Bassman aa864 Marshall channel

Tube or solid-state, this section goes to eleven!

Bassman aa864 Marshall channel

Postby spoontex » 23 Sep 2017, 13:07

Hello,

I'm build a Bassman aa864 head, and I read that some people convert the bass side to a marshall style channel. Anybody knows something about this? any shematic?

Thanks!!
spoontex
 
Posts: 13
Joined: 11 Feb 2014, 23:24
Has thanked: 0 time
Have thanks: 0 time

Re: Bassman aa864 Marshall channel

Postby bajaman » 23 Sep 2017, 14:59

not me sorry
be kind to all animals - especially human beings
User avatar
bajaman
Old Solderhand
 
Posts: 4107
Joined: 26 Jun 2007, 22:18
Location: New Brighton, Christchurch, NZ
Has thanked: 349 times
Have thanks: 826 times

Re: Bassman aa864 Marshall channel

Postby POTL » 23 Sep 2017, 18:38

spoontex wrote:Hello,

I'm build a Bassman aa864 head, and I read that some people convert the bass side to a marshall style channel. Anybody knows something about this? any shematic?

Thanks!!


Which model of marshall do you want to convert?

In this scheme, a tunneled fender is installed, you will need to rework the tone of the stack in order to make the marshall mod

the optimal bassman for creating the marshall is 5F6A - which is essentially the half-brother of Marshall JTM45 and in the receiving section they are identical.
POTL
Solder Soldier
 
Posts: 228
Joined: 26 Sep 2016, 04:11
Has thanked: 47 times
Have thanks: 48 times

Re: Bassman aa864 Marshall channel

Postby spoontex » 24 Sep 2017, 11:13

Really, I don't know. Only that some people convert the bass channel to high gain without modifcation the chassis, using the existing holes. The only shematic that I found is a mod that convert the bass channel into a trainwreck preamp. But I don't how it works, and how it sounds. What do you think about the shematic? Is factible?

The shematic is in this post:

http://www.tdpri.com/threads/bf-bassman ... od.756912/
spoontex
 
Posts: 13
Joined: 11 Feb 2014, 23:24
Has thanked: 0 time
Have thanks: 0 time

Re: Bassman aa864 Marshall channel

Postby POTL » 24 Sep 2017, 12:30

spoontex wrote:Really, I don't know. Only that some people convert the bass channel to high gain without modifcation the chassis, using the existing holes. The only shematic that I found is a mod that convert the bass channel into a trainwreck preamp. But I don't how it works, and how it sounds. What do you think about the shematic? Is factible?

The shematic is in this post:

http://www.tdpri.com/threads/bf-bassman ... od.756912/


without a complete replacement of the tonestack of the sound of the marshall is not achieved
POTL
Solder Soldier
 
Posts: 228
Joined: 26 Sep 2016, 04:11
Has thanked: 47 times
Have thanks: 48 times

Re: Bassman aa864 Marshall channel

Postby phatt » 24 Sep 2017, 14:22

As POTL has mentioned it's not going to work like a marshall but I'm guessing you just want more OD. If so then change the PI AT7 to AX7 that will give earlier distortion. Then insert a 50k pot in series with the existing 820R FB resistor and dial in the amount of OD you want, just watch out for oscillations. :twisted:

You can also insert the Normal channel *Before* the extra triode giving both channels access to the extra gain stage, just copy the the Bass circuit and don't forget to add the .1uF cap.

Another tweak would be to raise the value of the power supply drop resistors (being 1k and 4k7) replace 1k PI drop with 2K ~ 5k and the preamp value to ~10k. These are just ball park ideas so you will have to tweak it all to taste so make these alterations easy to change before you commit to anything.

