AB165 Bassman Mod Choices

[Tapp]

Yeah, I also replaced the 15K, but I think all I had in my toolbox was a 30 or 31K resistor; still too much bias voltage. I will parallel another 30K and see what happens.

I'm looking forward to hearing your mod. I assume it's crucial to change out the pots to 1MA's on the Bass channel side?

[billyz]

If you do the mod there are a few more tricks not on my website. Pm me .

[shocki]

Could you tell me what this mod sounds like? I have seen other mods where pretty much a Marshall preamp section was used. Your schematic looks much more simple to do. Could you explain to me what the changes in the third and fourth preamp stage do? The 470k is some kind of feedback that is removed. I am not really shure about the 10k and 150k resistor though.

Thanks alot.

[condorface]

Shocki,

The mod changes one channel to be more like a trainwreck express preamp. The sound won't be exactly the same as a trainwreck though, because the poweramp section is 6l6's. However, trainwreck clips on youtube may give you a better idea of how it would sound. The powersection is also modified to more of a blackface type powersection, and i think thenegative feedback has been reduced to make the poweramp breakup earlier.

Unfortunately, i have not heard it myself in person. I have a bassman that has been modded this way on the bench, but ran into a snag with a bad power transformer. Haven't had time to come back to it. If i ever get it running, i will post a clip.

[shocki]

Ahh now I see what it is. As far as i understand the parallel triode does add gain? Is this basicly the same as shared cathode like you see here in a Marshall amp? So is this also a parallel triode? Or is this different and the grids are not connected in this case?

http://music-electronics-forum.com/atta ... -1992u.gif

[condorface]

The marshall amp you link to does not have a parrelel triode. The cathodes are shared in the first stage of the marshall, but the anodes are not shared. As i understand it, The parrellel triode is mostly used for lower noise. I think it also provides lower impedence for the tonestack.

[shocki]

OK. I modded this to the trainwreck schematic and added a La Mar MV.
It makes the amp more versatile for sure. I don't know what a Trainwreck sounds like in "real live" but to me the amp still sounds like the same amp with more gain and a little more grit on tap. Slightly different voicing. I thought it would sound more "marshally" but it does not.

Changing the tone section to Marshall specs brought some changes.. But I did way less than I thought actually. I will add a trim pot to the feedback loop and play around with it.

Would a cathode follower or change of the tone stack position do a lot for getting a Marshall kind of sound?

I know its not the 6l6 since I played a Suhr OD100 with 6L6 and it really could do a very good Blackface amp and a Marshall sound at the same time.

If there is a reason that it cannot sound like a Marshall that is OK with me. I just would to know what the reason could be. If its the voltages and filtering it cannot be done I think.

[electrochimp]

What difference will changing the treble and bass controls from 250k to 1 M be? (on the Longhorn mod)

Thanks

[electrochimp]

I did the longhorn mod on my AB165 except I still have the 250k treble and bass pots in it along with the 6.8k mid resistor. The sweet spot at low volume is about Treble-2 and Bass-8. What to do to increase or balance sweet spot. I haven't been able to crank it to see if the pots would go towards the middle or...?

also I changed the 150k to 56k thinking it would be bluesier?

any suggestions?

Thank you

[Teleguy61]

Download the Duncan Tone Stack Calculator, and it will answer most of your questions.

[electrochimp]

Thanks TG, It looks like putting in a 1M bass pot with the 10k will get it more balanced control-wise.

Does the 150k resistor produce more gain/ tone difference than the 56K ?

[chief mushroom cloud]

a fun mod is removing neg feedback...way more fun amp to play
or add a 250k pot on back in place of 47k FB res

[electrochimp]

It sounds real nice now but it has an intermittent "tick" a little longer than a second apart when it ticks. It doesn't go up and down with volume controls and it's there when the volumes all the way down. I recently changed the filter caps and bias cap so it is quiet hum-wise.
Now that the amp has been on awhile it hasn't ticked for over 5 minutes.
This is an AB165 Bassman, no trem...just puzzling me

thanks

[Mant]

i did some simple mods to my AB165.
Love the regular channel so left that alone.
Blackfaced the phase inverter and redid the bias to adjustable instead of hum balance.
Re worked bass channel into a more tweed bassman values.
Added mid control, Defeatable tonestack and switchable neg feedback.
Sounds real good and pretty flexible.
Love the touch sensitivity with the no neg feedback.