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Yeah Platefire's got one in over my head but I will give it my best shot! Hope we got some SVT Classic experts around?

I easily found a manual and schematics on the net for it. I've been studying it trying to get a general understanding of how it works. The serial number is TCLDI800007. Customer said he thinks it was made in the 90's, he bought it from a friend. He said the problem is that shortly after being poweing up it automaticly goes into 'red' standby/Fault which means high voltage B+ is cut off becuse of a problem detected. I did my first test tonight with it hooked up to a single 12' 100watt speaker. It powered up fine with stanby on.

After I flipped stanby off it stayed red for a few seconds(no B+) and then went green(B+operating). I ran the amp quite a while and it never went into cutoff/Fault(red).

Checking the back panel, it has two bias pots with red & green LED's over each pot to set bias with. Bias #1 controls left three tubes, bias#2 the right three tubes. Bias #2 stays green which means correct bias setting but bias #1 has the red & green stays on all the time which the manual says it is a possible bad bias control or bad tubes. I checked the tubes on my TV-7A/U and one 6550 tested bad.

I re-installed with all good tubes but the red & green bias LED's stays on. So the amp seems to be running pretty good with no red plating and my guitar sound good through it. So---------- 1-Not getting it to go into cutoff like customer said was a problem. Could the difference be in my 1-12 cab and his large bass speaker cab?

2-On the bias, all tubes are good covered by bias pot #1---hard to believe the bias pot is bad? LooseChange Yep! I tried setting the bias. On side #1 that is showing the problem with a solid red & green(manual says 'Possible bad Bias Control or bad tubes')--if you turn the bias counterclockwise the red comes on and hum increases. The manual says a solid red only is 'One or more tubes not functioning'. OK-the customer gave me two new EH 6550's to use. I did find one of the existing TUNG-SOL's in the left three tubes under #1 bias with a bad reading on my TV-7A/U.

I did check the EH's also and they checked good. I put the two new EH's in plus one high reading TUNG-SOL's I pulled. No change in the in the bias readout on #1--still does what I described above.

I did try setting the #2 bias that is set at about 12:00 oclock showing a solid green(good). Hum increases if you go clockwise or counterclockwise from original position and the green light goes off but comes back when you return to 12:00. The problem side #1 is set at 12:00(same as #2) and hum increases if you go either way away from that---soundwise it's happy set same as #2. It also runs normal soundwise for my 30 minuet test at one time without going into cutoff on the standby like the customer said he was having a problem with. So according to the manual---if it's not a bad tube it's a possible bad bias control?

I haven't pulled anything yet, this will be a bugger to work on. I've never seen transformers so large!! This problem can occur in the CL if there is a fault in the output or bias circuit.

Something that commonly causes this is a bad screen resistor. There should be a 220 ohm, 1/2W, flameproof, 5% resistor on each screen. Older versions had a 22 ohm resistor, some had a diode in parallel. There was a technical note issued by Loud stating to remove the diode and install the 220 ohm resistor.

I'd check those as well as the plate resistors. Running the amp at a low level might not reveal the fault problem. If you have a 4 ohm 300W dummy load, it would allow you to turn it up. Hope that this helps. On Drawing 075419-11 Sheet 1 of 2 Power Amp Pictorial is says: 'TO ADJUST BIAS-Allow to warm up for 20 Minutes. Adjust bias so green LEDS are lit.