Fender Bassman 100

[DummyLoad]

looks like a 75-76 model - a friend left it with me to mod for him - he said it sounds like $#!^ - i do concur.

what would y'all do to a bassman 100 - not at all opposed to getting medieval on the the chassis to add bottle hole(s), etc..

it's kind of beat but overall in fairly good shape and it looks like i'm the fist guy to bust her open. it got bounced HARD though, one of the PT nuts was stripped off the bolt and the opposing nut backed out so the PT was flopping around like a clam-shell.

the iron is all good and the amp works. just looking at the schematic makes my ears hurt. he hinted around having two channels - i'm thinking that a matchless type front end for the "normal" channel (has 3 knobs) and a marshall-like or trainwreck-like pre for the "bass" channel. the owner did mention - no marshall 800 or 900 series mods... he already owns one.

thoughts? ideas? use it to start a new barrier reef?

as always, much thanks for yor time and guidance in advance.

[sluckey]

Plug a Jazz bass into it. Sound better?      

[DummyLoad]

Plug a Jazz bass into it. Sound better?      

weenie! yer no help.   :p

[PRR]

> it sounds like $#!^

How so?

http://www.el34world.com/charts/Schematics/files/fender/BASSMAN_100.pdf

The "Normal" channel looks like classic Fender.

The "Bass" channel looks like someone's concept of "Bass", but maybe not yours.

Fix what's broke, try other speaker, it may be a work-horse as-is.

> my ears hurt

Yes, it is the Chevy 454 of guitar amps. Ideal for hauling horses across Texas plains. Gross over-kill for a beer-run. Probably should preserve it for whoever may need a BIG amp for large PA-less gigs.

[topbrent]

Working on a 76 right now.  I will post some pics when I get it done.

The first thing I am going to try on the bass channel is a tweaked "superlite" style arrangement. 
The Orange AD-15- AD-30 - AD140 uses a similar preamp arrangement as the super lite.

This rearrange the 2 gainstages and puts the tonestack last.  Quite a bump in overall gain.

 

[stratman_el84]

Looking at the schematic here: http://drtube.com/schematics/fender/bassman100.gif

The things I would do first would be:

1. I would remove the two 2000pF caps going to ground between the .1uF PI coupling caps and the 1500 ohm 6L6 grid-stopper resistors.

2. Remove the 220K resistor and .01uF cap that connects between the top half (as it's drawn) of the PI and the PI cathode circuit.

3. Modify the PI cathode circuit closer to an AB763 BF Twin-Reverb: http://www.davidsonamp.com/sf/twn_rev_ab763.php

4. Not necessary, but I would modify the bias circuit from a "balance" adjustment only type, to a standard voltage-adjustable bias supply like the AB763 as well.

That should get it into a state where you can start tweaking the preamps & tone stacks (the fun part!).

Good luck!

Strat

[Willabe]

I'd try changing the PI R/C values to AB763 values. Mainly the 47K plate R's to 82K/100K and the 330K grid return R's to 1M.


           Brad   

[thermion]

i am a huge fan of this amp. for reasons i really can't get into...
+1 on what everyone said so far.
if you do all this and still hate it let me know and i'll buy it from you.

[worth]

I did all the ab763 changes to a Bassman 100  , plus raised the values of the main filter caps, and changed the non-working hum balance pot to two 100 ohm resistors.. sounded great. Pictures if you want.

[DummyLoad]

and i'll buy it from you.

thank you, but no. it isn't mine to sell...

Pictures if you want.

thanks worth, i would love to see them. 


--DL

[worth]

Pics from 2009. I used F&T 220uf 350V for the main series caps. I also soldered the 2nd channel components directly to the pots because of the long wire run.

[DummyLoad]

worth, thank you very much. appreciated.

--DL

[gldtp99]

I've serviced up a few of these Bassman 100's--- the studio across the hall has gotten yrs of use out of a Bassman 100 for both bass and guitar duties----- it makes a good platform for effect pedals as it is.
If you want to mod it out the possibilities are almost endless--- there's a lot of room for a new turret board, relays, optical switching boards, Jose Master Vol w/diode clipping circuit etc, etc.
Just keep the output and bias sections mostly stock (can make bias adjustable) and go wherever you want with the preamp---- there are plenty of schematics on this website to choose from---- i've found that high gain circuits that orig ran thru EL34's or KT88's sound fine when running thru 6L6GC's---since you already have the 6L6GC x 4 output section, don't mod this part of the amp.

[stratman_el84]

gldtp99, are you saying to keep the tone-sucking 2000pF caps from the power tube grid circuits to ground, the .01/220K PI feedback network, and the stock PI values?

I'm just wondering because, in my experience, those are the main weaknesses with this design and the things keeping that amp from sounding really good. The preamp is pretty standard Fender design fare, and ought to sound good with a good power amp section. The power amp and PI are really the only parts of the amp that have major design departures from standard Fender designs that sound great, with or without pedals.

Heck, if the PI and PA were modded to be closer to an AB763 TR, you'd effectively have a TR without reverb or trem.

Not saying your wrong here. There is no wrong when it comes to what sounds best to someone. I'm just trying to understand your reasoning in more detail, and I also might learn something. You're never too old to learn something new!

Strat

[gldtp99]

No, mostly i was talking about changing to different types of output tubes (advising against it)----- the Bassman 100's i've seen never had the 2000pF caps or they were removed before the amps came to me------- nobody ever complained about the tone from them with the stock PI values---some actually liked them better than my '67 BF Showman head (nice sounding Showman, by the way) because they sounded "tighter, crisper" to them---i prefered the Showman-type sound myself but that's what i expect out of a big, BF Fender---- to those without any expectations, some liked the Bassman 100 with stock PI values better.
Diferent people just hear things in different ways---- no amp or amp mod will please everyone so don't fall into the belief that your way is the only way.
I'm lucky enough to have players around who will test out my builds and not care what circuit it is or what components were used----they just plug in and play and give me honest opinions----- and they keep me honest..........................gldtp99

[stratman_el84]

Ah, OK. That clears things up for me, thanks.

Please don't misunderstand, I wasn't being critical. Like you, I understand that tone is all in the ears of the player. I just wanted to understand the situation and your reasoning more fully. I mights learn me sumthin'! 

Like you, when I build/mod an amp I have players I trust try it out and give me feedback. Heck, I often will take one of my amp builds to the Guitar Center a couple blocks away to get additional feedback (and some promotion!). I don't want to fall victim to self-confirmation bias. I understand how easy it is to live in your own personal echo chamber when it comes to these things.

Again, thanks for taking the time to explain!

Strat