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Post subject: Bassman ab165 pop and hum
Posted: Wed Apr 03, 2019 5:45 pm
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Hello. I’m new to using forums. I’ll do my best to do this right.
I recently acquired a 66 bassman ab165. I had typical issues. I’ve replace all the electrolytic caps, and this helped. I purchased new JJ 6L6’s, set the bias, but something is puzzling me. When I turn the standby switch on, the amp pops and hums, then the hum goes away after the amp is on for a few seconds. When setting the bias I noticed the meter bouncing around from 35-40ma. I tried replacing the grid coupling caps. I’ve checked the grid and screen resistors. I’ve cleaned the tube sockets. I’m running out of ideas, and any help would be appreciated.


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Post subject: Re: Bassman ab165 pop and hum
Posted: Sun Apr 07, 2019 9:21 pm
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Location: Province de Québec, Canada
On any second amp ; Check fuse, might be too powerful and amp is not protected.

About your noise, pop an hum;
1-bad solders ; reheat them
2- clean socket contacts,
3- re-tension them if needed.
4- replace bias filter caps by a 100 mfd 100 volts.
5- Power tubes bias ; 6L6 or 6L6GC ?

Coupling cap have nothing to do with your noise issue.
Replace all plate resistors; they came noisy with years.

schematic here;

https://el34world.com/charts/Schematics ... ematic.pdf


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Post subject: Re: Bassman ab165 pop and hum
Posted: Mon Apr 08, 2019 4:37 am
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stratele52 wrote:
On any second amp ; Check fuse, might be too powerful and amp is not protected.

About your noise, pop an hum;
1-bad solders ; reheat them
2- clean socket contacts,
3- re-tension them if needed.
4- replace bias filter caps by a 100 mfd 100 volts.
5- Power tubes bias ; 6L6 or 6L6GC ?

Coupling cap have nothing to do with your noise issue.
Replace all plate resistors; they came noisy with years.

schematic here;

https://el34world.com/charts/Schematics ... ematic.pdf


+1

Make sure you get the polarity correct on the bias supply capacitor.

Unless the circuit has been modified, bias adjustment is a bias balance on the AB165.

Welcome to the forums.


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Post subject: Re: Bassman ab165 pop and hum
Posted: Mon Apr 08, 2019 4:42 am
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stratele52 wrote:
On any second amp ; Check fuse, might be too powerful and amp is not protected.

About your noise, pop an hum;
1-bad solders ; reheat them
I checked the solder connections. All have been touched up.
2- clean socket contacts,
I have cleaned all the tube sockets
3- re-tension them if needed.
Tube sockets are good
4- replace bias filter caps by a 100 mfd 100 volts.
Done
5- Power tubes bias ; 6L6 or 6L6GC ?
6L6GC

Coupling cap have nothing to do with your noise issue.
Replace all plate resistors; they came noisy with years.
All Plate resistors have been replaced.
schematic here;

I starting to wonder bad choke or output transformer??


https://el34world.com/charts/Schematics ... ematic.pdf


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Post subject: Re: Bassman ab165 pop and hum
Posted: Mon Apr 08, 2019 5:59 am
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Did you re-check the resistors and soldering, under the doghouse? Could be one of the resistors in the power rail has issues. Also, did you check all soldering to the sockets? Including those tangs which are folded over and soldered together, like in the phase inverter?

Good luck! Keep us posted.


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Post subject: Re: Bassman ab165 pop and hum
Posted: Mon Apr 08, 2019 6:40 am
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vinyl wrote:
stratele52 wrote:
On any second amp ; Check fuse, might be too powerful and amp is not protected.

About your noise, pop an hum;
1-bad solders ; reheat them
2- clean socket contacts,
3- re-tension them if needed.
4- replace bias filter caps by a 100 mfd 100 volts.
5- Power tubes bias ; 6L6 or 6L6GC ?

Coupling cap have nothing to do with your noise issue.
Replace all plate resistors; they came noisy with years.

schematic here;

https://el34world.com/charts/Schematics ... ematic.pdf


+1

Make sure you get the polarity correct on the bias supply capacitor.

Unless the circuit has been modified, bias adjustment is a bias balance on the AB165.

Welcome to the forums.


Thank you. Yes, Polarity is correct on the Bias supply capacitor. The circuit has been modified from the bias balance to the blackface style bias system.


