Vox VT40+

[matt mincer]

Hey All,

I have a Vox VT40+ on the bench with a power problem. Basically, all of the LEDs go dead intermittently so the amp don't amp  . It's not the fuse and it doesn't appear to be the transformer. The filament for the 12AX glows so I know we are getting power. Still, it cuts off. I have tried flexing and poking at the board but so far that hasn't yielded any results. Anyone have any clues? Also, the schematic is proving hard to find. Anyone know where I might find that?

THanks
Matt

[terminalgs]

LEDs go dead intermittently

by 'intermittently',   do you mean when you using the amp the LEDs light, then dark, then light,  then dark?  or do you mean you can't predict when the LEDs will go dark, put once they do they never light up again?


schematics are great, but you can do a lot of trouble shooting without one:

• When amp is functioning,  check voltages and write them down:  B+ at the rectifier,  power tube plate,screen,grid (I'm guessing this is fixed bias). preamp and PI tube voltages as you can, as well as voltages at different filter caps. 
• Once the amp stops working, re-check voltages.
• Look for burned components or bulging caps.

[matt mincer]

You can't predict. When I first got it, it didn't work at all. I took it down to my shop (basement) and it did work. One time it flickered on and off several time quickly. It seems to be staying on for now. My guess would be something in the low voltage power supply.

Thanks
Matt

[92Volts]

This is mostly a digital amp which uses the tube for distortion, but not the same way as all-tube amps. Maybe we can help you, but there's a good chance this problem isn't related to the tube.

I found this service manual for an older/smaller Valvetronix: http://www.valvetronix.net/docs/AD15_30Servicemanual.pdf The exact schematic would be helpful. Not sure if it's available.

Based on that possibly-wrong info? The 12AX7 heater is run separately from all other parts of the power supply, so it's no surprise that it works while an unknown number of other things stop working.

Also, there aren't high/low voltages for the tube and SS parts, only 2 voltage levels in the power supply. The tube's non-heater parts run off the same "low" voltage of +/-20v (later lowered to +/-15v) as other discrete components, including the SS poweramp. The computer parts run off +3.3v.

So I'd inspect any part of those low-voltage power supplies that you can locate. Find somewhere to check their voltages. If you're lucky, it will be correct when the amp's working and zero (or very low) when it stops working. If it's just a power supply problem, that would confirm it. You can then check the rectifiers, capacitors, physical connections, and regulators in the power supplies. I'd expect the newer, larger amp you have uses different components for basically all of those functions, but a similar overall design.