Adding a gain stage to BF Bassman?

[Yoda]

Hello all,

So I’m learning about amplifiers and I’ve decided a neat project would be to wire up the un-used half of one of the 12AX7s in my Fender Bassman.

I’m thinking of adding a gain stage to the bass channel so then it would have 4 gain stages instead of 3 and I would leave the Normal channel completely stock for that clean blackface goodness and it would also bring the bass channel in phase with the normal channel so jumpering is possible.

So I’m just trying to understand what the bare minimums are for a new gain stage and my understanding of things is still minimal but I believe I need a 25uf/25v bypass cap + 1.5k cathode resistor, a 100k plate load resistor to bring in the B+, a 100k grid stopper resistor, and possibly a 470k grid leak to ground? I’m not sure on the grid leak as I’ve combed through a lot of information in the last weeks trying to come up with a simple gain stage design I can implement with minimal circuit modification and I don’t recall what role the grid leak resistor plays.

Also I will need a coupling cap after the plate before the signal moves on to the phase inverter but I see values of .1, .047, and .02 in different Fender schematics so I’m curious if there is a reason certain values are picked or if it’s a subject of experimentation.

Thanks in advance for any help:

[thetragichero]

all those values are determined by what you want it to do/how you want it to sound

[sluckey]

This discussion thread may be of interest to you...

     https://el34world.com/Forum/index.php?topic=26289.msg287322#msg287322

[Latole]

Don't mod a Blackface Bassman 

Don't learn on a amp who made history

[d95err]

I say mod away! But if you can, keep it reversible so it can be restored if desired. I.e. avoid drilling new holes if possible.

[Latole]

I say mod away! But if you can, keep it reversible so it can be restored if desired. I.e. avoid drilling new holes if possible.

A beginner must not learn/ work on a Blackface amp

With time he will learn he make a big mistake !

There are so many amps to learn with

[shooter]

beginner must not learn/ work on a Blackface amp

Google must have messed up, an amp owner has every Right to do anything they like to their amp.
I know the virus19-84 stuff kinda mess that up for awhile.......

[Latole]

beginner must not learn/ work on a Blackface amp

Google must have messed up, an amp owner has every Right to do anything they like to their amp.
I know the virus19-84 stuff kinda mess that up for awhile.......

Yes he has the right to do anything including ignoring the advice of experienced people.

[Yoda]

Well I expected to receive comments discouraging me from modding a BF Bassman, trust me I understand the sentiment but I think this is a relatively simple and reversible mod and it would allow me to enjoy this amplifier even more. The bass channel also lacks the same Fender ‘sparkle’ for lack of a better term than the normal channel so I’m willing to mod it a little bit to make it more fun and usable.

[sluckey]

The blackface bassmans are a very popular amp for all kinds of mods. Lots of good mod info for the AB165 and AA864.

[bmccowan]

I am in the go ahead and mod it camp. BUT, I suggest that those modes should not include drilling any new holes. I would argue that hand-wired amps were always designed to be modded. Want more gain - fine change 2-3 resistors. No big deal. And FCS, when the filter caps wear out, replace them! Save the old ones if you must, but a guitar amp is not the Mona Lisa, or the David. Its a tool that makes music.

[sluckey]

The bass channel also lacks the same Fender ‘sparkle’ for lack of a better term than the normal channel so I’m willing to mod it a little bit to make it more fun and usable.

You do understand that the bassman was designed to be used with a bass? No sparkle, just boom boom. Even the original speaker cab has no sparkle, just boom boom.

[bmccowan]

According to John Lee Hooker, that's Boom Boom Boom Boom

[66Strat]

According to John Lee Hooker, that's Boom Boom Boom Boom

Or, to quote Little Walter; "Boom, boom, out got the lights."

[Yoda]

So my basic idea is to just copy a gain stage into the normal channel, anybody see any reason why this shouldn’t work?

[dwinstonwood]

So my basic idea is to just copy a gain stage into the normal channel, anybody see any reason why this shouldn’t work?

