Blackfacing a 68/69 Bassman

[Norse]

I've done some Blackfacing to this 68 or 69 Bassman- more for fun than in search of the perfect amp. Like many of this era, the circuit sticker does not reflect the actual amp - the sticker says AB165, but the circuit values are all AA270.



So far I have done the basic things that Gerald Weber suggests:

Convert to fixed Bias, to include replacing the 100k resistors with 220k after the .047 caps feeding the power tube grids.
Remove the Power section extras - grid supressor caps and cathode ceramic resistors.
I Also converted the feed-back loop to the AA864 design using a 1.5k instead of the 820 ohm resistor.

The amp has a nice Jazz tone now at lower volume, but I have some more work to do for the higher volume blues tone that I want to end up with.


I am considering other values to go after next- in particular:

.047 Power section grid caps,
the presence resistor at the phase inverter ground point,
the 47k resisitors off of the phase inverter plate,
.1 cap feeding the phase inverter grid,
the electrolytic cap off of the Normal channel plate.
Suggestions greatly appreciated!

http://s423.photobucket.com/albums/pp319/ggkohler/?action=view&current=69BassmanAA270pic.jpg
http://s423.photobucket.com/albums/pp319/ggkohler/?action=view&current=101_0525.jpg
http://s423.photobucket.com/albums/pp319/ggkohler/?action=view&current=AA270Bassman.jpg
http://s423.photobucket.com/albums/pp319/ggkohler/?action=view&current=AA864schematic.jpg
http://s423.photobucket.com/albums/pp319/ggkohler/?action=view&current=AA270Bassman.jpg
http://s423.photobucket.com/albums/pp319/ggkohler/?action=view&current=AB165Bassman.jpg
http://s423.photobucket.com/albums/pp319/ggkohler/?action=view&current=69BassmanAA270pic.jpg


Greg

[Blue Strat]

Definitely try blackfacing the phase inverter if nothing else. That will help the amp overdrive earlier and should get you close to your goal.

Also try a Marshall tone stack for the bass channel with is typically useless for guitar in it's stock configuration.

I don't remember the AA270 of the top of my head, but if any of the plate load resistors (100K) have caps in parallel (as do the AB165s), ditch those caps.

[Norse]

Thanks Mike! I'm glad you mentioned the plate load resistor caps, I meant to ask about those as well.

I definitely agree with your assessment on the bass channel being useless for guitar. I'll do some searching on how to play with the Marshall tone-stack once I get the rest of the amp dialed-in.

Now considering:

--47k resistors @ PI plates (go to 82/100k?).
--cap values feeding PI and PWR section?
--addition of 100k "Presence" resistor at PI gnd?
--8uf cap at plate junction Norm/Vib chanel? (remove?)

Could use some advise on how to add pics too!

Many thanks!

[Blue Strat]

Thanks Mike! I'm glad you mentioned the plate load resistor caps, I meant to ask about those as well.

I definitely agree with your assessment on the bass channel being useless for guitar. I'll do some searching on how to play with the Marshall tone-stack once I get the rest of the amp dialed-in.

Now considering:

--47k resistors @ PI plates (go to 82/100k?).
--cap values feeding PI and PWR section?
--addition of 100k "Presence" resistor at PI gnd?
--8uf cap at plate junction Norm/Vib chanel? (remove?)

Could use some advise on how to add pics too!

Many thanks!

The first 3 fall under "blackface the PI". If the input R's to the PI aren't 1 Meg, do that too.

8uf cap....? Is that in the power supply? Just do whatever the BF version does there. That may stay as is. You can also bump it up.

[hasserl]

I just finished up one of those, came out really good if I do say so myself. I've been really enjoying it.

I did do the Marshal thing on the Bass channel, used the unused triode for a cathode follower and drove a Marshall style tone stack with it. But as for "blackfacing" it, don't. Well, go ahead and change the bias balance to a bias adjust, but actually having a bias balance is a great option, it'd be better to add a bias adjust pot and leave the balace pot alone. But I didn't do that either. Oh well. But don't go crazy with other changes, particularly to the PI, leave the 47k plate loads there as they are a better fit for the 12AT7 tube used as the PI. IMO the tone is larger and fatter and less squished than with 82k/100k plate loads.

