SF Bassman Gain Mods

[Jag]

Hey Satamax!

Did you do HTH's mods or are you referring to the mod you posted?

I've got 3 SF Bassmen (3 BF's as well but they're hands off), and I'm thinking of trying one of each out of curiosity.

[Satamax]

Hi Jag!

Well, that's my mod, not HTH's.

[Russ]

Bruce,

Would it be very hard to modify the PCB so that it accepts a stereo clif jack and so that you could set it up to have 2 gain settings depending on how far you push in the plug?

[Bruce / Mission Amps]

Yes too hard... it is what it is and a tiny PCB with no way to easily mod the thing.
However, it just goes to show you that somebody will always think of something else that they can't live without.

[steve_e_nixx]

Bassman Mods

here's what I'd do (to the normal channel)...

* change 6k8 resistor on bottom of tonestack to 68k (more crunchy mids/gain - this will bring out that AC/DC vibe you're after)

* make sure you have 12AX7s in all positions - no time for 12AT7s when we're talking AC/DC

* change the 1k5/25uF combo on V1 to 820/0.68uF and the same on V2

* add a 250pF cap across the 220k mixer resistor for the channel you're modding

* increase feedback resistor to 100k

* play with the tonestack - that 100k slope resistor ain't gonna give you those thick low-mids you want for AC/DC. change the slope resistor to 47k and the treble cap to 470pF

* I'd probably change out the 0.1uF and 0.047uF tonestack caps to 0.022uF to stop the bass from getting too tubby.

* finally, you're gonna want a master volume to take advantage of that gain without having to crank the amp to get it. I'd just add a 1M pot before the PI with a 100k resistor off the wiper into the PI.

Cool. Thanks for in info. Ive got my amp cracked and am hacking away!

>S<

[steve_e_nixx]

:
* finally, you're gonna want a master volume to take advantage of that gain without having to crank the amp to get it. I'd just add a 1M pot before the PI with a 100k resistor off the wiper into the PI.

I was going to do this and was wondering if the pot should come before or after the .1 uf coupling cap before the PI? Linear or audio taper?

THanks again!

>S<

[Fred G.]

Steve_e,

BEFORE the .1 uf cap going into the PI. While you're at it, change that cap to a .01 uF. Use an Audio taper pot for the MV.

Fred G.

[steve_e_nixx]

MV

Fred,

Thanks. Ill be doing that tonight.

>S<

[Jag]

Putting it before the .1uf will put DC on the pot. I would put after the cap myself, no scratchy DC noise that way when turning the pot.

Changing that .1 to .01 is kind of a user preference. The .1 will allow more bottom through. Some people like it some don't. I usually kepp the .1 there YMMV.

Yes, you want audio taper. I prefer post-PI cross-line master using a 250K no-load audio taper. Again, that's a user preference.

[Fred G.]

Jag,

My bad.

I just looked at the schem of Steve_e's amp and I see what you're saying - I put mine in my Bandmaster right from the schem of a silverface Twin Reverb, but both those amps have an extra coupling cap coming off the 2nd gain stage into the 220k resistor. The AB165 Bassman doesn't have that - it's a whole different critter.

The post-pi master is another animal altogether. Steve_e might indeed want to look into that, but I'd suggest he try it later. Personally, I've tried every variant of the post-PI master, and ended up yanking them out sooner or later out of sheer frustration. All that variation in the NFB loop at different volume settings just got too troublesome (for me, anyway). YMMV...

Fred G.

[drewl]

FWIW I installed a PPIMV in my ProReverb....used the trem Intensity pot hole and fixed the Intensity at half way up with resistors.

It sounds really good and can get some good grind when cranked.

Bruce, great little idea there, reminds of the little "Orange Squeezer" compressors that plugged right into the input jack.

[boyer]

Re: Master vol

Hi all,

On my SF Bassman I added a relay to switch channels, and used the ground switch hole for the footswitch jack. I wired the inputs up so that the Bass ch input 1 is the only input hooked up.

I removed the first normal channel jack and put my MV pot there.

If you don't want to add channel switching, and only use your first input, you can remove the 2nd input on the bass channel, and move the first input of the normal channel to the 2nd input of the bass channel. You'll then have two free holes on the panel.

Since I never use the deep or bright, I have my deep switch hooked up like the gain mod for the Blues Maker by John McIntyre (google for the "john mcintyre blues maker").

The Bright now functions as a channel switch in absence of the footswitch.