SF Dual Showman Ultralinear blowing 6L6's

[ValveBytes]

Hi,

This Dual Showman has blown two 6l6's and 5A fuses over the past two days, now the amp doesn't seem as loud as it used to I'm mostly concerned that the OT might be damaged what is the best way for me to check it out?

Detail and possible cause:
The amp had been running fine dimed, cranked whatever for the past 5 months, two things have changed recently, firstly new tubes they were running for about 2 weeks untill this happened secondly and more recently the 4ohm 12x4 was changed to different one that I later found out has a transformer in it so you can adjust impedance from 4/8/16 my theory is that the tx in the cab got saturated and reflected back the load on to the other side of my OT tx in the Showman and blew a power tube, it blew one in the middle left then one in the middle right. Static dissapation of the first set was about 17w and the second a cool 13w.

I remember checking out transformers with a variac to get the ratio, also I seem to remember doing something with a bulb flicking it on and seeing if it lights up on the other side of the OT?! its been a while since I built an amp so bit rusty

[techineer]

Ohmmeter test the two halves of the OT primary... they should be about equal. Also check for shorts from any winding to the case or frame, and between windings.

Voltage test:
Unplug the amp and remove the output tubes.
Feed a signal (preferably a signal generator at about 200 Hz, 1VRMS, although 60Hz will do) into the speaker jack. Measure the AC voltages from the primary center tap to each end. They should be very close to equal, and probably about 20-30V. Any significant imbalance indicates a defect.

If you REALLY understand how to use an ammeter, monitor the current of the signal source. It shouldn't be very much. If it measures more than ~100 mA I would suspect a short.

[vintagekiki]

See bias or cathode current of each individual 6L6. Approximately 25-30 mA per tube.
Set bias by change bias resistor 15k on OTM TrimPot .
Check .1 uF couple capacitors from the anode 12AT7 to g1 6L6

schematics:
http://www.schematicheaven.com/fende..._100_schem.pdf
http://www.schematicheaven.com/fende..._135_schem.pdf

[pdf64]

Have you confirmed that the screen grid resistors are good, particularly on the sockets where the tubes blew out?
These SF UL amps are very hard on the 6L6 screen grids, their dissipation when cranked can be several times over their 5 watt rating.
Consider increasing the value of the screen grid resistors. In these amps I would use 2k2 as a minimum, 4k7 if it's going to get cranked hard (2 watts in both cases), unless you can stretch to a set of sylvania str387, which seemed to be able to cope with 470R. Peter.

[vintagekiki]

If 6L6 are correct, if the right tube sockets (not broke pin2/pin3), set a negative bias approximately 25-30 mA per one tube.
Screen grid resistors are 470 ohm / 5W.