'67 Bassman amp, modded!

[ckim715]

First off, I want to thank you guys for indirectly helping me do this. Before I learned how to build pedals and read schems, the simple thought of working on my amp intimidated me. Thanks to Brian and all of you kind folks who have helped me out, I finally attained enough knowledge to do this! Anyway, I digress.

I'm just about finished modding it to my tastes. I reworked the circuit to get the normal channel back to close to original blackface fender circuitry, and replaced all the electrolytic capacitors. I did these mods:
http://acruhl.freeshell.org/mga/main/AA864_vs_AB165.html

All electrolytic capacitors were replaced with Sprague Atom caps.

The "bass" channel is where it gets really interesting. I rewired and replaced some values to bring it closer to a Marshall-esque flavor. he tone isn't exactly plexi, but it gets close. So now, I have an amp that will do Fender cleans, and Marshall crunch if I choose. More specifically, the tone stack caps were swapped from .1uf to .022uf. The deep switch is no longer wired to a .1uf, but a 2200pf cap. The 250pf treble cap was swapped for a 500pf, and the slope resistor was changed from 100k to 33k. Last but not least, the 6.8k resistor on the bass pot was changed to a 22k to let more mids through.
The best part is, I can even run the two channels in parallel, and get some crazy thick tones. PICS! Enjoy!

[bigmufffuzzwizz]

Super awesome!! I bet that amp rips now! My buddy had a '65 bassman that he got that marshall mod done and it sounded very very good. There was much more color and there was more gain than the other channel. His tech told him to play it through a 4 ohm cabinet to get the best results from the amp.

[stevewire]

Hey ckim715,
Awesome job.  Very impressive.  Although not as involved or even cool, I just upgraded the coupling caps in my Chinese AC30CC a couple of weeks ago.  I would also say that building pedals gave me the courage to take this on.  I have a Mercury Magnetics output transformer waiting to be swapped in, but I am a little under educated on how to carry that out.



This PCB really pales in comparison to the Bassman's turret board, carbon comp resistors and such.  Your pictures look so cool.

[ckim715]

My turret board may have all the mojo going for it, but you've definitely got the cleaner and easier amp to work on! Tracing under-board wires was not fun at all. What all is involved in swapping the transformer? The more I delve into this stuff, the more I get interested in it

[madbean]

Looks great. Boy, I sure do love amp guts.....

[pandadandan]

Looks great. Boy, I sure do love amp guts.....

The guts of my amp are pornographic they're so beautiful...

Some pretty capacitors in those photos above, though!  Love 'em.

[stevewire]

My turret board may have all the mojo going for it, but you've definitely got the cleaner and easier amp to work on! Tracing under-board wires was not fun at all. What all is involved in swapping the transformer? The more I delve into this stuff, the more I get interested in it

I was thinking it looked a lot easier to get in and work on your amp.  There was so much that I had to unsolder and take apart before I could get to things.

As for the output transformer, Mercury Magnetics sent a little sheet with a few things marked on it such as "primary" and "secondary", but I am just not really getting it.  The stock transformer has different colored wires and everything is braided so it's difficult to figure out how things are connected.  I've read online about so many people doing this, but I can't find the information how to do it.

[ckim715]

The thing that helped me out a lot was to find a layout AND a schematic for my amp. That way you have the schematic, and you can visually reference where it is on the layout, then look at it on the amp. Cross reference your original OT, as I am fairly positive there is a datasheet on it on the internet, then match the wires with the wires on the Mercury. That's how I'd go about it, anyway hehe

[pandadandan]

Your primaries are for the high voltage portions of the amp circuit.  The AC from this is rectified into DC (usually by diodes or a rectifier tube) which then hits the plates of the tubes.
The secondaries are for the heaters of the tubes (the parts which glow a nice yellow colour).
The CT is grounded.

Here's mine prior to DE-modding.  Some idiot had stuck in a PPIMV to get the amp to break up early.  It's not designed to break up early!  Had to buy some fancy vintage mustard caps and fix the shitty cap job the previous owner had done on it.

[ckim715]

aaaaaaaaand now I'm embarrassed by the messiness of my board 

At least I can blame Fender for the majority of it, lol

What amp is that? That wiring is ridiculously clean!

[pandadandan]

aaaaaaaaand now I'm embarrassed by the messiness of my board 

At least I can blame Fender for the majority of it, lol

What amp is that? That wiring is ridiculously clean!

It's just the Fender wiring style; perfectly cool.  All that cloth wiring rules.
Mine is an old Hiwatt, wired in the Harry Joyce manner (although not by him directly) which is known as "mil-spec" style.  Old Hiwatts are famous for this; they are regarded as the ultimate clean wiring amps for the way they're put together.  It actually looks better now than in that picture; the amp tech sloshed a load of silicone goop over a load of parts for no real benefit.  It's now back in original shape.  And loud.  Ungodly loud.