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Post subject: Re: Bassman ab165 pop and hum
Posted: Mon Apr 08, 2019 6:49 am
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BMW2002Ti wrote:
Did you re-check the resistors and soldering, under the doghouse? Could be one of the resistors in the power rail has issues. Also, did you check all soldering to the sockets? Including those tangs which are folded over and soldered together, like in the phase inverter?

Good luck! Keep us posted.


I looked over those yesterday. Everything looks good to me. I put the amp on my scope yesterday, and noticed something interesting. I turned the amp on and have a steady signal, and about every 30 seconds the signal on the scope goes completely crazy (lightning storm). The signal returns to normal, then this behavior repeats. Could this be something discharging??


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Post subject: Re: Bassman ab165 pop and hum
Posted: Mon Apr 08, 2019 9:26 am
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Did you change the voltage leveling 220K-ohm resistors in the first stage of the PSU? They are under the two 70mfd/350VDC caps of first PSU stage.

I remember a Showman reverb head amp which acted kinda like the way you described. One of the resistors was bad. I changed them with metal oxide 2-watt resistors --- problem went away.

Worth a look. :)

Image


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Post subject: Re: Bassman ab165 pop and hum
Posted: Mon Apr 08, 2019 9:46 am
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TomL82 wrote:
When I turn the standby switch on, the amp pops and hums, then the hum goes away after the amp is on for a few seconds.


A power-supply issue might be the cause. Have you checked the diode stack in the rectifier circuit?

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Bassman ab165 pop and hum
Posted: Mon Apr 08, 2019 10:47 am
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Did the amp do this originally? I think one of your new power tubes are bad. Not much else would affect the bias like that. The hum is from an uneven bias on the power tubes.
Put in your old tubes or try pulling one power tube at a time to see if it goes away. Do the same for the preamp tubes too. Could be a cathods to filament short in one of them.


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Post subject: Re: Bassman ab165 pop and hum
Posted: Mon Apr 08, 2019 12:41 pm
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BMW2002Ti wrote:
Did you change the voltage leveling 220K-ohm resistors in the first stage of the PSU? They are under the two 70mfd/350VDC caps of first PSU stage.

I remember a Showman reverb head amp which acted kinda like the way you described. One of the resistors was bad. I changed them with metal oxide 2-watt resistors --- problem went away.

Worth a look. :)

Thank You!

Definitely worth a look! Resistors are cheap.

Image


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Post subject: Re: Bassman ab165 pop and hum
Posted: Mon Apr 08, 2019 12:45 pm
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Retroverbial wrote:
TomL82 wrote:
When I turn the standby switch on, the amp pops and hums, then the hum goes away after the amp is on for a few seconds.


A power-supply issue might be the cause. Have you checked the diode stack in the rectifier circuit?

Arjay




Thank you.

I haven't checked the diodes. I will look into it.


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Post subject: Re: Bassman ab165 pop and hum
Posted: Mon Apr 08, 2019 12:48 pm
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TimsAudio wrote:
Did the amp do this originally? I think one of your new power tubes are bad. Not much else would affect the bias like that. The hum is from an uneven bias on the power tubes.
Put in your old tubes or try pulling one power tube at a time to see if it goes away. Do the same for the preamp tubes too. Could be a cathods to filament short in one of them.



I recently acquired this amp. It has always made noise and had bias issue. Its doing this with the old tubes and the new tubes. However, I will still try what you suggest.

Thank you.


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Post subject: Re: Bassman ab165 pop and hum
Posted: Mon Apr 08, 2019 1:29 pm
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Check out that converted bias pot while it's on your scope. Give it a tap or two and some slight adjustment. Just a thought.

UN


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Post subject: Re: Bassman ab165 pop and hum
Posted: Mon Apr 08, 2019 3:46 pm
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Thank you all for the feedback. So far I have checked the diodes in the recitifier, they seem fine. I have replaced all the resistors in the power supply. Tapping or adjusting the bias has no effect when connected to the scope. I’ve tried pulling all the tubes one at a time. All left me with the same issue. I checked the filter choke, it measures at 95ohms. The output transformer measures at .7 ohms on secondary side. Primary side measure 48 ohms and 51 ohms off power and 99 ohms total. Again thank you for your help.


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