Amp SVT whoa's

[navdave]

Been stumped on this 2002 SVT classic for a while. It all started with the bias LED's not working then eventually blew a power tube and some screen resistors and the power board ribbon. Ive replaced all screen resistors and hardwired the power board ribbon using 22 gauge solid core wire. Also replaced all the 10 ohm cathode resistors which all tested good. If I power on the amp without the power tubes in and test the voltage on the power tube sockets all seems well voltages look good plate, screen, negative voltage on pin 5 is fully adjustable. Heres a video of the AC power board relay flipping on and off like a type writer. Any help is greatly appreciated.

[jbrew73]

I’m guessing that the light bulb limiter is causing the relay to drop in and out.  What does it do without the limiter

[navdave]

 I’m pretty sure you’re right JB I’ll have to try again tonight hopefully that’s all it is. I’m scared to turn this damn thing on without the lightbulb limiter after all the work I’ve done in there.

[Williamblake]

Get a higher rated light bulb then.

[navdave]

This ones a 200 watt. I’m a risk it and fire it up without the bulb.

Get a higher rated light bulb then.

[shooter]

while you were soldering did you install 1 ohm bias R's?
do you have the bias set for MAX neg?
I'd probably do BOTH before firing it up just in case you missed a shorted gonna smoke all the big pieces problem

[Ritchie200]

navdave

You mentioned you blew a power tube?  Was it caused by a short possibly at the socket?  Did you check close for carbon tracking on the sockets?

Jim

[tubeswell]

Is this a SVT-CL? In those amps, the terminal spade connectors to the output tube board for the main heater wires (from the filament PT) can cause problems when the connectors get oxidation build up. Oxidation causes resistance which = high voltage spikes (especially at startup when the filaments are cold. Even with the heaters warmed up, these terminals see over 11A and the startup surge current is orders of magnitude higher). Oxidation (and resistance) results in these terminals getting charred so it’s pretty evident if this has occurred. The ‘problem’ with this is that voltage spike transients go back thru the filament PT to the other windings (and the filament PT also supplies power for the bias comparator and other circuits). If the transients are high enough, they will blow out the diodes and caps (and even transistors) in those other circuits, so you need to check everything that comes off that filament PT if you see charring on those output tube board terminals. Once you have replaced any shot parts, you really need to clip off any charred terminals and solder the filament PT wires direct to the output tube board terminals. This prevents that failure mode recurring. Not saying this is your problem exactly, but it’s a good place to start looking. (Also, if you don’t replace any blown Rg2s, the whole thing won’t run properly.)


Did you satisfy yourself that the tubes are okay?

[navdave]

No carbon just tubes that died and blew the screen resistors.

navdave

You mentioned you blew a power tube?  Was it caused by a short possibly at the socket?  Did you check close for carbon tracking on the sockets?

Jim

[navdave]

Ok it’s all fixed and running great! I preformed the standby switch relay bypass mod and that did the trick thank you Jbrew73. The AC voltage must of been running below 112v with the light bulb limiter in circuit this caused the relay to bug out. Lots of work put in to make this beast solid again thank you all cheers🍻