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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Wed Mar 16, 2011 11:40 am
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BMW2002Ti wrote:
For coupling caps, I disconnect the lead of the cap going to the next grid stage. You'll notice that one end is usually hooked to the last stage's anode plate and the other to the next stage's signal grid.

Be careful here: you are going to measure voltages with the amp on. Amp on, not in Standby mode, no inputs, volume(s) at zero, speaker(s) connected.

Hook the red wire of your DVM to the lifted cap lead. The black wire to the chassis ground. Set meter to read Voltage-DC.

With amp on you should only "see" a few mV. If you are reading close to or above 1VDC... you can consider the cap "leaky." If you read on the other lead of the cap (the one that is still soldered to the circuit). You prolly "see" a few hundred VDC. Maybe, somewhere between 100-300 VDC. Best to check schematic, to find what VDC should be read, at which points of the circuit.

This is why it's best to choose good quality, high VDC rated caps (400-600VDC). They have less chance of breakdown and going bad or leaky. You can see the consequences of a leaky cap. Least consequence is resetting the idle bias level of next stage way outta linear region. Worst consequence is a fried tube, when a few hundred VDC is past onto a grid designed for mV of DC.


OT: I still don't get why the mod calls for 0.001 mfd coupling cap on the PI, in place of the stock 0.1mfd. Hard to believe you'd get any signal passing through such a setup. Oh well...

Good luck on the project!


+1

And caution while performing any dynamic diagnostic testing cannot be over-stressed.

Arjay

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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Wed Mar 16, 2011 12:34 pm
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been thinking that WHAT IF that new volume pot is defect? It is what ties the rest of it in the circuit... I wont have a chance to dive into it at least until Monday, bummer. I will probably check those caps as well, thought about it but wanted to check everything else before I had to start doing things with the power ON :shock: !!!


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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Wed Mar 16, 2011 1:05 pm
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You can do continuity test on the volume and tone pots, with the amp off--- to check for bad soldering, broken wires, or bad grounding point. You may end up having to do voltage checks with amp on, at marked places corresponding to the schemo and the layout.

Anyhow, have a good weekend! Good luck come, next week.


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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Wed Mar 16, 2011 8:46 pm
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Thank you for all of your help at advice! Have a Happy St. Patty's Day!!! And, a great weekend! Be back on Monday hopefully with some good news... :D


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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Tue Mar 22, 2011 11:47 pm
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Well I finally got a chance to get back to the Bassman tonight. I tried swapping out the cap on the normal channel and no change. Hmmmmm..... I looked at the layout and talked myself through what was going on in the circuit and decided that it HAD to be that 1M Vol pot!!!! It is a NEW Alpha pot that I put in... I have a Sovtech Mig 50 head laying around and pulled one out of it and swapped it out and BAMMMMMM!!! It WORKS! :evil: @$%*! I should have known not to trust a new part from working on cars for 10+ years but I assumed that it would be different and was eating my own words. the only other thing I found was that I had a 68K resistor to ground on the treble pot instead of the correct value of 6.8K. Stupid mistake again... BUT, it's up and running now and sounds BEAUTIFUL! I thought it sounded great before but was never quite "satisfied" with it over all. Now it is everything I could have hoped for. The highs aren't shrill and ice picky but chimey and sweet that doesn't muddy up when you turn up the bass. Very 3 dimensional with very subtle bite that makes me want to play and play and play but Its late and I have a new job that a start in the morning so I'm really kind of bummed out that I can't and don't really care about the job after playing that amp... :roll: oh well gotta pay for new gear some how, right?

88


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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Wed Mar 23, 2011 3:04 am
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Congrats on finding the problem(s)!!

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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Wed Mar 23, 2011 8:05 pm
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Congrats! You measure that new Alpha Pot? Was it in the wrong range, or just didn't work?


