Fender Bassman Silverface 1968 AB165

[analogean]

Hello, I have a Bassman that looks pretty original (including the RCA tubes and Mallory caps) and the bass channel works but the Bass tone poti does not seem to do anything. The Normal channel is not working at all except some flubby fluctuating sounds comes through at high volume and then fade away after a few seconds.
You might be saying, "so what's the problem, change the power supply caps in the doghouse, and the bias cap, check the tubes, change if needed, and off you go!". Well yes you are right, and here in Vienna Austria I have found a shop that sells tubes (JJ) and he also had some caps from the same company, namely a 47Mfd for the bias and 2x 80 Mfd for the power. The 3x 20Mfd, and the 16Mfd seem to be more difficult to find, and there is also a 25/25 Mfd filter cap that i will have to search for as well as a few others.
Any tips on finding good caps??
My main problem though is reading the schematic! The original fender schematics that one finds on the internet seem to be hand written and there are no high resolution scans. How do you guys read these?? I mean they are practically illegible!
So my question is; has anyone out there redrawn these schematics in a modern fashion or does anyone have a legible high resolution
scan of a Bassman AB165 circuit??
My Bassman is very similiar in date and design to Statelejazzuers post about his 68 bassman
http://www.offsetguitars.com/forums/vie ... 10&t=36784
In fact almost identical, the main difference being that mine is an export model with changeable voltage.
There are also things in there that are not on the schematic which I have read is not unusual but still makes you scratch your head.
Here are some pics, please have a look,





sorry for the bad photos..I can link to some better ones if necessary.
Cheers
Edit: the largest schematics I could find were by Kbapps but still hand scribbled.
http://www.kbapps.com/audio/schematics/ ... ab165.html
Also a very informative link about modding the ab165 circuit:
http://bwilliamson.home.mchsi.com/bassman.htm

[al30]

Most of the Fender schematics floating around the net look just like that. I've never had any problems reading them.

The Fender Amp Field guide has some more info and also layouts. Maybe the layout will help.

http://www.thevintagesound.com/ffg/bassman_bf.html" onclick="window.open(this.href);return false;


AL

[analogean]

HEELLPP!!
Okay I recapped the bassman and replaced the old mullary 25+25 cap from the normal channel. Now both channels are working
and the normal channel sounds pretty good, the bass channel is a bit dull and lifeless and missing mid-tones. So I was considering doing a mod as so many folks have done with their SF bassmans. After reading around I decided to try to convert it to AA864 specs,
ala the "Allen Mod"
but wait a minute, maybe I should really check what is in there first, since I have read that the charts aren't always correct, and maybe somebody modded it already, who knows...
It took a while of staring at schematics and layouts and finding resistor values, and what did I find?
I am not really sure and it is starting to confuse the heck out of me but it looks like
a cross between an AB165 and a AA371 !!!
I found that the 2 plate resistors coming off the 12AT7 are both 47k instead of 100k.
the 2 caps between the plates of the 12AT7 and the 2 6L6GCs are .047 instead of .022
and there are a few other things that indicate that this is an AA371. Some other parts in the circuit however
are more like the AB165.
So what to do now??

[al30]

HEELLPP!!
So what to do now??

It's your amp. What do you want to do? Amps that are a combination of different circuits are common. You've already traced out the differences from the sounds of it. It's a simple thing to switch out those components - and it's a simple thing to put them back if you don't like it.

AL

[analogean]

hi Al and thanks for your response.
What do I want? Well at the moment I have a lot of very clean tube amps,
an Echolette M40 with the NG51 tape delay, an Echolette BS40 and an Echolette Showstar. They are all wonderful sounding tube amps
but like i said very clean. So with the Bassman I was hoping to get something a bit dirty and grittier, I want some nice tube distortion, but at lower volumes so that the plaster doesn't fly off the walls.
Here is what I have done so far.
I removed the 500pf and .01 caps across the100k plate load resistors.
I removed the cap .01?? that was bridging the feedback resistor to the PI.
I put in new power supply and bias electrolytic caps. For the bias cap i used 47uf (is that too low?, the original was 50)
That is all for now. There is more distortion now but still very loud.
The bass channel has really opened up and sounding much crisper.
It is labeled as an AB165, however after the second stage and before the PI it turns into something more like an
AA371 circuit, then at the power tubes there are some more strange things going on that look similar to the AB668 in a Pro Reverb. To be specific there are 150 Ohm 7w resistors going from the cathode (pin to ground. There is also a mallory 5Mfd/25v cap running between both cathodes of the 6L6GC tubes. There are also 2000pf caps going from both tubes grids (pin 5) to ground. I have read that the 2000pf caps are for filtering high frequencies, but what do the 150 Ohm resistors and the 5uf cap between the cathodes do??
Many thanks
Lucas

[Stratelejazzuar]

Haha, here's the thread you posted that i suggested you post, but not before i looked to see if you had posted a thread. Nice! Next time I'll have to look for posted threads before I suggest posting threads.



I still need to re-cap my Bassman. And maybe clean the "deep" switch on the bass channel, as it seems finicky.