Help with low DC voltage on 12ax7 preamp tube

[smackoj]

ola amigos; thanks in advance for your suggestions/opinions.  I have a retrofitted Bogen amp that now is a Proluxe build. It has been working fine and then it started shocking me last night at worship team practice. I had almost no volume from it also. I popped the lid this morning and visual inspect did not turn up any glaring booboos. I did resolder the fuse holder which looked a little questionable. I warmed it up with my multi meter handy. No shocks and the volume seemed back to normal. I decided to check B+ voltages. Power tubes fine at 345vdc. It uses one preamp tube at input with 1/2 used for channel one and the other half for channel 2.  Channel 2 looking good with 250vdc but Ch. 1 no good at 59vdc.  I tried re-doing the solder connections and changing out the bypass cap and cathode R but nothing changes. Checked the voltage divider and see both sides using a 100K R.

Is it possible for Ch. 1 side of the preamp tube to be bad?  I ran out of time testing so thought I would post here while I go to work.

gracias amigos.   

[jjasilli]

Is it possible for Ch. 1 side of the preamp tube to be bad?    Yes, always suspect tubes first.


However, because you got shocked, there may be a partial short to the chassis, leaving some B+ on the Ch1 preamp tube plate.  If so, the chassis could be live and dangerous.  You should check for that.


Is there a "death cap" at the power chord?

[sluckey]

If so, the chassis could be live and dangerous.  You should check for that.

+1

Anytime you get a shock just from touching strings or amp chassis there will be a faulty ground involved. This may mean you don't have a three prong power cord properly connected to chassis, or the house wiring has a faulty ground.

The death cap is a non-issue if you have a proper ground system in place inside your amp AND in the building.

[supro66]

here is a good test I saw it on  youtube and it works
I had an old amp with only a two prong plug
check it no light reverse the plug it lights up
put the tester on the guitar strings


it detects AC voltage on a circuit used to check live wires
like on the outside jacket of electrical cable


You can see haw it works here it works on everything
https://www.youtube.com/watch?v=306neu3iDyM

[smackoj]

thanks amigos; I am not certain what a death cap is?  I am not 100 percent pos. that the 3 wire power cable is in proper but I will double check that. I believe it's in there like it should be.

So if something is amiss on Ch. 1 side of preamp tube the voltage can be down 200 volts? 

Test I have done; I attached the ground side of the multi meter to the chassis and then touched the pos. lead to diff. parts of the chassis and checked for dc and ac. I did not find either on the chassis now.

thanks, jack

[HotBluePlates]

thanks amigos; I am not certain what a death cap is?

It's the cap from the Ground Switch (if present) to ground. Used to reduce hum on a 2-wire system. On a proper 3-wire system, it should be absent, or disconnected if the amp was refit with the 3-wire cord.
 

I am not 100 percent pos. that the 3 wire power cable is in proper but I will double check that. I believe it's in there like it should be.

I had a Princeton Reverb with a properly-fitted 3-wire refit, yet was still shocked when touching the guitar strings with my left hand and stepping up to a mic. The amp was plugged into a 3-wire outlet in one part of the venue, while the mic was connected to a PA plugged into a 3-wire outlet on the other side of the venue. Apparently, there was not a good 0v connection among the neutral/ground for outlets throughout the building.
 
Point being, your amp can be perfectly fine and you can still get a shock if the house wiring is faulty.
 

So if something is amiss on Ch. 1 side of preamp tube the voltage can be down 200 volts?

Yes. Something is causing that triode to pull excessive current, which flows through the plate load, creates bigger-than-normal voltage drop, and results in lower-than-normal plate voltage.
 
Check (by unsoldering) the cathode bypass cap first. Then measure volts across the cathode resistor (if not shared with the Channel 2 triode working properly). Look for solder blobs on the top socket pins and/or mounting points of the components for that stage.
 
I wouldn't suspect the grid for that triode to be the issue, because a dead-short to ground at the grid should just kill the signal but not change tube bias/plate current. A dead-short to ground at the plate should cause 0v at the plate, unless the short is not to ground but some other voltage level. So a too-low cathode resistance (maybe due to a shorted bypass cap or shorted/partially-shorted cathode resistor) is the prime suspect.

[smackoj]

double checked all the suggested points. could not get the voltage corrected with changes to the cathode resistor but I did replace the bypass cap and then, after all other attempts without change, I replaced the preamp tube. turned it on and it's working good, plenty of volume. checked the plate voltage again and found 133vdc on Ch. 1 and 127vdc on Ch. 2.  I believe that is low by approx. 100 volts but I'll await your input as to whether I should make changes? I want to play it on the 'clean' side not the dirty, raspy side. 

thanks   

[shooter]

Don't have schematic with volts, so what is the node volts for that tube, what size is your plate R's for each, and cathode R's. those things should be a good indicator if it's "right".  Do the math using the cathode R volts drop and that should also help, like if it's 1mA vs 3mA

[HotBluePlates]

... could not get the voltage corrected with changes to the cathode resistor ... I replaced the preamp tube. turned it on and ... checked the plate voltage again and found 133vdc on Ch. 1 and 127vdc on Ch. 2.  I believe that is low by approx. 100 volts ...

You went from 250v+ on one plate and 50v on the other, to roughly-same voltage on both plates. What makes you think this new voltage level is not correct?

What is the voltage of the filter cap supplying this tube?

[smackoj]

what I was trying say is that 133vdc is low compared to say 250vdc that some amplifiers run on the plates. I changed the resistor feeding the filter cap for this tube and replaced the 5Y3 recto with a 5U4 and now I have 173v and 181v on the preamp tube's plates.  It is sounding nice and strong with good tone so I'm satisfied with the outcome.

thanks gents   

[HotBluePlates]

what I was trying say is that 133vdc is low compared to say 250vdc that some amplifiers run on the plates.

Please be careful to say all you're thinking! After all, the thread was about low plate voltage due to some unknown fault, so...

[sluckey]

I wonder what happened with the shocking issue? Did you find a cause? Is it resolved?