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Post subject: 1973? Bassman 50
Posted: Thu Mar 05, 2015 6:31 pm
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Recently picked up what I believe is a 1973 Bassman 50, at least that's what it looks like based on the transformer codes. I've attached some pictures of the circuit. Does anyone know (a) what circuit this is and (b) if this looks stock? The component caps look largely stock to me, and the filter caps are almost certainly stock (a couple bubbling on the positive side). But I'm wondering if someone mucked around with this circuit at all. Some of the cabinet screws show wear from being taken out and put back in, and there is some Sharpie writing behind the second input on the bass channel.

I bought this head for pretty cheap and was looking for something to do some low-to-medium volume bass practice and recording at home. I was going to pair the amp with a 1 x 12 modern 4 ohm cabinet. I wanted an old tube amp with a vintage type sound for home practice on my Precision Basses.

http://smg.photobucket.com/user/SirMike1983/library/Bassman%2050?sort=3&page=1

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Post subject: Re: 1973? Bassman 50
Posted: Thu Mar 05, 2015 6:37 pm
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It's had a resistor replaced on the V6 tube socket but beyond that it appears to be unmolested. It's definitely ready for new electrolytics in the filter stage (recommend F&T's for performance and durability). It should perform its intended mission well once the maintenance issues have been addressed.

Arjay

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Post subject: Re: 1973? Bassman 50
Posted: Fri Mar 06, 2015 4:47 am
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Retroverbial wrote:
It's had a resistor replaced on the V6 tube socket

Arjay


+1

This new resistor is 1/2 watts and must be 1 watt minimum


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Post subject: Re: 1973? Bassman 50
Posted: Sat Mar 07, 2015 1:42 pm
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This looks like an AA371 on further inspection and comparison to a board diagram. Good catch on the V6 resistor. I think I will change grid screen resistors on the power tubes. I would like to keep them at 470, but maybe up the wattage to 3 watts? Does a change in wattage change the effect of the grid screen resistors on the power tubes? I think I'd just spring for flame proof, wire wound resistors.

Are F&T preferable to Sprague Atom for caps? The Spragues cost more, but I have heard the F&T are closer to the values advertised.

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Post subject: Re: 1973? Bassman 50
Posted: Sat Mar 07, 2015 1:57 pm
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Sprague recently had some serious QC/QA issues and for that reason alone, I choose F&T as my sole source for electrolytic caps. Older Spragues (pre-2011) should be fine though. I would not over-spec those screen-grid resistors on V5 and V6. A 1-watt/470-ohm metal oxide type will likely outlast your ownership of the amp. The only mod I'd genuinely consider would be to re-wire the bias-balance control to a conventional symmetrical configuration (a la AA864).

Arjay

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Post subject: Re: 1973? Bassman 50
Posted: Mon Mar 09, 2015 7:04 am
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Here's a good site to ponder mods. I set my AB165 to AA864. Better NORMAL channel with guitar. I haven't tried a bass on either channel.


http://tone-lizard.com/Bassman_Modifications.htm


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Post subject: Re: 1973? Bassman 50
Posted: Mon Mar 09, 2015 12:29 pm
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Thank you for the insight- I did not know that about Sprague caps. I have the F&T Caps and some resistors on the way for this project. I need to get a bass cabinet for it, and I think something modern for bass is a better idea than the old type cabinets. Most of what is out there is 8-ohm. Does anyone know if the Fender Rumble 2 x 10 cabinet is wired in series or parallel by default? If it's in series and at 8-ohm, changing the wiring to parallel should give a total 4-ohm load, which is what the Bassman 50 is set for. I haven't seen a 1 x 12 4ohm cabinet that I have liked so far.


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Post subject: Re: 1973? Bassman 50
Posted: Mon Mar 09, 2015 12:32 pm
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F & T are great I use them most of the time


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Post subject: Re: 1973? Bassman 50
Posted: Mon Mar 09, 2015 10:20 pm
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Have you considered building an enclosure?

Arjay

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Post subject: Re: 1973? Bassman 50
Posted: Tue Mar 10, 2015 11:16 am
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I would love a modern 1 x 12 cabinet for this that has a vintage theme externally, but don't have the cabinet making skills to build my own.


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Post subject: Re: 1973? Bassman 50
Posted: Sat Mar 14, 2015 8:33 pm
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I located a cabinet for it. Yhat is on the way later this week.

In the meantime, I replaced the electrolytic caps, except for the two bias area caps. Those are the larger, white caps. The diagram calls for both to be 50uF, 70v. My amp has one at 80uF 75v (the one on the smaller, separate board), and another at 80uF, 50v (the one off the bias balance pot). Hoffman Amps indicates that some Silverface amps used 80uF caps in those two bias positions, and it looks like mine is one of them. Any suggestions on what to use as replacements?

I've never seen 80-50 and 80-75 in new caps, just 50-50 and 100-100. So I guess my choices based on what I can find are 50-50, 100-100, or leave the existing caps in for now.


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Post subject: Re: 1973? Bassman 50
Posted: Sat Mar 14, 2015 9:47 pm
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I'd go for the 100/100 values, Mike.

Arjay

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Post subject: Re: 1973? Bassman 50
Posted: Sun Mar 15, 2015 1:12 am
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Retroverbial wrote:
I'd go for the 100/100 values, Mike.

Arjay


+1


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Post subject: Re: 1973? Bassman 50
Posted: Sun Mar 15, 2015 3:46 pm
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Sounds good- I ordered two 100-100 caps. The rest have been replaced, so that's all that is left for this cap job.


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Post subject: Re: 1973? Bassman 50
Posted: Wed Mar 18, 2015 12:31 am
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Do you have the speaker cabinet in hand yet?

Arjay

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