Low output Marshall 100W TSL

[beasleybodyshop]

So buddy brings me his JCM2000 TSL, and complains that it isnt loud enough. I fire it up and the loudest it can get is slightly above raised voice level.

I test all the EL34s, all of them test strong, i test all four 12AX7s. two were iffy. I swap in two known working 12AX7s.

Fire up again, still no increase in volume level. I check bias, adjust to 67mv each pair (bias per tube is half that value) no difference. Clean channel seems kinda farty. I hook up my O-scope leads to the speaker terminals and run a 1Khz signal through the front end. I set in on the clean channel, TMB and gain/volume controls at 12 o'clock. Output signal isnt super clean - it has this little jagged hump thing, but its not very big. For the experts out there, what does this tell you? I have also attached a picture of the signal before its sent into the amp.

I want to believe that this is an output transformer issue, ive been told that you can determine this by looking at the output signal and if it's asymmetrical, OT is the culprit. I really want to get better at using a scope to troubleshoot problems, to really understand what is going right and wrong inside an amp - but first i need to know how to interpret the squiggly lines

[beasleybodyshop]

Amp has been on and with the standby off for over an hour, bias hasn't shifted. So its not the drifting bias problem this amp is known to suffer from.

[Leo_Gnardo]

Amp has been on and with the standby off for over an hour, bias hasn't shifted. So its not the drifting bias problem this amp is known to suffer from.

Aw rats. I've also run across bad OT's (shorted speaker winding) and miswired OT primaries. With the push terminals on the OT, it's easy enough to disconnect primary leads and try another OT with clip leads. Doesn't have to be a Marshall, just a good OT with 50-100W capability. Don't sweat the impedances now, just a rough test. If you get significantly more power then suspect the OT.

[beasleybodyshop]

Amp has been on and with the standby off for over an hour, bias hasn't shifted. So its not the drifting bias problem this amp is known to suffer from.

Aw rats. I've also run across bad OT's (shorted speaker winding) and miswired OT primaries. With the push terminals on the OT, it's easy enough to disconnect primary leads and try another OT with clip leads. Doesn't have to be a Marshall, just a good OT with 50-100W capability. Don't sweat the impedances now, just a rough test. If you get significantly more power then suspect the OT.

Thanks leo, I will try to find another OT and sub it in.


And does the output waveform give any kind of details?

[Leo_Gnardo]

Amp has been on and with the standby off for over an hour, bias hasn't shifted. So its not the drifting bias problem this amp is known to suffer from.

Aw rats. I've also run across bad OT's (shorted speaker winding) and miswired OT primaries. With the push terminals on the OT, it's easy enough to disconnect primary leads and try another OT with clip leads. Doesn't have to be a Marshall, just a good OT with 50-100W capability. Don't sweat the impedances now, just a rough test. If you get significantly more power then suspect the OT.

[xtian]

Scope the signal before and after the PI. A bad OT can make the signal look bad at the PI output, but make sure the signal is strong going in.

[beasleybodyshop]

Ok, so i attempted to check the turns ratio on the OT. I removed all tubes, and removed the HT fuse. then i jumpered a wire from one heater winding to the 8 ohm tap of the OT. I completely disconnected the primary wires from the OT (marshall uses these spade connectors on their iron, making this easy) but left everything on the secondary side hooked up. I powered up the amp, and was getting 3.15v to the jumper. now when i connected it to the tap, and then measured again, i was getting no volts AC. my common connection on my meter was clipped to the ground on the secondary. when i hooked my meter up to the outside windings of the PRIMARY and turned the amp on, i was getting something like .7vAC

did i do this test correctly? nothing burned up or exploded so at least I didnt do any harm

[beasleybodyshop]

Ok. so now i completely disconnected all wires from OT. clip my 3.15v to the 8ohm tap, measure voltage with meter ground connected to chassis. now im getting 3.15v into the secondary.

When i measure across the winding on the primary.....i get zero volts AC. and every wire connected to the primary is disconnected.

So this means my OT is bad, yes?

[Stevem]

If you see the idle current get up well into the 120 + ma range when driving a signal into the amp, then the output tubes are doing there thing driving the OT, but the OT has gone south Bro, its that simple!
I`ve changed out a ton of these and your buddy needs to confirm that he is using a speaker cord and not a shielded cable to connect things up, and also that the connections are rock solid as also you need to confirm the connections in the amp.
One great thing to add to this or any amp is a 25 watt 250 ohm resistor across the highest impeadence secondary tap of the OT. with this mod if the outoput circuit does go open for whatever reason the OT will not arc and take itself out which is what most of these OTs fail from!

[beasleybodyshop]

If you see the idle current get up well into the 120 + ma range when driving a signal into the amp, then the output tubes are doing there thing driving the OT, but the OT has gone south Bro, its that simple!
I`ve changed out a ton of these and your buddy needs to confirm that he is using a speaker cord and not a shielded cable to connect things up, and also that the connections are rock solid as also you need to confirm the connections in the amp.
One great thing to add to this or any amp is a 25 watt 250 ohm resistor across the highest impeadence secondary tap of the OT. with this mod if the outoput circuit does go open for whatever reason the OT will not arc and take itself out which is what most of these OTs fail from!

Ugh, Steve, why do you have to be so damn right all the time I don't have a spare OT for this laying around, so im going to tell him to order another one. Thanks everyone!

[Stevem]

If The the guy has the added green, a Mercury OT is a nice sonic step up on that amp!
Thanks for the kind words, but I ain't right all the time by any means Bro, but always glad to help!