Marshallising bass channel Fender aa864

[spoontex]

I just finished building a Fender AA864 amplifier. Everything works perfectly, but I think the bass channel is kind of pointless. I’m not going to use it, and I was thinking of repurposing it somehow. I’ve seen that it’s quite common to use the second half of the V2 to do something similar to a Marshall 2204. While searching, I found this:

https://www.premierguitar.com/diy/ask-amp-man/fender-bassman-mods

And interestingly, an amp manufacturer called P2P does something similar. In fact, on their website they reference the 2204:

https://www.p2pamps.com/aa864-custom-bassman

He also mentions that he has modified the 'Deep' control and turned it into a 'Mid-Boost'.

But I have some doubts. What do you think about the modifications mentioned in the first link? Don’t you think the Tone Stack should also be modified? If so, how can I implement a modified Tone Stack and a Mid-Boost switch?

I’ve also found some references to modifications made by someone named David Allen, and I think they’re along the same lines. Does anyone know exactly what he used to do?

Hope somebody can help me.

 

[Jon Snell]

I am surprised p2p are allowed to leave the UL Tested sticker on the repurposed Bass amp.
Wild noises will errupt if the signal is too great as the V2 mod increases the gain of mid to high frequencies more than lower frequencies; 680nF capacitor as in Marshall 2203/4 lead mods.

 

[spoontex]

Here are the schematics. The first one includes the extra gain stage based on the instructions from the Premier Guitar article. The second schematic modifies the tone stack to a Marshall-style layout and repurposes the 'Deep' switch as a mid boost. The idea is to replace the second input of the bass channel and add a gain potentiometer. I look forward to your suggestions.

 

[Jon Snell]

Not very accurate. First mistake is circled in blue.
Use a clean amplifier and a pedal.

 

[spoontex]

Not very accurate. First mistake is circled in blue.
Use a clean amplifier and a pedal.

Sorry, I forget the 1m resistor to ground...

 

[spoontex]

Thanks for the warning — is there anything else I should watch out for?

 

[sds1]

Thanks for the warning — is there anything else I should watch out for?

R35 is too big... 33k is plenty. Bigger=more hiss

R39 should come before R35 unless the voltage divider you've drawn is intentional

 

[MrBassmans]

@spoontex

I built a AA864/AA165 clone with a 59 tweed/JTM bass channel based off of this schematic posted by "chocopower" (an amp tech from Spain) in the early 2000's. It appeared on the EL34 World Hoffman & TGP sites.

I gig with this head all the time, installed a Mercury Magnetics 4/8/16 ohm multi tap OT for various cabs I'm using. A 12AY7 in V1 gets more of the tweed tone, a 12AX7 a bit more Marshally.

The bass channel is internally "jumpered" at V1. This mod may not have enough gain & drive for you, but it can easily be converted to a Marshall 2203 type channel later on.

 

[MrBassmans]

Concerning tone stack values, the JTM45 250pf/.02/.02 with a 56K slope resistor as shown works well with 2X12 & larger cabs.

I primarily use a 1X12 so I'm using the '59 values: 250pf/.02/.1 with a 100K slope resistor. Less gain but reduces the "boxiness" and pronounced mids of the 1X12.

I didn't add the presence control, used the stock 820 ohm feedback resistor.

 

[spoontex]

Concerning tone stack values, the 250pf/.02/.02 with a 56K slope resistor as shown works well with 2X12 & larger cabs.

I primarily use a 1X12 so I'm using the '59 values: 250pf/.02/.1 with a 100K slope resistor. Less gain but reduces the "boxiness" and pronounced mids of the 1X12.

I didn't add the presence control, used the stock 820 ohm feedback resistor.

Thanks for the info. The idea is to use a 250pF/0.02µF tone stack, a 33k slope resistor, and the "Deep Switch" for a mid boost — 0.02µF cap in parallel with a 6k8 resistor to ground.
Also, I'm planning to use a gain control in place of the second input jack.

I don't know if it's better 1M or 250k for the volume pot. Marshall has 1M and fender 250k.

 

[MrBassmans]

Thanks for the info. The idea is to use a 250pF/0.02µF tone stack, a 33k slope resistor, and the "Deep Switch" for a mid boost — 0.02µF cap in parallel with a 6k8 resistor to ground.
Also, I'm planning to use a gain control in place of the second input jack.

I don't if it's better 1M or 250k for the volume pot. Marshall has 1M and fender 250k.

I suggest going with the AA165 filtering scheme (also shown in chocopower's schematic) if you're going with a V2 cathode follower.

The 27K dropping resistor along with the 16uf filter cap will bring V2 down to a safer 150-180 volts. Otherwise the V2 CF circuit may be too hot with the AA864 filtering scheme.

