Dumbassbob 63 Bandmaster Crackling on Normal Channel

[dumbassbob]

The amp has been sounding great..I play it every day.I took out the extra 220K resistors,but for the last 2 weeks after 2 hrs it starts to sound crackly..If I lightly tap the top anound the normal channel you hear it,also the volume pot was scratchy,but that seems to of stoped... I have a DVM and I bought a VTVM,but was wondering about some of the resistors drifting.. would someone give me some tips to test resistors and trouble shooting. I know you need to undo the one side of the coupling caps and turn on the amp to test with a VTVM Bill52 told me to start a new thread...THanks DAB

[Leo_Gnardo]

after 2 hrs it starts to sound crackly..If I lightly tap the top anound the normal channel you hear it,also the volume pot was scratchy,but that seems to of stoped... I have a DVM and I bought a VTVM,but was wondering about some of the resistors drifting.. would someone give me some tips to test resistors and trouble shooting. I know you need to undo the one side of the coupling caps and turn on the amp to test with a VTVM

Good to see you've been getting some use out of that Bandmaster.

First thing I would do is give the pre tubes a pull & a push. Just re seat them, while you have 'em out one at a time examine the pins for oxidation. If any look shabby, clean 'em up with a little 600 grit paper. NOT steel wool! (Leaves conductive bits behind, don't find out the hard way.)

If that doesn't stop your crackles start looking around for defective solder connections. Your resistors may have drifted and you can check that easy enough with your DVM, but resistors don't usually respond to tapping on the chassis. Poorly connected resistors - do. Break out a chopstick, bright light, maybe magnifying glass, and poke around the preamp. Carefully.

Not much need to undo one side of coupling caps - it doesn't tell much - even a perfectly good cap will develop a charge with a test like that. If you see significant voltage on the low voltage side of your coupling & EQ caps, say northward of 50 millivolts, you either have a leaky cap or leaky circuit board. But the sort of noise leaky caps & boards make also doesn't much respond to chassis tapping.

[Chuck H]

Could be bad plate resistors, a bad tube, a cold solder joint, etc.

If I were me (and I am) I would check these things first. If I were you I would try a known good preamp tube, and if that didn't work I would send the amp back to Skip. He'll fix it. Let this amp be yours. Experiment on other amps. With symptoms that are ambiguous there is a possibility of creating new problems if you don't KNOW what needs fixing. For that you'd need bench equipment and experience. Skip has both and has proven himself in your case.

JM2C

[dumbassbob]

Could be bad plate resistors, a bad tube, a cold solder joint, etc.

If I were me (and I am) I would check these things first. If I were you I would try a known good preamp tube, and if that didn't work I would send the amp back to Skip. He'll fix it. Let this amp be yours. Experiment on other amps. With symptoms that are ambiguous there is a possibility of creating new problems if you don't KNOW what needs fixing. For that you'd need bench equipment and experience. Skip has both and has proven himself in your case.

JM2C

I was going too send it up to skip,but I thought if all it was a 100K plate resistor that had drifted.I would put them in..If I drain the amp do I just put the common lead to ground and Positive probe to resistor?.I changed out the preamp tube and sprayed Deoxit on the socket.But now it only does crackling after it has been on for a hour or more..I thought the plate (R) I was not going to do any more than that....The amp has been very good until now...When you think of how much G.Webers techs shotgunned this poor thing

[52 Bill]

I was going too send it up to skip,but I thought if all it was a 100K plate resistor that had drifted.I would put them in..If I drain the amp do I just put the common lead to ground and Positive probe to resistor?.I changed out the preamp tube and sprayed Deoxit on the socket.But now it only does crackling after it has been on for a hour or more..I thought the plate (R) I was not going to do any more than that....The amp has been very good until now...When you think of how much G.Webers techs shotgunned this poor thing

A drifted resistor will not make noise when you tap it. If the noise is caused/stopped/altered by physical stimulation, then the problem is mechanical in nature and should be investigated as such.

If you want to shotgun the plate resistors then go ahead and replace them. It couldn't be any worse than what some of the other techs have done.

Which 220K resistors did you remove?

