Fun filament rectifying flames!

[drewl]

Submitted for your amusement:
I have a Crate VC50 with a big black mass of burnt fried muck where 4 diodes and 2 100ohm resistors used to be!
This used to be the rectifier circuit for the 1st two preamp tubes.
The customer wrote on the tag that something happened while he was "experimenting"

So far I've removed the melted parts, scraped off all the fried bits of pc boardand cleaned all the mess up.

Luckily the pwr trans is still good. I'm gonna mount a bridge somewhere and all should be well.......
What the heck did this guy do with his "experimenting"?

possibly fried tube.

[loudthud]

This is just a guess based on my experience. When PC boards are used in preamps, AC heater supplies cause excessive hum because the wires can't be twisted and must be close to sensitive audio circuits. So, DC is used to light up the tubes.

I don't have a schematic of your amp, but many of the contemporary amps I've seen use the same scheme. The preamp heaters use the series connection from pins 4 to 5 and two tubes are in series requiring 25.2V. The heaters are run from a regulated DC supply of 30V (+/-15V) with a series resistor to drop extra voltage. These supplies are also used to power various solid stage circuits for channel switching, effects loops and whatever else the amp has.

Usually, three terminal IC regulators are used like 7815/7915 etc. The one thing that will kill these chips is the application of voltage of opposite polarity to the output. The way to prevent this is a diode across the output of each IC to clamp the reverse voltage to ground. Many times these diodes are not included in the design. So, any momentary short across pins 4 and 5 of one tube will take out one or both regulators which could lead to rectifier failure.

[drewl]

Althought it is a VC50, it's very similiar to the VC30.
The dc filament circuit was pretty striaghtforward, pins 4&5 tied together and pos, pin 9 being neg.
and running at 6v.
I removed a section of fried pc board, filed away the "carbonized" sections, mounted a bridge and the 100ohm-ers to ground and it works fine.