Well I was looking around the chassis of this amp and there are a few things that are starting to look out of place here. There are a couple "cement blocks" and a orange Sprague Atom hanging out near the 6L6 socket. Also, the two 220k resistors from the local negative feedback loops appear to be gone. Can anyone tell me what mods appear to have been done on this amp? I imagine it'll function fine after tubes and caps, but I'm just curious as to what might have been done. Plus, I know you guys like gut shots.
Looks like a completely stock AC568 Bassman. The cement blocks are the 150R/7W cathode resistors; this version doesn't have the 220K local feedback resistors; the orange Sprague is the 5uF/25V non-polarized cap. It looks like one of the 2000pF suppression caps on the 6L6 grids has been replaced.
That bias cap should be more than 50V; the circuit puts at least -55VDC on it. 50uF/100V or 100uF/100V.
I had one of these in for repair doing something similar last night. It ended up being a loose socket connection on pin 8 on the V1 tube. Amp would cut out and in when the grid made good contact. I had cleaned the sockets but ended up having to retension it to get it to work properly. The normal channel worked pretty good and was as loud on 3 as the Bass channel on 10.
Thanks for the info guys. It's interesting that it's completely stock. My first thought was it's an AB165 that someone tried to convert into something else. I used to work on cars for a living and can sometimes tell when someone has been "into" something before me. This amp is starting to give me that vibe, but I could be (and probably am) wrong.
My plan is to change out the filter caps and bias cap (and increase its voltage, thanks Firestorm) and then give the amp a listen. I want to hear it healthy first, before I go thinking about getting the circuit changed. I know this is considered the least favorable circuit, but who knows... I might love it.
Get rid of the death cap and bypass the ground switch like this diagram shows.
The first three prong diagram shows the ground switch and courtesy outlet totally ignored and not in the circuit, while the second one shows the outlet wired and the ground switch is bypassed but is used for a connection point.
Your choice but be aware that the outlet is not grounded.
Excellent diagrams, Structo. Thanks a lot! I have no problem with bypassing the courtesy outlet, so that will A) make things safer by not having an ungrounded outlet and B) make things easier on myself with less wiring.
I'm just waiting on a couple more caps. It's like a little kid waiting for Christmas.
Well just thought I'd post a little update. I finally got all of the old power caps changed and put a new three prong cord on.
The amp sounds incredible now. No volume loss and just the right amount of overdrive. The only issue is a noticeable popping noise through the speaker when the volume on either channel is above 5. I'm guessing I have arcing in a preamp tube somewhere. I still need to replace them (I know, bad on my part).
I know I'm still in the honeymoon phase with this thing, but man. I don't think I'll need to modify a thing.
I'd guess that internal tube arcing is rare. It is more likely that one of the small electrolytic caps you didn't change isn't behaving well. When you did the cap job, did you get all the e-caps or just the big filters. You should have replaced the cathode bypass caps in the preamp section, too.
![[IMG:150:113]http://img51.imageshack.us/img51/2638/sdc10337x.th.jpg[/img]](http://imageshack.us/photo/my-images/51/sdc10337x.jpg/){kind=link}
![[IMG:150:113]http://img7.imageshack.us/img7/8062/sdc10328yo.th.jpg[/img]](http://imageshack.us/photo/my-images/7/sdc10328yo.jpg/){kind=link}
![[IMG:150:113]http://img18.imageshack.us/img18/421/sdc10329d.th.jpg[/img]](http://imageshack.us/photo/my-images/18/sdc10329d.jpg/){kind=link}
![[IMG:150:113]http://img263.imageshack.us/img263/7324/sdc10330n.th.jpg[/img]](http://imageshack.us/photo/my-images/263/sdc10330n.jpg/){kind=link}
![[IMG:150:113]http://img12.imageshack.us/img12/3965/sdc10334t.th.jpg[/img]](http://imageshack.us/photo/my-images/12/sdc10334t.jpg/){kind=link}
![[IMG:150:113]http://img30.imageshack.us/img30/641/sdc10335c.th.jpg[/img]](http://imageshack.us/photo/my-images/30/sdc10335c.jpg/){kind=link}
![[IMG:150:113]http://img585.imageshack.us/img585/6544/sdc10336v.th.jpg[/img]](http://imageshack.us/photo/my-images/585/sdc10336v.jpg/){kind=link}
