1967 AB165 Bassman

[echo8287]

Hello all, new here. I've been reading a lot of the posts here for several weeks and I think I've landed at the right forum. I am an electrician/plumber by trade and enjoy playing the guitar for the past 45 years(old guy). Around thirty years ago, I bought this Fender Bassman for $135.00, thinking I would eventually try to make something of it. Serial number stamped on the back was A-29487. I've check the transformers and found them to be Schaumacher>Main(606-2-40)Choke(606-6-30)Output(606422). I have ordered new electrolytic caps,fuses, and a few resistors that are out of spec for it. I have never tried to fire it up. Only thing I remember from the guy I got it from was he had been playing it, that he had a tech add 2 more power tubes for more power and a couple of years after that, it had a problem and it popped the fuse. He did get a couple of caps changed,but it has many of the old brown caps still in it. I have pictures that I will try to upload. What I had in mind was to add the new caps and a few resistors and then try and troubleshoot it. The only thing I noticed looking at it,looks like a resistor might be burned up on the bias board. I was hoping someone here might help me with a schematic of an amp with 4 6L6's and there are only 3 preamp sockets. I didn't know if a bandmaster,showman,or a twin reverb might be more like the circuit I should be looking at. Thanks for any assistance, David

[Justin Thomas]

I will openly admit, I'm biased. I own a BF AB165, mostly stock circuit-wise. I <LOVE> it. I even converted my Bassman 100 preamp to match my Bassman. Your fourth preamp tube hole was converted for an extra 6L6. So, going forward... first, 3-wire cord, ground plug, etc. Does the amp actually work and sound good as-is? If so, that's a good start. If the Power tranny & Output Tranny are indeed "stock," are they up for the task of powering a 100W amp? I used to know for sure, but I don't have info handy. One way to tell is checking the 6.3 heater voltage - does it sag much below 6V with all four power tubes? You <REALLY> don't want to blow a tranny up.

I don't see any burnt resistors on your bias board. So,if it was <MY> amp... I'd see about adding a 5V transformer for a tube rectifier. I'd convert the other extra 6L6 back to a 9-pin socket with an adapter. Then I'd build the stock AB165 preamp, and twiddle it from there. I'd also add a bias adjust along with the bias balance. Then I'd see if I liked it.

My personal amp had a few mods when I got it... the OT is from a Twin Reverb, so it's bigger than stock but not nearly as big as my Bassman 100's. The 220k feedback resistors on the 6L6s are gone. I may have changed one cap in the tone stack... I also jump the channels, since on only the AB165 and later Bassmans are the channels in phase, which I consider a big plus.

Follow the layout for the model, along with the schematic. I'm sure others will say otherwise, and I realize I'm in the minority in favoring this particular model of Bassman (and all others that followed it)... if you have any questions, I'll answer as best I can. As far as 100W/3 preamp tubes, Bassman 100 (boring as-is) or Marshall non-mast we r volume ( a good time, but don't change to EL34s w/out extensive tranny tests.).

Justin

Edit: your transformers are from 64/74, 62/72, and 66/76. Are there any other readable numbers on them? If they're stamped with Fender part numbers, we can figure out if they're up to your desires.

[g1]

It looks like the chassis has been cut & holes drilled to accommodate larger transformers, but the other numbers Justin asked for will be helpful.
The part you said looks burnt appears to be the bias diode, not a resistor. Can't tell for sure from the pic whether it is burnt or not, they are mostly black.

[mozz]

I second the checking of the power transformer. You can google the part numbers to see what amp it is from, or if it's original. Last year i worked on a friends bassman that was used as a test bed frankenstein. The previous owner burned out the filament winding using tubes which drew a lot more than the stock 6l6's. It does happen and you will be out a original fender transformer.

[echo8287]

First let me thank all of you for your replies. I have Kevin O'Conner's Ultimate Tone and another book #3 and a couple of books by Gerald Weber, but I just started reading them again.I bought them a long time ago. So I am justing getting somewhat up to speed and don't fully comprehend a lot of this stuff. It is looking simpler every day though.
I have just started looking at this amp, so I never have put the tubes in it to try it. It has had the cord changed to a 3 prong, and the death cap removed. The resistors on the tube sockets are newer ones. I figured it needed the electrolytic caps changed first. The part on the bias control was black and I didn't see any colors on it like a resistor. It might not be burnt. Can I check that part with an ohm meter? I'll try to take a better picture of that. I called Schmacher about the transformer numbers,but the guy that might know is on vacation. He'll be back next week. It's hard to remember but I thought the guy I got it from did mention that the trans was from a Twin or something other than normal. I have the tubes he gave me from 35 years ago and I have an old Hickok tube tester, I'll have to dig that out.

Other numbers on the transformer>
KO 22756, CSA 827, 606-2-40 these were stamped into the trans
On the choke>
125 C1A, 022699, 606-6-30 these were stamped into the trans
On the output trans>
125A32A, 606422 also had 76(was like a red stamp on the side)

On the inside of the cabinet it has the AB 165 sticker.

I was thinking if I could get it going, I'd see what it sounded like before changing it. I have a SF Twin, a Princeton with no reverb, and a Vibrolux all from the early 1970's. I really don't need 100 watts of power. 50 or less would be fine, so I might just change it back to near original,Mr. Justin.I normally play blues,classic rock, and some of the grunge stuff to keep up with 2 daughters. Thanks for all the assistance, David