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BF Bassman - which model the "best"?

Chris Scott

Member
Messages
9,268
Just got an AA165 BF head, and as it's been modded for harp, I am going to put it back to stock condition, to be used only for guitar.

My question is this - I remember reading somewhere that there's a model that is considered the "worst sounding" of all the BF/SF Bassman heads, and this begets the question; which is the "best" of the existing circuits to put it back to?

...and fwiw, no, I don't wish to get too involved, (5F6 circuit, for example is pretty easy to do, as most all the holes are already there;)) just the overall best-sounding orig. Bassman head circuit.

Thanks!
 

jay42

Member
Messages
7,083
You'll have to check whether it really is an AA165. Fender kept messing with the BF Bassman and used up old tube charts after revisions instead of purging stock. The AA165 is a good one, btw. The AB165 is a bit of a turkey, however, with some rework, is just fine. They did a strange thing with the bias supply that has to go bye-bye.

V3 is similar to the reverb recovery/mix stage in blackface reverb amps. I prefer the Normal channel to mix before V3, not after, for that reason. It doesn't have to be that way, though.

Read me
http://bwilliamson.home.mchsi.com/bassman.htm
 

AR-305

Member
Messages
974
I just took a 68 drip edge AB-165 and rewired to AA-864, true blackface. Much improved to my taste. And just for the hell of it I rewired the bass channel to a marshall 2203 master volume preamp. With an A/B switch this is a real monster. The marshall pre is operating off higher voltages so has a character all of it's own into the BF PI and output. I made the bright cap switchable like on the Fender channel and it is totally useful and good.
 

Jonny D

Member
Messages
1,283
I have used a very lightly modified AB165 for many years and I get compliments all of the time.

a 5751 tube in the "normal channel" and a nice higher gain tube in the bass channel gets you lots of options with an ABY switch, enabling you to switch between or combine the two channels.

Some do not like the bass channel, but you should try it before you change it. There are some that would not touch the bass channel at all because of the throaty and mojo filled alternative sound you can get there. Try a good "woman tone" setting on that bass channel, and you may never touch it again.

If you do truly have the AA165 circuit, then restore it and keep it that way.
 

Prairie Dawg

Member
Messages
1,928
Trust your ears and that only.

Each one sounds a little different even if they're the same model. For my taste the AA864 and AA165 have too much negative feedback so every one that comes in here gets a 33k nfb resistor and the owners love it for the most part because their amps sound muddy and it cleans them up. Maybe it's a local thing, I dunno.

Discard all you've heard about this or that circuit sucks the tone out of an amp. It's bollocks unless and until you have heard it with your own ears.
 

Chris Scott

Member
Messages
9,268
Thanks everyone.

I've decided to put it back to original AA165 spec, then "season to taste".

This one's been gone over pretty good, and I'm just glad they didn't toss the transformers in their quest for harp nirvana...

Great ideas - thanks again, and keep 'em coming!
 

silver surfer

Member
Messages
1,970
Mark Norwine modded my 66 so that it had the AA864 circuit on one channel and the AA165 on the other. Amazing tones.
 

jay42

Member
Messages
7,083
For my taste the AA864 and AA165 have too much negative feedback so every one that comes in here gets a 33k nfb resistor and the owners love it for the most part because their amps sound muddy and it cleans them up.
Those amps have the 820||100 pair. Do you mean the AB165?

I had a spate of BF Bassman amps come through recently. The stock AB165 biased at about 20mA with the fixed + balance circuit.
 

teleman1

Silver Supporting Member
Messages
15,362
Mark Norwine modded my 66 so that it had the AA864 circuit on one channel and the AA165 on the other. Amazing tones.
This is the idea! Two amps in one and both sound unique and stellar. Then there's jumpering.



I had a 68 Bassman and had every intention of converting it. The bass channel sounded bleh. But it had a mod to the Normal channel and its sounded fantastic. It was odd that the bass and treble channels acted differently than other Fender amps. They also seemed to add or take away volume in their frequency range.
 

mark norwine

Member
Messages
17,200
Mark Norwine modded my 66 so that it had the AA864 circuit on one channel and the AA165 on the other. Amazing tones.
Almost.

The mod is to leave the preamp (all of it) as AB165....but revert the inverter & power section to AA864.

I also changed the (useless) bass channel to be just like the guitar channel.
 

Blue Strat

Member
Messages
30,241
Almost.

The mod is to leave the preamp (all of it) as AB165....but revert the inverter & power section to AA864.

I also changed the (useless) bass channel to be just like the guitar channel.

+1000. I doubt many of these are converted to full 864's which involves rerouting wires under the board.
 

Chris Scott

Member
Messages
9,268
Almost.

The mod is to leave the preamp (all of it) as AB165....but revert the inverter & power section to AA864.

I also changed the (useless) bass channel to be just like the guitar channel.
Based on what I've learned, this seems to be a good way to go.
 

Blue Strat

Member
Messages
30,241
Using either the Marshall tonestack cap values or increasing the midrange resistor for the bass channel gives a little versatility. An A/B switch can be used for channel switching.
 

smolder

Platinum Supporting Member
Messages
14,502
+1000. I doubt many of these are converted to full 864's which involves rerouting wires under the board.
exactly... if you remove the original board and rebuild it (which I had to do) the full 864 conversion in the power section is pretty simple.
 

Blue Strat

Member
Messages
30,241
exactly... if you remove the original board and rebuild it (which I had to do) the full 864 conversion in the power section is pretty simple.
I've never seen the need to go under the board for the PI or power section 864 conversions, only the preamp conversion.
 

mark norwine

Member
Messages
17,200
Mike.....the AB165 is the offending circuit. Look at a layout diagram: there's an under-board wire from the inverter's NFB junction.

True, I have seen later ('67, early '68) specimens where the connection is an over-board wire, but all of the earlier ones had the underboard. It needs to be considered when modding the inverter circuit to BF (864) specs.
 

Blue Strat

Member
Messages
30,241
Mike.....the AB165 is the offending circuit. Look at a layout diagram: there's an under-board wire from the inverter's NFB junction.

True, I have seen later ('67, early '68) specimens where the connection is an over-board wire, but all of the earlier ones had the underboard. It needs to be considered when modding the inverter circuit to BF (864) specs.
It's been a while since I've done one, but I'm pretty sure I was able to accommodate that connection with component layout.
 

Prairie Dawg

Member
Messages
1,928
Those amps have the 820||100 pair. Do you mean the AB165?

I had a spate of BF Bassman amps come through recently. The stock AB165 biased at about 20mA with the fixed + balance circuit.
Well, the AA864 and AA165 have (I think) an 820 ohm NFB resistor which means more nfb that the AB165 which has a nominal 47k nfb resistor. For my taste, they've too much NFB and it makes them sound muddy. Stable but muddy.
 






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