Marshallising bass channel Fender aa864

[sds1]

Lots of interesting things going on in that schematic that are neither typical Dumble nor Bassman!

 

[Supertwang]

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!

I'll beg to differ. I modded my '72 Pro Reverb normal channel with Marshall spec components on V1 and I gotta say WOW WOW WOW. I changed 4 components: cathode bypass cap & resistor, plate resistor, grid resistor?? And now it's like have 2 amps. I have a BF Fender amp into the tremolo channel and some sort of very touch sensitive, firm & authorative Tweed Bassman or Marshall on the normal channel. The normal channel also makes a pretty good bass amp

 

[Supertwang]

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.

I followed RobRob's directions for Marshal-izing the normal channel of AB763 amps,...I love it

 

[spoontex]

https://reverb.com/item/1092548-fender-dumble-bassman-mod-1971-silverface

Love how this sounds!!

 

[telemnemonics]

I'll beg to differ. I modded my '72 Pro Reverb normal channel with Marshall spec components on V1 and I gotta say WOW WOW WOW. I changed 4 components: cathode bypass cap & resistor, plate resistor, grid resistor?? And now it's like have 2 amps. I have a BF Fender amp into the tremolo channel and some sort of very touch sensitive, firm & authorative Tweed Bassman or Marshall on the normal channel. The normal channel also makes a pretty good bass amp

Clearly we all hear “Marshall” differently!
The Pro Reverb is to me the epitome the ills Marshall fixed around 67-68.
Flabby bottom due to weak power filtering weak voltages and undersized OT that saturates on bass notes before the plain strings distort.
A preamp can add sizzle to the plain strings but cannot add authority to the bottom when the power section cant keep up.

Of course many like crunchy bass and clearer highs.
Or run at low volume the bass doesn’t fall apart as badly.
But what Marshall brought after the JTM45 saggy bottom blues tone was a hard slam of full range roar.

Dumble adds more low volume gain and can be used for snarl while keeping the bottom tight, but that’s a whole different sound.
I’ve done mods to BF Fender amps and in the end I feel what I want from Marshall is not hiding inside BF architecture.

But since we may all hear different sounds when thinking Marshall it may not matter what is scientifically possible with a weak power section driven harder.
Plenty are happy with the sound of a MIAB into a PRRI…
Dayum.
What are they thinking!

I had a Twin reverb modded by Harry Kolbe who did a huge number of modded Marshall’s before the 800 series when they were all over pop rock records.
Much was changed and it had more bottom slam but was still not reaching Marshalldom.

Modding a Bassman still makes tons of sense given the useless bass channel too though.
Expecting some Marshall spec changes to get a Marshall channel is worth holding up to closer scrutiny though.
There are others paths to a clippy side.
Not to forget using the second channel just for added controls over the main channel instead of keeping two separate channels.
GP mag had a mod in the 90s I think that used the bass channel for some gain controls and lost the dual channel for single channel with gain added.

 

[Supertwang]

Clearly we all hear “Marshall” differently!
The Pro Reverb is to me the epitome the ills Marshall fixed around 67-68.
Flabby bottom due to weak power filtering weak voltages and undersized OT that saturates on bass notes before the plain strings distort.
A preamp can add sizzle to the plain strings but cannot add authority to the bottom when the power section cant keep up.

Of course many like crunchy bass and clearer highs.
Or run at low volume the bass doesn’t fall apart as badly.
But what Marshall brought after the JTM45 saggy bottom blues tone was a hard slam of full range roar.

Dumble adds more low volume gain and can be used for snarl while keeping the bottom tight, but that’s a whole different sound.
I’ve done mods to BF Fender amps and in the end I feel what I want from Marshall is not hiding inside BF architecture.

But since we may all hear different sounds when thinking Marshall it may not matter what is scientifically possible with a weak power section driven harder.
Plenty are happy with the sound of a MIAB into a PRRI…
Dayum.
What are they thinking!

I had a Twin reverb modded by Harry Kolbe who did a huge number of modded Marshall’s before the 800 series when they were all over pop rock records.
Much was changed and it had more bottom slam but was still not reaching Marshalldom.

Modding a Bassman still makes tons of sense given the useless bass channel too though.
Expecting some Marshall spec changes to get a Marshall channel is worth holding up to closer scrutiny though.
There are others paths to a clippy side.
Not to forget using the second channel just for added controls over the main channel instead of keeping two separate channels.
GC mag had a mod in the 90s I think that used the bass channel for some gain controls and lost the dual channel for single channel with gain added.

Not all Marshalls are JCM 800 or 900 amps like the Marshall purists seem to think while prostrating themselves to their Marshall gods. I did the "Marshall mods" after a total service and completed a "blackfacing" which someone started before me. I also increased the filter/dropping resistors to 10K + 10K ohms while increasing the filter caps to 40uf + 30uf + 30uf + 30uf and swapping the rectifier tube to a 5V4 and adjusted the bias down to 30ma-ish. V1 and V2 got 5751 tubes installed, V3 and V6 went to 12au7 and may go back to 12at7 and maybe not. My scheme/theme was to REDUCE the B+ and output wattage where ever possible,...and in doing so the smaller PR output TX is no longer "under-sized",...I'd say it's just about the perfect size for a 30-35w amp. My Pro Reverb isn't flabby on the bass,...in fact with judicious speaker selection it sounds great amplifying my EB-0 bass on the "Marshallized" normal channel. My other AB-763-esque Fender amps,...SF Super Six Reverb, SF Vibrolux Reverb, BF Super Reverb are all close to stock but since the Pro Reverb was previously modded when I bought it I felt free to mod it however I see fit. IMHO,...the modded Fender Pro Reverb maybe the best of my BF and SF Fenders,...mostly due to the 2x12" layout without the 85w output of the Twin Reverb. IDK however because I also love the 4x10" lay-out. I also have a modded 5e3 Tweed Deluxe, 5f6 Tweed Bassman waiting repair, 6L6 5f10 Tweed Harvard also waiting repair, and a 6L6 5f2a Tweed Princeton on the bench waiting for me to come back and finish the build. I guess I'm kinda a Fender amp enthusiast but I do own other brands as well like Vox, Ampeg, Valco and a Marshall 1974 clone. I think a Hiwatt or Reeves might be the next addition.

