SF Bassman Gain Mods

[steve_e_nixx]

Hi, first post here . . .

I have a SF AB165 bassman and want to get lots more gain out of it. I dont want to use an attenuator or distortion pedal.

Can anyone suggest some good mods to crack up the gain? Id like to keep the mods to reversable ones. I play in an AC/DC type rock band.

THanks!

Steve

[tubeswell]

Well I've been doing quite a bit of experimenting with Pre-amp gain stages myself in the last month.

If you're past subbing different valves, then this little page has a few good tips (at the end)

http://www.diystompboxes.com/pedals/tubedummy.html

Things I've tried (and am still trying - I took my amp aprt and put it back together five or six times the day before yesterday) include changing to smaller grid stopper resistor value(s), changing to less attentuation/padding in the coupling stage(s) - Just watch if you are trying to alter the Grid leak resistors that you don't get too high (as the man says), lowering the cathode resistors (820 Ohms was the lowest I safely got to in my C30), changing the cathode bypass caps to 22uF or 25uF values, Upping the Plate voltage (by putting in smaller value supply resistors), increasing the Plate resistor values (just be wary subbing a pot there), ...

Of course when you start doing all this you pretty much end up also having to muck around with the tone (different value coupling caps between gain stages etc) to get the thing sounding nice once you've got all that distortion the way you want it.

Its a mucky iterative journey

Good Luck

[Satamax]

Steve, try to adapt this one to the bassman

http://satamax.free.fr/bandmastermod.jpg

[Bruce / Mission Amps]

Not to turn this into a sales brochure, but have you considered this?:

http://www.missionamps.com/turbojax.shtml

[Steve A.]

Not to turn this into a sales brochure, but have you considered this?:

http://www.missionamps.com/turbojax.shtml

Bruce:

That looks really slick! Did you design that yourself or do you just sell them? I like the way that you can adjust the gain trim pot from outside the amp with a small screwdriver...

Without giving away any proprietary secrets, how does it work with a 70-300vdc power supply? Is there a voltage regulator in there so that the circuit gets a constant voltage?

Thanks!

Steve Ahola

[hasserl]

Dang Bruce, I like that idea! (your's too Max, but I've seen that here before, this is the first I've seen Bruce's). Finally, something to do with the #2 jack. I never use them now.

[Bruce / Mission Amps]

Yes... I had my buddy Randy Linenberger draw up the PCB from notes I gave him.
It isn't anything outrageous, a decent FET preamp that is all clean boost.
The 70-300v is termindated into a voltage divider with a 5w 18v zener across the last grounded resistor in the divder and a filter cap... all on the PC board.
The adjustable pot accessable through the Cliff jack just came to me about the same time we were working on the final PCB layout...
It works really good and is very clean... the band pass can be altered by using different value source bypass caps and coupling caps.

[Bruce / Mission Amps]

Dang Bruce, I like that idea! (your's too Max, but I've seen that here before, this is the first I've seen Bruce's). Finally, something to do with the #2 jack. I never use them now.

Me either, that's why we did the Turbo Jax system.
You move the TJ with the Cliff jack to the old high gain jack location and the old high gain jack to the low gain jack hole... now you have someting to work with in any low gain amp and just plug into the old jack for the same as it was before.

[HTH]

Hi, first post here . . .

I have a SF AB165 bassman and want to get lots more gain out of it. I dont want to use an attenuator or distortion pedal.

Can anyone suggest some good mods to crack up the gain? Id like to keep the mods to reversable ones. I play in an AC/DC type rock band.

THanks!

Steve

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.

[Satamax]

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.

Love teh sound of this! Only prob, i don't want to drill holes in a BF bandmaster

[Jag]

You could take out the Ext. Speaker jack and put the master there. I did that to install a cross line (my fav) MV in one of my SF Bassmen. Easily reversible.

[Russ]

Bruce,

Very clever design. Have you considered using a stereo cliff jack so that you could have two gain settings, depending on how far you push in the plug?

Russ

[Satamax]

Well guys, i was playing my moded bandmaster earlier on, this amp is mean, real mean. Guys, i think it's real worth checking this mod! I'll try to practice a bit my playing and put up a sample.

Bye.

Max.

[Regis]

Love teh sound of this! Only prob, i don't want to drill holes in a BF bandmaster

Max, if you have replaced the power cord with a grounded one you don't need the ground switch, you could put it there. Or, as was said you could put it in the extension speaker hole.

[Satamax]

Yep grounnd switch was planned to become either mid control or variable NFB. But i'd like to keep the stock sound on at least a channel! Certainly will make 68K switchable in series with the 6K8! with the bright switch on normal chanel!