Help! No Sound! Valve 3 circuit problem!

  • Thread starter Thread starter Me
  • Start date Start date
Hello again. I'm sat here now with a function generator, multimeter, oscilloscope and what-not. I've re-measured now I know what the pins do, and measured relative to the valve cathodes using a DC multimeter. Should make more sense now...

All preamp heaters are at 6.84V roughly.

V1
Half A:
Anode to cathode: 178.2V
Grid to cathode: -1.381V
Half B:
Anode to cathode: 178.5V
Grid to cathode: -1.239V

V2:
Didn't bother as it doesn't work on any channel and this is for Ch3 only right?

V3
Half A:
Anode to cathode: 399V
Grid to cathode: -5.93V
Half B:
Anode to cathode: 10.56V
Grid to cathode: +0.9V

V4
Half A:
Anode to cathode: 222V
Grid to cathode: -1.625V
Half B:
Anode to cathode: 193V
Grid to cathode: -1.253V

V5
Half A:
Anode to cathode: 205.4V
Grid to cathode: -1.183V
Half B:
Anode to cathode: 199.3V
Grid to cathode: -1.146V

V6
Half A:
Anode to cathode: 168.5V
Grid to cathode: -2.471V
Half B:
Anode to cathode: 157.6V
Grid to cathode: -2.435V

Is the grid in the 2nd half of V3 supposed to be +ve relative to the cathode??? :confused:

I'll try to follow the signal next with an oscilloscope but it's tricky to get a cathode ground reference using the little croc clip on the oscilloscope. If you don't hear from me again I'm probably dead! :lol: :LOL:
 
I couldn't follow the 1kHz signal I put into it without the circuit diagram, and whilst only able to see 1 side of the board. Tried a different tack. Stuck a different ECC83 in V3. Anode voltage is now only 5V. I checked the other side of R32 (the anode resistor) and it's 400+ volts. I assume then than something's funny that's making the 2nd half of V3's grid +ve and pulling the voltage wrong, or the cathode bit has a problem? I'm kinda stuck. What now? Peter? Help! I don't know anything about tube amps apart from what I worked out tonight!!! :cry:
 
Just realised I was changing V4 last night, not V3 as it was upside-down, back to front, and I was very tired. I'll try again with another V3 and let you know...
 
It's now working with a different V3 in. I'm gonna leave it plugged in running into my Palmer load box the rest of the day with a 1kHz tone going through it and see what happens...

I had changed V3 before, from the original ruby, and retubed all the preamp with new tubes, except switched V2 - a Harma STR Cryo (which couldn't have been the problem because it was on all channels), and moved it into the V3 position, which then worked for a few hours, until it stopped again. So it still doesn't make logical sense unless the amp's destroying the V3s, but I'll see what happens after running it all day. :confused:
 
Had a thought. Is the grid bias voltage come from the same place for V2 as the second half of V3, but separate from the other tubes, and separate from the first half of V3? If so this ECC83 could have been causing a problem on the V2, which also buggered the bias voltage for V3. This would cause all channels to fail because of V2, even when it's not on channel 3. Then when I switched it moved the problem to the V3 position.

Still seems odd moving it made it work for about 3 hours though before it went duff again, and then continued to have the same problem from cold again though. Could do with a circuit diagram really! :confused:

Maybe I should have bought one of those s#!t-sounding solid-state Randalls! :lol: :LOL:
 
Here's a tip. Stop over thinking !!!

Get some flux, solder and reflow all the solder joints for the preamp tubes on the PCB.
It's most likely a cold joint and why you cannot get it to behave erratic enough to figure it out.

Occam's Razor. :)
 
Mark2C, they've all been throughly wiggled, poked, and prodded, at all temperatures. I don't think that's it.

Anyway, this amps been sat behind me all day and not a single thing has happened with it, it's been running fine continuously. I think I've just had INCREDIBLY bad luck with 2 tubes. I'm gonna reflow the joints as suggested, and then put it all back together with all new preamp tubes (I'm not putting any pre-used favourites in ANY position this time). Then I'm gonna play my guitar as much as I possibly can, which was my new plan before this happened anyway, and see if it happens again. I'm hoping not. :thumbsup:
 
Thanks Peter. I just can't make it go wrong againafter running it all day again, and since it can't have been a dry joint on a tube socket or less current would have flowed, not extra cathode to grid current, I can only think it's been just VERY bad luck with tubes again.

Played it again a bit tonight and it's all fine still. I think I must be the unluckiest person I know! :doh: Last thing I want is to send my amp to germany wasting your and my time and money again, only for you to find nothing wrong with it, and just retubing different parts. Since last time when I had power tube problems and you couldn't find anything wrong I ended up thinking it was just bad luck with tubes. Can't believe I had such bad luck again, especially since it's always in a foam padded flight case, I don't get to play it as much as I'd like to, and I'm particularly careful with it when it's hot. :aww:
 
BTW, getting back to a good few thousand posts ago in this thread, ;) the 12AT7/ECC81 as phase inverter seems to make it more controllable at quiet volumes at home. Only tried it tonight though so I'm waiting to wind it up a bit and see how it barks at higher volumes. :thumbsup:
 
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