If I can finish up the schematic for it I'll post it for you.

[RedMan]

One of the best BF improvements you can make to a SF IMO is the smaller cap feeding the PI and larger ones feeding the power tubes.

[Blue Strat]

Opinions abound. Try everything for yourself and decide. THIS is the real answer. ;)

What's "squishy" at one volume to one person is perfect at another volume to another person.

[Norse]

Got to mess with the amp tonight and it is sounding very nice - I completed the black-face mods mentioned above and was rewarded with happy sounds. I was not able to turn up to club volumes tonight but I did press my luck a little :)

Mike - The parallel caps on the plate load resistors are now gone - the PI is completely BF - Thanks for all your input (Marshall channel too)!

Richard: Marshall tone-stack on the Bass channel intrigues me - I would love to have something useful on that channel and utilizing the other half of the 2nd tube sounds good to me. Your SR is sounding really nice - as are all the clips I sampled! Now that I see you do this for a living, I am hesitant to ask for your schematic!

Redman - glad you mentioned the PI input cap - I've been wondering about that. I changed the .047's to .1's feeding the power tubes and may try a 500pf to the PI if higher-volume needs more tweaking.

My Jazz tone is very nice right now - very thick, detailed, and spacious.

Thanks again to all for the input!!!

[chocopower]

Just for share it..

i did this to a friend´s AB165. Basically, a JTM45 with V1 fully paralelled...

http://i101.photobucket.com/albums/m74/chocopower/BASSMANHYPERON.gif

[Mike9]

Don't forget the "Rule of Sixes" - quoted from another site here:

There is the old guidance that Fender Blackface amps are best set on the "rule of sixes"... volume = 6, treb = 6, Mid (if you have one) = 6, bass = 3, reverb = 2 (3x2 = 6).

[ChickenLover]

For another idea on the Bass channel I installed a 6G6-B circuit (last revision of the Blonde Bassman) there but I tweeked it for guitar. It's pretty much just like the 6G6-B Bass channel except for the following:
220k Slope resistor
.1u Bass cap
.047u Mid cap
8k2 Mid resistor
Treble control is simply a series .01u cap and a 1M pot to ground right off of the plate (same spot as the 6G6-B Treble control...except 'before' the coupling cap...probably doesn't matter if it's before or after it though)
Also added a switchable 100pF bright cap
I also tweaked that 'fixed' treble portion of the first tone stack to 82k/22k instead of 47k/10k but there's a lot of 'tweek' to that voltage divider...might wanna play with that.

The other channel is just plain jane Fender BF Normal channel and the power section/PI is all BF'd (with 12AX7 in PI) and the bias has both adjustment and balance.

IMHO, this new second channel does the 'BF Fender thang' better than the Normal channel. The tone stack is more responsive (lol...it better be with all that loss!). It's also stellar with pedals (again...better than the Normal channel imo). One reason I think it's better with pedals is because it has more sparkle without using the Bright switch...and using the Bright switch can often ruin the tone with overdrive/distortion pedals.

But it's kind of a weird looking circuit so I wouldn't blame anyone for not trying it. But imo it sounds awesome. Imagine a Fender clean tone...with a little bit of Vox clean tone mixed in. That's not really accurate but the best I can come up with.

EDIT: and low-ish voltages to the new 6G6-B channel...like around 260vdc-280vdc supply.

[hasserl]

For those interested, here's a schematic of the one I just finished up.
http://members.dslextreme.com/users/rlhasse/Ampdocs/HasseSFBassmanmodschem.pdf

I'm playing it now through a 2 x 12 cab loaded w/ a pair of Weber ceramic California 12's w/ paper dust cover. The Normal channel sounds just great through those speakers, just as you would expect it to. But I'm surprised with how good the Bass/Marshall channel sounds through them also.

Maybe I shouldn't be too surprised as Duane Allman used JBL's with his Marshall; and though these aren't JBL's they were designed with to be in the vein of those speakers.

[Norse]

That's what I'm talkin' about! Can't wait to jump into the Bass channel, thanks for the schematics :BEER

Here's what I've got so far:
http://www.soundclick.com/bands/default.cfm?bandID=690518

The volume tips-over around 3 on the dial. Just under 3 and you have nice jazz tone, 3 & above and you have decent blues tone now, a bit boomy so I'll be replacing the PI input cap next.

