Vox - Night Train - Nice distortion tone - Circuit discussion

[LooseChange]

Hi Guys!
I have a Vox Night Train on the Bench and it's has some troubles... When played the guitar sound is splatty and just not right.... More importantly, it appears I may have found the problem but would like some opinions before I start ripping the amp apart. (PC board and very tight inside).

V2 seems to be the problem. Voltages for Pins 6 (73v), Pin 7 (5.8v) and Pin 8 (5.8) are not right.
All tubes are good.
All the resistors around this tube (Plates and cathode... R11, R25, R28 and R27) all read within range.

Any advice or need any additional info?
(Schematic attached)
Thanks for your help!

[birt]

V2 is a powertube. are you sure you meant that tube?

if it's V4A i would replace that tube. cathode and grid have the same voltage.. so that looks like a short. internal? or on the PCB?

[sluckey]

Lift C22 and see if the voltages look normal.

[LooseChange]

OOPS!!!
V4A

[LooseChange]

I paralleled R11 with a 100k and the voltage went up to 125v. Does that provide any clues?

Sluckey... This amp is such a pain just to lift components without lifting the board and removing all sorts of stuff... How about if I ground Pin 7... Will that give any clues.

[LooseChange]

One more clue... When I power up the amp the voltge on the plate (V4 - pin 6) starts high and drops very slowly (a couple of minutes). Seems to drop much slower than the actual time it takes for the tube to heat up.

[birt]

you can take out the preamp tubes and see if there is a DC voltage at the pin7 of the socket. then you know C22 leaks. you can also check if the plate voltage is right.

[sluckey]

Grounding pin 7 should put the tube voltages closer to what they should be, but it won't necessarily tell you if C22 is leaking. However, if the tube voltages still remain the same, you'll need to look elsewhere. C22 is the #1 suspect on my list. I wouldn't try anything else until it's eliminated. If it's an axial lead cap, just snip one lead in such a way that you can repair the broken lead without pulling the board. If you can't get to the leads, I'd be tempted to crush the cap body, leaving the lead stubs in place so you could temporarily tack solder another cap in place. If that is the problem, I'd then set about the unpleasant task of replacing it correctly.

[LooseChange]

C22 was the problem.  Replaced it and all is good.
This is a great sounding amp. The full setting gets some really freat distortion sounds.

Thanks for the help!!!

[LooseChange]

Take a look at the schematic in the first post.
The Thick setting sounds really good. Great distortion sound.
- Bypass the tone control with a 4700pf
- Lift the tone stack
- Remove that 47p bright cap on the volume pot (I would remove it altogether)
- Change the first gain stage (V3a) cathode resistor (made a bit smaller)
- Add gain to V4A by reducing that cathode resistor too.

All make sense... What do you think?

[birt]

this is all stuff i have tried in marshall amps. and some of this is used in mesa's too (the use optocouplers though). all guitar amps are basicly the same :p

[HotBluePlates]

Maybe I'm looking at it cross-eyed, but there appears to be some positive feedback going on between V4A and V3A, through the connection between cathodes through resistors. It also appears that the input signal is applied to both of these stages unless there is a high-gain/low-gain switch that selects which tube the signal is routed to. If there is, it doesn't appear to be indicated on the schematic.

Anyway, in the bright setting, the tone circuit rides on the "positive feedback" which in this case would cause negative feedback because the tone stack is fed by the V3B plate signal, which of course inverts the signal, and makes it negative feedback. That gives a pretty advanced implementation of a high-gain/low-gain setting.