 

[telemnemonics]

Actually as a “Marshall purist” I snub all MV / 800/ 900 and only believe in the gospel of four inputs!
Ive had a dozen vintage Marshall’s mostly in the 1990s and also have some clones.
I just spent 10-15 years trying to make BF amps work for me including modding them and when I moved to Marshall for Marshall sounds it was a huge relief.
BF amps used to be so cheap and I had four Super Reverbs that came close to being lovable.

Plenty of players get what they want from BF amps with some mods or pedals, but I am so happy to not be trying to make them work for me.
Tweeds are cool but again lack the brutal low end punch and clarity.

 

[spoontex]

I followed RobRob's directions for Marshal-izing the normal channel of AB763 amps,...I love it

Can you post the link? Please?

 

[Supertwang]

Can you post the link? Please?

I’ll try to remember to add a link tomorrow when I’m on a desktop. But if you go to Robrobinette.com and search there for “lead channel mod” or “ab763 lead channel mod” you should find it

 

[Supertwang]

Can you post the link? Please?

AB763 Mods

AB763 Blackface Modifications

robrobinette.com

 

[Supertwang]

Can you post the link? Please?

I’d probably recommend stiffening up the power supply filter caps to get the full effect.

 

[Kevin Wolfe]

I’d probably recommend stiffening up the power supply filter caps to get the full effect.

Definitely.
This was always a standard upgrade to these Bassman amps. Two 100uF caps for the ones in series. When folks were still using these as bass amps, and I still do, we’d even go bigger. The fattest ones you could fit in the doghouse. You could even decrease the preamp dropping resistors and get more volts up front.

 

[Supertwang]

Definitely.
This was always a standard upgrade to these Bassman amps. Two 100uF caps for the ones in series. When folks were still using these as bass amps, and I still do, we’d even go bigger. The fattest ones you could fit in the doghouse. You could even decrease the preamp dropping resistors and get more volts up front.

I usually increase both the dropping resistors up 10K ohms too. Fender amps usually have plenty of B+ voltage and I tend to prefer to drop the B+ hopefully under 400v B+. With a lower plate voltage the bias mA could be increased and still be under max PD wattage,…but usually I prefer the sound around 30mA for 6L6 and around 20mA for 6V6 and EL-84 even with more conservative plate voltages. I think tubes sound better when they aren’t pushed so hard,…and Fender tended to push them very hard. A bonus to cooling down the power supply and bias is basically you are getting a free “virtual upsize” in the transformers and caps,…I hope that makes sense. You will lose output wattage going this route but it seems almost everyone has plenty of output wattage to spare.

 

[spoontex]

After looking at different options, I’ve decided to go with a 2204-style build. I don’t want to do the “Theseus Amp” and do a complete rebuild. I like how the clean channel of the AA864 sounds; I just want to make minimal changes to get a more versatile Lead channel. Tomorrow I’ll start with the modifications. Here’s the schematic.

 

[spoontex]

Today I completed the modification, and I also added a Master Volume PPIV. I like how the Lead channel sounds! Now, the problem I have is that this extra gain is bleeding into the Normal channel. When I’m on the Normal channel and turn the gain knob, the noise goes up or down in the Normal channel. I imagine this is normal when doing this type of modification. Any advice on how to improve that?

 

[sds1]

You did Rob's lead channel mod? I wonder if you just didn't notice the noise bleed before the mod. I would have to guess it's bleeding thru the power supply. It could be mitigated by decoupling the preamp stage power supplies, on any Fender 2- channel amp the preamp stages share the same power supply node. Not sure this is worth the trouble?

 

[spoontex]

I followed the schematic I posted in the previous thread. I'm sure it didn’t make any noise before the mod.

 

[spoontex]

I followed the schematic I posted in the previous thread. I'm sure it didn’t make any noise before the mod.

It’s true that for the tests I didn’t use shielded cables, and some of the capacitors in the tone stack are close to their voltage limit — it’s just what I had at home to try out before buying the proper components. Maybe when I build it properly I’ll be able to draw better conclusions.

 

[sds1]

I followed the schematic I posted in the previous thread. I'm sure it didn’t make any noise before the mod.

Correct me if I misunderstood but you built a standard bassman aa864 then modded it with Rob's lead channel correct? 4 part swaps I believe?

My thoughts are this:
1) I believe the only bleed path is via the power supply, and this doesn't change in any way with the lead channel mod
2) You didn't really increase the gain that much. I would think prior to mod you would have to turn the gain pot up a bit more to hear the same label of bleed noise, but it was still there

 

[spoontex]

Correct me if I misunderstood but you built a standard bassman aa864 then modded it with Rob's lead channel correct? 4 part swaps I believe?

My thoughts are this:
1) I believe the only bleed path is via the power supply, and this doesn't change in any way with the lead channel mod
2) You didn't really increase the gain that much. I would think prior to mod you would have to turn the gain pot up a bit more to hear the same label of bleed noise, but it was still there

No, I don't use the Rob's mod. I use this one that I modified myself. Yes, it's a aa864 build. I add extra gain stage with unused triode on the bass channel.