Flakey Reverb on a BF Deluxe Reverb

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PhatTele
Junior tube assistant
Posts: 14
(4/11/04 7:37 pm)
 Flakey Reverb on a BF Deluxe Reverb
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 I'm working on a BF Deluxe Reverb which has a flakey reverb issue. With the reverb On (and no instrument plugged in), the reverb works normally from 0-3 on the dial. Above, 3.5 a high pitched squeel starts in (something dogs would appreciate). The more you turn the dial, the more squeel. Also, as the squeel kicks in, the power tubes start to heat up and glow abnormally. I noticed that the squeel stops when the footswitch plugs are removed (the reverb seems to work normally without the footswitch).

I've swapped both the driver and recovery tubes. I swapped the reverb transformer. I tried a different foot pedal. I hooked it up to a another reverb tank. I shortened the lead from the reverb output jack to Pin 2 of the recovery tube. I repositioned the ground from the reverb transformer. The amp has already been completely recapped. I checked the leads to the filter caps. Everything seems solid and the leads look clean/neat, yet we're still getting the squeel and glowing power tubes when the foot pedal is plugged in and the reverb is above 3 on the knob.

Aside from telling the owner "just don't use the foot pedal," where would I look to diagnose this squeel? What would I measure and how? Thanx...


 
punkykatt
Junior tube assistant
Posts: 3
(4/11/04 8:23 pm)
 Re: Flakey Reverb on a BF Deluxe Reverb
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 Did you try cleaning or replacing the foot switch jack?
 
Tiny Daddy
I will work on all amps
Posts: 611
(4/11/04 9:10 pm)
 Re: Flakey Reverb on a BF Deluxe Reverb
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 "glowing power tubes" That shouldn't even be happening. Have you installed 1-ohm resistors on the output tube cathodes so you can measure the tube current?
 
PhatTele
Junior tube assistant
Posts: 15
(4/12/04 7:26 am)
 Re: Flakey Reverb on a BF Deluxe Reverb
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 Right - Glowing power tubes didn't sound like a good thing to me either.

I've got a BiasRite from Weber. I'll use that for the measurements. I'm assuming I'll be measuring at idle and with the reverb low or off and then measure again as I raise the reverb level.

I'll post the numbers here this evening hopefully.
 
OldHouseScott
Posts: 19
(4/12/04 8:37 am)
 Re: Flakey Reverb on a BF Deluxe Reverb
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 Sounds like you're getting an oscillation in the reverb recovery tube when the footswitch cable is plugged in. Maybe try a grid stopper resistor between the junction of the reverb return/footswitch/220k grid load resistor and the grid pin on the tube itself. If you have a resistor substitution box, you can start with a high value (50k) and work your way down until you get the smallest resistor that still kills the oscillation, then sub that value in.


OldHouseScott
 
PhatTele
Junior tube assistant
Posts: 16
(4/12/04 9:32 am)
 Re: Flakey Reverb on a BF Deluxe Reverb
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 Thanks. Good suggestion. I'll try that. Given the location of the jack to Pin 2 of the recovery tube, I'm guessing I could just replace the wire lead with an untrimmed resister.

Is that "resistor substitution box" a type of tool? I haven't heard of those. I've heard of the adjustable resisters (with a little slide contraption on them)...but you still have to hook those in and then measure them once the correct position is reached.
 
OldHouseScott
Posts: 20
(4/12/04 12:07 pm)
 Re: Flakey Reverb on a BF Deluxe Reverb
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 Yes, a resistor substitution box is a kind of diagnostic tool. It is just as its name implies, a box with a bunch of resistors wired to switches that let you play with resistance values in-circuit. You use combinations of switches to get virtually any value between dead-short and usually several Mohms. They range in price and performance from about 40 to a couple hundred dollars. There are also similar boxes for capacitors, usually in the pF to nF range.


OldHouseScott
 
PhatTele
Junior tube assistant
Posts: 17
(4/17/04 9:54 pm)
 Re: Flakey Reverb on a BF Deluxe Reverb
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 Well, I'm not sure I solved the problem as much as I suppressed the problem. It turns out that the Tremolo pedal jack was not grounded properly. I replaced the jack with one that had a solder lug. I ran the ground wire to a soldered ground source close by. Now, instead of an oscillating, microphonic squeal from 3 and above, the microphonic sound doesn't start until about 8 or 9 on the dial....but only when the foot pedal is attached. Without the foot pedal, no squeal...and yes, I tried swapping the connectors. In fact, I tried three foot pedals. 2 were reissues and one was original from a '69 Bandmaster Reverb. No squeal at all with the old Fender pedal. That's how I traced back the problem to the Tremolo jack.

Also, the speaker had a loose connection which seemed to add to the problem. Once those were soldered, some of the other gremlins that were plaguing this amp went away...at lease the amps seems stable.

Anyway, thanks for everyone's help.
 
 
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