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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Wed Mar 23, 2011 9:32 pm
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one side of the resistor was bad but the other side worked. It had continuity so I passed it up not thinking anything of it... :roll: I even shot contact cleaner in it to see if it was just gummed up but... no dice. I'm glad it is finally finished so I can turn my attention towards my Ampeg SVT-2pro. It blew a tube and I have to make sure it is just the tube and not an internal problem :cry: AND THEN... Finally get on to my next project

La Cabronita clone!!!!! 8)

oh and here are some pics of the build

http://s881.photobucket.com/albums/ac11/jaoldscut86/blueSkull88/


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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Thu Mar 24, 2011 2:39 am
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Nice custom job on that Fender (though it might have Leo flipping in his grave! I hear he was pretty conservative about colors) :D

Good luck on that Ampeg SVT. I have a special place in my heart for those big Ampegs. I played on the Tungsol 6550 loaded SVT going into that monster 8 X 10-inch cab. Loud is not the word. More like "capable of internal organ damage" loud. But what a tone! (if you stand 30 feet away).

I can't believe that Elliot Randall played the solo in SD's "Reelin In The Years" with an SVT with full cab full blast in the WB studios. What I read is the engineers put the mikes all the way down the hall away from the speakers. Slowly moving them about, during each take to get the right sound.

The SVT and V-series Ampegs I've worked on seemed to last the longest with MATCHED grid stoppers and screen resistors for each push-pull pair of tubes. I used 5 watt Mills & hand measure each one to get the two that come closest in resistance. Seems that if you can keep the pp pairs in harmony, all is well. Once any tube, of any pair starts to go out of whack... there's still horrendous cascade effect that ends up taking out a lot power tubes and sometimes the OPT (UGH!!!!).

Those "avalanche" diodes off the power tubes are very important. I usually replace all of them. They are there to prevent OPT flyback, when you turn the amp off. That humongous OPT is capable of many amps of flyback current.

Hey! Good luck. A worthy amp of restoration.


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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Thu Mar 24, 2011 2:08 pm
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BMW2002Ti wrote:
The SVT and V-series Ampegs I've worked on seemed to last the longest with MATCHED grid stoppers and screen resistors for each push-pull pair of tubes. I used 5 watt Mills & hand measure each one to get the two that come closest in resistance. Seems that if you can keep the pp pairs in harmony, all is well. Once any tube, of any pair starts to go out of whack... there's still horrendous cascade effect that ends up taking out a lot power tubes and sometimes the OPT (UGH!!!!).

Those "avalanche" diodes off the power tubes are very important. I usually replace all of them. They are there to prevent OPT flyback, when you turn the amp off. That humongous OPT is capable of many amps of flyback current.


+1

Virtually every component in the V6's output stage -- from the phase inverter to the speaker jacks -- is über-critical for proper performance. Same as if it were a Fender 400PS or a Sunn 2000S. Corner-cutting is not recommended for these amps.

Arjay

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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Mon Feb 13, 2012 12:40 pm
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I have a question for the group--

What actually is suppose to be wrong with the AB165?
Are the proposed changed suppose to bring the amp back to an earlier design, or to improve the existing design?
I have a "Blackface" version of that amp which I bought a many years ago, and it has worked and sounded pretty good to my ear.
I have also listened to some of the YouTube demos, and they all seem to also sound OK.
There certainly were a lot of the BF/SF AB165s sold and used.
I don't use the amp much as I use smaller easy to carry amps, but when I listen to it, or use it, I don't get complaints of poor sound.

Thanks for your answers.


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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Mon Feb 13, 2012 8:43 pm
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Hi Lotus, it's not that there's something wrong with the AB165. It was just another design to achieve a cleaner bass amp. For some guitar players, it can seem a little stiff. I redesigned the pre amp section on my AB165 to suit my preferences. Also changed the power section to an AB763. But that is not, at all necessary, if you are happy with your amp!!! BTW, I enjoy trying different things with that Bassman. Art

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