Cause of high plate voltage on 12ax7

[jordan86]

As I continue to tinker with an old amp, I am checking more things. Recently discovered the plate on my second gain stage measures way high.

Stage 1 (pin 6): 240V
Plate load resistor measures 101K

Stage 2 (pin 1): 320V
Plate load resistor measures 108K

They share the same B+ source. Is there any other culprit besides that plate load resistor I should check for? Already tried different 12ax7's to make sure it wasn't the tube. Got about the same values within ~1%.

Schematic attached.

Bonus question. All my voltages are a little higher than this schem. I noticed this schem uses the 115v primary side of the Hammond PT instead of the 125v, which mine was wired with. Seems weird. Would switching down to the 115v primary wire increase the voltage on the secondary side? Assuming wall voltages are the same.

EDIT: Post corrected for wrong pin numbers.

[sluckey]

First off, pins 2 and 7 are grids, not plates. The value of the cathode resistor has a big impact on the plate voltage.

Yes, switching down to the 115v primary wire will increase the voltage on the secondary side

[jordan86]

Thanks, Sluckey! UGH. Yes, Pins 6 and 1. Just when you think you can start doing things from memory, you get humbled. Post fixed. My numbers were wrong but my measurements and pins were correct and accurate.

Is changing the plate load resistor the first/only solution?

[2deaf]

Low plate current also results in high plate voltage.

[jordan86]

Low plate current also results in high plate voltage.

Ahhhh! Would a higher value cathode R cause that. I turned that into a cold clipper stage ala JCM800. Currently have a 15k installed.

[sluckey]

Changing the plate load resistor is about the last thing to do. To understand what's going on you need to measure the voltage on the plate, grid and cathode. While you're at it, measure the resistance of the cathode to ground and also grid to ground (One probe on the tube socket pin, the other probe on chassis). Knowing these other measurements will help to pinpoint your problem.

But, based on what you have told us so far, I suspect the reason the plate voltage is so high is because the tube current is really low. The most likely cause for this is the cathode resistor has significantly increased in value or has a bad connection to ground. I bet pin 3 reads much more than 1.2K to ground.

Ahhhh! Would a higher value cathode R cause that. I turned that into a cold clipper stage ala JCM800. Currently have a 15k installed.

Telling us this kind of shit would save a lot of typing!   

[jordan86]

Will do. Sorry, Sluckey! Didn't realize cathode R affected plate voltage as well. That makes complete sense now though. I know just enough to be dangerous. :)

Will measure plate, grid, and cathode.

[sluckey]

No biggie. You did notice the wink, right?   

[jordan86]

I did 

[tubeswell]

As I continue to tinker with an old amp, I am checking more things. Recently discovered the plate on my second gain stage measures way high.

Stage 1 (pin 6): 240V
Plate load resistor measures 101K

Stage 2 (pin 1): 320V
Plate load resistor measures 108K

They share the same B+ source. ...

Pray tell - what is the voltage at the B+ source?


If you know that, you can work out the tube (plate and/or cathode) current. (B+ minus plate voltage) divided by plate resistor value.


As you've found out, bias affects tube current (and hence plate voltage).

[jordan86]

Pray tell - what is the voltage at the B+ source

If you know that, you can work out the tube (plate and/or cathode) current. (B+ minus plate voltage) divided by plate resistor value.

B+: 372v
Plate Resistor: 108K (measured)
Pin 1: 319v
Pin 2: measured 0v to ground dc and ac
Pin 3: 3.78v
Cathode resistance measures 15K to ground. Same as R, which is not bypassed.

So: 372 - 319 / 108,000?

.00049

Is that .049ma?

[tubeswell]

Pray tell - what is the voltage at the B+ source

If you know that, you can work out the tube (plate and/or cathode) current. (B+ minus plate voltage) divided by plate resistor value.

B+: 372v
Plate Resistor: 108K (measured)
Pin 1: 319v
Pin 2: measured 0v to ground dc and ac
Pin 3: 3.78v
Cathode resistance measures 15K to ground. Same as R, which is not bypassed.

So: 372 - 319 / 108,000?

.00049

Is that .049ma?

yep. Hardly any current at all. Which is why the plate voltage is high.

[shooter]

so 3.78 / 15000 isn't good math?

[tubeswell]

so 3.78 / 15000 isn't good math?

Yes it seems odd doesn't it. As if 1/2 the plate current is being siphoned off somewhere. (Mind you, its way down in the cold region of grid curves. Trying to see that on a chart, the bias point is on the x-axis, so hard to predict bias voltage in that region. Looks like a rectifier diode)

[jordan86]

The original schematic is somewhat close to a JCM800 pre, so I was attempting to get back to that for a reference. I didn't have a 10K resistor though. Would it be worth paralleling a 27K in on the cathode and seeing how things measure at 9.5K cathode R? It's not MUCH warmer, but a bit.

[2deaf]

so 3.78 / 15000 isn't good math?

Yes it seems odd doesn't it. As if 1/2 the plate current is being siphoned off somewhere.

It is being siphoned off through his volt meter.  In addition to that, the plate voltage with the meter connected is different than without it connected so that the cathode voltage isn't 3.78V when the meter is connected to the plate.

[jordan86]

Would it change anything (be more accurate) to measure across pins 1 and 3?

[2deaf]

The original schematic is somewhat close to a JCM800 pre, so I was attempting to get back to that for a reference. I didn't have a 10K resistor though. Would it be worth paralleling a 27K in on the cathode and seeing how things measure at 9.5K cathode R? It's not MUCH warmer, but a bit.

Lots of things change when you change the cold-clipper cathode resistor.  It just depends on what you are trying to achieve.  39K is another popular cold clipper.

 

[2deaf]

Would it change anything (be more accurate) to measure across pins 1 and 3?

Measure the voltage across the 108K plate resistor then subtract that from the B+ voltage.  Still has an error, but a much smaller one.

[jordan86]

Measure the voltage across the 108K plate resistor then subtract that from the B+ voltage.  Still has an error, but a much smaller one.

Brilliant, 2deaf!  I only measured -24v across that stage 2 100k plate resistor (measures 108K). The first stage plate resistor (also 100K) measures -98v across it.  They are fed from the same B+ tap and turret. I take that as a failed plate resistor?

Which explains why the voltage from Pins 1 to 3 still measures 328v.

[sluckey]

So: 372 - 319 / 108,000?

.00049

Is that .049ma?

No.     .00049 is .49mA

[2deaf]

I only measured -24v across that stage 2 100k plate resistor (measures 108K). The first stage plate resistor (also 100K) measures -98v across it.  They are fed from the same B+ tap and turret. I take that as a failed plate resistor?

Plate resistor is fine.  The voltages are fine.  Nothing is wrong.

Attached is a loadline like the one that tubeswell posted except with the updated data you posted.  You show 348V on the plate and the calculator shows 345V.  You show 3.78V on the cathode and the calculator shows 3.8V.  These are really close.

[tubeswell]

So: 372 - 319 / 108,000?

.00049

Is that .049ma?

No.     .00049 is .49mA

.Ooops My bad


Take 3 

[jordan86]

Thanks for all the help here. I tried some different K R values and couldn’t get the plate voltage below 300. So I abanded the cold clipper jcm800 approach.

I put that second stage back to 1.2k and paralleled the first two stages instead of putting them in series.

More super lead spec now. It’s sounding great. Going to change the second stage power supply resistor from 1k to 10k for a little less voltage. But very happy with where it’s at.

Thanks again.