Unhappy 12AX7 circuit and negative voltage on the cathode

[Toxophilite]

Hello all and Happy New Year!
Let's hope for less war in the world this year!



So I was wiring up another Akai unit and I made two stupid mistakes , working too fast.


- when wiring the 12ax7 circuit I connected the output to the plus side of the 25mfd electrolytic cap instead of the negative side where it should be


- also (yay!) I neglected to attach the 8 ohm load resistor to the speaker output


I was testng in very brief on/off spurts (looking for smoke etc.)
I actually had the e circuit working but it had piles of gain so for some reason I thought I'd try moving the output to the other side of the cap...a mental disconnect!


Anywho the volume pot now starts a high pitched squeal as it was turned up, there is sound but only at the top of the range and very distorted, the VU needle doesn't move or is pegged.


I moved the connection back to where it should be and wired up[ the speaker output properly
the volume pot is very noisy like it has some DC on it


The cathode measures negative -1.2 V. The plate voltage seems to go up and down with the volume and is a 2 volts with the pot turned down.
The first section of the same 12ax7 measures normal


I'm a little stumped though. I have been remelting solder joints, trying different 12ax7s


I'm worried I hooped the OT or something else big


Here is the schematic with the cap in question circled in red

[pdf64]

Maybe the global feedback loop has been inadvertently wired up positive rather than negative?
See if breaking the loop resolves the DC issues, ie break the feed from R21//C18 that returns to the 2nd stage cathode.

[shooter]

The cathode measures negative -1.2 V.

are you using the circuit ground or chassis ground when measuring?
double check your meter against small volts, just to make sure it can read  1 volt.
measure the same point, only measure VAC instead of VDC.

[WimWalther]

Since there isn't a negative voltage supply, it would be extremely unlikely that you'd see a negative voltage anywhere in this circuit. I'd agree that you are probably using the wrong point as your ground reference.

Put the black meter probe on the center tap of the PT secondary or negative terminal of the first filter cap coming off of the rectifier (C26), use that as the common / ground point, and take another measurement.

The squeal is an oscillation, if it hasn't already been made clear.

[Toxophilite]

Thanks! Advice taken. Hooked up negative to main filter cap etc. star ground and now read 37V at the cathode. This voltage goes down as I turn up the 500k volume pot as the plate voltage goes up.

[WimWalther]

Thanks! Advice taken. Hooked up negative to main filter cap etc. star ground and now read 37V at the cathode. This voltage goes down as I turn up the 500k volume pot as the plate voltage goes up.

That's not right. Typical cathode voltage on a 12ax7 section is around 2V or less.

Just a quick sanity check - we're discussing V2B, the right-hand section of the 12ax7 yes?

If it's changing with operation of the vol pot VR1, it sounds like C9 is shorted. Do you read any DC on the pot VR1? Should be none..

Check into that for sure, something is *way* off.

[Toxophilite]

Yeah, it had/has me worried. By accident I found a kind of solution though whether it's indicative of another problem I'm not sure.


The grounds for the preamp are hooked to one side of the chassis and the the grounds for the power supply and power amp are hooked to the other. I was trying a ground on the power amp side to get the high voltage reading and I thought I'd try a ground on the preamp side simultaneously and the voltages normalized. 1.2V  on the cathode and 128V on the plate and the unit seems to be working as it should. Connecting the two sides together with a clip of course accomplishes the same thing.

[Toxophilite]

This is perhaps best explained by a picture. The two sides of the amp are screwed to the faceplate, normally running across the board and joining the two sides is a thin plate to shield the tube sockets . I have it  rotated up in this picture but it was completely removed for the conversion and the face plate screws were loose. I put the plate on and tightened up the screws and remove dthe ground clip and no problem. Would it be bad to connect the two sides with a line on the back of the board as well so one could remove the plate to troubleshoot what have you?


(the old M68 in the background is being used for the XLR ins for testing as they would normally be attached the the back plate of the bock this will inhabit with it's right side.)

[WimWalther]

Briefly, it sounds as if one of your chassis ground points has gone high-resistance. Inspect them both carefully for cold / cracked / loose joints.

Measure (low ohms range) from the metal chassis itself to a point on the ground wire where it joins the circuit. Needs to be near to zero as measurable.