transformer help needed on L&K 50 watt amp from 60's

[smackoj]

hello again amigos; I scored this L&K (made in Denver) amp from a guy who gave this report, "I owned this amp and it was working good. I traded it to a friend who used to hang around the shop in Denver where these were built. He called recently and said 'It turns on but no sound'. I took the amp back from him and turned it on. Before I could get my meter leads into it to check voltages, the power trans smoked." I asked if all the tubes were with the amp and he said, "not the power tubes (6L6) they are bad". He also said he had replaced all of the power filter capacitors.

So, I did the usual visual inspection and found no burnt resistors, no exploded caps, no charred spots on the power tube sockets. I took all the tubes out (he sent along a set of used power tubes with the amp) and powered up using a current limiter. Bright light! I unsoldered the two red HV leads from the diodes and checked voltages with 237vac on one side and 380 vac on the other. I checked the negative bias voltage and got -55vdc. I put the HV leads back on and tried the amp thru the limiter again. Bright light. I unsoldered the HV line from the power switch to the output trans and powered up again. Shows running normal with dim light. I checked the soldering job and leads from the power filter caps to the circuit board, all looks good. I powered up again without the current limiter and all tubes installed (OT still disconnected) and checked voltages. Not looking good from what I can tell. Only 308 vdc on the power tubes, the PI has about 140v on both sides and the first preamp tube has 210v on both sides.

Any suggestions on where to go from here? I don't want to buy a different PT and OT until sure that's what I need? I do not have a schemo but this amp is almost a dead copy of blackface fender.

thanks amigos        

[sluckey]

I unsoldered the two red HV leads from the diodes and checked voltages with 237vac on one side and 380 vac on the other.

Those voltage readings mean the PT is bad.

The fact that the lamp limiter glows bright with no tubes installed but glows dim with the OT disconnected does not necessarily mean the OT is bad. But it does indicate a short to ground in the OT primary plate circuit. Check resistance from pin 3 of each output tube to chassis. Should be open circuit with OT primary center tap disconnected from B+. If you get any resistance reading other than open (infinity) then disconnect OT leads from tube sockets. If either pin 3 still shows resistance to chassis, then look closely at tube sockets or possibly protection diodes if present, or plate to ground caps if present. If pins 3 no longer show resistance to chassis, check OT pleat leads for resistance to chassis.

Show us a good pic of the output tube sockets.

[smackoj]

Thanks SL; adding pics of output sockets.

[DummyLoad]

as stated - PT is bad. try a fender super reverb/bassman perhaps even tremolux replacement. measure the mounting centers to ascertain fitment.


--pete

[smackoj]

Both of the output pin 3s show resistance to ground with OT primaries connected. With both primary leads unsoldered, no resistance from either pin 3 to ground.  Does this mean the OT is bad?

thanks

[sluckey]

Does this mean the OT is bad?

Probably. With all three OT primary wires dangling in the air Measure from each wire to chassis. There should be no resistance.

[smackoj]

doesn't look good. 2 of the 3 wires on the primary side show resistance to ground.

thanks, jack

[smackoj]

I am adding a few more pics for those who may want to know more about the L&K tube amp made in Denver, CO sometime in the 60s. It looks a great deal like a Fender blackface from the same era.

Thanks again Sluckey and Dummyload.