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Post subject: NAD - Pine Cabinet '68 Bassman
Posted: Mon Jul 30, 2012 8:16 am
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So! I recently purchased and received a 1968 Bassman through eBay! I'm still waiting on the Webers to arrive (one 10f150, one 10a125) so I haven't had the chance to hear her yet, but she is a beauty!

Click on pictures for full sized images

Image

Image

With baby Sylvia Vincenza, checking to make sure everything is in good condition
Image

The head was already mounted in the pine cabinet; the extension cabinet was built by Carl's Custom Guitars

The tube chart says it is an AB165, and the serial number is A40047, so the tube chart very well may be correct.

Though I bought it without playing it first, I haven't met a 60s Fender that I didn't like (A Bassman 10 was one of my favorites actuallly). I've bought high quality tubes (JAN 12AX7WAs, a Mullard 12AT7, and TAD 6l6WGC-STR short bottles), Weber speakers, it looks amazing, it's easy to carry, and the whole thing cost less than buying a new, stock DRRI. Think it was worth taking a risk on this.

I bought it through a highly reputable shop, Real Guitars, in San Francisco, where my brother lives, and to save on shipping costs, my parents agreed to deliver the amp to me on their recent visit. I paid for the amp then called my brother up (whose nickname is Amp) and said, "Hey, I need you to do me a favor..." He is a musician himself so he was all too happy to help! I'm happy the amp has a story behind it.


Unfortunately one of the power tube getters (Ruby 6l6GC-STRs are installed) broke in transit. The ring is literally rolling around inside the bottle. Is this tube still useable? As mentioned I will have TAD Short bottles installed soon (and adjustable bias installed), but I'd like to keep the Ruby's as backup-- they were installed new by the store. If it is not useable, I can still run the amp (just for a test) with one power tube, correct? Which position should the tube be installed?

Installed preamp tubes have a picture of a rocket- any clue what they are? The 12AT7 is a GE.


As well, there are a few articles written by Jeff Bober (Buddha amplifiers) providing simple ways to 'enhance' a Bassman. These are not huge modifications; I'm simply interested in anything that can add to the richness, depth or detail of the sound. Anyone know whether these are worth doing?

http://www.premierguitar.com/Magazine/I ... an_10.aspx
http://www.premierguitar.com/Magazine/I ... an_50.aspx
http://www.premierguitar.com/Magazine/I ... _Cold.aspx


:mrgreen: !!


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Post subject: Re: NAD - Pine Cabinet '68 Bassman
Posted: Mon Jul 30, 2012 9:11 am
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Ignore the tube chart as far as the specified circuit goes -- the amp could be one of two revisions, contingent upon what month in 1968 it was built. If made prior to May, it's the AB165. Beyond that, and it's the AC568. There are specific differences which the schematics will reveal, especially within the power supply and the output stage.

The busted tube is a non-starter, throw it in the trash immediately and do not use. For testing purposes, any known-good 6L6GC may be installed. However, the amp really wants a matched pair. They operate on a fixed bias with a bias-balance control to maintain symmetry with the output tranny. Modding to a true adjustable bias circuit is possible with a minimum amount of muss-and-fuss though it's not a necessity for many.

Best of luck, HTH

Arjay

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Post subject: Re: NAD - Pine Cabinet '68 Bassman
Posted: Mon Jul 30, 2012 3:48 pm
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+1...If Arjay gives you advice, follow it to the T.

Double D...That is a sweet looking rig, and we see you are already raising the next generation correctly...well done!

T2

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Post subject: Re: NAD - Pine Cabinet '68 Bassman
Posted: Mon Jul 30, 2012 5:27 pm
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Yep, agree with T2, on all counts!!! I too, am fond of the finished pine cabs. Very nice rig. Art

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Post subject: Re: NAD - Pine Cabinet '68 Bassman
Posted: Wed Aug 01, 2012 7:26 am
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Thanks guys!

Tube chart has no date code letters/stamp, and there is no stamp that I can see on the outside of the chassis. There is a 6 digit number at the bottom right corner of the tube chart (047928) but this does not resemble any date code info that I can find.

There is also a little 67 printed on the bottom right corner of the back panel, though I have no idea what that means. Even if it indicates that the back panel was printed in 1967, the amplifier could still have been assembled in 1968.

Assuming serial numbers were assigned sequentially, given that Bassmen #s A39000 to A49000 were printed in 1968, my Bassman (#A40047) was built in the first quarter of 1968, meaning it's an AB165.

If it does turn out to be an AC568, I'll be curious to hear what makes this circuit so unpopular!

Hope to pull the chassis by the end of the week, but dear lord is it hard to find time with two little kids!! Not complaining, just saying. This will be my first time pulling out an amp, so I'll wear rubber gloves and I won't touch ANYTHING for fear of electrocution. If I manage to get it done, I'll take lots of pictures so we can get this beast tagged and released. If not, the speakers and tubes should be arriving early next week, and I'll see what the local (experienced) tech can tell me.


