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Post subject: Vintage Fender Bassman questions
Posted: Tue May 29, 2012 2:46 pm
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Hi all - new member here. I've got some experience with a soldering iron with amp mods and stompboxes, and am relatively unafraid of projects... but with vintage amps, I'm reluctant to just start clipping and swapping components. Anyway - I got a Fender Bassman head in a trade. I normally prefer to get my gain from the amp itself, and run FX in the loop. On a whim though, I hooked this thing up and ran with dirt pedals... and am really liking what I can do just running everything into the front of a good, clean amp. I had it looked at by a local tech, and the power tubes replaced. Here's that amp:

Image

I've been running this amp with 2 Mesa LoneStar 1x12 cabs with Black Shadow speakers in them. To my ear, it sounds fantastic. I run a few pedals, one of which is a Line6 M9 for mods / delay / reverb, at the end of my chain before going to the amp input. After awhile, I thought man... I wonder if I could score another Bassman head and run a stereo setup. Would people in clubs realize I was playing in stereo? Probably not at most venues... but I would. :D And I always find that the happier I am with my sound overall, the better I feel I push myself to play.

Anyway... long story short, I found one. Local pawn shop where I know the guys. They'd had it for awhile, and wouldn't split up the set, but I worked them on the price and they were cool about it:

Image
Image

I don't know that I'm really interested in using the cab in the future, but I guess you never know. I haven't opened the cab to see what kind of speakers are in it - there's a ton of screws on it! It's practically showroom new though, complete with the original cover. Are these cabs desirable in the vintage market?

The point of my post is this: this new amp (to me) has no power tubes. I pulled the ones out of my working Bassman just to make sure it worked. It seems noticeably quieter than my existing Bassman. I know this could be preamp tubes, it needing service, a whole slew of things... so I ordered a whole new set of tubes for the amp.

I saw a couple of other threads on here about maintaining / fixing up old Bassman heads... I'm probably going to be the next in line with this new amp of mine. I figured swapping tubes for new ones is the first thing to do before heating up a soldering iron. Is 'biasing' these amps literally just adjusting that 'Hum Balance' pot inside the amp?

Anyway... looking forward to my replacement set of tubes arriving so I can hopefully turn some bandmates' heads with ping-pong delays! ;)


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Post subject: Re: Vintage Fender Bassman questions
Posted: Tue May 29, 2012 4:19 pm
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StratJeffMN wrote:
I haven't opened the cab to see what kind of speakers are in it - there's a ton of screws on it!


Have you ever heard of Makita's "cordless screwdriver", son?

You can be in and out of that enclosure in under five minutes.

As for biasing those Bassmans, the "hum balance" circuit is totally isolated from the bias power supply -- it's an independent sub-system. You'd do well to have the entire amp bench-checked by someone familiar with vintage Fender architecture before committing an all-new set of glass to it. The AB165 and subsequent revisions do not have a traditional biasing system. One power tube is factory-set (not adjustable via a pot) while the other tube's bias voltage may be varied to match the fixed value of the other bottle.

HTH

Arjay

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Post subject: Re: Vintage Fender Bassman questions
Posted: Tue May 29, 2012 5:55 pm
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Quote:
Have you ever heard of Makita's "cordless screwdriver", son?

You can be in and out of that enclosure in under five minutes.

Indeed I have, and if the thievin' jerks at my apartment building hadn't stolen the majority of my tools from my storage locker, I'd have been in and out of there already! ;)

Quote:
As for biasing those Bassmans, the "hum balance" circuit is totally isolated from the bias power supply -- it's an independent sub-system. You'd do well to have the entire amp bench-checked by someone familiar with vintage Fender architecture before committing an all-new set of glass to it. The AB165 and subsequent revisions do not have a traditional biasing system. One power tube is factory-set (not adjustable via a pot) while the other tube's bias voltage may be varied to match the fixed value of the other bottle.

I called my amp tech (the guy that serviced my original head) and he said he could go through it, but he is backed up right now. I've been in an amp or three before, so I figured it would be a good learning experience to go through myself. I have looked at the schematic and layout of the AB165 circuit, it doesn't look like my 5150III head does. :) I realize it may deviate from the AB165 circuit, even though the tube layout sticker claims that's what it is - but that's half the fun of this that I'd like to see. :)


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Post subject: Re: Vintage Fender Bassman questions
Posted: Tue May 29, 2012 6:18 pm
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The tube chart inside an amp's cabinet is the last thing I rely on when IDing a Fender circuit revision. The build date must be definitively determined before proceeding with servicing or repairs.

Arjay

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Post subject: Re: Vintage Fender Bassman questions
Posted: Tue May 29, 2012 7:08 pm
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Retroverbial wrote:
The tube chart inside an amp's cabinet is the last thing I rely on when IDing a Fender circuit revision. The build date must be definitively determined before proceeding with servicing or repairs.

Arjay

Well, if my understanding is correct with dating via transformer codes, then:

Power:
8316805 = 1968, 5th week.

Choke:
606-7-36 = 1967, 36th week.

Output:
606804 = 1968, 4th week.

So, my guess would be the amp was assembled sometime mid-1968... correct?


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Post subject: Re: Vintage Fender Bassman questions
Posted: Tue May 29, 2012 7:44 pm
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Nice amps, Strat! You might want to get yourself a bias probe and see what the power tubes are doing. JMHO Art

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Post subject: Re: Vintage Fender Bassman questions
Posted: Tue May 29, 2012 10:33 pm
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StratJeffMN wrote:
Well, if my understanding is correct with dating via transformer codes, then:

Power:
8316805 = 1968, 5th week.

