OT TUBE: the tale of the Fender amp

[aron]

As you guys know I have a 64 blonde bassman. Just recently, it developed the usual "Fender amp" pop when you jar it  syndrome - except it was worse than usual. When I hit low notes it would "pop or make a scratchy sound". I thought it was my sockets so I replaced those - I wanted to do this long ago. Then I thought it was maybe my power tube sockets - they didn't look too good. They didn't look any better after I found out that my ceramic sockets I ordered (Fender type) didn't fit! argghhhhh

Then I figured bad solder joint or more insidiously conductive circuit board.

I tapped inside with the chopstick and yes, I did isolate the part on the board that was making the sound - except there's nothing on the board at that point. I assumed it was a bad solder joint so I redid the solder joints around that area. No go.

The confusing part was that if I tapped sideways from the back of the amp, it still made the sound - so whatever it was - it was easily heard. I noticed one of the screws that held the board was loose - I tightened that... should work... nope.
Then I figured it's the conductive board - I reasoned that I needed something in between the bottom board and the top one - like formica or something to isolate the two boards. That was yesterday.
Today as I tapped away - I noticed that yes, if I tapped that exact same spot, it was clearly heard, BUT if I tapped at another spot far away it was also clearly heard. What?

What runs the entire length of my tapping, but is not the board? Then it hit me. The power supply caps underneath! I opened the pan up and saw that my work still looked OK after all these years. I started looking for a bad solder joint.

Long story short - one of the capacitor leads had come off - bad soldering. Solved (I think!)

[Ripthorn]

Sounds a lot like what I have been running into with my submini tube amp that I am finishing.  I noticed a fair amount of hum and realized I didn't put any smoothing caps on the outputs of my voltage regulators for my relays, heaters, etc.  So I put in a couple caps and reassembled.  Then I noticed a huge amount of hum.  After a couple hours, I realized that where I had my input grounded was right by where those caps were grounded, so the hum was getting dumped from the regulator outputs and right into my input signal.  Grounded the input right near where it enters the chassis and now the thing is one of the quietest high gain amps I have ever heard.

Thanks for posting, it's always fun to hear stories of how people figured things out.

[jacobyjd]

CHOPSTICK'D!

Nice find!

[aron]

OK, it's fixed. The amp is totally silent when I bang on the top of it. I wonder how long it had that bad solder joint? I realized some of the electrolytic caps are over 45 years old. I am going to replace them with high quality electrolytics.

BTW: Thanks R.G. - this is the amp you helped me with long ago.... I built the bulb limiter and you helped me through the process. It's my go to amp after all of these years!

[kurtlives]

Sounds a lot like what I have been running into with my submini tube amp that I am finishing.  I noticed a fair amount of hum and realized I didn't put any smoothing caps on the outputs of my voltage regulators for my relays, heaters, etc.  So I put in a couple caps and reassembled.  Then I noticed a huge amount of hum.  After a couple hours, I realized that where I had my input grounded was right by where those caps were grounded, so the hum was getting dumped from the regulator outputs and right into my input signal.  Grounded the input right near where it enters the chassis and now the thing is one of the quietest high gain amps I have ever heard.

Thanks for posting, it's always fun to hear stories of how people figured things out.

How is that going btw? I see you mention it here every once in a while. Saw your progress pics at ampgarage a while ago, look like your almost there.

Nice find Aron. I have had tons of wonky stuff like that happen to amps I have built/repaired.

[Ben N]

It's my go to amp after all of these years!

It'd be mine, too--if I had one!
I'm glad you got it sorted out, and also that you shared it with us to file away in our "what to do if" cabinets. That is, those of us partial to obsolete technologies!

[Ripthorn]

Chris - I literally have to wire the impedance selector switch and the speaker jack, but with my 3 month old now living in my former electronics room, an hour or two to work on it is hard to come by, but I will update my thread here and ampgarage when it is done.

Aron - is there any difference in the circuitry for the blonde or is it just the covering that differs?  I want to build a Fender preamp, and if yours sounds so good, perhaps I will give one a go, so that I can have more crazy stuff happen.

[Gus]

Aron  are you sure you should change the caps?  Are they leaking out the ends?

I would not be surprised if they still measured good for value, DA,  ESR, leakage at working voltage.

[aron]

Brian, I think the 6G6-B is slightly different but maybe not a huge difference from a standard plain Fender amp.

http://diystompboxes.com/amps/bassman_6g6-b_schem.gif

Gus, do you really think that these paper dual electrolytic caps that I have in there are still good? Now I do have my Sencore tester. I could pull them and test.

Aron

[Gus]

I have measured some old amp caps maybe 30 or more years old that still measured good for value, leakage at working voltage and ESR.  I have also measured some that were bad so I test before replacing.  The thing is you need a good tester.  What sencore a LC102?
Does the amp sound bad with the caps in it now?

[aron]

I have an older Sencore capacitor/inductor tester. I will check the model number. Yes, it sounds ok but I figure it's worth a test.

[drewl]

FWIW the caps in my '67 Pro Reverb are original and still fine.
The Blond circuit was slightly different than the later Blackfaces (which you knew) and is one of the best sounding, but short lived unfortunately.
The Beatles had a '64 Blonde Bassman that's on more recordings than most realize and was used longer than any other amp, it's owned by the Harrison estate.

[Ben N]

I don't recall which of the 6G6s it is, but that is Brian Setzer's amp of choice. Great, great amp.

[aron]

Yep, that's the one I have - I believe Brian uses the same Bassman. The Sencore I have is the LC-101.

[Rob Strand]

I'm sure your report will save someone a lot of time in the future!