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Post subject: Fender Bassman AB165\AA864 issues (120hz/attack/lows)
Posted: Wed Dec 13, 2017 6:54 pm
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Hello forum.
!Short questions in the end of this post!
So i've got a problem with my amp. Originally it was an AB165 blackface bassman with some minor mods (like cut 220k feedback and some 0.01 (?) caps). I didn't like the sound because of HUGE amount of highs, amp always set to treble 0, bass 8.

Then i bring it to an amp tech. He screw the amp a bit. It sounded better on a bass channel, but have a strong 120 hz (100 actually, but it doesnt matter cuz i have 50hz in my country) buzz/hum, whatever you call it. It doesnt change with any volume and disappears with pulling out an output tube

So i opened it up, cut 220k resistors feedback - noise disappeared. Now i have a question - why it was there? Maybe that's a key for another question.

Ok, so i made an Andy Ruhl mod. There started another (and still MAIN) problem: when i hit strings hard, amp pushes a speaker, and it makes low pop, or boom, or whatever. Something like your amp want to breakup, but it told to stay clean. It happens mostly on bridge single pickup of my strat with 3rd string. Also when it happens, the 6l6's are glowing blue. I know thats normal, but it happens mostly when speaker pushed forward by amp.

Also i cut two wires from the bottom of the board: one from 0.1 cap in PI, and another from 47k resistor in PI (according to a chematics). That gave me a bit more volume and overall sound, but the problem is still here

I wanted to bring amp to the basics - AB165, but i dont have needed parts for now, and i think the problem is not that mod. Maybe i'm wrong. who knows

Things that have been done:
1. Changed caps
2. Changed resistors
3. Changed output tubes
4. Cleaned sockets
5. Tested all kinds of cables and guitars
6. Changed diodes
7. Added mid knob for normal channel (just for my taste)

Once again, my main questions:
1. Why there was a 120 (actually 100) hz noise/buzz, when 220k feedback resistors was connected?
2. What can cause the bass pop-boom (also pushing the speaker which looks really scary) and staying clean instead of breaking up when i hit my strings REALLY hard (not touching magnets thou) , mostly on bridge pickup? Could it be a burned ele-cap in a doghouse (and i dont know its dead or something)? Maybe some wrong wiring under the board, which is too hard to fix? I don't know, help me please ;D

Thanks for your posts, guys. I have seen same problems from other guys, but they havent found the answers. Maybe you can help not only me, but also this folks.

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Post subject: Re: Fender Bassman AB165\AA864 issues (120hz/attack/lows)
Posted: Tue Dec 19, 2017 2:49 am
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I don't know about the mods you are talking about.

A great amp like this one should be kept with original circuit.

1- I will rebuilt it as original blackface
2- I replace cheap 'lytic capacitor for good F & T caps
3- I do a complete wires dress. As it is now sure you may have noise
4- Find a qualified amp tech

Show picture under the dog house.
Where do you dowload picture, it work fine


Last edited by stratele52 on Tue Dec 19, 2017 10:16 am, edited 1 time in total.

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Post subject: Re: Fender Bassman AB165\AA864 issues (120hz/attack/lows)
Posted: Tue Dec 19, 2017 8:33 am
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Bassman's are a super popular platform for mods. IMO nothing wrong with that. They weren't really optimized for guitar, and doing one channel for Fender sounds and the other channel for Marshall can result in a killer guitar amp. Andy Fuchs built his highly regarded reputation on modded Bassmans.

I might draw a line on drilling holes in the chassis or other irreversible mods, but you haven't done anything like that.

However, you've done mods on top of mods. And you say one tech did some mods badly.

A Fender (or a Marshall voice) should be bright, but not so overbearing that you have to turn the treble knob to zero. (!)

I'd tend to agree that you should start off by returning it to stock and get it working perfectly. I don't think a bad filter cap fits any of your symptoms, but if you don't know when the last time the doghouse was opened it would be a good idea to check them. Lead dress is sloppy and should be cleaned up but again doesn't seem to be the likely cause of your problems.

Once it's healthy, if you feel the need to mod it you won't be chasing your tail trying to track down problems caused by previous mods.


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Post subject: Re: Fender Bassman AB165\AA864 issues (120hz/attack/lows)
Posted: Tue Dec 19, 2017 8:43 am
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Re-reading the first post:

Pop boom speaker pushed so far forward it looks scary?

That sounds like DC leaking into the output. A good way to kill a speaker.

Not sure what you mean about not having the parts to return it to stock? Some caps, resistors, maybe some wire? It's not necessary to use exact replacement parts -- doesn't have to be carbon comp resistors or blue molded caps, just the original values/ratings.


