Bassman Servicing

[rockgardenlove]

So, I have a 1967 AB165 bassman that still has 39 year old electrolytic capacitors(or so it seems). I turned on the amp, and sure enough, smoke. Fun stuff. ;-)

Anyways, I'm a bit unclear on what needs to be changed...

I know all the power supply stuff does need to be swapped out, but on the main eyelet board there are 2 three legged electrolytic caps and some other ones...

http://img361.imageshack.us/img361/609/ab165electros9xo.png
Here are the ones I'm a bit confused about.

Here is what they look like:
http://img.photobucket.com/albums/v503/crg23/DSC01020.jpg

http://img.photobucket.com/albums/v503/crg23/DSC01021.jpg

And if the 3 legged ones do need to be changed, where can I get new ones? I've never seen any other 3 legged ones in my life.

And then also, somebody has gone and jumpered both pin 8's on the 6L6's with two 5uF caps in serias...what? I really don't know what is up with this. Here's a pic:
http://img.photobucket.com/albums/v503/crg23/DSC01029.jpg
Do they need to be changed?


Thanks!

[Marc]

Hey Rockgardenlove,

It would probably help if you looked at the schematic that is closer to your amp compare this:
http://schematicheaven.com/fenderamps/fender_bassman_ac568.pdf

to your amp. It appears you have the cathode resistors and fixed bias scheme with the 5 mfd cap strung in between. If you leave it set up that way I'd consider replacing the cap with a film type. This was usually a non-polar cap, but sometimes people would put them back to back for essentially the same effect.

The bias cap should be changed, The three legged caps are the preamp cathode caps, they can be replaced with individual units and just twist the ground leads together.

The blue ones probably do not need changing, unless you measure them leaking DC.

Have you checked the caps in the doghouse?
What smoked?

Marc

[rockgardenlove]

I'm quite sure that it's an AB165...it has the Silver around the grill, so it has to be from '67 to '69, and it's states it on the tube chart...?
And where can I find a nonpolar 5uF cap(this is the bias cap right?)?

The power supply caps are smoked, no issues there at all. I can fix those.


And what is the "ground" switch for?

EDIT:
http://plexipalace.com/plexiboard/viewtopic.php?p=327881&sid=a40059a6180e7a789472469132801313
Seems like it must be a different circuit, this guy has the identical amp as me it seems...
Cool!

[Carl / Zwengel Amps]

Im not exactly sure what you're asking here. Also, the images from the plexi palace post and the ones you've got here seem to be from different amplifiers which has me somewhat confused as well.

Generally, if you want to be anal about it, replace every single E-lytic in the amp. If you cant find an exact match such as the one you've got circled in the bias supply you can always use a cap with a higher voltage rating without any adverse effects.

As for the dual caps(big green ones with three leads) on the preamp board, they're fairly easy to come by and can be ordered from most of the restoration and tube amp parts suppliers. Alternatively, you can use two single caps in those positions as well.

Last note with regard to the ground switch. The purpose of this switch is to reverse the polarity of the AC signal coming into the amp. Its basically a throwback to the days when polarized wall outlets weren't codified. If you're getting a lot of hum from the amp you'd just change the position of the switch. If the amp quiets down then the wall outlet was wired backwards...simple as that! Contrary to internet mythology, this is NOT a death switch or cap, and there's really nothing inherently unsafe about this switch in it's STOCK form so unless you're absolutely certain there's something wrong with the switch, dont even mess with it.

Hope that helped you out in some way or another

-Carl

[rockgardenlove]

I see my pictures have been resized...not very helpful.
http://img239.imageshack.us/img239/3568/dsc010201gy.jpg

http://img73.imageshack.us/img73/9809/dsc010211ni.jpg

http://img239.imageshack.us/img239/7608/dsc010299zv.jpg

The question is which ones ARE electrolytic? I'm not even sure. :)

I know the tan colored paper ones are, but what about the others?

Thanks!

[Carl / Zwengel Amps]

The large metal one on the small bias board IS an E-lytic, as well as the small white mallory to the far right and the two yellow ones under the mains fuse. The blue cap in the middle of the board is a film cap and shouldnt require replacement unless it's leaky which is actually a pretty rare event for these caps.

-Carl

[rockgardenlove]

Ok, fantastic. Thanks very much for the help. Really appreciate it.

So should I take it that the "ground" switch is dangerous in the wrong position?

And if I'm installing a polarized 3 prong powerchord, should I hardwire the ground switch? What exactly do I need to do?

Thanks much.

[Carl / Zwengel Amps]

No is not dangerous in any position if it's wired as it left the factory. Therefore, do exactly what i said....if there's nothing wrong with the switch, DON'T TOUCH IT! As for doing any hardwiring with a 3 prong cord the answer again is DO NOTHING! Can't make it any more plain than that.

As for directions as to how to install a 3 prong cord, I would strongly recommend taking it to a professional tech in a professional shop. He'll charge you probably 50 bucks and you'll know you got it done right! Why do I suggest this? Because I have a sinking feeling you dont have the proper equipment or understanding to do a proper job. And if you make a mistake then you potentially could have a dangerous situation.

Do I come off sounding like a bit of an asshole? Probably. But when it comes to raw AC it's no joke and not someplace you want to make a mistake.

