"bass friendly" AB763 ?

[worth]

I'd like to use my Showman for guitar AND bass... what can I do , using a SPST switch to make the normal channel more "bassy" ?

[bigsbybender]

Well, there wasn't really any major difference between Leo's Bass amps and his Guitar amps. Without making the AB763 into something completely different you can only cut some range of frequencies. Most modern bass tone is very much midrange "scooped" meaning they really cut out the midrange, leaving the bass and treble intact. That said, the standard AB763 has a pronounced midrange scoop as well. To create the sound of a bass boost in a "loss" type tonestack as Fender uses, you have to cut treble and mid. It would be completely possible to do this and Fender did in the bass channel of amps like "Bassman Ten". 

What really differentiates a Bass amp from a Guitar amp though is the speakers used. Guitars use musical instrument speakers that are 'tuned' nearer to 70-100hz resonance as opposed to speakers closer to the 50hz (and sometimes even lower) mark which are more suited for bass. Bass cabinets are often very deep and also ported to assist in better bass tone. Popular guitar cabinets are relatively narrow in depth and either completely open back (Fender Combo, Vox) or sealed (ala Fender Piggyback or Marshall 1960)
I have a Super Twin that I altered the tonestack for more bass friendly tone, but it still works for guitar but I change the speaker cabinet for use, for me that's the biggest difference.

j.

[firemedic]

I'll second that. Look at a lot of the bass amp schematics here; they're almost exactly the same tone & coupling cap values as guitar amps, all the way down to the PI. Larger cap values tend to make the tone muddy. You may get better bass response from larger filter caps.

For fun one day I ran my bass into my BF Vibroverb build, using 1-15" bass speaker instead of the 2-12s I use for guitar. It sounded awesome & very bassy. You should be able to simply switch speaker cabs for bass with your Showman, with no mods necessary.

[HotBluePlates]

I'd like to use my Showman for guitar AND bass... what can I do , using a SPST switch to make the normal channel more "bassy" ?

Are you really limited to using a SPST switch? Maybe you're thinking about using just the existing Bright switch to be a "Bass switch"?

If you do nothing else, you could copy the Deep switch on the AA165 Bassman. This connects a 0.1uF from the tone stack to ground, throwing away highs to get the impression of more bass. I can tell you it works, but may or may not be the sound you want. You could always put alligator clips on the ends of the cap and try tacking it into the circuit without soldering to see what it sounds like.

If you keep the Showman's Vibrato channel for guitar, are you really gonna miss having the Normal channel also? Many guitarists never even plug into the Normal channel on their reverb-equipped Fenders (may not apply to your Showman, as the Normal channel may have better isolation from the vibrato circuit's signal loss).

If you could do away with having the Normal channel for guitar, you could make changes to the tone stack and coupling caps similar to the AA165 Bassman, and keep that channel as a dedicated bass channel. You might want to skip the sillt T-filter stuff between the 2nd and 3rd stage of the AA165 Bassman's bass channel. Do notice the small-valued caps across the plate resistors in the bass channel; these remove some treble in each of those stages, while you can use a big coupling cap (like 0.047uF or 0.1uF) to reduce bass roll-off between stages. The bigger caps in the tonestack of the AA165 may or may not be to your liking; try them and see what you think.

I'd suggest (if you go the dedicated bass-channel route), to try one change at a time, then listen and evaluate. Keep or remove the change (depending on your opinion) before moving on to the next. If you change 4-5 things at one sitting, you won't be sure what's doing whatever you do/don't like about the resulting sound.

[P Batty]

A Dual Showman was THE standard bass amp in the mid 60's for rock groups.  It had to have the D-140F speakers, though, not the D-130F speakers. With the DS looking at 4Ω, it could easily handle 2 cabs, which gave a very nice tone and volume.  The only other alternatives were Ampegs, and they were not as loud.

[phsyconoodler]

Don't change anything,just use a bass cabinet.And change the slope resistor to shift the range of the tone stack towards the bass end of the spectrum.Done.

[Willabe]

change the slope resistor to shift the range of the tone stack towards the bass end of the spectrum.

Which way up or down in value?

I never under stood this. I always thought that if you change the 100K slope R to 56K that would let more signal go to the bass and mid pots, a little less to the treble pot, so more bass. But I read something recently that seemed to say I had this backwards.


                   Brad       

[6G6]

^ What P Batty said.
I was there.
Before the Bassman 100, DS with D140s was THE big bass amp.
Two cabs, if you were rich.

[saxetc]

I am rebuilding my 65 Showman and planning to use the AA864 tone stack on the normal channel. I am also building a 1-15" tone ring cabinet using a Weber Bass speaker. I am open to any constructive comments and recommendations.

[sluckey]

I am rebuilding my 65 Showman and planning to use the AA864 tone stack on the normal channel. I am also building a 1-15" tone ring cabinet using a Weber Bass speaker. I am open to any constructive comments and recommendations.

The AA864 will require an additional preamp tube. Where will you put it? I did the same thing way back in '76. The amp is long gone. This schematic is all I have left...

     http://sluckeyamps.com/misc/76_Amp_Project.pdf

I used the head with a 2x15 CTS bass cab that I built in '69. The cab was covered in silver sparkle roll 'n tuck, similar to the popular Kustom amps of the time. The head in this pic is a Sunn Sonora clone.

[tdvt]

I am rebuilding my 65 Showman and planning to use the AA864 tone stack on the normal channel. I am also building a 1-15" tone ring cabinet using a Weber Bass speaker. I am open to any constructive comments and recommendations.

I can't speak to the tone-stack mods, but as far as a cabinet, I would build it to the "vintage" dimensions for aesthetics, but pick a specific bass driver & port the cab for that speaker.

There is a lot of accessible information available now (along w/better drivers) that will give you a ported design that will far out-perform the tone-ring cabinet for bass. Plus they don't give those tone-rings away.

I have a '61 Showman & play it through an EV TL606 cab when I use it (which is not that often)

[AmberB]

The AB763 amp has a 250pf, a .1uf, and a .047uf cap in the tone stack.
The bassman amps of later design such as the Bassman 50 or the Bassman 100 have a either a 500pf, a 330pf or 250pf, depending on the model and which channel, and 2 of the .1uf caps in the tone stack on some of them, a .1uf and a .047uf on others.
My old AB165 Bassman 50, an early silverface model, had a 250pf, and 2 .1uf caps in the "bass" channel tone stack plus a .1uf for the deep switch.  The "normal" channel had a 250pf, a .1uf, and an .047uf in the tone stack plus a 120pf from the input to the volume control to the bright switch.
The AA864 Bassman had pretty much the same configuration as the AB165.
There were variations in the Bassman 100 and the AA371, but they weren't all that different.

[pdf64]

I’ve always found the regular black panel normal channel very bass friendly.

[saxetc]

Thanks to everyone regarding my Showman rebuild. I bought the amp from the original owner in 1971 for about $200. I stupidly tried to convert it to a Boogie overdrive without knowing what the hell I was doing back in the 70's. It has been unused since then. I acquired a 15" tone ring, a Weber 15 bass speaker, and plans for a tone-ring cab. I am starting by rebuilding the showman from the ground up using Hoffman eyelet boards and modern components (metal film/oxide resistors, caps, etc.).  Also I'm building an EV TL.. cab for an EVM-12L for the Vibrato channel for guitar. If I wanted  to maximize the bass to modern standards I would purchase one of the modern bass amps but I want to use what I have on hand as much as possible (I'm a cheapscape!).