The Twreck idea might work also but up to you to make your own judgement of the outcome.
Jope it helps,,Phil.
phatt
Diode Debunker
 
Posts: 949
Joined: 04 Aug 2010, 06:17
Location: Morayfield SE Qld AU
Has thanked: 0 time
Have thanks: 232 times

Re: Bassman aa864 Marshall channel

Postby spoontex » 25 Sep 2017, 14:43

spoontex
 
Posts: 13
Joined: 11 Feb 2014, 23:24
Has thanked: 0 time
Have thanks: 0 time

Re: Bassman aa864 Marshall channel

Postby POTL » 26 Sep 2017, 02:09



OK
replace all 220K resistors at 470K
add parallel to them capacitors 470p (as in the marshall scheme)
Potentiometer 250K up to 1M
In the half of the tube going to the tone of the stack, replace the resistor by 1.5k at 820m, the 25uf coder at 250uf.

This will give you something similar to the sound of a marshall super lead
But consider the differences:
1) In the scheme of the fender, another tone of the stack, it always cuts off the middle frequencies, the marshall has a lot of mediums (in this, the charm of their sound).
2) The tone of the stack in the marshall scheme is at the end of the preamp circuit, in the fender it is in the beginning - it strongly affects the sound
3) The Marshall has 2 gain sections and a buffer before the equalizer, in the Fender 3 gain section.

In any case, you will not get the sound of the marshall, just the illusion of the placebo effect :?

and yes, if you ask such questions, most likely your work experience is not great.
Remember that in tube devices there is a very high voltage that can kill you.
Be careful and good luck
POTL
Solder Soldier
 
Posts: 228
Joined: 26 Sep 2016, 04:11
Has thanked: 47 times
Have thanks: 48 times

Re: Bassman aa864 Marshall channel

Postby phatt » 28 Sep 2017, 11:50

OK having read this again,, Looks like you just want to find the schematic for the marshall mod?
Well I can't find it with google fu but I doubt it's hard to work out. :wink:
Just re arrange the tone components to marshall values then simply insert a pot between V2 and V3.

There is a pair of 220K resistors there forming a voltage divider so half the input voltage is shunted to ground and half is sent to the input grid of V3.
Leave the series 220k resistor and insert a 500k pot which is then the master. Obviously the extra master pot goes into one of the old input socket holes hence very easy mod and YES it will give you the extra gain you want,, just tweak tone values to your taste. :thumbsup
Don't forget to remove the 1nF cap which shunts higher freq to ground when used for a bass guitar. 8)
Cheers, Phil.
phatt
Diode Debunker
 
Posts: 949
Joined: 04 Aug 2010, 06:17
Location: Morayfield SE Qld AU
Has thanked: 0 time
Have thanks: 232 times

Re: Bassman aa864 Marshall channel

Postby okgb » 28 Sep 2017, 18:11

It sounds like you should continue researching this so you can better " understand " what parts and why you are changing them

Until they became channel switching most Marshalls of the 70's & early 80's used only three preamp tubes , one for the phase splitter
and two for the preamp [ the first tube , one half for the bright ch and the other half for the normal ch ] so it is possible to get a
Marshall sound with 1 & 1/2 tubes [ 3 gain stages ,the bogner Xtc uses 3 gain stages for it's classic marshall channel ]
but 2 whole tubes will get you more gain like the 800's. If you have enough available tubes and space
[ you may need to drill or find holes for separate tone controls ] you can have a fender channel & a marshall channel, otherwise
you have to decide what changes will make it most marshall like, marshall's have brighter first stages where as the fender is a little more fuller,
as pointed out the tone stacks are a little different , but as importantly in different spots of the circuit , the fender usually right after the first
gain stage [ tube half ] and the marshall after the 4 th gain stage and off of the cathode instead of the fender plate.
If you have a confident friend, rewiring a couple of tubes shouldn't be so hard , unless you are trying to jam it into a certain space, you could use one of the input
jacks as a volume control and the other three as tone controls if there is no mid and you want to add it, you are probably kits so take your time and good luck It's doable!
okgb
Diode Debunker
 
Posts: 748
Joined: 16 Mar 2010, 04:58
Has thanked: 181 times
Have thanks: 39 times


Return to Amplifiers

Who is online

Users browsing this forum: Bing [Bot] and 7 guests