Bandmaster Reverb crackle..fixed!

steveb

Member
Messages
559
I have a '72 Bandmaster Reverb that started crackling & popping today. I tried tapping & wiggling all of the tubes, but the noise never changed with any of that. I shut it off for a while, & when I turned it off of standby, there was a series of crackles & pops that settled down after a second or two, but randomly kept crackling. I pulled the phase inverter tube & the crackle was still there, so I'm thinking maybe the power section? I tried other 6L6's & rectifier tubes, & it made no difference. I did notice that when I take it off of standby, one of the power tubes flashes a bit, even with the other set I tried.
Just to be sure, I plugged it into another speaker cab with a different cable, & the crackle was still there.
Thanks in advance,
 
Last edited:

WaltC

Member
Messages
2,126
sounds like it's time for a trip to the tube amp doctor. DIY repair is not for the beginner because of the potentially fatal voltages inside a tube amp, so I'd not recommend that for someone who's not familiar with the dangers and reasonably well versed in amp repair.
 

steveb

Member
Messages
559
Thanks Walt, I've done all the work on my amps for quite a while now, & I'm well aware of the danger. I'm just looking for clues.
 

WaltC

Member
Messages
2,126
that said, I'd take a close look at the screen resistors (on the power tube sockets between pin 6 (a "terminal strip" point) and pin 4 (the screen). See if one (or more) is cracked and check the solder joints on both. If they're both still old 1 watt 470 ohm carbon comp resistors, you might want to change them to 470 ohm wire wound 3 watt or 5 watt or even 500 ohm cement 5 Watt'ers.

also check those 1/4 watt 1500 ohm grid stoppers on pin 5 of each power tube and make sure they're intact and both have the same negative bias voltage on them. Wouldn't hurt to replace them while you're at it. Replacing with the same size and type should be fine.

Also check (re-flow or re-do) all the solder joints on both power tubes. Then see what's what.
 

steveb

Member
Messages
559
Thanks, Walt. I got this amp in 2009, did a cap job & converted the hum balance to a bias adjustment. I also got rid of the 2000 pf caps on the power tubes, but never changed those resistors, which are probably original.
 

steveb

Member
Messages
559
I happened to have some 470 ohm 2 watt flameproof resistors in my parts bin. Replacing with those did the trick.

Thanks again, Walt.
 

pdf64

Member
Messages
7,756
If they're both still old 1 watt 470 ohm carbon comp resistors, you might want to change them to 470 ohm wire wound 3 watt or 5 watt or even 500 ohm cement 5 Watt'ers
An alternative rationale is to use 470 ohm 1 watt, on the basis that whilst regular resistors do not have specified fusing characteristics, they do fuse when subjected to overload and 1 watt resistors are very likely to fuse before those of higher wattage.
A common failure mode of power tubes is for the screen grid to short across to other elements within the tube.
If the fault current has to pass via a 1 watt resistor, it tends to blow the resistor, whereas higher rated resistors may be able to withstand the fault current for a prolonged time.
It may be seen to be preferable for the screen grid resistor to blow, rather than for the power transformer etc to withstand potentially damaging fault currents for as long as it takes the line fuse to blow, which may be 30 seconds or more, depending on the magnitude of the current.
It is very beneficial for flame retardant resistor types to be used, eg most metal oxide types, to avoid the collateral damage often evident when carbon comp resistors are heavily overloaded.
Pete
 




Trending Topics

Top