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Post subject: Bassman AB165 Hum and low volume
Posted: Thu Mar 24, 2011 9:36 pm
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I have just finished moding an AA371 back to AB165.
Before I started I had a hum and low volume.
Followed schematic, also have a working and sweet sounding 68 Bassman AB165 to use for comparison
Mod went smoothly, all new filter caps, changed all resistors and caps to correct values to match AB165 schematic. New tubes.
The amp works sort of BUT, I have a fairly loud hum and volume is low, sound a little distorted as well.
Chassis has some vibration (felt with fingers) in the power cord area which coincides with the hum, less apparent at other spots on the chassis.
Checked all voltages side by side with the working AB165, all are fairly close.
Checked grounds to make sure they are intact.
I'm obviously missing something but I'll be darned if I can find it.
Any guidance would be greatly appreciated. Thanks, DB.


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Post subject: Re: Bassman AB165 Hum and low volume - more info
Posted: Thu Mar 24, 2011 9:45 pm
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More info - I've tried the other speaker cabinet (known working) and same results. Changed speaker cable with known good one, same results. Not the cab or cable.


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Post subject: Re: Bassman AB165 Hum and low volume
Posted: Thu Mar 24, 2011 10:46 pm
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Did you make any mods to or backdate the biasing circuit in any way? The AB165 Bassman's biasing circuit isn't a true bias level adjust at all -- more akin to the "output tubes matching" arrangement on the larger silverface platforms of the era. You might also check the polarity of those filter caps you recently installed. Whatever it is, I'm guessing it's related to a power supply problem.

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 Hum and low volume
Posted: Fri Mar 25, 2011 3:50 am
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I did indeed do a bias mod making it a bias adjust instead of just a hum adjust. Same exact mod I did on the 68 which went perfectly. I'll check the caps again but I figured if I put one of them in with the polarity wrong it would have blown pretty quickly and spectacularly. The new ones are all Tube Amp Doctor parts.


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Post subject: Re: Bassman AB165 Hum and low volume
Posted: Fri Mar 25, 2011 5:09 am
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JOC, does the new circuit have NO center tap on the heater line off the PT? And the old AA165 used a CT heater line? If you try using 100-ohm resistors to make an "artificial" CT on a tap that has a CT, you may inject 60Hz hum into the signal.

These 100-ohm resistors usually sit off the lamp connection, in the older amps.

AB165 (CT)--- see line off PT to heaters.

http://www.kbapps.com/audio/schematics/ ... ab165.html

SUPER REVERB (NON CT):

http://www.kbapps.com/audio/schematics/ ... ab568.html


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Post subject: Re: Bassman AB165 Hum and low volume
Posted: Fri Mar 25, 2011 6:06 am
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you said it had the problem BEFORE the mods. Maybe not something you did. You didn't mention if you tried swapping out tubes?? Power or PI maybe??
trannys can "vibrate" if not up to par. Pull all the tubes and see if you still have the vibration. That may possibly give and indication its before the power supply in the circuit.
Good info here:...http://www.blueguitar.org/new/schem/fen ... bmtips.pdf
mentions something about reversing speaker leads???

double check the diodes in the rectifier circuit too...
Good Luck!!


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Post subject: Re: Bassman AB165 Hum and low volume
Posted: Fri Mar 25, 2011 5:44 pm
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How are the filament wires ran on the output tubes.It should be the pair of wires on pins #2 and #7 most output tubes should be wired that pin #2 of one of the output tubes should terminate on pin #2 of the adjacent tube.Like wise with the wire on pin #7.It should be really easy on most fenders it's the green wire that connects the tubes.


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Post subject: Re: Bassman AB165 Hum and low volume
Posted: Fri Mar 25, 2011 11:40 pm
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Good idea to check the wiring on the power tubes (#2 & 7 pins). Also, while you are at it, check the voltage of the heater wiring. Gotta be a good 6.3 VAC.

My guess is: it is a wiring issue. Do you have a photo of the chassis?


Thanks!


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Post subject: Re: Bassman AB165 Hum and low volume - update
Posted: Fri Apr 01, 2011 1:40 pm
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So now I've gone even farther, hum is gone but volume is still low. I've replaced all the electrolytics, replaced any resistor testing out of spec, did the bia mod, replaced the choke (which eliminated the hum), and checked every single voltage and ground compared to both the schematic AND a working AB165 Bassman - everything checks out but I still have almost no volume. I've cheched resistance across the taps on both of the transformers, once again using the working Bassman as a reference. This thing is driving me nuts as I don't know what else to try other than replacing tube sockets - there's pretty much nothing else left! Any ideas?


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Post subject: Re: Bassman AB165 Hum and low volume
Posted: Wed Aug 17, 2011 5:13 pm
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Problem solved - some idiot had put a wire where it shouldn't be - I traced every single line on the layout/schematic and found it. Removed it, 100% working amp. All I can guess is it was a poor way to try and attenuate the amp for in-home use.


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Post subject: Re: Bassman AB165 Hum and low volume
Posted: Wed Aug 17, 2011 6:22 pm
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Book 'em, Dano......murder one!

Congrats on isolating and repairing the problem -- sometimes that's what it takes to locate some miscreant's "improvement".

RAWK ON!

8) 8) 8)

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 Hum and low volume
Posted: Sat Apr 04, 2015 1:29 pm
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Ok, danoHSS ... How did you do the bias modification and how did you replace the choke... I have a similar problem..


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