Choke:
606-7-36 = 1967, 36th week.

Output:
606804 = 1968, 4th week.

So, my guess would be the amp was assembled sometime mid-1968... correct?


Closer to February of '68, Jeff.

Should be the AB165 circuit revision (same as most of the blackfaces). You're in good shape I think, assuming no mods or other monkeyshines have occured to the amp. And -- as you've already discovered -- they're great-sounding platforms.

Arjay

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Post subject: Re: Vintage Fender Bassman questions
Posted: Wed May 30, 2012 6:54 am
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Retroverbial wrote:
StratJeffMN wrote:
Well, if my understanding is correct with dating via transformer codes, then:

Power:
8316805 = 1968, 5th week.

Choke:
606-7-36 = 1967, 36th week.

Output:
606804 = 1968, 4th week.

So, my guess would be the amp was assembled sometime mid-1968... correct?


Closer to February of '68, Jeff.

Should be the AB165 circuit revision (same as most of the blackfaces). You're in good shape I think, assuming no mods or other monkeyshines have occured to the amp. And -- as you've already discovered -- they're great-sounding platforms.

Arjay

Monkeyshines? ha!

I did open it up and look inside. There are some components that appear to have been replaced, some metal film capacitors in the area of V2. There's one cap that was replaced as well on the rectifier board. There is also a line on the normal channel volume to V3 that was replaced with an insulated cable. All of this looks like it was done well, not hacky. I was going to take pics of it, but it was late and I decided that poking around is best done when alert and awake... so I put it away for the night.


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Post subject: Re: Vintage Fender Bassman questions
Posted: Wed May 30, 2012 9:28 am
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Post some pics if/when you get a chance.

Arjay

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Post subject: Re: Vintage Fender Bassman questions
Posted: Wed May 30, 2012 10:46 am
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Okay - here's photos of the inside:

Input side:
Image

Power side:
Image

Closeup of the changes around V2:
Image

I forgot to mention that in addition to the resistors being changed, there are a couple of electrolytic caps that were replaced there as well. Whoever installed them did it with the values on the underside - I can't easily read if they are direct replacements to what was there. Also, connected to some of the updated resistors looks like some smaller ceramic caps - do those look original? All other caps in the amp are much larger than these.

I was tracing the schematic / layout from V1 over and values look 'correct' to the AB165 circuit schematic up to the replaced components indicated. I'm hoping to look at it more tonight!


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Post subject: Re: Vintage Fender Bassman questions
Posted: Wed May 30, 2012 11:18 am
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Examining the pin-out of your power tube sockets, I see the snubber caps and concrete resistors which were hallmarks of the AC568 revision. And the ink-stamped build date on the chassis itself would seem to confirm this. These components were added as an attempt to minimize/prevent subsonic oscillations and give the amp more clean headroom.

Arjay

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Post subject: Re: Vintage Fender Bassman questions
Posted: Thu May 31, 2012 12:11 pm
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Ah the memories !!!
check out my thread for a lot of great info on AB165 Bassman stuff

viewtopic.php?f=12&t=68513

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Post subject: Re: Vintage Fender Bassman questions
Posted: Tue Jun 05, 2012 9:08 pm
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StratJeffMN wrote:
After awhile, I thought man... I wonder if I could score another Bassman head and run a stereo setup. Would people in clubs realize I was playing in stereo? Probably not at most venues... but I would. :D And I always find that the happier I am with my sound overall, the better I feel I push myself to play.



Yes. You can have fun with two amps. Not sure if "stereo" is the best word for
running two amps, but I have been doing this type of setup, lately. Lot of fun.

Benefits:
1) you automatically have a backup amp.
2) you can point the speakers in more directions with two sets of cabinets.
3) try NOT to match the tones and have one amp with a bit more or less of treble or bass.
Experiment. Just have fun. The hard things to matchup are reverb and vibrato, which
your amps do not have.
4) you can mess around with the bright switches
5) have one amp plugged into the NORMAL channel inputs and
the other into the INSTRUMENT channel inputs. Mix them up. Have fun.

Maybe you might have one amp set brigher on the bass player's side
and the other amp be more bassy on the opposite side.

Take care. Toppscore :)

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Post subject: Re: Vintage Fender Bassman questions
Posted: Tue Jun 05, 2012 9:12 pm
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Toppscore wrote:
StratJeffMN wrote:
After awhile, I thought man... I wonder if I could score another Bassman head and run a stereo setup. Would people in clubs realize I was playing in stereo? Probably not at most venues... but I would. :D And I always find that the happier I am with my sound overall, the better I feel I push myself to play.



Yes. You can have fun with two amps. Not sure if "stereo" is the best word for
running two amps, but I have been doing this type of setup, lately. Lot of fun.
Also, you have two different Bassman amp heads from two different years.
There will naturally be different tones/sounds with different tubes, caps & resistors.

Benefits:
1) you automatically have a backup amp.
2) you can point the speakers in more directions with two sets of cabinets.
3) try NOT to match the tones and have one amp with a bit more or less of treble or bass.
Experiment. Just have fun. The hard things to matchup are reverb and vibrato, which
your amps do not have.
4) you can mess around with the bright switches
5) have one amp plugged into the NORMAL channel inputs and
the other into the INSTRUMENT channel inputs. Mix them up. Have fun.

Maybe you might have one amp set brigher on the bass player's side
and the other amp be more bassy on the opposite side.

Take care. Toppscore :)

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