Valve amp crackling

[AScheck9]

Sorry to keep asking silly questions but...

 

My fender bassmans second channel is playing up again. I thought the problem had rectified itself however it is back! 

 

Anyone come across this before? I've changed the valves in it a couple of times, but to no avail. Vintage channel is fine (and luxurious) but the second channel- no good! 

 

Cheers

 

 

 

Edited December 21, 2022 by AScheck9
Vid swap

[itu]

Is there anything like dirt in the speakers? Could be on either side of the cone. Or the dust cover is loose?

[Eric.C.Lapton]

Clean the switch, see if the contacts are furred or corroded, that would be my first check 

[Happy Jack]

Crackling is usually caused by coating with olive oil and sea salt, and then whacking the temperature up very high for the first half-hour.

 

Hope that helps.

 

[BassmanPaul]

You Porker Jack!!  LOL

[AScheck9]

3 hours ago, itu said:

Is there anything like dirt in the speakers? Could be on either side of the cone. Or the dust cover is loose?

Thanks for the answer, but no the speakers are clean and SUPER clear. 

[Bill Fitzmaurice]

Clean everything with contact cleaner, including the pots, tube sockets and tube pins.

[bassace]

Could it be a dry joint in a valve base? It happened to me with a home built preamp - there weren’t too many bass specific in those days. I was working behind a very highly regarded female sax player - OK, it was Kathy Stobart - who turned round to me in frustration and asked ‘What do you think you’re playing, a Geiger counter?’

[BassmanPaul]

It could be almost anything but careful diagnosis will reveal the problem. I do caution you that a tube amp contains LETHAL voltages even when switched off and unplugged. 

[Dan Dare]

16 hours ago, BassmanPaul said:

It could be almost anything but careful diagnosis will reveal the problem. I do caution you that a tube amp contains LETHAL voltages even when switched off and unplugged. 

 

This. Stay well away from the power supply, especially. The reservoir caps hold lethal voltages for a long time. If in doubt, take it to a tech'.

[Bill Fitzmaurice]

Not necessarily lethal, and it's not the voltage that kills, it's the amperes. Still, getting zapped with even 450v isn't a pleasant experience. One thing to do is to turn the power and standby switches on, with the amp unplugged, before removing the chassis from the case, especially the standby. When the standby switch is off the power supply caps will hold their voltage for a long time. With it on they'll discharge, albeit slowly.

[BassmanPaul]

The suggestion of taking the thing to a tech is a valid one. His/her bill will be far less than what your life is worth.

[itu]

4 hours ago, Bill Fitzmaurice said:

Not necessarily lethal, and it's not the voltage that kills, it's the amperes.

That's not exactly true. Low voltage and lots of amperes (like car battery) is as safe as high V and low A. Maybe even more comfortable than the spark. But when both are on the high side, beware!

 

If you meant that when amperes (a lot!) run through body, patient is in danger, yes, that's true.

[Bill Fitzmaurice]

Not that I recommend it, but if you connect the terminals of a 9V battery, the small type, with a screw driver you'll get a small spark. Do so with a 12v car battery and the screw driver will suffer serious damage. It can even get arc welded to the terminals. The difference is due to the ampere capacity of the two. Consider a taser. They run at 30 to 50k volts. They tend to be non-lethal because they have very low ampere capacity. A static shock you receive on a dry winter day can be an upwards of 75,000 volts, but the current is so small that all you might feel is a slight annoying zap. Like most electricians I seldom bothered to turn off the breaker to swap out an outlet or switch, and as a result got zapped dozens of times over the years. Smart? Maybe not, but that's what we do. I once stuck my finger in the wrong place in a Fender Bassman and got the full 470v. Unpleasant, but not injurious. Still, that experience educated me to the value of using a bleeder resistor to drain the caps before messing around. 😳

[AScheck9]

You guys have scared me enough already! 😂 I'll find a tech to sort it, hopefully! Cheers for the heads up. 

[paul_5]

Just now, AScheck9 said:

You guys have scared me enough already! 😂 I'll find a tech to sort it, hopefully! Cheers for the heads up. 

A wise move. 

[BassmanPaul]

5 minutes ago, AScheck9 said:

You guys have scared me enough already! 😂 I'll find a tech to sort it, hopefully! Cheers for the heads up. 

Thank you for my Christmas Present!!  

 

Please keep us informed of results. 

[agedhorse]

13 hours ago, Bill Fitzmaurice said:

Not necessarily lethal, and it's not the voltage that kills, it's the amperes. Still, getting zapped with even 450v isn't a pleasant experience. One thing to do is to turn the power and standby switches on, with the amp unplugged, before removing the chassis from the case, especially the standby. When the standby switch is off the power supply caps will hold their voltage for a long time. With it on they'll discharge, albeit slowly.

It's the voltage across the body resistance that causes the current to flow. 450 volts can easily cause enough current to flow through the body to kill (if you are moderately unlucky). The power supply has plenty of current available (>100mA) so yes, it is dangerous.

 

Since this is a newer version (designed by Matt Wilkins, he worked under the VP of R&D at the time who was a good friend) which is safety approved to IEC 60065, there are bleeder resistors within the power supply that will safely discharge the power supply when power is removed regardless of the position of the standby switch (I verified this, it applies to all factory stock Fender Bassman 300's). This doesn't mean that the bleeders haven't been damaged, or some hack tech removed them because of some cool trick they heard about on the internet, so a professional tech will ALWAYS verify that the voltage has been discharged before doing any work. 

 

On 21/12/2022 at 19:17, Bill Fitzmaurice said:

Clean everything with contact cleaner, including the pots, tube sockets and tube pins.

