Not Seeing High Voltage in Preamp Tube Pins

[Matt-O]

Hello there -

I'm new the forum, and I'm a relative noob at amp wiring (although I've done a lot of simple amp mods and a lot of pickup harness wiring). Recently, I decided to get into more advanced amp wiring by overhauling my Fender Pro Jr. with Hoffman's turret board kit. I completed the wiring a few days ago, and I've double-checked (more like quadruple-checked!) all of the connections, resistor values, etc. using the method described elsewhere on this site (i.e., using a highlighter to mark off connections after verifying that they're correct). I've also gone through all of the links on this page https://el34world.com/Forum/index.php?topic=19361.msg199897#msg199897 in my effort to troubleshoot. I'm ready, willing, and able to review any additional information, but I thought I'd let everyone know that I have done some due diligence prior to posting. ;)

The issue is this: With the tubes NOT installed, I'm not seeing high voltage on pins 1 or 6 of my 12ax7 (Tung-Sol reissue) preamp tubes. I am, however, seeing high voltage to pins 3 and 7 of the EL84 power tubes (also Tung-Sol). Does this scenario suggest any obvious issue that someone new to amp circuits might miss?

[sluckey]

I'm not seeing high voltage on pins 1 or 6 of my 12ax7 (Tung-Sol reissue) preamp tubes.

Do you have a 22K resistor installed between points Y and Z? Is point Z accidentally grounded? Check for resistance between point Z and chassis. Must not be low resistance.

I am, however, seeing high voltage to pins 3 and 7 of the EL84 power tubes

Pin 3 is the cathode and must not have high voltage applied. Pins 7 and 9 are the only pins that should have high voltage present.

Pictures would be very helpful.

[Matt-O]

Thanks for the reply, sluckey! I actually meant pins 7/9 on the power tubes - that was pre-coffee:)

I do have a 22k 3-watt resistor between points Y and Z, and I'm seeing 9.6k ohms between Z and chassis. Is that considered low resistance in this context?

[sluckey]

I do have a 22k 3-watt resistor between points Y and Z, and I'm seeing 9.6k ohms between Z and chassis. Is that considered low resistance in this context?

The resistance should be very high, hundreds of KΩ or MΩ. Unsolder the positive lead of that 22µF cap that's attached to the point Z buss. Does resistance go way up?

[Matt-O]

Here are a few photos (I'd be happy to take more):

[Matt-O]

And a few more pics...

[Matt-O]

I do have a 22k 3-watt resistor between points Y and Z, and I'm seeing 9.6k ohms between Z and chassis. Is that considered low resistance in this context?

The resistance should be very high, hundreds of KΩ or MΩ. Unsolder the positive lead of that 22µF cap that's attached to the point Z buss. Does resistance go way up?

Unsoldered the positive lead of the 22uF cap and checked the resistance. It actually went down to 2.3 Ohms.

[sluckey]

I see the problem. See pic...

[Matt-O]

Doh, how did I not see that?! Thank you so much for the assist!

[Matt-O]

Well, I moved the black wire to the proper turret connection. I also replaced that 22k/3w resistor. Looks like that 22k resistor is still getting really hot - the color bands have already darkened like on the previous resistor.

Also, still no high voltage to the preamp pins.

[sluckey]

Repeat everything you just did. I'm betting there is still a ground on that Z buss.

[Matt-O]

Will do.

My ignorance might be showing here, but aren't there multiple pathways from the Z bus to the main ground (see pic)?

[PharmRock]

Is your under-the-board ground wire for the filter caps in the right place?  Sometimes when flipping the board over and doing the under-board wires, its easy to put them on the wrong side.  could the ground wire be on the (+) of the filter caps instead of the (-)?

[Matt-O]

Is your under-the-board ground wire for the filter caps in the right place?  Sometimes when flipping the board over and doing the under-board wires, its easy to put them on the wrong side.  could the ground wire be on the (+) of the filter caps instead of the (-)?

This is possible. I did check the under-the-board wires before I installed the board, but it's possible I wired it incorrectly. I'm going to check that tonight when I get home from work.

[sluckey]

My ignorance might be showing here, but aren't there multiple pathways from the Z bus to the main ground (see pic)?

Apparently there are in you amp! But there should be ZERO paths to ground in a properly wired amp. I already told you what resistance you should measure. Keep looking. There's at least one more duh huh in there.

[Matt-O]

Ok, checked the under-board connections, and they're all correct. I verified this by testing continuity between the corresponding turret points on top of the board.

One thing I noticed when I was doing some continuity checks is that I'm getting continuity between the 22uF cap that connects to the Z buss; I don't think I should be. Think it might be a bad cap, which would explain how the Z buss is connected to ground.

[sluckey]

Well, unsolder the positive lead and see if your resistance between Z and chassis goes way high.

[Matt-O]

I believe I've figured it out! I think the Z buss wires were in contact with a screw that holds the turret board to the chassis. I had covered the screw with two layers of electrical tape, but the buss wire seems to have been in contact with it somehow. I moved the buss wire up toward the top of the turrets on either side of the mounting screw and voila - high resistance at Z, no more ground on the Z buss, and I'm seeing high voltage at pins 1/6 of my preamp tubes.

[sluckey]

Haha. I had to blow that pic up really big to be able to see that tape. Good job. A nylon screw might be a good idea.

[Matt-O]

Haha. I had to blow that pic up really big to be able to see that tape. Good job. A nylon screw might be a good idea.

A nylon screw would have been a great idea. If it becomes a problem down the road, I'll switch it out.

Thanks so much for your help! I feel like I know a little bit more about this circuit and how to troubleshoot.