Ampeg SVT-CL Thermal shut-down?

[nevetslab]

I have a client's Ampeg SVT-CL Bass Amp on the bench, which came back to me from repairs in April, which then had shut down during shows. I had replaced power tubes, driver tubes, cleaned up some solder joint issues, and other routine service, since it hadn't been serviced since 2017. It came back a couple weeks ago, once again it shutting down during a show.

This morning, I had powered up the amp, now both chassis removed from the cabinet to check connections in the preamp, and again have a look at the power amp chassis wiring. All seemed nominal, but after having left it in Standby Mode, powered up for a bit over an hour, when I switched out of Standby, it didn't light up. No difference in AC Mains Wattage consumption. And the Heater/Bias Power Xfmr on the chassis was VERY HOT. I powered down for an hour and tried again. Xfmr core was now only moderately warm to the touch. It powered up without any issues this time. I powered back down, removed the hold-down clamps & power tubes, removed the power tube cover, and then unplugged the wires to the power tube PCB. The right angle 1/4" female Fast-on Terminals vinyl covers were quite dark from years of heating (these connections being the source of the 6.3VAC into the amp's Heaters).

I slid the covers back, checked the fit of the terminals to the blades of the power tube PCB, and they felt on the loose side. Closed the contacts down a bit to get a nice tight fit, so you really have to push them on firmly to get them into place. (Yeah, I know soldering them into place is best). I've now had the amp running under Burst Pink Noise for an hour and a half & counting, pulling 250W AC mains during burst, and 210W at idle between bursts, and haven't yet had the power amp shut down as it had earlier.

Has anyone out there found their Ampeg SVT-VR or SVT-AV thermally shut down during operation? I've never seen any mention of there being resettable internal thermal switches in the Heater Pwr Xfmr.

[SoulFetish]

I know on most of the SLM mains transformers I’ve had experience with had resettable thermal fuses. Sometimes it’s labeled somewhere on the transformer.
but another thing to check is the connection were the heater leads connect to the magnet wire at the transformer.

[nevetslab]

Today, after having run this SVT-CL for close to four hours, 1 hr at Idle / Standby, then three hours under burst pink noise, pulling 250W AC Mains Power from idle power of 210W @ 120VAC, I never had the amp shut down. I didn't set up for measuring the temp rise of the heater xfmr's Primary, but just using my Fluke 51 Temp Meter with a surface probe, the laminations of the heater/bias xfmr was 61 deg C, and the Pwr Xfmr's laminations were 53 deg C after 3 hrs. Never shut down, so I think the heater input connections was where the problem was. Seems fine again. Missed that detail two months ago.

Onward.