34Ls so he ordered some and we threw them in. Warmed up, checked the bias on all tubes @ -40V, OK, with all of the knobs all the way down I hit the GO switch to check the output on a scope. I soon noticed a peculiar smell emanating from the power section somewhere. Took a look at the output tubes and noticed the plate on tube#1(closest to the bias switch) was red hot! :shock: I turned the amp off imediately thinking that we may have gotten a bad tube, I threw in one of the Svetlanas in its place and let the amp warmup again. Hit the GO switch and now the Svetlana was the one glowing. I took that tube out and left that socket empty to see if it was just that socket but when I turned it on again Tube#2 started glowing. I took out Tube#2 and turned it on yet again and tubes 3 and 4 looked normal and showed no signs of plate glow.

So that means it's probably something specific to that side of the transformer, inbetween the phase splitter and the output jack(yes, I had a dummy load hooked up and reverified those connections). I removed the power amp board looking for any burned components but have found none. I continuity checked the 8-pin sockets and they looked good, and there are no shorts across any of the resistors although I haven't unsoldered them to check the values of them yet. The electrolytics in the power supply look fine(not exploded) and all the diodes in the power section seem OK. I thought that DC may have been getting across the coupling caps coming off of the phase splitter but they measured open like they should and I noticed no difference in grid1 voltages from one side to the other. I measured 97ohms across the choke, fine.

All this lead me to the OT. I'm not sure what I'm supposed to be reading on the primary but I am getting 27.7ohms on the problem side and 36.2ohms on the OK side. It seems iffy to me that they are not equal but I have never had to troubleshoot a transformer before. I've heard from some people that the primary windings are rarely exactly equal due to the order in which they are wound but I didn't think it would be by that much.

May be it's OK and I need to start taking out components to test them? May be I am overlooking something obvious. Any ideas/answers would help me immensely. Sorry about the novel. :roll:

UPDATE: 9/10/08 - After thinking about this all week and checking out the GeoFX tech page, here is what I did: With the transformer isolated, ramped up AC to about 60VAC through the primary using a variac and there are no shorts (ie: current very low). OT is fine except for the slight primary DC resistance mismatch, which is probably negligable. Cranked down the bias adjustment to ~-45V and threw the new tubes back in. Whad'ya know, no plate glow. I'm an idiot. Now with the o-scope hooked up I was able to make my adjustment and ended up at around -42V. Still no plate glow. Sounds like it should, and no funky smells.

I need to get one of those bias-rite meters so that I can more acuratly measure the bias current on tubes. I use the shunt method in other amps but I didn't even attempt it in this amp because I could not find a good place to hook up aligator clips to either the plates or the primary center tap. Anyway, hope somebody can learn something from all of this.