Note that the placement of the 27K resistor is a little different in the AA165 compared to the AB165.

 

[mountainhick]

Here are the schematics. The first one includes the extra gain stage based on the instructions from the Premier Guitar article. The second schematic modifies the tone stack to a Marshall-style layout and repurposes the 'Deep' switch as a mid boost. The idea is to replace the second input of the bass channel and add a gain potentiometer. I look forward to your suggestions.

I have done something similar as a mod on a Bassman ten, though there are marked differences:

-The extra gain stage is placed as the third stage, not the first.
-Extra gain/drive control is placed before the third stage.
-Tone stack is after first stage as usual, more or less trainwreck style.
-Have not yet changed the cathode, coupling and tone stack values. It does need the tighter values to reign in the wooly bass response.
-It is quite noisy. I have not had the time to troubleshoot this yet.

Watching this thread!

 

[MrBassmans]

@spoontex @mountainhick

Here's an old video of an AA864/2203 '67 conversion. Sound quality isn't the best, done on an iPhone with the amp up loud.

Some players weren't impressed, sounds more like a high gain Fender than a Marshall. I think the tele sounded the best thru the 2203 channel. Speakers are celestion 65's.

Keep in mind going way back we didn't have the bunch of great sounding MIAB dirt boxes we have now. Hence the bassman head as a mod platform. With my AA864/'59 tweed head clone I found it better to go with the sound of a non master volume big tweed amp with MIAB's, for my needs.

Kudos to the OP for experimenting with a clone build, rather than tearing up an old head. Players nowadays have high regard for a serviced, unmolested AB165.

 

[spoontex]

@spoontex @mountainhick

Here's an old video of an AA864/2203 '67 conversion. Sound quality isn't the best, done on an iPhone with the amp up loud.

Some players weren't impressed, sounds more like a high gain Fender than a Marshall. I think the tele sounded the best thru the 2203 channel. Speakers are celestion 65's.

Keep in mind going way back we didn't have the bunch of great sounding MIAB dirt boxes we have now. Hence the bassman head as a mod platform. With my AA864/'59 tweed head clone I found it better to go with the sound of a non master volume big tweed amp with MIAB's, for my needs.

Kudos to the OP for experimenting with a clone build, rather than tearing up an old head. Players nowadays have high regard for a serviced, unmolested AB165.

I will try and experiment with booth options, your mod and the other one that I upload ( but I need to check again ).

Your mod have 3 eq section on clean channel?? Where you put the middle pot?

 

[MrBassmans]

Your mod have 3 eq section on clean channel?? Where you put the middle pot?

The chocopower circuit has the normal channel with a 6.8K resistor in place of a mid control, same as the AA864/AA165, bandmaster, pro reverb etc. Acceptable for most playing, however at low volume it would be nice to lower a mid control (like a twin reverb).

The '59/JTM bass channel does use a 25K mid pot, however. Since my clone has a generic Ebay chassis, I just drilled a hole in the back for the mid control. I ran the ground to the front thin brass control plate (not the middle potentiometer case).

 

[telemnemonics]

Modding amps is cool, but I think attempts to mod the preamp of a BF Fender for a Marshall channel when a classic Marshall is not a preamp sound; ain’t it.
Instead why not something like a Dumble type mod?
Since it is designed by a really good circuit designer to work with a BF power section it does probably a better nasty sound than trying to get a Marshall sound.

Many designers have added tweaked dirty preamps in BF amps including Dumble, Zinky, Rivera, Harry Kolbe.
A Marshall preamp in there seems obvious yet is mostly hobby stuff and just not really a great marriage in my opinion.
If you have 5f6/8a then it may make more sense but even then it is missing the voltages transformers and filtering that makes a Marshall sound.

Again of course it is fun and you can reverse it if not great sounding.
But look at some master designers instead of trying to Brutish-ize this oh so American amp British style.

Sorry if I sound negative!

 

[spoontex]

It doesn’t sound negative. The thing is, I want to make the most of the bass channel so that it’s more versatile and can be used as an extra channel. I'm not a designer, so I tend to go for the easy route — searching for information online and trying to implement it. If you have any interesting schematics from the manufacturers you mentioned that I could implement, they would be very welcome!

 

[telemnemonics]

Some Dumble circuit mods for Bassman here

Dumble Modded Bassman Layout/Schematic - The Amp Garage

 

[telemnemonics]

Not familiar with this guy but he did a series on a Dumble circuit mod Bassman build.

 

[spoontex]

Not familiar with this guy but he did a series on a Dumble circuit mod Bassman build.

I will take a look! thanks!