1968 (?) Bassman Drip-Edge - Blackface circuit

[jfab]

Hey All! Been working on the drip-edge Princeton Reverb and the SF Champ lately, but I got the opportunity to work on my fiancee's Bassman head and thought that I would show a pic or two while I had it open getting my parts list.

It is a drip-edge, so I am guessing '68. There are some numbers which start with 68 and some which start with 69 on the chassis, so I am not sure. This is the Blackface circuit, as it says "AB165" on the tube chart, and it seems to match with the layouts from ampwares.

I checked the 470 Ohm resistors on the power tube sockets, and they are just fine. I am going to order the caps this afternoon. Looks like I will be getting the following:

1 - 50uf @ 70v
5 - 25uf @ 25V
1 - 5uf @ 25V

In the doghouse, I have:

2 - 70uf @ 350V
3 - 20uf @ 500V
1 - 16uf @ 450V

So, it's off hunting caps now.

My fiancee does play bass, and her grandfather left this amp to her when he passed on. She doesn't really ever use it, but I thought that I would put some new caps and a three-prong plug on it to make sure if she (or I) wanted to play it, it was going to be good to go. She has the large cab (2x15s?) which goes with it -- no pics. Sorry.

Anyway, here are the pics. Forgive the quality. I am at work now, and only have my phone with me for pics.









JFab

[phantomman]

Schweet!!!

Send it to me an' I'll give her a good home.

[Cygnus X1]

2nd dibs!

Very nice.

[chuckocaster]

me 3!

i love those old drip edge bassmans, they are awesome amps. restore it back to OG specs, fo' sho. bass through it is okay, great for low volume, but they sound KILLER through a 4x12 with celestions and nice guitsteel.

best of luck.

[jfab]

Ack! I am the victim of Fender using up the old tube charts before switching! I looked at the schematic, and it actually appears to be the AC158 circuit instead...

A check on the Fender website put this transformer during the second week of 1969, and adding about six months on makes this a July 1969 final manufacture or so... I guess that this will be called the "Woodstock" Bassman around here from now on!

[steve25]

That amp has the dredded fixed bias/ part cathode bias output stage, its time to put that back to a AB165 schematic!

[ES350]

Lose the chocolate drops too...

[Cygnus X1]

So does this mean it has a dreaded high B+ power transformer?

Swapping out those brown turds will only cost a couple dollars (OK maybe 10 or 20).
I tried Sprague orange drops in the DR, and didn't like them at all.
Mallory 150's sound better, and are cheaper.

[steve25]

No, Fender did not start upping the PTs a/c output untill the mid 70s when they brought up all the 40 watts to a very optomistic 70 watts.
In my fully recapped 76 pro reverb, even with the very best new GE7581A tubes I have never seen more than 64 watts rms of output!

[steve25]

And try removing those ciraimic disc caps off the output tube sockets also as most times the amp is stable with out them.

[jfab]

Thanks for the info. I already ordered my caps, but I will order replacements for the "brown turds" next. There are always more parts to order, it seems. :) The quest for audio mojo continues. Thanks for the great advice, guys. I look forward to hearing this thing after I have given it some TLC.