Random Reverb Pops (BFDR)

[H Chandler]

Once again I come to the well for knowledge.

This times it's a BF Deluxe Reverb and it's about random pops. It happens with both channels volumes down all the way. It's weird because it's quiet for a while, then it'll get a random sputter and an occasional loud pop, followed by the decay of the reverb springs. Kind of sounds like water dripping in a tiled bathroom. There seems to be no curing it. I've isolated it to the 12AT7 driver (I think). I've pulled both V1 and V2 so there's no signal coming from them. Still happens.

It goes away when I pull the reverb plug that sends the dry signal to the pan. Is this just random noise generated by the 12AT7? What can I do about? I don't know if this is a factor, but the only mod to this circuit is that the normal channel has been mixed with vibrato channel with 220K resistors, just before the 500p cap that feeds the grids of the 12AT7.

Any suggestions are most appreciated.

Thanks

[Tom Phillips]

...Is this just random noise generated by the 12AT7? What can I do about?...

That would be my call.
I would try a different 12AT7 in the reverb driver position. Also make sure that the tube socket is clean. If you don't have another 12AT7 handy then try swapping the reverb driver and the phase inverter tubes to determine if the reverb pops go away. IF it cures the problem you may even be able to live with the noisy tube in the PI.
Good luck,
Tom

[H Chandler]

Thanks for the comments.

I've tried 1/2 a dozen 12AT7's. I've settled on what seems to be the quietest, but it still does it every now and then. Are there 12AT7's known for being quiet. Is it possible to get one specially tested for this kind of noise?

I haven't tried cleaning the socket yet. Not sure how to do it though... other than spraying tuner cleaner/lubricator and repeatedly insert and remove a tube. Do they make tiny pipe -cleaner-like wire brushes?


Thanks again.

Howard

[bob p]

Well, now that you've eliminated a bad reverb driver tube as the cause of your problem, you'll be progressing from Level 1 to Level 2. Now things get trickier.

Tubes are the easy thing to fix. Now you've got to look at other parts that can cause these types of problems. These can be simple things that are visible, like arcing over on the tube sockets, or things that can be really hard to see, like bad components.

It could literally be all sorts of things, like intermittent failure in a coupling cap or a resistor, a bad wire, a bad reverb pan, transformer, etc.

Aside: where's @stokes when you need him?

[Euthymia]

Snap, crackle, pop

Swap out the 2K2 resistor and the 25uF cap going from the cathodes of the 12AT7 reverb driver tube to ground.

That kind of crackly-pop usually comes (if the problem is not the tube) from resistors with higher voltage on one side, like plate resistors, but it's not unheard-of for it to be a cathode resistor.

Can you hear this stuff with the Reverb control all the way down? My guess is that you can't.

Does tapping the reverb driver tube with a stick have any effect? If so, retension the socket.

[H Chandler]

I think I got it!

Well Bob P.

As I was writing my reply (not posted) as to why it must be isolated at the tube since there are no plate resistors on the reverb driver, I started to thinking about what was attached to the plates. Well, this led me to notice how the plates run to the reverb xformer which goes right to the PS. So, I figured I'd just check it out. Well, as I'm lifting the pan I get a little nip from the residual charge on one of the caps. Well, whoever did the last cap job jammed a bunch of oversized caps in there. They barely fit and, I notice a bit of oxidation on the cap lead that goes to the choke/reverb-xformer etc. So, I push the leads in as far as I can, and put electricians tape on the sides of the pan. Fire it up, no more noise. I need to spend a bit more time with it to be sure, but I think this must be it.

Note to self: When cramming oversized caps in a PS doghouse, cover the leads with some good heatshrink.

Anyway, thanks for the help and encouragement.

Howard