[Chuck H]

Other things it could be include a leaky cap or the dreaded conductive circuit board. Any problem can come on after the amp has warmed up though some seem less likely than others to do so. If it's a conductive board I'm sorry to say that it would probably be of the permanent variety. That is, not just from moisture. In this case the only fix is to replace the entire board . Things it could be:

Bad plate resistors
Loose tube socket connections
Cold solder joint
Conductive board
Leaky coupling capacitor
Bad potentiometer
Microphonic capacitor
Dirty jacks
Bad filters (decoupling) causing intermittent positive feedback between nodes
Intermittent short (from a stray wire strand, board grime, gobby solder mass, etc.)
Intermittent fault (from a lead or component with an internal failure)
Failing tube (preamp OR power)

Hell, it could even be the speaker going bad, a bad speaker connection, a bad rectifier or something other than the amp like your guitars volume pot or the instrument cable.

Finding the exact problem to fix is critical to avoid a repeat of your previous experience. An experienced and capable tech is less likely to cause new problems that confound further troubleshooting and repair. Make the problem go away in the most efficient way that's KNOWN to be effective and be DONE. Don't call Gerald, don't replace your own plate resistors on a guess, do not pass go and do not spend 2000 dollars.

[dumbassbob]

Other things it could be include a leaky cap or the dreaded conductive circuit board. Any problem can come on after the amp has warmed up though some seem less likely than others to do so. If it's a conductive board I'm sorry to say that it would probably be of the permanent variety. That is, not just from moisture. In this case the only fix is to replace the entire board . Things it could be:

Bad plate resistors
Loose tube socket connections
Cold solder joint
Conductive board
Leaky coupling capacitor
Bad potentiometer
Microphonic capacitor
Dirty jacks
Bad filters (decoupling) causing intermittent positive feedback between nodes
Intermittent short (from a stray wire strand, board grime, gobby solder mass, etc.)
Intermittent fault (from a lead or component with an internal failure)
Failing tube (preamp OR power)

Hell, it could even be the speaker going bad, a bad speaker connection, a bad rectifier or something other than the amp like your guitars volume pot or the instrument cable.

Finding the exact problem to fix is critical to avoid a repeat of your previous experience. An experienced and capable tech is less likely to cause new problems that confound further troubleshooting and repair. Make the problem go away in the most efficient way that's KNOWN to be effective and be DONE. Don't call Gerald, don't replace your own plate resistors on a guess, do not pass go and do not spend 2000 dollars.

I think it is bad solder joint, dirt,Leaky cap,or plate resistor...It was working fine until a few weeks ago I cleaned the V-1 socket with DeOxit,and it went away for a few days...I also think I need a new Opto-coupler...A 50 yr old anp that when through all the shit this thing did..I was just trying to save $60.00 to ship it to Mr Skip Simmons....He even said he wrote a story about the amp and its history,Getting it from the HBowl in 1971 and it being stolen for all that time,and getting it back,and all the crap that Kendricks did and charged me over $1500.00 all it was a bad ground and the power tube plate wires were switched around..They shotgunned the hell out of it..Glad I got all the parts back,they even tried to keep them too

[Tom Phillips]

I think it is bad solder joint, dirt,Leaky cap,or plate resistor...It was working fine until a few weeks ago...

Bob,
One of your guesses may indeed be correct. However, they are just guesses. Also note that everything is working fine right up to the moment when it is no longer working fine. Therefore, unless there were particular events or a sequence of failure symptoms that you observed then we just need to follow traditional troubleshooting techniques to diagnose the problem.

I encourage you to re-read the advice given my the forum members. Otherwise, I see you starting down one of your old paths. For example, you mention that you think the opto-isolator is bad but you don't give any symptoms to support that thought. The approach should be thoughtful troubleshooting to isolate the problem. Not guessing and parts changing.

Cheers,
Tom

[dumbassbob]

Bob,
One of your guesses may indeed be correct. However, they are just guesses. Also note that everything is working fine right up to the moment when it is no longer working fine. Therefore, unless there were particular events or a sequence of failure symptoms that you observed then we just need to follow traditional troubleshooting techniques to diagnose the problem.

