Fender Bassman 59 reissue - mystery random static

[kdawg]

Hello - I'm trying to figure out issues with a bassman 59 reissue, the noise file is attached. I've completely switched tube sets, no change. I did find a bad 22uf/500v filter cap and replaced but is still doing this randomly. I've used capacitor meter on all the coupling/bypass caps, they read OK. Tapped and prodded everything, it doesn't seem to be a wiring or solder joint issue. It does seem to happen more when warming up.

Noise doesn't seem to happen if all preamp tubes are in, it does occur with just phase inverter in place. Any ideas?

Thanks
-kdawg

Memo.m4a

[MWJB]

can't open the sound file, but I would suspect the 1W FP screen grid resistors, the PI plate resistors or the 100K resistor feeding the cathode follower (R14?). These can make odd noises as they fail, the screen grid resistors can crack the solder around the eyelets, causing intermittency.

[Enzo]

Did I read this right, it DOESN'T HAPPEN with all the tubes installed???? it only happens with JUST the phase inverter installed?

If that is correct, then what is the problem?

And just to be sure, does it happen when the amp is plugged in somewhere else, like in a different building?

ANd try this too, set a little AM radio near the amp and tune it to a weak station or even between stations. When the noise comes from the amp, does it also come through the radio?

[kdawg]

Sorry typo - Noise happens with power tubes and phase inverter in, or with all tubes in - but doesn't seem to happen if I pull all preamp tubes out.

Yes it does show up on radio, high voltage type noise I suppose? It has happened in two different locations.

[Enzo]

Actually, the radio test was to see if there was noise in the environment, like the amp could be picking up noise from something like a sump pump motor or air conditioning system or whatever. An example is that if someone flips on the lights in the next room, my amp makes a pop sound when the switch is flipped.

[kdawg]

Yea it's not from the environment, definitely in the amp. I won't be able to get to it for a few days... I'm not sure if I should just replace all those resistors, I'm wondering if they'll probably still read good since its working.

One thing is it did have a solid state rectifier instead of a 5AR4 - could that have some kind of degrading effect on particular resistors/caps? I've switched it back to a new 5AR4 since I got it, I thought since there was no adjustable bias on these that it may be off a bit with different voltages of a solid state rectifier.

Thanks
-kdawg

[MWJB]

No, the SS rectifier is fine, voltages & bias will depend on specific tubes fitted, you can adjust the bias by adding resistors in parallel with R40 to cool plate current, or adding in parallel with R41 to increase current...or, replace R41 with a Cermet trim pot & tweak R40 to give desired sweep.

[Alex R]

If one filter cap failed it might be worth changing the others too. It could be a filter cap. The funny thing is there's kind of a sawtooth rectified ripple kind of sound in there. But yep it might just be interference from something external to the amp.

[kdawg]

Well I had abandoned this project for a long time, kept trying and trying with no results... replaced basically every component, resoldered every joint, etc. - still no change. Random arcing sound, except I had it on my bench recently had been testing it with a different speaker cabinet, and -no- static. I decided to check each individual speaker, 3 measured 8 ohms and 1 measured 160 ohms! I had originally only checked them as a final load, not individually. The speaker must have been doing something weird to the load.

So lesson learned, its almost always the simplest things. Blown speaker causing me days of grief!

-kdawg

[tubeswell]

I know this sounds a little off-the-wall, but if you clip one of your MM leads to the chassis and the other lead to the input grid of the PI, does the noise go away? (If it does I'm thinking it could be a bad presence pot - just a hunch)

[J M Fahey]

The speaker failure points to cracked/almost cut flexible wires, where copper braid is all broken but inner cotton thread core keeps ends close by.
The ideal spark generator.
Next time please post a more classic audio file, such as MP3 or WAV or something.
For unknown reasons Firefox insists on "recognizing" it as a ".m4a.html" (it adds the .html at the end on its own) and shows a garbled text screen.
I had to download it and force Quicktime (for Windows) to read it as an audio file.
Oh well.
PS: are you a Mac guy?

[tubeswell]

Oops I didn't read the last post before mine properly - its most likely what JM said. BTW if you are converting files in iTunes and want an .mp3 converter to show up in the 'advanced' menu, choose 'preferences' from the 'iTunes' menu, and under the "General" tab click on the 'Import Settings' button and then choose the encoder.