I literally just finished doing that exact thing: https://el34world.com/Forum/index.php?topic=26289.msg287322#msg287322

It works great! I totally recommend doing it. Here's my schematic and sluckey's layout for the project:

[Bieworm]

I'd start with smaller size caps. .68uf cathode bypass cap and a .0047uf coupling cap. And work up in value from there.. that 25uf cap will go boomboom tooooooo boomboom

[tubeswell]

Another idea to use that spare triode, is you could wire the other channel as a 59 tweed Bassman with a TB Tonestack,

I.e. input stage>vol>2nd stage with DC-coupled cathode follower>TB tone stack>PI


Or a 62 Blonde Bassman bass channel

[mresistor]

Also you will be able to jumper the channels.

[Yoda]

Yes! Jumpering channels is one of my biggest reasons for doing this. It always sounded like crap jumpered so I didn’t run it that way for many years until recently when I started learning about these things and the light bulb went off in my head.

Also the normal channel has more overdrive than the bass channel despite having one less gain stage, I’m curious what components may be responsible for that. I’m still learning so bear with me but I can’t tell if that’s a voltage divider right before the third triode in the bass channel.

[bmccowan]

So my basic idea is to just copy a gain stage into the normal channel, anybody see any reason why this shouldn’t work?

Sure - it'll work. And the AA864 musings project is a good example of a one channel project.
I have modded a SF Bassman 50 in similar fashion by modding the bass channel rather than the normal channel.
But IMO the normal channel of the BF bassman is a good sounding guitar channel - you could tweak that one to your liking making minor cap and resistor changes. For mine, I made that Normal channel nearly identical to the AA864 Normal channel.
I think the bass channel is the one to modify in order to make good use of that channel for guitar. I think the reason you hear less gain from that channel is that it sends a lot of the guitar friendly frequencies to ground in order to make it bass friendly. What I did for that channel in mine is almost identical to what DWinstonwood did. Almost because I have not added a gain pot and I experimented with different voltage dividers upstream of that 3d stage until I had what I wanted. At some point I will likely add a gain pot. As Sluckey suggested in that thread - swapping different preamp tubes in V1&2 provides a pretty good range of tone choices. Right now I have a 5751 and a 12Y7 in mine. Going up in gain too much introduces too much noise for my taste.

[dwinstonwood]

...As Sluckey suggested in that thread - swapping different preamp tubes in V1&2 provides a pretty good range of tone choices. Right now I have a 5751 and a 12Y7 in mine. Going up in gain too much introduces too much noise for my taste.

Same here! I'm using these tubes and it really smoothed out and richened the tone while lowering the noise to almost nothing at moderate volume settings:

[Yoda]

I’ve also discovered that the 6G6 Bassmans have a bass channel with 4 gain stages and people rave about the brownface tone so I’m considering just copying the bass channel of the 6G6, it makes use of that fourth triode.

[echuta13]

One of the stages is a cathode follower which is intended to drive the tone stack in the 6G6 era amps.  They bring their own quality to these amps. 
http://www.valvewizard.co.uk/accf.html
http://www.valvewizard.co.uk/dccf.html

[bmccowan]

I’ve also discovered that the 6G6 Bassmans have a bass channel with 4 gain stages and people rave about the brownface tone so I’m considering just copying the bass channel of the 6G6, it makes use of that fourth triode.

Yes - many including me like the Brownface tone. But in terms of the 6G6 Bassman - for guitar, most are raving about the normal channel, not the bass channel. If you search for Brian Setzer amp/tone you will find some interesting articles on this subject.

[bmccowan]

Further - Since my Bassman 50 was sitting open waiting for the gain pot, this afternoon I converted the normal channel from the AA864 circuit  (previous mod) to the normal channel 6G6B circuit. It is a great sounding circuit. So now I have the AA864 with the extra gain stage as channel 1 and the 6G6B normal channel as channel 2. I used the feedback circuit from the 6G6B and will add either a presence control or an adjustable feedback. For now, I just used a pair of resistors that gets me to the 6G6 presence turned up to about 5.
Before I stop experimenting with this amp, I may try a pentode in the V2 position and configurate it similar to the Matchless Clubman and/or one of the HoSo56 circuits from this Forum.

