bass amp builds....

[briane]

ok - I have a lot of people asking me about bass amps - I've built a number - rather than send out the drawing 5 times, here it is.

Sorry - I dont have a full drawing, and no time to sketch. Heres the pre-amp and cathode follower, just put the drawings together and add the power section.

I build a marshall 1986 100w or 200w version - but essentially add a cathode follower, 12ax7 - similar to ampeg svt - the 1986 schematic can be found at ceria**** - so I wont post that here - just insert what I have drawn here for the new CF, post PI, and add a 12ax7 to the 1986 schematic.

Then make it a single input/2 volume knob configuration (bridge v1). add GNFB 7.5k/82 switchable (ie aggresive switch), and local NFB to v1 aka dumble 124, 20m/.005/20m (mallory) to enhance the smoothness. This works well.

Run 2x6550 min, kt88 also sound nice. I use big ass bassman 100w transformers for the 100w build, webers seem to work nice for the prices. Youll have to double up the transformers or get some bigger iron for the 200w version.

That should be enough to suss it out. The Cf drawing - follow the red marks to the power amp....Thats what I did.

cheers, have fun!

[briane]

uploaded build pics....

[LeftyStrat]

Thanks!

[jbrew73]

Couple of questions if you don't mind.

At the junction of the 82k and 100k PI resistors is that a 10k balance pot?

Your chassis picture seems to show two resistors from the first stage coupling caps to ground. Is that correct? What value?

The local NFB, is that actually on v2?

Can you explain the switchable gnfb?

Sorry for so many questions and thank you for the drawings and pictures!

[briane]

no problem - been a while since I've been in that amp...

At the junction of the 82k and 100k PI resistors is that a 10k balance pot?

yep

Your chassis picture seems to show two resistors from the first stage coupling caps to ground. Is that correct?

no - I think should be one aka 1986 pre-amp - 1.5k? maybe

The local NFB, is that actually on v2?

yep - been a while

Can you explain the switchable gnfb?

I use a 8.2k +- as the min, add in another 75k via switch for more aggressive sound. Gives a few options with the dual volumes voiced a tad differently.

[jbrew73]

Does anyone else have any experience,or advise to offer on the post pi cathode follower?

I've read quite a lot on the matter but its mostly theoretical information. I'd like to hear from anyone else that has implemented this into a 100-200 watt amp.

Also any comments on the use and level of nfb in a bass amp. If I remember correctly mesa boogie doesn't use nfb on some of their tube bass or power amps.

Opinions welcome.