Troubleshooting: High plate voltage on V2B

[dancemyth]

Hi Gang

I need some help troubleshooting my new build. Full disclosure:
1) This is a Ceriatone kit. I know what that means to many of you (although I didn't when I bought it).
2) I'm a pretty new amp builder. (This is my 3rd amp). I read schematics and understand the concepts.

I have what Ceriatone calls an OTS50, which is, from what I understand, a high-plate non-HRM skyliner.

The clean channel works OK. The problem is with the overdrive channel. I get a very very quiet, highly distorted, non-musical sound. V2A checks out OK (I think): Pin 1 = 194v ; Pin 3 = 2.1v.

The problem is V2B. Plate voltage (Pin 6) = 309v. B+ = 355v, so I am getting some current across the load resistor. Pin 8 = zero v.

Here's what I've already tried/checked:
- Switched tubes (all 12AX7s)
- Tested the socket for shorts
- Tried removing the snubber cap, just to simplify the circuit.
- Tested the load resistor and the cathode resistor. Checked the polarity of the cathode bypass cap. Checked continuity and proper resistance from pin 8 to ground.
- Checked for continuity from the grid (Pin 7) all the way back to the coupling cap from V2A. Found proper resistance, including the Level pot.
- Found a very small DC voltage on the grid (pin 7). Just 1.8 mv. I don't know if this means anything, or if it's even real. It's not a great multimeter.

Any ideas/suggestions? Thanks in advance,
Dan

[M Fowler]

As your well aware of, there is a hell of a lot of connections on the board, pots, relay, and back of chassis switching when talking about V2.

Man it could be a host of things. I would suspect bad solder joints poke with chop stick, ground wire issue both involving the tone board and the coaxial cable runs to pots, especially how they were grounded.

Signal tracing probably only way to find the troubled spot.

Mark

[norburybrook]

have you got a good ground from the cathode cap -ve end? pin 8 shouldn't be 0.v that's why the voltage is high.

I'd reflow the joints on the valve pins too.


Sorry,that's all I can offer.


Marcus

[sluckey]

The problem is V2B. Plate voltage (Pin 6) = 309v. B+ = 355v, so I am getting some current across the load resistor. Pin 8 = zero v.

Since pin 8 is zero volts and the cathode cap/resistor are OK, no current is flowing through the tube. If there really is a 44V drop across the load resistor, then there must be a resistive path from the plate to ground.

However, if you have a cheap meter with a 1M input resistance then 309V is exactly what you would get if the load resistor is 150K. You have a simple voltage divider of 150K and 1M (meter) across 355V. Do the math and you will get 309V across the 1M meter. I bet this is the case and there really is no voltage drop across the 150K (until you connect your meter).

I better way to check for a voltage drop across the plate resistor with a cheap meter is to put the meter leads directly across the resistor.

[norburybrook]

Steve,

that's a good point and makes great sense. Is this common with cheap meters?



M

[sluckey]

Steve,

that's a good point and makes great sense. Is this common with cheap meters?



M

Not necessarily a characteristic of all cheap meters, but it's certainly a valid point with ANY meter that has a 1Megohm input. A lot of cheap meters do have a 1M input resistance. I just saw another example of incorrect voltage readings due to a cheap 1M resistance meter over at Hoffman's forum within the last couple weeks. Meter loading is real. Heck, even a 10M input meter can't accurately measure the grid voltage of a LTP PI tube.

So, to anyone using a 1M input meter just be aware that all your plate voltage readings will read lower than they really are.

[dancemyth]

Well my amp doesn't work, but at least I've learned something. I couldn't figure out: where was that plate current going, if not through the tube? Answer: through my meter! Now I know what I'm getting for Christmas (maybe a little early).

Still frustrated. I've tried your (collective) suggestions. Checked connections and grounds. Looked for shorts. Reflowed socket connections. Just can't get any current to go through that tube! I'm putting up a couple of pictures in case anyones can spot anything. It's very crowded around V2, so it may be hard to make out the connections. The extra switch was so I could fool around with alternate snubber cap values without having to mess with the actual cap soldered between plate and cathode. (I've tried disconnecting this sub-circuit altogether. No difference.)

Thanks for your help.

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[sluckey]

Check resistance of pin 7 to ground and pin 8 to ground. I know you have done this but I want you to do it in a specific way. Connect one meter lead to chassis. Now pull the tube and put the other meter lead in the socket pin on the tube side, not the bottom side of the socket. What are the numbers?

[dancemyth]

Thanks! Problem solved!
Didn't make sense to have infinite resistance from grid to ground. (Thought I checked this already.) OD Level pot wasn't grounded. (doh!)
Thanks again. I never thought I'd get this much help this fast.