Bassman AB165 help needed

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Bassman AB165 help needed

Postby BetterOffShred on Thu Dec 31, 2015 3:28 pm

Hey fellas, so first let me say I really love this bulletin board, and I've been enjoying reading up on all the amps I wish I owned!

So here's a little background on this amp. I bought this bassman, which I believe to be a 1969 Bassman AB165 in 2004 for $250 USD, and I was 24 at the time, and foolishly decided I was going to convert it to an AA864 circuit. The amp had the filter caps replaced at the time of purchase and it sounded pretty good. At the time I wanted something that I could crank up and get overdrive out of and I didn't really appreciate it for what it was/is. All of the work I did was based off the official schematics and layouts which I still have and printed, so at least I sort of had a game plan..

Now I'm 35, and I've been building my own effects pedals, and I'm thinking "Man, this OD pedal would be so amazing in front of that bassman if only it was running.."

I am not super familiar with all the parts of this circuit, but I've included pictures here of what I've done to the circuit (11 years ago in my foolish zeal). There is a fairly recent post here with an image showing which parts to remove from the AB165 circuit and what the layout looks like after the AA864 conversion. I looked this over, and mine is basically that (exactly that I think, minus the .1 caps shown in the last picture, which I am going to replace). Back when I was doing this conversion somewhere I read that you had to swap two wires from one of the transformers, but I cant remember which wires.. Anyway, I need some assistance with this project if I ever want it to live again! Does it look like It's workable in its current condition? I think I may have fried a couple of the preamp tubes back when I was trying to do this, I fired it up without the .1's you can see missing in the left side of the board. I think I also didn't have the power tube resistors soldered when I did that.. I put in the power tube resistors, and last time I fired it up (with two matched 6l6's out of my triple rec) it didn't make any noise, but when I turned it all the way up you could hear the guitar signal with super saturated overdrive but extremely low volume. Looking at the board, I didn't have the .1's in that time either.
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Re: Bassman AB165 help needed

Postby BetterOffShred on Thu Dec 31, 2015 11:24 pm

Sorry for the incomplete post guys, it was my first post and it wasn't letting me attach all my pics. Now that it has been approved my power has gone out at the house!! So the other 4 pics will have to wait with the rest of my story. So sorry for the inconvenience. This is the third time in a month the power has gone out here in the Spokane Washington area ..

-Brett
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Re: Bassman AB165 help needed

Postby stratele52 on Thu Dec 31, 2015 11:40 pm

I'll follow your post.
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Re: Bassman AB165 help needed

Postby BetterOffShred on Fri Jan 01, 2016 1:04 am

Ok so power came back up before we left for the new years party, so I'm posting these pics!
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Missing the .1uf's there on the right side of course..
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and here with the .1's where they are supposed to go (but not soldered).. I think these were originally blue barrel caps like the others too?
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Yeah that's it so far. The holes match up to the AB165 sheet, but the layout diagram has a little standoff board coming from the 6l6's with some resistors, this chassis never had that (while i owned it). I can provide any other pictures anyone might want to look at.

I plan on removing most of the metal film resistors and replacing them with carbon comps in the AA864 values, and replacing that ceramic disc (the new one) with the 500p value in an orange drop or something. I also plan on ordering an entire new set of tubes for the amp, but I don't want to power them up till I'm sure i'm not going to burn them up.

I know I messed up and I should have left it alone, but I didn't and now I've taken it upon myself to get it going and I would love some advice. I've come pretty far on the AA864 conversion and I'm thinking I should just forge ahead and complete what I've started there, but I'm extremely open to experienced opinions!

Thanks for looking

-Brett
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Re: Bassman AB165 help needed

Postby stratele52 on Fri Jan 01, 2016 9:23 am

Last photo ; a red capacitor is not soldered at one end.
Some mess up but not bad this amp.

Just follow the layout you want to build to finish/fix the amp ( AB165 ? )

Afraid to burn tube ; test with no tubes in the amp fist time you power ON or after some mod / work if you are not sure of your work.

1- If you have no more than 7 volts AC at each tube's heater NO tubes in amp ; heater voltage is good and tubes are safe.
With tubes in ; 6.3 volts AC
2- If you have -30vdc ( minus 30 volts DC ) and more at pin 5 on each 6L6 NO tubes in amp , tubes are safe.
With tubes in , same volts.
Less than -30 vdc turn bias pot

No need carbon comp resistor , but it is up to you.

Be patient , take your time , look and check 3 times what you do and you will have a great amp.
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Re: Bassman AB165 help needed

Postby BetterOffShred on Fri Jan 01, 2016 5:08 pm

Hey thanks for looking!
Yeah the last picture is worthless I guess since we can see from the schematic and layout that the .1s go there..