No, please don't do this, it's a common cause of additional damage to amplifiers. It's probably the quickest way to ruin a set of pots, and likely not the cause of the original problems anyway. When I was working for Fender, I saw the results of "magic cure-all spray" and is always cost the customers more (and if the amp was under warranty, this damage was not covered). The authorized service centers hated working on such amps because there was so much to clean up after.

Edited December 24, 2022 by agedhorse

[agedhorse]

7 hours ago, Bill Fitzmaurice said:

Not that I recommend it, but if you connect the terminals of a 9V battery, the small type, with a screw driver you'll get a small spark. Do so with a 12v car battery and the screw driver will suffer serious damage. It can even get arc welded to the terminals. The difference is due to the ampere capacity of the two. Consider a taser. They run at 30 to 50k volts. They tend to be non-lethal because they have very low ampere capacity. A static shock you receive on a dry winter day can be an upwards of 75,000 volts, but the current is so small that all you might feel is a slight annoying zap. Like most electricians I seldom bothered to turn off the breaker to swap out an outlet or switch, and as a result got zapped dozens of times over the years. Smart? Maybe not, but that's what we do. I once stuck my finger in the wrong place in a Fender Bassman and got the full 470v. Unpleasant, but not injurious. Still, that experience educated me to the value of using a bleeder resistor to drain the caps before messing around. 😳

With the battery examples, the amount of current delivered to the load is a function of the open circuit voltage the internal resistance of the battery, and the resistance of the load (which in this case is 0 ohms). The 12V car battery has an internal resistance of about 0.02 ohms, the 9v battery has an internal resistance of ~5 ohm. It's not the capacity, but the internal resistance that limits the current. Generally, higher capacity batteries rated for higher discharge rates have lower internal resistance.

 

The current delivered to the short circuited 12V battery would be 12V/.02 ohm = 600A

The current delivered to the short circuited 9V battery would be 9V/5 ohm = 1.8A

 

Some folks are more tolerant to getting shocked, but some are not tolerant at all. Because of this variability, the safety recommendations are absolutely valid and are also "best practice" in the service field. While bleeder resistors are required for all amps with an IEC60065 or IEC62368 safety rating, a wise and experienced service tech will ALWAYS verify that there is no voltage present before attempting service.

 

Do you know what the biggest danger from shock is? It's either falling off of a ladder (in the power field) or getting knocked off your stool as the amp falls on you after you react to the shock. Once you have a tube amp fall on you, you may wish you were dead. 

[Chienmortbb]

20 hours ago, Bill Fitzmaurice said:

Not necessarily lethal, and it's not the voltage that kills, it's the amperes.

It is the current. Current is measured in amperes.

[Chienmortbb]

7 hours ago, agedhorse said:

It's not the capacity, but the internal resistance that limits the current.

Yes, as a battery depletes, its internal resistance rises.

 

7 hours ago, agedhorse said:

Some folks are more tolerant to getting shocked, but some are not tolerant at all.

Correct, I used to align CRT TV monitors, the kind used in TV studios. We would often get a 1.5Kv (1500 volt) shock from part of the line output circuit. I hurt, drilled a small hole in your finger and gave your nervous system a kick up the ..... (insert slang or expletive to your taste). Although it was 1500V, it was a very short duration pulse and there waa very little curret, around 1mA or 1/1000th of an amp. Funnily enough, the jolt to the nervous system meant you felt more alive for the rest of the day. However one word of caution. If you put your hand into a valve amp, put the other hand in your pocket. Do not make a circuit with your heart in the middle.

 

7 hours ago, agedhorse said:

While bleeder resistors are required for all amps with an IEC60065 or IEC62368 safety rating, a wise and experienced service tech will ALWAYS verify that there is no voltage present before attempting service.

Oh, yes. Bleeder resistors are useful and will discharge the cap after switch off, but they are a compromise.

[agedhorse]

4 hours ago, Chienmortbb said:

Yes, as a battery depletes, its internal resistance rises.

 

Correct, I used to align CRT TV monitors, the kind used in TV studios. We would often get a 1.5Kv (1500 volt) shock from part of the line output circuit. I hurt, drilled a small hole in your finger and gave your nervous system a kick up the ..... (insert slang or expletive to your taste). Although it was 1500V, it was a very short duration pulse and there waa very little curret, around 1mA or 1/1000th of an amp. Funnily enough, the jolt to the nervous system meant you felt more alive for the rest of the day. However one word of caution. If you put your hand into a valve amp, put the other hand in your pocket. Do not make a circuit with your heart in the middle.

 

Oh, yes. Bleeder resistors are useful and will discharge the cap after switch off, but they are a compromise.

At a high enough voltage, wounds become self-cauterizing… 

 

The main tradeoff with bleeders is the loss of efficiency as voltage increases.  This is an import consideration in small Switchmode power supplies meeting the EU’s eco-directive.

[umcoo]

I'd be interested if you manage to get this resolved. I have an old Peavey valve amp which is making a similar noise, and previous techs have not found any issue with it. Rather frustrating.  

[agedhorse]

50 minutes ago, umcoo said:

I'd be interested if you manage to get this resolved. I have an old Peavey valve amp which is making a similar noise, and previous techs have not found any issue with it. Rather frustrating.  

Have you confirmed (beyond any doubt) that the problem is actually with the amp?

 

I see a fair number of claimed problems end up being misdiagnosed as an amp problem when in fact it ended up being something else.

[goingdownslow]

 

 

This was the cause of crackling in one of my amps...