Carvin X60 OT and other issues

[Delta362]

I am working on a Carvin X60 that the owner asked for new input jacks and basic pot cleaning. Upon getting the amp on the bench, I first tried out both inputs and was able to get sound. The jacks felt solid. I have a call to find out why the input jacks were requested to be replaced.

hen I turned off the amp and began removing the tubes before removing the chassis. One of the power tubes was cold to the touch. I turned off the lights and turned the amp back on to standby. I could see only one of the two power tube filaments lighting up and getting warm. I powered down, swapped power tube locations and turned it back on to standby. The lack of filament followed the tube. Apparently this has been running on a single power tube since one is definitely a goner.

I confirmed the tube was bad by not getting a resistance across pins 2-7 as well as put the pair on my tube tester. I also tested the preamp tubes on its tester and they are about shot as well. Looks like a retube is in order.

I pulled the chassis and did some preliminary visual checks. R132 was burned up and unreadable. From the schematic, it feeds the +15V to the opamps and is a 22ohm 1/2W resistor, while R133 is also 22ohm 1/2W and feeding -15V. I replaced R132 with a 22 ohm 1/2w resistor (per the attached schematic) and put it on standby. I've got -14.5V for the 15V rail, but my positive rail is down around 13.3V. Subsequent versions lowered the value to 10ohm, I'm guessing to allow for more higher loading of the 15V rail.

After replacing the resistor and while on standby, I checked all of the opamps for any shorted outputs or rails and did not find any problems with any of the opamps. None felt like they were getting hot to the touch when the amp was on for a few minutes.

I then grabbed new preamp tubes and a new matched pair of power tubes and put them in (with my bias probes installed). I powered on with a speaker, and noticed that one side of the power tubes was pulling a lot more mA that the other side did. I swapped tube locations and noticed the same side running hot. I powered off, pulled the tubes, then measured the resistance of each leg of the OT. The blue side is reading 91 ohms while the brown is reading 82 ohms. The brown leg side is the one that's running hotter.

Would running a single side power tube (because the other tube wasn't operating) cause damage to the OT? What is a good way to test this transformer to make sure it isn't damaged?

[The Dude]

The OT is likely fine. A slight deviation in resistance readings from one primary side to the other is normal. Are both screen resistors good?

[Delta362]

Thanks for replying, Dude! This amp has a single 350ohm 5W that is common to both output tubes.

Edit: I also confirmed the grid stopper resistors and bias voltage to each was the same..

[The Dude]

So yours doesn't have the 10 ohm screen resistors shown in the schematic?

[Delta362]

Just looked at the amp. R113 and R115 are jumpers. I found this other schematic that doesn't show those resistors. This one seems more like what I have (subtle tweaks from the first one I posted.)

[Delta362]

What's interesting is the OT wiring on Terminal H2 matches my original schematic pins (5, 6,7), but R113 and R115 are definitely jumpered and not 10 ohms. So I'm thinking the board for mine matches the original schematic, minus whatever components they jumpered.

[The Dude]

I'd also check sockets, especially given that you had an open filament on one tube- possibly caused by an arc. Retension socket pins and look for any burn marks.

[Delta362]

The offending socket looks like tubes have been attempted to be inserted with the key out of position a few places. I had cleaned and retensioned the sockets which were definitely loose and dirty, but I will pull the board to look better at where the sockets connect to the board to see what might be lurking there. Thanks for the suggestions! I'll let you know what I find.

[g1]

I also tested the preamp tubes on its tester and they are about shot as well.

I'm not sure what this means. Have you also tested new tubes and found a big difference?

[Delta362]

I used a maxi-preamp II tester that I've had for several years now. The preamp tubes from this amp give numbers (especially the transconductance) that are significantly lower than any other good tubes I have tested. I installed new

I pulled the power tube sockets and gave them a hefty cleaning then reinstalled them. I then installed a different matched pair in there and they are biasing within 1mA of each other so the first pair I tried was clearly not matched well and perhaps the socket has some residual buildup on it that also impacted the previous readings. I'm not sure how that first pair I tried ended up as a set in my stash, but they're no longer a pair.

The amp is now back to functioning and sounding as it should.

Thanks for the help and guidance!