Is anyone familiar with this 'conversion' at all? Someone told me I needed to swap two leads from a transformer. . Which is consistent with the layouts.. the AB165 has a spot where the blue yellow and black come out of a hole in the chassis, black grounds, and blue and yellow go to the board... in the AA864 it appears to be a different layout. I'll look at the schematic when I get home.

Thanks again for looking

-Brett
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Re: Bassman AB165 help needed

Postby stratele52 on Fri Jan 01, 2016 9:46 pm

Wich transformer are talking about ? Power or output ?
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Re: Bassman AB165 help needed

Postby BetterOffShred on Sat Jan 02, 2016 3:35 am

So I have been doing a ton of research and found a schematic and layout done by a gentleman that looks just like the current scenario I have. I found out you have to swap the green and black leads (output transformer) at the speaker and ext. Speaker jacks since I fooled around with the power inverter section of the circuit making it look like the AA864.

I ordered all the resistors and caps to finish the job and even threw in some of the reproduction knobs. So I'll update in a few days when the parts are in and soldered up.

Thanks for looking
-Brett
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Re: Bassman AB165 help needed

Postby stratele52 on Sat Jan 02, 2016 9:24 am

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Re: Bassman AB165 help needed

Postby keithb7 on Sat Jan 02, 2016 4:27 pm

Thanks for posting. I am having fun looking at your amp gut shots. I was curious about the B+ cap board. The last 20uF cap is on a slight angle.
Interesting how the eyelets for it are shorter distance apart than all the others. The resistor on a 45 deg, I had not seen that before on a vintage Fender amp.
Usually they are all lined up the same. (All I have seen anyway). Seems odd. Interesting...

For example:

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Re: Bassman AB165 help needed

Postby BetterOffShred on Sat Jan 02, 2016 5:24 pm

Thanks for the schematic link Statele52, I have both schematics and layouts printed from back when I first attempted this mod. It's nice to see I have the correct ones!

As for the B+ board, I think it's been completely redone at some point. It's not the same paper material as the main board, so someone probably just didnt get factory spacing. This amp worked fine when I bought it, but it was a little harsh and was crazy loud. I've been building pedals for a while now and I'm excited to try a couple different drive pedals on it.
Thanks for looking!
-Brett
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Re: Bassman AB165 help needed

Postby Dean Jr. on Tue Jan 05, 2016 6:54 pm

Fwiw, the circuit that that amp had when it was new was not an AB165. Fender used tube charts until they were gone....adn sometimes the old charts were at the bottom of a bin and 'came up' after newer charts had been used. A 1969 Bassman would be a '68 circuit, iirc.
Thet said, imho, that amp needs to be gone through thoroughly. I do believe in taking the biasing circuit and power supply back ressitances to the AA864 scheme. I do however NOT believe in taking the rest of the circuit back to the '64 scheme. I prefer the 3 gain stages in the Normal channel as all of the AB165 and later Bassman amps...except for the BAssman 10 and Bassman 100 amps...had. That makes for a really nice, hot guitar channel....as K. Fischer realized when he designed the Trainwreck Express. That Express is almost identical to the AB165 Normal channel but for some tweaks....and a stiff first stage of filtering.
good luck with it.
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Re: Bassman AB165 help needed

Postby Gasstationman on Tue Jan 05, 2016 11:38 pm

Very close to this one: https://www.gbase.com/gear/fender-bassman-1968-black-1
Both have altered poweramps to older specs. I go for a AB165 as I can't see any signs of cathodebias.
Check out the inkstamp 17 12 68 in both amps and the best... Still some blue molded caps.
Good luck with the project.
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Re: Bassman AB165 help needed

Postby Naked Clarke on Wed Jan 06, 2016 7:30 pm

It is a late March 1968 AB165 amp.
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Re: Bassman AB165 help needed

Postby BetterOffShred on Thu Jan 07, 2016 12:08 am

Thanks for looking guys :D

Yeah the 220k feedback resistors from the AB165 schematic were not present when I bought the amp, and of course after reading up on them, they are not going back in! The bias section appears like it has been converted to the AA864 specs too.

I'm planning on getting the "normal" channel going by finishing the AA864 conversion on that channel, and once we're up and running there I'm going to mess with the bass channel and probably run the 5F6A preamp section through the extra gain stage on that channel. I think it's popular as the "Allen" mod, and a couple guys on el34world did it with the deep switch going between two different values of caps for a "JTM45/5F6A" switch, apparently with great success. Someone was nice enough to publish a schematic of this mod, and I've been looking it over while I wait for my parts and I think it's doable.

I'm also going to do the 820 ohm resistor on the Negative feedback loop (AA864 spec), and put in the 10k pot for Variable negative feedback.. :dunno: why not?

-Brett
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