1967 Bassman build

[Randall]

Got a 67 or 68 Silverface Bassman head that is totally toast. I want to tear it down, sandblast the chassis, paint it, make a new head cab and build it back up. I want a classic Normal channel, but want to do something cool with the Bass channel. The trannys check out good, albeit pretty rusty. It has the blue coupling caps, and a pair of big bottle RCA 6L6s, but I don't yet know their condition.

Anyone have opinions on what is the best 60's Bassman circuit, and what would be cool to try for the Bass channel? Maybe more gain, without going crazy, still keeping a Fender. I'm looking for a blues and classic rock tone, something that will play nice with pedals.

So far I think the AB165 is a bit odd in some places for a Fender, so I'd probably want to move away from the CBS engineer's idea of how a guitar amp should act, and get back into the nitty gritty.

[Justin Thomas]

Shameless plug for the AB165... sounds awesome! Mine has been my Desert Island amp for going on 7 years, and is pretty much stock except for the 220k feedbacks, a Twin Reverb OT, and a bias adjust added. Look for Machine Gun Amps article on what to ditch & what to keep, by Andy Ruhl. I opted to keep more than leave. One thing - leave the channels in phase - it's FAT. Also, layout is quite picky as far as wiring layout near the controls.

PS this thing is plenty gainy - seriously. Mine is, anyway. Just do some cap tweaks to cull bass. The Normal Channel is better for Bass, anyway.

Good luck!

Justin

[Leo_Gnardo]

An easy conversion to the 5F6A tone stack on the bass channel is mighty nice. Lose input #2 & put a mid control there.

That amp doesn't look awfully rusty anyway. Hope you found a nice new home for the ants.

[Randall]

Justin, when you say leave the channels in phase, is this for channel jumpering?

[Justin Thomas]

Yup. Unlike every other 2-channel Fender of the time, the AB165 Bassman has in-phase channels that can be combined without phasing problems. Same for every Bassman that came after, too.

I like that shared third "compressor" stage (Andy Ruhl's term) so much I added one to my Bassman 100.

And it really does make for a fat blues/rock/jazz amp.

Justin

Edit: does this work?
http://acruhl.freeshell.org/mga/main..._vs_AB165.html
I don't agree with EVERYTHING he says, but it's a starting point. And keep in mind, mine has a Twin Reverb OT in it, so I run an 8R load...

[Randall]

Yes, I have read that article before. I wonder which parts you would recommend. Pretty sure I will put a "normal" bias circuit in it with a 100uF/100v cap. Never understood why you would put a 50v cap in a circuit that supplies -50v.

I am drawn to Leo's idea of a 5F6A in the bass channel. No mid control in the input jack for me, tho. I never have liked that. Maybe in the back though, where there are already two extra holes.

[bob p]

Anyone have opinions on what is the best 60's Bassman circuit, and what would be cool to try for the Bass channel? Maybe more gain, without going crazy, still keeping a Fender. I'm looking for a blues and classic rock tone, something that will play nice with pedals.

Transatlantic Bassman.

[Randall]

"Transatlantic Bassman."

Not if it means new transformers, which it seems it would.

[cjenrick]

punch out the chassis for some octal sockets and run RCA JAN 6SL7 prreamp tubes

[Randall]

"An easy conversion to the 5F6A tone stack on the bass channel is mighty nice."

Leo, if one was to do this, where would the two channels mix? The AB165 mixes before the third gain stage, but the 5F6A tone stack is after the third gain stage, and the last thing before the PI input. Are these two points compatible?

[Leo_Gnardo]

"An easy conversion to the 5F6A tone stack on the bass channel is mighty nice."

Leo, if one was to do this, where would the two channels mix? The AB165 mixes before the third gain stage, but the 5F6A tone stack is after the third gain stage, and the last thing before the PI input. Are these two points compatible?

If that's where the Bassman was mixing its channels before, it'll do fine for your modded channel. You needn't add a cathode driver for the EQ stack, just swap out the existing caps for a couple 0.022's, a 33K or 56K slope resistor, and 500 pF or 1000 pF treble cap. Replace the 8200R with a 25K pot for midrange & there you go.

[Randall]

So, if I understand you, keep the modded tone stack before the third gain triode? Somehow, I thought it being after that stage was part of it's particular sound?

And btw, why is the cathode driver there in the 5F6A? Is it to de-couple the plate voltage without using another cap before the stack?

Would it be easy or desireable to utilize the unused triode in V2?

[Leo_Gnardo]

So, if I understand you, keep the modded tone stack before the third gain triode? Somehow, I thought it being after that stage was part of it's particular sound?

And btw, why is the cathode driver there in the 5F6A? Is it to de-couple the plate voltage without using another cap before the stack?

Would it be easy or desireable to utilize the unused triode in V2?

Good questions. Fender was trying out a couple ideas from the "better engineering" page - presumably the cathode follower could drive the tone stack better, without "loading down the plate" although as we can see in their later 60's era amps they dispensed with the cathode follower in all except the white/brown Bassman, and nobody complains about their tone. Also the early 60's Bassman's bass channel tone stack is exceptional, in that it does use very large caps and smaller value pots so loading down, aka drawing current from, the driving circuit has to be taken into account. I think you'll find you can skip the cathode follower. I've put this circuit into hundreds of Fenders & nobody's complained "wah wah wah it doesn't sound exactly like a tweed bassman ." Close enough for rock n roll is plenty close enough. Plus you can select values for the contour resistor & brite cap to suit your own taste. An advantage is you can use a simple A/B switch externally to select channels, no ground loop hums like you get with using 2 separate amps. Just make sure you use a DPDT switch in your A/B to mute the unused channel feed. Finally, you have that extra triode so you can try the cathode follower drive if you choose. Or find a way to insert an extra gain stage. Choices, choices....

[Randall]

"Also the early 60's Bassman's bass channel tone stack is exceptional, in that it does use very large caps and smaller value pots so loading down, aka drawing current from, the driving circuit has to be taken into account. I think you'll find you can skip the cathode follower."

I'm still a little fuzzy on this, so tone after the first triode like the AB165, but with different values as you suggested?

And I am interested in utilizing the extra triode on one of the channels, just not sure how to do that.

[Leo_Gnardo]

tone after the first triode like the AB165, but with different values as you suggested?

That's all, just 3 caps, a resistor and a pot.