[Yoda]

Well out of curiosity I thought I’d try the parallel triode mod first since it’s the simplest. I don’t hear much difference in gain but it does sound more “musical” actually so it’s a nice mod for the bass channel so far.

[bmccowan]

That's interesting. I have used parallel gain stages but only as the first stage. With that, I might suggest changing the "deep switch" to a "bright switch" just the way it is done on the normal channel. And then I would experiment with removing the .001 and .22 caps and the 1M & 4.7K resistors and replacing with a 470K/470K voltage divider and leave the .047 cap or experiment with other values.
Honestly I think 3 gain stages is enough for breathing fire, but adding a forth in parallel fashion will produce less hiss that cascading 4 stages. Changing that deep switch to a bright switch makes a big difference IMO. A nice thing with these bassmans is that these mods and experiments can be done without drilling any new holes. The bad part is that the eyelet boards get pretty scummy after repeated soldering.

[Yoda]

So I was wondering if somebody could help me understand what exactly is the purpose of the components between the second and third gain stages of the bass channel, I can’t tell if it’s a voltage divider or an RC filter.

[PRR]

> what exactly is the purpose of the components between the second and third gain stages of the bass channel, I can’t tell if it’s a voltage divider or an RC filter.

All passive filters are dividers. I have a theory that everything is a voltage divider.

But what this does, I dunno. It is a midbass valley and a top-cut and taken all together it seems to be a really-really mild +/-4dB S-curve around unity gain. Did Leo sit up all night with a tray of caps and a bottle of California Red "perfecting the tone", and didn't look at it sober before putting it in production?

[72Blazer]

That's interesting. I have used parallel gain stages but only as the first stage. With that, I might suggest changing the "deep switch" to a "bright switch" just the way it is done on the normal channel.

Hope I did this right..

Can someone elaborate a little more on modifying channel 1 wrt changing the "Deep Switch" to a "Bright Switch"?  I'm looking for more treble in the Bass channel.  My amp is an AA864 and I added a Master Volume.

Vr
J

[AlNewman]

I think what they meant was to remove the bottom .1 uf cap and switch from the tone stack, and rewire the switch to bypass the volume control with a low value capacitor.  Looks like they used 120pf in the other channel.

[sluckey]

Can someone elaborate a little more on modifying channel 1 wrt changing the "Deep Switch" to a "Bright Switch"?  I'm looking for more treble in the Bass channel.  My amp is an AA864 and I added a Master Volume.

Completely remove the Bass channel tone stack. This means all components including bass, treble, and volume pots. Replace with tone stack that's identical to the normal channel. New pots of the correct value. Scratch through the word "Deep" and write in the word "Bright".

[72Blazer]

Thanks.  I just got done restoring back to normal after as I realized (although it took me awhile and several component swap outs) that I was going to have to replace all the pots too.  That is when I decided not to mess with it.

[AlNewman]

Ahh, yeah.  The aa864 is a lot different than the schematic posted here.

[bmccowan]

Scratch through the word "Deep" and write in the word "Bright"

Blazer; there are many threads here about the AA864 to check out.
When I was in High School my Dad bought a 69 Blazer - 4 speed. I swiped it whenever I could. That thing was a blast.

[dwinstonwood]

I've built two modded AA864's, and I'm working on my third one. The first one uses 6L6's, the second uses 6V6's, and my third version will be single-ended with a 5881. I removed the .001uF Treble bleed cap after the 220K/220K voltage divider, and added a 500K "Gain" pot in front of the third triode.
Sluckey linked to my first build up in post #2. He more or less designed it for me.

 
Here is my second, 6V6GT version. I borrowed some component values from a Trainwreck Express schematic. The Express seems, to me, to be derived from the AA864. This one sounds great. I still need a cabinet, and I think that those cabinets for the Ceriatone OTS 20 Lunchbox will work with Hoffman's Stout chassis. They are 15.5" W x 8.5 T" x 9" D.

https://el34world.com/Forum/index.php?topic=28777.0