ampeg svt 3 pro tube drive question

[supawah]

hey all, i always thought that the tube drive on an svt 3 pro changed the cathode or something, looking at this schematic it seems to control the plate voltage, would ampeg really put 220v through a cable, tyco connection, onto a pot? the one i'm working on has 220v at the pot. is q1 tip 50 bad? thanks.

[supawah]

schems attached

here's the schematics

[Enzo]

I would probably expect closer to 300v on that Plate Voltage pot when it was all the way up. Look at the schematic. Power supply A comes into Q1 collector, and also through R9 and R81, through pin 11 to the control. Since the control is like 1 meg, when it is up fairly high those two resistors won't diminish it a lot. That pot then controls Q1, which in turn controls the current through the tubes. So 220v sounds reasonable to me.


Is there a symptom you are trying to repair?

[supawah]

I would probably expect closer to 300v on that Plate Voltage pot when it was all the way up. Look at the schematic. Power supply A comes into Q1 collector, and also through R9 and R81, through pin 11 to the control. Since the control is like 1 meg, when it is up fairly high those two resistors won't diminish it a lot. That pot then controls Q1, which in turn controls the current through the tubes. So 220v sounds reasonable to me.


Is there a symptom you are trying to repair?

yeah, it pulses or drops signal every now and then, only ting i can find weird is a glitch in that pot that cause signal drop, and the fan is on full blast from startup

[Enzo]

First suspect there would be the power amp in jack cutout contact. Clean that well, or just plug a cord from preamp out to power amp in and see if that cures it.

Stuck fan may or may not be related. The odd fan supply off +65 also serves the tube heaters in series. The fan speed control circuit is straightforward. Figuer it out to see if it is related.

[nevetslab]

As Enzo said....I'd suspect the Power Amp input jack, it getting signal via Normal contact from the Preamp. Patch cable bypasses the Normal, though you may also have the typical solder fracture problem that plagues so many of the Ampeg SVT Pro Series amps. I see from my service records I haven't yet been inside our two SVT3-Pro's yet, but find on the Ampeg SVT4-Pro's, SVT-CL's and others, Ampeg uses rear panel jacks and pots to support the back end of their large PCB assemblies. Radial solder fractures around those jacks is a very common problem. Pain to get at, as ya have to extract the main board to get to the bottom of it.



A couple photos of typical rear panel phone jacks with solder fractures.....these were from an SVT-CL.

Odd Fan Control circuit, having the two preamp tube's heaters in series with the fan's pass transistor, where, in normal operation, the heater voltage varies to some degree as a function of the amplifier's heat sink temperature. Their controlling circuit TS1 (LM35) could be bad. It's a precision Centigrade Temperature Sensor, so it's in direct contact with the heat sink....probably via thermal grease, and using the TO-92 package (haven't seen it yet)

[drewl]

Yeah, in my notes I have the wiring layout for pulling the main PC board for easy reference!