-Carl

[rockgardenlove]

I'm quite sure I'm capable of it, I've built a 5e3 tube amp from scratch.

This my understanding about adding a three prong cord:

Before:
http://img.photobucket.com/albums/v503/crg23/Picture4.png
After:
http://img.photobucket.com/albums/v503/crg23/Picture4edit.png

If that's wrong, I'd be VERY suprised, and I'll never work on another amp in my life.

[Ampclutz]

Well, you had better not work on another amp ever again in your life because the diagram posted doesn't have a three wire cord... Only a two wire cord coming into the amp. You have just showed how to add a three wire receptical to the amp and it isn't grounded to the wall plug.

clutz

[Regis]

I have three Bassman heads, two Silverface and a Blackface. If that amp were mine, I would do a complete recap on it including replacing the turd brown caps with orange drops or yellow mallorys, it will make a huge difference in the tone of the amp. I also would rewire the bias section to emulate the AA864 Bassman (http://schematicheaven.com/fenderamps/bassman_aa864_schem.pdf). I have done this on my SF tops, they sound damn good. Those snubber caps and all that other shit around the power tubes are making your amp sound like ass. Plus you would get rid of that "Bias-Balance" crap and be able to adjust the bias of the power tubes as needed.

I disagree about the death cap, if it shorts out in any way you will have 120v AC connected to the chassis of the amp. It's not an internet myth, Get Rid Of It. You can rewire the power cord using this diagram (http://www.regiscoyne.com/tech/3prongconversion/). Doing so eliminates the need for a ground switch, other than using it for a tie point.

About those dual caps, as was stated you can replace them with two separate caps and twist the ends together. Here is a page I did on my Bandmaster with pics where I did exactly that. (http://www.regiscoyne.com/Blackface_Bandmaster/)

You can check the date codes on the pots to determine when the amp was built, here is how to do it. (http://www.provide.net/%7Ecfh/pots.html)

This is a great amp, it has the potential to sound awesome, good luck!!

[rockgardenlove]

Well, you had better not work on another amp ever again in your life because the diagram posted doesn't have a three wire cord... Only a two wire cord coming into the amp. You have just showed how to add a three wire receptical to the amp and it isn't grounded to the wall plug.

clutz

Ooops, it was late at night and I just drew the ground on the receptacle instead of the AC input.
Now I feel like an idiot. :(


As for doing a major rewire, I'm not going to do one yet as I'm leaving town in a week and I don't have enough time to get started on such a big project. I'll probably do it later at some point though.

[Enzo]

It may SAY AB165 on the label, but that doesn;t mean that's what is inside the chassis. I see date codes on the dual caps from 1968, not 1965. The AC568 has the cathode resistors and caps like yours, not the AB165.

Fender often used old tube chart stickers rather than throw them away.

The dual caps are just that, I always replace thenm with a pair of singles. If you look at a Fender layout drawing of the era, you'll see it drawn as two caps rather than one dualie.

I wouldn't bother with the ground switch when I mount a three wire cord. If the "death cap" shorts, in one position it shorts neutral and ground together which won't hurt anything, and if it is switched to the hot side, the cap will pop the breaker in the wall. Usually the thing is fine, and teh cap doesn't short.

[Joe L]

No is not dangerous in any position if it's wired as it left the factory. Therefore, do exactly what i said....if there's nothing wrong with the switch, DON'T TOUCH IT! As for doing any hardwiring with a 3 prong cord the answer again is DO NOTHING! Can't make it any more plain than that.
-Carl

Carl is right, he can be an asshole but when it comes to AC, let the pros handle the job. In the late 60's I was 17 years old and helping a best friend mop the club after the Friday night gig before and fired up the bands Telecaster/Fender Twin amp combo and started playing some Creedance until one of the uncut strings ("you'll put your eye out kid") contacted an obviously grounded post on the bandstand. Flames flew and I shut that thing off as quick as I could - naturally the band showed up minutes later but I had to bite my tounge and not tell the guitarist the problem or give up that I had been cranking his rig. There is no way that PT leakage alone could have melted that string like it did.

And I disagree. Unless that post was connected to a hot leg of a 117v circuit, the death cap had shorted and I would have gotten a good jolt if I had leaned against that post. Get a pro to wire in a 3 wire grounded plug and the issue is put to rest. Why risk your life on the integrity of a few mils of 40 year old polyester?

Also, just a few years back, one of the regulars here reported that his bassplayer had fell through a temporary cover over a swimming pool and was electrocuted. That couldn't have happened on a properly grounded rig - or wireless.

[rockgardenlove]

I'm back.
With some more questions. :)

That post above recommends that I remove the "snubber caps" and stuff.
Would somebody mind explaining what those are? Is it the 5uf NP cap bridging the two cathodes?

I also read somewhere else that it would be advisable to "replace the coupling caps." I think I have a clue what that means, but if somebody could point them out on the schem I'd be very grateful. Also, from the same source I read to "replace the plate feed resistors."
Now when I read that I start looking for resistors off the plate(pin 3, right?) I don't see any resistors off the plate though...some help guys?


Now some of the carbon comp resistors were drifting a bit, some as much as 50%. Do they all need to be replaced? What is the tolerance for drifting? Where does it matter the most?
I'd rather not go and redo them all, but I will if it's really important.


Thanks for any assistance that might be offered. :)