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Post subject: Re: Fender Bassman AB165\AA864 issues (120hz/attack/lows)
Posted: Tue Dec 19, 2017 10:13 am
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strayedstrater wrote:
Re-reading the first post:

Pop boom speaker pushed so far forward it looks scary?

That sounds like DC leaking into the output. A good way to kill a speaker.

Not sure what you mean about not having the parts to return it to stock? Some caps, resistors, maybe some wire? It's not necessary to use exact replacement parts -- doesn't have to be carbon comp resistors or blue molded caps, just the original values/ratings.

Hey! Thanks for your reply.

Now i unsoldered all the weres from the board, took out most of the underwires, soldered it all again on top of the board (to not think of that under-part again). Also now my amp looks (and soldered) like a AB165. Sounds great, but still a bit too trebly. Volume 3-7 (depends), treble 0, bass 6-8, mid knob depends.
I liked the way it sounds, but it needs to be balanced. Ordered new pots and tube panels/sockets, also set of new 0.1mf and 300pf caps.

Checked doghouse - there is new el caps, and i changed resistors to new 2w.

There is just one problem - bass channel doesnt seem to work. I guess i have to doble check all wiring again.
SO generally i'm waiting for new parts and working bass channel, and THEN i'll try to mod amp closer to aa864


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Post subject: Re: Fender Bassman AB165\AA864 issues (120hz/attack/lows)
Posted: Tue Dec 19, 2017 11:02 am
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After researching if DC can get into the output of a tube amp, apparently it can't.

But found references that weak filter caps can cause poor bass response and loss of gain.

Could be that weak caps are causing you to turn the treble so low and the bass high to make up for weak bass response, and low gain is why it doesn't break up when you hit the strings hard.

Perhaps the boom/pop on big signals is the early stage of motorboating? (Also weak filter caps.)

Brand new caps can be bad, and it's certainly possible for relatively new caps to go bad long before they should.

Also make sure the polarity of each cap is correct. I'm not intimately familiar with Bassmans, but many Fenders have 4 hooked up one way and 1 hooked up the other way. Fairly common mistake for people to hook them up all the same way.


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Post subject: Re: Fender Bassman AB165\AA864 issues (120hz/attack/lows)
Posted: Tue Dec 19, 2017 3:45 pm
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strayedstrater wrote:
After researching if DC can get into the output of a tube amp, apparently it can't.

But found references that weak filter caps can cause poor bass response and loss of gain.

Could be that weak caps are causing you to turn the treble so low and the bass high to make up for weak bass response, and low gain is why it doesn't break up when you hit the strings hard.

Perhaps the boom/pop on big signals is the early stage of motorboating? (Also weak filter caps.)

Brand new caps can be bad, and it's certainly possible for relatively new caps to go bad long before they should.

Also make sure the polarity of each cap is correct. I'm not intimately familiar with Bassmans, but many Fenders have 4 hooked up one way and 1 hooked up the other way. Fairly common mistake for people to hook them up all the same way.


Thats a good info! I will test all filter caps. Also, they are wired in a right way - 1 reversed.


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Post subject: Re: Fender Bassman AB165\AA864 issues (120hz/attack/lows)
Posted: Sat Dec 23, 2017 8:30 am
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Did you take into account that you need to place the Global Negative Feedback on the other side of the phase inverter, when you blackface a typical Bassman AB165? This era Bassman had their feedback inverted (for reasons, I can't remember).

If you do not reverse the GNFB position onto the PI, you'll get the craziest howling, buzzing noise (you are actually applying positive feedback).


Tone Lizard explains it well, in following link --- about 1/2-way down the page.

http://tone-lizard.com/bassman-mods/


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Post subject: Re: Fender Bassman AB165\AA864 issues (120hz/attack/lows)
Posted: Sat Dec 23, 2017 2:19 pm
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BMW2002Ti wrote:
Did you take into account that you need to place the Global Negative Feedback on the other side of the phase inverter, when you blackface a typical Bassman AB165? This era Bassman had their feedback inverted (for reasons, I can't remember).

If you do not reverse the GNFB position onto the PI, you'll get the craziest howling, buzzing noise (you are actually applying positive feedback).


Tone Lizard explains it well, in following link --- about 1/2-way down the page.

http://tone-lizard.com/bassman-mods/

Hey! You mean the wiring on speaker outputs? If so, i made it right. In a wrong position there is no useful sound.
If not - i'm probably doing something wrong :mrgreen:


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Post subject: Re: Fender Bassman AB165\AA864 issues (120hz/attack/lows)
Posted: Sat Dec 23, 2017 7:04 pm
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No, you actually NEED to reverse the anode plate leads and reposition the global negative feedback wiring. Check this schematic. Explanation is next to this schematic, on Tone Lizard site.

http://i0.wp.com/tone-lizard.com/wp-con ... dAB165.gif


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