69 Bandmaster reverb -oh the crackling noise!

[benindavis]

Hey guys/gals!

Here is the problem. If I plug into the clean side, I can crank it and its beautiful.Smooth warm tone and BLASTING powerful sound.On the reverb side when I get it up past, say 3 on the volume, I have to crank the bass waaaaaaay down or it sounds like the speakers are cutting out. I have it narrowed down to the reverb knob. I E. If I have the reverb on a very low number say 1 or 2, there is no problem with cranking the volume and the bass. The speakers are brand new Jensen C12Q's. At first I thought I had blown them!

So here is what I am wondering. Is this problem most likely a bad tube (valve, for those of you on Sir Paul's side of the ocean ) Or do you think it might be the switch itself???

Thanks for the input!!

Ben

[cbarrow7625]

Going out on a limb here, but....the fact that your problem seems related to the reverb pot setting, my first inclination would be to think that the 220K grid leak resistor for the gain make-up stage (after the reverb / 3.3M resistor) is no longer connected.

The 220K grid leak keeps the makeup gain for both the reverb & dry siganls relatively controlled. If the 220K resistor was not connected, then the 470K reisitor + 100K reverb pot would become the new grid leak reistor for this 3rd gain stage. As you move the reverb pot, the gain would fluctuate more than it is designed to (because you are changing the voltage divider values). It should be 3.3M series into 220K shunt voltage divider (if the 220k resistor is grounded), if the 220K resistor is not grounded, now you have 3.3M series into a 470K - 570K shunt. That will give you far more voltage at the grid of the gain make-up stage than the amp is designed to handle.

Second thought is possibly a shorted cap on the reverb return / gain make-up stage (this is a shared cathode resistor / cap for both stages) just causing too much gain period. Although this is really highly unlikely.

Chris

[pdf64]

On the same theme maybe the coupling cap (0.022uF) feeding the 3M3 resistor is leaking dc, so upsetting the bias at the mixer stage by putting dc on to the grid? Peter

[mykey]

Hey guys/gals!

Here is the problem. If I plug into the clean side, I can crank it and its beautiful.Smooth warm tone and BLASTING powerful sound.On the reverb side when I get it up past, say 3 on the volume, I have to crank the bass waaaaaaay down or it sounds like the speakers are cutting out. I have it narrowed down to the reverb knob. I E. If I have the reverb on a very low number say 1 or 2, there is no problem with cranking the volume and the bass. The speakers are brand new Jensen C12Q's. At first I thought I had blown them!

So here is what I am wondering. Is this problem most likely a bad tube (valve, for those of you on Sir Paul's side of the ocean ) Or do you think it might be the switch itself???

Thanks for the input!!

Ben

change the plate and cathode resistors in the preamp that is having problems.

[PeaveyBandit]

I wonder if you could have a bad reverb pot, bad cap or lifted ground in that one section. I fixed a farting overload kind of issue like that on one of my amps lately. I was trying to find the bad component or bad connection and finally noticed one end of a bypass cap at the cathode of one of the preamp tubes had some white discoloration or leakage. The ones that are usually 25/25 value and in pairs. That was my issue but I also had to replace the reverb pot a long time ago and it was causing the amp to pitch fits. You couldn't even use the amp that pot was acting up so much.

How long has it been since the amp had the electrolytic capacitors replaced? By the way if you do recap the amp dont replace the tone caps. Just the bypass and filter caps only. I still defer to the higher level of expertise many others on this forum have. Just wanted to comment. It could be anything from a bad connection on a wire component or one of the grounds that come off the board and solder to the chassis, bad potentiometer control or anything. Doesn't sound like a tube to me but anything is possible.

[benindavis]

Over my head

Hey guys thanks for the advice and technical talk.. I guess its time for me to learn a thing or two about electronics,cause.. man........ I got NO IDEA what any of the replies you gave me mean.. !!!! Time to hit the books.. but.. I want to play.......... WAAAAAH!

Got any recommendations for reading for a unedyukated guy like me?