I encourage you to re-read the advice given my the forum members. Otherwise, I see you starting down one of your old paths. For example, you mention that you think the opto-isolator is bad but you don't give any symptoms to support that thought. The approach should be thoughtful troubleshooting to isolate the problem. Not guessing and parts changing.

Cheers,
Tom

After almost 2 hrs it started the crackling noise....I know it is just some simple "thing"I switched V-2 with V-3,and it stopped for today.. They are building 3 houses right across from me,and the afternoon wind at the beach might be getting micro dirt in the amp..I keep everything covered and clean but gale winds everyday...I am going to send it back to Mr.Skip PS as for the Opto coupler the 10meg and 100and 220 resistors spark when I pug in to the RCA jack

[Tom Phillips]

After almost 2 hrs it started the crackling noise....I know it is just some simple "thing"I switched V-2 with V-3,and it stopped for today...

These type of things can be finicky and intermittent. Sometimes anything you do can make it stop or start.

...I am going to send it back to Mr.Skip...

Based on your past experience he should know what to do.

...PS as for the Opto coupler the 10meg and 100and 220 resistors spark when I pug in to the RCA jack

I'm not getting what you mean by that but, again, Mr Skip should know what to do.


I'm glad that you have been getting enjoyment by playing the amp.
Cheers,
Tom

[Leo_Gnardo]

as for the Opto coupler the 10meg and 100and 220 resistors spark when I pug in to the RCA jack

I think you're seeing the reflection of the neon lamp.

[km6xz]

There is no suggestion a 3rd party can offer that will be more valid than a wild ass guess because we do not know what you are hearing. There are many types of noises and an experienced tech can differentiate between any of them. For example crackling sound can run the range from a hf dominate snits and burbling sound, high level crack, low frequency dominate rumbling crackle, intermittent white noise, or pink noise based on whether it has a reactive component involved, level of peak and valley,for example a crackle could be a level shift of a few db sharply or a 60 db abrupt shift in level, and 200 more types of "crackle". Plate resistors are often the source of crackle but those are the easiest to prove or eliminate without any test gear or unsoldering, if they are carbon composition type.
If there is a oxided sockets or tube pins, do NOT use sandpaper or any abrasive on mating contacts unless designed to be soft metal types like power contactors. Use a de-oxidizer, and very little. Using abrasives increase contact resistance, point contact heat and increase the rate of corrosion so replacement of the socket will be required in the future.

You have so much invested in this amp, and found a competent tech, why risk that investment in an amp you want to keep, by shotgunning and guessing? Anything you do to it will only make it harder for a tech to find all the real problems plus those you may introduce.

[dumbassbob]

There is no suggestion a 3rd party can offer that will be more valid than a wild ass guess because we do not know what you are hearing. There are many types of noises and an experienced tech can differentiate between any of them. For example crackling sound can run the range from a hf dominate snits and burbling sound, high level crack, low frequency dominate rumbling crackle, intermittent white noise, or pink noise based on whether it has a reactive component involved, level of peak and valley,for example a crackle could be a level shift of a few db sharply or a 60 db abrupt shift in level, and 200 more types of "crackle". Plate resistors are often the source of crackle but those are the easiest to prove or eliminate without any test gear or unsoldering, if they are carbon composition type.
If there is a oxided sockets or tube pins, do NOT use sandpaper or any abrasive on mating contacts unless designed to be soft metal types like power contactors. Use a de-oxidizer, and very little. Using abrasives increase contact resistance, point contact heat and increase the rate of corrosion so replacement of the socket will be required in the future.

You have so much invested in this amp, and found a competent tech, why risk that investment in an amp you want to keep, by shotgunning and guessing? Anything you do to it will only make it harder for a tech to find all the real problems plus those you may introduce.

It was a weak 7025 tube, dirt from a house they are building across the street and salt from living at the beach,How do I get all the crap off the tube pins? I got a shoebox of tubes from a man next door,but they have been sitting at the beach for 45-50 yrs NIB

[The Dude]

Deoxit both socket and pin. Work in and out a few times.

[Chuck H]

I think some tube pins may actually be loose by now. They could be re-tensioned if possible. One less thing and no one gets hurt.