Bassman AB165

[punkykatt]

Hello all, Im restoring the electronics on one of these amps and  looking at the schematic  http://www.el34world.com/charts/Schematics/files/fender/BASSMAN_AB165.pdf   there is a 220k resistor connected between the plate of each 6L6 and each plate of the 12AT7 PI. The 12AT7 PI already has its 100k plate resistors coming from the PS. I have never seen this before.  What is the function of the two 220k resistors between the plates?  Thanks in advance.  Punky

[LooseChange]

Feedback circuits.

[HotBluePlates]

And it works just like the 470k local feedback circuit on the 3rd stage of the preamp.

Notice also that the feedback from the OT secondary is injected in the opposite side of the long-tail than is standard. If you undo that injection point (make it like other Fender amps), you will need to swap OT wires to avoid the positive feedback howl.

[Tiny_Daddy]

Save this mod before it disappears off the web:
http://archive.ampage.org/threads/0/ampgen2/012234/AB165-AA864_mods_lvery_long-1.html

[punkykatt]

Thanks for the info guys.  Tiny Daddy, On the Ampage AB165/AA864 mod they spoke about, I assume they are removing those two 220k  feedback resisters between the power tube and PI plates? . thats why you have to swap the speaker wires? YES?  NO?  He doesnt actually say to remove them when he talks about the power amp section.  I found the read a bit confusing, he mentioned PI grids when I think he ment to say plates.  Whats your take on it?  Thank you for your time.  Punky

[PRR]

> removing those two 220k feedback resistors between the power tube and PI plates? thats why you have to swap the speaker wires? YES?  NO?

No.

The changes from the older conventional amp are extensive.

You may remove the new-to-AB165 220K 6L6-plate-to-12AT7-plate resistors with no other change.

Look at the feedback from speaker-tap to driver. It normally goes to a cathode or to the "other side" of a longtail. In AB165 it goes to the driver INPUT. That's opposite signal polarity. To keep it negative-feedback (non-howling) you must also (as HBP said) change polarity somewhere else. Some guys swap at 6L6 grids. Most guys swap the OT plate leads. If the amp has been mucked-around, you may have to experiment to get the polarity right. (Disconnect NFB, test for OK but loud/brash, connect NFB. If it howls, swap polarity somewhere and try again.)

AB165 also applies NFB around the mixer stage. This makes a more-theorectical mixer. Also sets overall gain, which could be too-high with four gain stages. Also reduced mix-stage THD.

I think Leo (or CBS?) got on the wrong track with this model. That it tries to go in the direction of "High Fidelity", but doesn't quite get there, and hi-fi may not be what guitarists want. The older models are widely copied; this one aint. There is a steady business converting (more or less properly) AB165 amps to the older circuit.

[punkykatt]

Thanks PRR,  Makes  sence now. I didnt look to see where the NFB was connected to the PI input.  I asked the question to soon.   I should have look closer.  Thanks for the informative  reply.

[FYL]

Save this mod before it disappears off the web:

http://www.blueguitar.org/new/schem/fender/modnotes/mga-aa864_vs_aa165.pdf

[punkykatt]

Sorry guys, I must apologize,  I was having  senior moments yesterday(brain farts).  My mind was in my butt.  After a good nights rest and re reading the posts I feel real stupid