AB 165 mods - NFB & Mid control

[angelodp]

Hi, I have a nice old 1967 Bassman AB165, It is in fact the published schematic and layout. I have yet to replace the doghouse caps and a few of the bypass caps are the older dual caps, otherwise its pretty much stock. It has a grounded three prong plug done correctly.

I removed the death cap and two prong service AV outlet ( saved with no destruction), I also removed the Polarity ground switch (saved).

It place of these two components - I added a 50k pot for a Midcontrol and linked it to the 6.8k R on the output of the Bass pot. I used shielded cable and grounded one end. It works well and does not add noise, but does provide added dynamics on tap.

In place of the Polarity switch I placeed a 250k pot. I relocated the wire that connects to the NFB R (110k on this amp) over to the input of the 250k pot. Then I ran a shielded wire over to the output jack. Pot is as a variable resistor. This works well to dial in additonal gain, but it does raise the noise floor a fair amount. The .1 cap is as it should be in place.

I suppose I do need to change out the caps before trying to trouble shoot the 120 hum ( I think its 120). Does the arrangement I described seem like an idea that has merit.

Best Ange

[martin manning]

Recap it for sure if you want to play it. I'd expect the hum to diminish and the low-end to get stronger. Mid pot and variable feedback mods are common, if they are useful to you, then it's all good! Many people disconnect the local feedback in the power stage (220k's). Lot's of info here: http://fenderguru.com/amps/bassman/

[angelodp]

Martin,nice to hear from you. I'll read up some more.

Best Ange

[angelodp]

Re-did the Normal Tone-stack w/Mid-pot per a nice diagram from Martin, works great. Proceeding to recapping.

Question- The original caps are 2-70uf in series then 20uf's and an 8uf - I am thinking

70uf - either 80uf or 100uf - would that make the amp too stiff

20uf - 22uf - no big change there

8uf - to 22uf as well - any issues with that?

Amp sounds very good and just trying to get the noise floor down to lowest possible level. Cathode caps are gonna get replaced as well. New tubes coming.

Wonderful platform for a few choice pedals. I jumped the bass channel into the normal channel and had my trem on the bass channel with Blues driver, Reverb and Boost on the normal.... nice.

[martin manning]

I think I’d replace 70 with 80, 20 with 22, and 8 with 10, but a 22 there would probably not make any real difference. Post that tone stack diagram you mentioned, I’m curious to see which one you are referring to.

[angelodp]

Hi Martin, this one.

[angelodp]

If any eyes could check this arrangement. I know the 50k pot on Mid is a bit excessive.

[sluckey]

I like to use a 250K-A mid pot. The first half turn acts like a regular mid control and the second half turn acts like a raw control (effectively lifting the tonestack) that gives a boost with a bit of growl.

[angelodp]

Hi, Ok, so maybe experiment with a RAW option might be interesting. As far as the layout I posted, it seems ok though?

A

[sluckey]

yes

[angelodp]

I just tried bridging over the 470R LNFB with alligator set up and the amp went very quiet. I was expecting the amp to get much louder?? Ok, mistaken approach. I lifted the 470K and now it roars, literally raised the db by a factor of two, I think. Its damn loud with lots of gain. Maybe too much.

Ange

[angelodp]

Here is the amp as it is now. I added a 120pf to the NFB R.

A

[Paul G.]

Filtering: These amps can take more filtering. I send 'em out 80/80/22/22/22/16. My personal amp has 220/220/22/22/22/16 under the dog house.
NFB: I changed mine to AA864 -- pain in the you know, but the amp feels looser. Doesn't add tons of gain, but enough for me. You've got to lift the board to remove a jumper. Schematic here: http://fenderguru.com/wp-content/upload ... u-mods.gif

and photo of my board for an idea of what it looks like after the mod. You're pulling a .1uf cap which leaves an unused eyelet to connect the 100R tail resistor and then jump it to ground.

changes.jpg

[Roe]

I prefer at least 50uf on the mains of these amps. even 110uf works well

[Paul G.]

I prefer at least 50uf on the mains of these amps. even 110uf works well

Yup 220/220 in series gives me 110uf and I prefer the amp this way. I've had 35uf (oem), 50uf and finally ended up with 110uf. Overtones are never harsh, and the bass never gets sloppy.