Perplexing Plexi Problem...Solved...It Lives!

[Blindog]

This thing was pissing me off, so I shelved it to build my Dumble Bluesmaster HRM clone. After finishing that amp, I went through this thing and cleaned up several weak joints on some of the flying leads from the board, and generally tidied things up. It was one of my first projects, so I did a lot of soldering/unsoldering trying new things out.

I also built a listening amp to try and help isolate where I'm getting the distortion. Everything sounds great until the PI. The signal after the entrance to the PI all the way through the rest of the amp is super distorted, fuzzy.

So before I completely rebuild the PI section, my question is what do you all think may be going here?

Many thanks for all the input so far!!

Mark

[martin manning]

Get voltages all around the PI. Supply, plates, and cathodes. Don't measure at the grids, but get the voltage at the bottom end of the bias resistor.

[Blindog]

Get voltages all around the PI. Supply, plates, and cathodes. Don't measure at the grids, but get the voltage at the bottom end of the bias resistor.

Hey Martin, thanks for the input, from a fellow Buckeye (transplanted now for me) and your last name's not so bad either Here's what I got:

PI supply 319V

1. 215V
2. 33V (from end of 1M)
3. 34V
6. 215V
7. 33V (from end of 1M)
8. 34V

Thanks again man!

Mark Manning

[martin manning]

Those voltages look entirely reasonable cuz, so I wouldn't tear up the PI just yet.

You are listening to the signal and find it going distorted and fuzzy at some point. The problem must be a bad connection or component right there. See if you can isolate the exact point in the signal path where that happens.

[Reeltarded]

I have an idea for a TV show called Too Many Mannings. It's like Gilligan's Island but no island.

[Blindog]

Those voltages look entirely reasonable cuz, so I wouldn't tear up the PI just yet.

You are listening to the signal and find it going distorted and fuzzy at some point. The problem must be a bad connection or component right there. See if you can isolate the exact point in the signal path where that happens.

You know, I've suspected the problem was in the PI somewhere for a while. I've probed all around that area but can't seem to isolate. I've suspected 2 original signal caps left in this thing, both .1uf, one in the PI and the other on the presence pot. I left them in 'cuz they were old mustard looking Holland's...I know I clipped a good cap across the one in the PI one time, but I'll take it out and try again. Wondering if I should try disconnecting the negative feedback as well...

[Blindog]

I have an idea for a TV show called Too Many Mannings. It's like Gilligan's Island but no island.

Miles, you gotta be the funniest dude in this place...Ima hafta make the 4 hour drive to Savannah some time and buy you a beer...or a Coke...or a Crown & Coke?

[Blindog]

Dicked around with this thing all day, rebuilt the PI, disconnected NFB, swapped out the OT *again* to be sure I wasn't crazy the first time I did...no change. Pretty much gave up. So I started searching for posts on conductive boards, considering this thing was built in '68, and it's a fiberboard. I came across this post:

https://tubeamparchive.com/viewtopic.ph ... d+symptoms

What he describes is pretty much the same situation I'm having..amp plays fine for over a year, suddenly craps out, no real solution. He fixed his problem by shortening the grid wires from the 220ks to the power tubes, swapping positions on the board with the plate resistors to the PI. Guess it's worth a try, I've just about given up!!

Mark

[martin manning]

Above I said that the problem has to be a bad joint or component, but it could indeed be a parasitic oscillation in that area, and most often the problem is lead dress. Maybe post a picture or two of that area. Did you say you have a 'scope? Maybe it's time to learn to use it?

[Blindog]

Above I said that the problem has to be a bad joint or component, but it could indeed be a parasitic oscillation in that area, and most often the problem is lead dress. Maybe post a picture or two of that area. Did you say you have a 'scope? Maybe it's time to learn to use it?

Yep, guilty as charged The thing is, I get the basics of troubleshooting with a scope...feed a signal and view the waveform, in this case for clipping, right? I guess I'm not sure where I need to probe specifically. I need to spend time with my scope, for sure, no time like the present...all I have is a function generator app on my Galaxy Note...

Mark

[martin manning]

Yes, connect a dummy load to the speaker output and feed a small signal into the input, maybe 200mV. Set the controls for a clean tone and look at the signal through the amp, say at the output side of each coupling cap. Look for something other than a reasonably clean sine wave, clipped or otherwise distorted, maybe with some fuzz or a discernible oscillation riding on it.

[Blindog]

So after trying just about everything, I decided that the problem HAD to be a conductive board... after all, this thing is pushing 50 years old. I had Ken Watts from Watts Tube Audio make me a new board, and I rebuilt the entire circuit. I also reworked the layout to accommodate the 33/33 filter cap on the board better than I had previously. Fired up, did a quick voltage check down the B+ string, set the bias and plugged her in. She's back! I'm freakin' beside myself man!...like I said, this was my favorite build and my go to amp. Now to pull the junk test preamp tubes out and have some fun tomorrow...too damn late to jam in the hood! I'm running KT88's, and this baby is LOUD!!

Big thanks for everyone's help and input!!

Mark Manning

*Added pics with PPIMV