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Post subject: Re: NAD - Pine Cabinet '68 Bassman
Posted: Wed Aug 01, 2012 11:22 am
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Likely there will be a date ink-stamped inside of the chassis, adjacent to the tag board for the bias supply or the power transformer. It'll consist of a letter followed by six digits, the last four of which will represent the year and the week within that year in which the amp was built. If the head cabinet is original there may also be a date on it, periodically stamped on the inside of the bottom.

When removing the chassis be sure to dismount the strain relief holding the AC power cord to the inside of the cabinet. Use the output tranny as a "handle" to support the chassis as you pull it out of the box. You may wish to remove the power tubes before you begin (use caution and be gentle when loosening them from their sockets).

Best of luck, HTH

Arjay

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Post subject: Re: NAD - Pine Cabinet '68 Bassman
Posted: Tue Aug 07, 2012 7:01 am
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Hey guys I managed to pull out the chassis last night and made a pretty cool discovery!

Check out the EIA numbers:
Choke: 606 7 36
Power Transformer: 606 7 44
Interior Chassis Stamp: T404867

Looks like it's a 67!

Here are some pictures:

Image

Image

Image

The output transformer is not original, it is a smaller replacement, similar to what you would find in a Pro/Bandmaster/Vibrolux (according to the seller). If what I've read is correct, this may result in lower wattage and possibly slightly earlier breakup?

Arjay, thanks for mentioning the strain relief, I could have easily missed that. Pulling the chassis was easier than it seemed. I'm amazed at how large the power transformer is- getting that last screw tucked behind the PT is very tricky, it's a good thing I have fairly small hands!

Very glad I did this, it's the first step to one day being able to service this myself.

Speakers and power tubes should arrive any day and I'm getting very excited. It's going to be a fun weekend!


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Post subject: Re: NAD - Pine Cabinet '68 Bassman
Posted: Tue Aug 07, 2012 7:57 am
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Cool! Looks like some caps have been replaced too. Maybe another good sign that it has been serviced.

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Post subject: Re: NAD - Pine Cabinet '68 Bassman
Posted: Tue Aug 07, 2012 8:50 am
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+1

It's nice to see these aging warriors receive the TLC that they've earned.

I see great moments in this amp's future.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: NAD - Pine Cabinet '68 Bassman
Posted: Tue Aug 07, 2012 9:38 am
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Very glad to hear it passes a quick visual test! Thanks for the help. My first impressions too were that everything looked in good order. And it DOES look like a few components have been replaced-- they just look relatively 'new.'

I plan to take her to a tech the day I receive the tubes to have them properly installed.
However the most experienced tech in town is currently backlogged.
I found another tech who I trust, however I'm wondering, is there anything he needs to consider given that this amp has (if I'm not mistaken) a bias-balance set up, rather than a standard bias set up? Anything I should be sure that he knows?

Or, are there any minor changes he should make while the hood is open? It does look like the amp was well loved by its past owner, so I'm guessing that answer is probably 'No.'


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Post subject: Re: NAD - Pine Cabinet '68 Bassman
Posted: Tue Aug 07, 2012 10:08 am
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I would have your tech pull that excess power cable through the chassis's strain relief so it's not looped over the power tranny. Excessive exposure to heat is the primary reason that neoprene insulation dehydrates and becomes brittle.

HTH

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: NAD - Pine Cabinet '68 Bassman
Posted: Tue Aug 07, 2012 10:10 am
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+1 Retroverbial

Depending on the particular tubes that you have chosen, if the bias is too high or too low, he could change a resistor value to correct the idle current if necessary. Otherwise, just adjust the balance pot for minimal hum.

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Post subject: Re: NAD - Pine Cabinet '68 Bassman
Posted: Thu Aug 09, 2012 12:21 pm
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Speakers and power tubes should arrive, if not today then definitely tomorrow. According to USPS they arrived at the local post office this morning, so we'll see if there's either a box or a delivery notice when I get home today.

Of course, of the four amp techs in town, two are on holiday, and two have a turn around time of over a week. :lol:

I'm thinking of popping in the TAD's without biasing for a quick low volume sound check, but won't play with any volume (or for any length of time) until I can get the amp properly biased. If this is a seriously bad idea let me know but from what I've read (and according to a local tech) a quick test without rebiasing should be okay.

Again, the amp had a set of Ruby 6l6GC-STR installed at the shop. One tube broke in transit and so I am replacing them with a pair of TAD 6l6WGC-STR.


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Post subject: Re: NAD - Pine Cabinet '68 Bassman
Posted: Thu Aug 09, 2012 12:58 pm
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:mrgreen:

Arjay

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Post subject: Re: NAD - Pine Cabinet '68 Bassman
Posted: Sat Aug 11, 2012 6:49 am
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:lol:

Amp is up and running!!!

Only able to play with volume @ 1 for the moment (quiet enough to play in the apartment!), but the sound is just right. It's warm, full and beautiful. I'll get it properly biased this week to make sure everything is set right, but judging by the tone everything sure SOUNDS right!


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