The preamp tubes look original, and the power tubes are Realistic long life's. Haven't messed with them yet - just set the bias at -48v.

The sound file was taken while my brother Chris was test driving the amp on a Kramer Striker (bridge pickup), amp settings:
V-3, T-6, B-4, going through a 2x12 closed-back cab (1 Wizard, 1 G12 Heritage).

[RedMan]

For those interested, here's a schematic of the one I just finished up.
http://members.dslextreme.com/users/rlhasse/Ampdocs/HasseSFBassmanmodschem.pdf

I'm playing it now through a 2 x 12 cab loaded w/ a pair of Weber ceramic California 12's w/ paper dust cover. The Normal channel sounds just great through those speakers, just as you would expect it to. But I'm surprised with how good the Bass/Marshall channel sounds through them also.

Maybe I shouldn't be too surprised as Duane Allman used JBL's with his Marshall; and though these aren't JBL's they were designed with to be in the vein of those speakers.

Don't you have any problems with the 6.8k engaged in the feedback loop going to the hot side of the PI? Just curious.

[hasserl]

Don't you have any problems with the 6.8k engaged in the feedback loop going to the hot side of the PI? Just curious.

No, that is the stock location for the nfb in the SF amps. You can move it to the other side of the PI if you want to, but then it become positive feedback, not negative, so you also have to switch the secondary wires on the OT. But I found no reason to make the switch, it works fine like it is and sounds great. The nfb does it's job and all is well.

The 6K8 resistor was added to increase nfb, I put it on a swtich so I can revert to the stock 47k when I want to. I run it mostly with the 47k only, but the added nfb adds some stability and calms the amp down a bit for when I want a milder amp. It comes in handy with a particular Strat I have that otherwise sounded too loose and wooly with this amp. With the 6k8 resistor engaged I get a much cleaner Fenderish tone, very silky and smooth. I spent an evening with a decade box playing with many different values there before I settled on 6k8. Though I may want to open it up and play with that some more.

[RedMan]

I'm familliar with the SF feedback arrangement. My concern wasn't with the polarity, it was more with the frequency response with what looks like a very low pass filter youve got there.

[Norse]

For those interested, here's a schematic of the one I just finished up.
http://members.dslextreme.com/users/rlhasse/Ampdocs/HasseSFBassmanmodschem.pdf

I'm playing it now through a 2 x 12 cab loaded w/ a pair of Weber ceramic California 12's w/ paper dust cover. The Normal channel sounds just great through those speakers, just as you would expect it to. But I'm surprised with how good the Bass/Marshall channel sounds through them also.

Maybe I shouldn't be too surprised as Duane Allman used JBL's with his Marshall; and though these aren't JBL's they were designed with to be in the vein of those speakers.

Thanks for the schematic, Richard! I ended up going for your design since it was the quickest/simplest for me to try, and I am very pleased with the results. The power section was taken to AA864 entirely, except for 1500 ohm value on the NFB loop, and the preamp now has a FAT switch replacing Bright for Ch2. Ch1 still has its 1500/25-25 Cathode bypass setup across the board. The bottom is tame, so I'm going to run it like this for a while.
For the PI coupling cap, I tried .1, .01, .02, .047, .005, and .001 values, ending up keeping the .047.
Slope resistor on Ch1 started out at 58K but that had too much bottom end wooliness so I ended up with 118K there.
This is really a nice amp now - I can't thank you enough for providing the schematic! My final schematic is here along with a few pics - I still can't show images, so here is the link...
http://s423.photobucket.com/albums/pp319/ggkohler/?action=view&current=68BassmanHasseBasschan-aa864-aa270.jpg
http://s423.photobucket.com/albums/pp319/ggkohler/?action=view&current=68BassmanHasseBasschan-aa864-aa270.jpg
Clip of the amp in action...Clean - low volume (2.5).
http://soundclick.com/share?songid=7167633

Many Thanks!!!
Greg

[hasserl]

Very cool Greg, glad it helped you sort yours out and come up with your own version. I didn't realize I had left the slope resistor value out on the schematic, I think I used 56k there.

Sounds really nice too. Congrats!