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Post subject: Re: Bassman ab165 pop and hum
Posted: Mon Apr 08, 2019 4:04 pm
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Here is a video of what I’m hearing.

https://youtu.be/RUrk1pgteIk


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Post subject: Re: Bassman ab165 pop and hum
Posted: Mon Apr 08, 2019 4:24 pm
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TomL82 wrote:
The circuit has been modified from the bias balance to the blackface style bias system.


Your problem may lie here. The bias-balance control *IS* a blackface style circuit, introduced in January of 1965. If someone modified that layout to a true symmetric bias control then you need to reference the AA864 schematic for component values and servicing. This circuit preceded the AB165 revision but was only used for +/- four months.

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 5:30 pm
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Do you have a photo of the new cap setup, under the doghouse? And a photo of the board holding the power supply & bias supply diodes, cap, and resistors?

This is what I'm hearing. Which means you have a PSU issue, post rectification. IMHO.



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Post subject: Re: Bassman ab165 pop and hum
Posted: Mon Apr 08, 2019 5:36 pm
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Retroverbial wrote:
TomL82 wrote:
The circuit has been modified from the bias balance to the blackface style bias system.


Your problem may lie here. The bias-balance control *IS* a blackface style circuit, introduced in January of 1965. If someone modified that layout to a true symmetric bias control then you need to reference the AA864 schematic for component values and servicing. This circuit preceded the AB165 revision but was only used for +/- four months.

Arjay



Thank you Arjay!! I found it!!

I reviewed the bias section of both the schematic from the AA864 and from the AB165, and found my amp was a hybrid of both. I wired my amp to match the AA864 schematic, and now the hum is completely gone. Thank you all very much!


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Post subject: Re: Bassman ab165 pop and hum
Posted: Mon Apr 08, 2019 6:17 pm
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Pay my receptionist on your way out.

:mrgreen:
Glad I could help.

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 8:43 pm
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TomL82 ,

You said everything look good! With your eyes you can't say.
What are your experience in electronic ? On amp repair.
With not lots of experience, you may not do good test

I read on s many forum and after few weeks of question and answer, the OP did not do the right tests.... :(


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Post subject: Re: Bassman ab165 pop and hum
Posted: Tue Apr 09, 2019 6:51 am
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TomL82 wrote:
Retroverbial wrote:
TomL82 wrote:
The circuit has been modified from the bias balance to the blackface style bias system.


Your problem may lie here. The bias-balance control *IS* a blackface style circuit, introduced in January of 1965. If someone modified that layout to a true symmetric bias control then you need to reference the AA864 schematic for component values and servicing. This circuit preceded the AB165 revision but was only used for +/- four months.

Arjay



Thank you Arjay!! I found it!!

I reviewed the bias section of both the schematic from the AA864 and from the AB165, and found my amp was a hybrid of both. I wired my amp to match the AA864 schematic, and now the hum is completely gone. Thank you all very much!


Cool beans! Arjay is our Fender Guru!

:mrgreen:


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Post subject: Re: Bassman ab165 pop and hum
Posted: Tue Apr 09, 2019 10:46 am
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Yea, that's a good one. It is a very unusual bias balance circuit.
But aside from that issue, I'm looking at the posted schematic and seeing something I have never seen before on any amp.
There are 220K resistors going from the plates of the output tubes to the plates of the Phase inverter.
Since its a bass amp, I gotta assume its a negative feedback compressor circuit to lower the gain of the main stage. There is 225 VDC on one side and 425VDC on the other. While it still uses the normal loop from the speaker back to the phase inverter, this loop is local to the power tubes only. This gives it more accuracy and speed of response without much phase shift.
I wonder why this concept wasn't done in other Bassman amps.
It seems like a simple mod to do to other amps to lower gain.
I'm just sketchy about hooking plates to plates. Seems like your inviting DC stability issues and signals phase shifting into oscillation. But there it is in that Bassman. Built. Stable and sounding good.
I think I'm gonna try that on my Thrust amp and see what it does.


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