Suggestions on further preamp troubleshooting

[bradicusmaximus]

I have an amp on the bench right now that has been giving me more grief than I think I've ever had and I've been staring at it so long, I think I'm seeing weird visions in the wiring.

It an old Garnet amp that all point to point. Looking at it, it reminds me a lot of an old hammond organ amp. It's actually a very cool amp, but it's become the bane of my existence. Owner brought it to me for some problems in the power stage - no biggie, bad cap, and got it replaced. Amp worked at that point but was noisy as all get out. So after some discussions, we decided to replace the caps in the preamp stage. I normally don't do that as a rule, but these were all ceramic discs and some old small value electrolytics. Got the caps replaced and nothing. No signal is passing from the input jack to the output.

Put on the troubleshooting hat...

I've retraced my steps. Verified the schematic. Compared it against intake pictures to ensure everything was where it was supposed to be. Nothing, nada, no go. So out comes the scope and all the other test equipment cause things are getting serious.

Here's the interesting part. When I move the controls, I can hear the crackles and the frequency sweeps. I know the tone stack is working, but it's not getting signal that's usable. If I put in an audio feed (basically so I can hear something that's not a sine wave) I can hear it very, very, very low when the volume is all the way down. In the interest of disclosure, the volume control is hosed and is slated for replacement as well, but I need to get this worked out first.

So, I fire up a 1khz sine wave and put it into the system. I get a clean wave on the scope at the jack, and at pin 7 of the 12ax7. Pin 6 is dead quiet, which is where I would expect to pick it back up again on the output. I've got good heater voltage and have tried with a couple different tubes. The socket looks in good condition as well.

At this point I'm stumped and have given myself quite a headache.

Anyone have any suggestions of places to look. At this point, I'm afraid this could easily fall into a situation where things get changed out of desperation and I'd much rather keep this surgical and intelligent.

Thanks

[xtian]

So, I fire up a 1khz sine wave and put it into the system. I get a clean wave on the scope at the jack, and at pin 7 of the 12ax7. Pin 6 is dead quiet, which is where I would expect to pick it back up again on the output.

Please post the schematic.

Is 6,7,8 the first triode? So you see your sine wave on the grid, pin 7, but not on the plate, pin 6? You're right, that should be easy to solve. What are the voltages at 6 and 8?

[bradicusmaximus]

So, I fire up a 1khz sine wave and put it into the system. I get a clean wave on the scope at the jack, and at pin 7 of the 12ax7. Pin 6 is dead quiet, which is where I would expect to pick it back up again on the output.

Please post the schematic.

Is 6,7,8 the first triode? So you see your sine wave on the grid, pin 7, but not on the plate, pin 6? You're right, that should be easy to solve. What are the voltages at 6 and 8?

Here's the basic schematic. There are some minor variations I've discovered, but the difference are relatively inconsequential.

The voltages I have around the 12ax7 are:

Pin 1 - 297
Pin 2 - 0
Pin 3 - 285
Pin 4 - heater
Pin 5 - heater
Pin 6 - 308
Pin 7 - 0.2 (input signal)
Pin 8 - 287
Pin 9 - Heater

As I say, I can see the sine wave right to pin 7 and then it's gone. The other half of the tube is being used for the reverb circuit.

[xtian]

The voltage on your cathodes, pins 3 and 8, should be around 1.5 volts. NOT hundreds of volts!

You replaced the cathode e-caps? With what? Did you reverse the polarity? Can we see some closeup photos?

[bradicusmaximus]

The voltage on your cathodes, pins 3 and 8, should be around 1.5 volts. NOT hundreds of volts!

You replaced the cathode e-caps? With what? Did you reverse the polarity? Can we see some closeup photos?

Everything is the proper polarity - though I will say I'm thinking about swapping two new e-caps in. Here's an annotated picture of the section.

[JazzGuitarGimp]

The cathode resistor is more responsible for keeping the cathode down around 2VDC. I would make sure you haven't accidentally lost the connection to one end of this resistor. It also wouldn't hurt to verify the cathode resistor's value, given the age of the amp.

[bradicusmaximus]

I replaced the two cathode caps and when I fired it up it looked pretty decent. I was getting about a volt on pin 8. However, with in a minute or two, I was getting 250v on both sides of the cap. Considering these are only 25v caps, I'm not thinking this is good. So, now I need to figure out why these caps are getting slammed with so much voltage. My guess is the problem on the first input stage is a symptom and not the problem.

Back to the drawing board.

[bradicusmaximus]

The cathode resistor is more responsible for keeping the cathode down around 2VDC. I would make sure you haven't accidentally lost the connection to one end of this resistor. It also wouldn't hurt to verify the cathode resistor's value, given the age of the amp.

That's a great point. Thanks!

[JazzGuitarGimp]

I replaced the two cathode caps and when I fired it up it looked pretty decent. I was getting about a volt on pin 8. However, with in a minute or two, I was getting 250v on both sides of the cap. Considering these are only 25v caps, I'm not thinking this is good. So, now I need to figure out why these caps are getting slammed with so much voltage. My guess is the problem on the first input stage is a symptom and not the problem.

Back to the drawing board.

250V on both sides of the cap has a net effect of zero volts across the cap, so no worries there. But this is a good indication that you've lost the ground connection at the bottom end of the cathode cap//resistor.

[bradicusmaximus]

I replaced the two cathode caps and when I fired it up it looked pretty decent. I was getting about a volt on pin 8. However, with in a minute or two, I was getting 250v on both sides of the cap. Considering these are only 25v caps, I'm not thinking this is good. So, now I need to figure out why these caps are getting slammed with so much voltage. My guess is the problem on the first input stage is a symptom and not the problem.

Back to the drawing board.

250V on both sides of the cap has a net effect of zero volts across the cap, so no worries there. But this is a good indication that you've lost the ground connection at the bottom end of the cathode cap//resistor.

That makes a lot of sense to me. I've verified all of the resistors in the amp - even the new ones that got changed out and they all read the expected values within tolerance. All of the caps meter out well. I'll trace the ground coming off of that and see what comes.

It would be really nice if it was just a bad ground. (We need an emoticon for crossed fingers)

[JazzGuitarGimp]

Use an ohmeter and measure from the cathode pin on the tube socket to ground. You should see 1.5K. If it's open-circuit, you've got a disconnect somewhere.

[bradicusmaximus]

Use an ohmeter and measure from the cathode pin on the tube socket to ground. You should see 1.5K. If it's open-circuit, you've got a disconnect somewhere.

After looking at everything, I realized that the entire control stack had no tie to ground. For kicks, I tied a jumper from the grounded lug of the volume pot to the ground post on the terminal strip and what do you know, I had sound.

So, there are a couple things I find interesting. First, the amp worked, albeit noisily, after I fixed the power amp section. After I changed out the caps, the amp didn't work. I didn't remove any ground wires and the ground tie ins on all the controls are tight as can be. Second, I don't see anything obviously missing or cut. But, this is all me just thinking out loud.

Unfortunately, my proof of concept has left me with a very nasty hum, which I attribute to a ground loop. So, I guess now I'll try to track down where the original ground was supposed to be and try to quiet this beast down a bit.

Thank you very much for your help. Sometimes, after beating your head against the wall for so long, you forget to check the obvious things and it takes some fresh eyes to put you back on track.

[xtian]

Some builders rely on the pot chassis being grounded, but this is unreliable, as you discovered.