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Post subject: '68-'69 Fender Bassman 50, 215 cab
Posted: Sun Apr 07, 2013 12:46 pm
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I bought this amp with cab back in January, I got a really good price on it, but I've been having some problems with it.

I got it from a guy who acquired it through a closed recording studio, and it's in pretty pristine condition. It had the original paper work, and after having a guy from a local shop look at it, he told me it contained a lot of '68 parts (even though it's deemed a '69).
I believe the head and cab have been together for their entire life.

There was a lot of crackling and break up when I first bought it.
My first step was to replace the Power tubes and upgrade the speakers (one was dead). I had Tube Doctor 6L6GC's put in it, and I replaced it with some old JBL's (the kind that were an upgradable option). I was told the speakers were originally in a PA system, but were taken out because the cabinets were pretty beat up. I was surprised to find this out because they speakers looked brand new. It sounded pretty great after doing just those two things, but the sound has sort of degraded over time.

I thought it may be the pre-amp tubes so I bought a JJ Tesla 12AT7, and 3 of the Svetlana 12AX7 (matched trio). I was sure this was my problem, but it didn't really seem to fix it. The bass channel actually cleared up a bit, but the normal is pretty much just as bad as before.

The problem I am having is a sort of "fuzz". When I turn the amp up past 4 or so it sort of loses all clarity and sounds like I have a fuzz on it. It used to be a lot more clear and I feel like I shouldn't be losing this kind of headroom. What could my problem be?

I should say I primarily play bass through. And when when the amp is tuned below 4, and I strike the string hard enough I get the sort of break up/fuzz that I get all the time when above 4.

Thanks for any tips or pointers anyone has to offer.

I should also probably note that the 215 cab is vertical.
Could anyone also maybe tell me what kind of speakers I took out of it? They aren't marked with a brand. They are just silver metal with a sort of serial stamp on it.


Thanks!


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Post subject: Re: '68-'69 Fender Bassman 50, 215 cab
Posted: Sun Apr 07, 2013 1:09 pm
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inmyfruitcup wrote:
They are just silver metal with a sort of serial stamp on it.


Do you have that "serial stamp"?

Arjay

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Post subject: Re: '68-'69 Fender Bassman 50, 215 cab
Posted: Sun Apr 07, 2013 3:33 pm
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The serial is "108506" it also reads "67-6916"


On the cone there is another stamp, "1553".


Thanks!


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Post subject: Re: '68-'69 Fender Bassman 50, 215 cab
Posted: Sun Apr 07, 2013 4:32 pm
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The speakers were made by Eminence in the 16th week of 1969.

Arjay

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Post subject: Re: '68-'69 Fender Bassman 50, 215 cab
Posted: Sun Apr 07, 2013 8:33 pm
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Thanks for that!

Any clue what could be causing the weird fuzz problem?


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Post subject: Re: '68-'69 Fender Bassman 50, 215 cab
Posted: Sun Apr 07, 2013 9:20 pm
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Likely the amp is ready for some long-overdue maintenance.

Arjay

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Post subject: Re: '68-'69 Fender Bassman 50, 215 cab
Posted: Sun Apr 07, 2013 10:56 pm
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Can you check the bias on those new TAD 6L6GCs? It maybe they are running a bit too hot. Esp if the old tubes were in the amp a long time. Ppl sometimes crank the beejeebers of the pot, in the bias supply, to get every bit of life outta old tubes.

Then, any new tubes will run WAY TOO hot, with same settings. Early OD is one of the signs of too hot biasing.

Good luck!


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Post subject: Re: '68-'69 Fender Bassman 50, 215 cab
Posted: Mon Apr 08, 2013 2:49 am
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BMW2002Ti wrote:
Can you check the bias on those new TAD 6L6GCs? It maybe they are running a bit too hot. Esp if the old tubes were in the amp a long time. Ppl sometimes crank the beejeebers of the pot, in the bias supply, to get every bit of life outta old tubes.

Then, any new tubes will run WAY TOO hot, with same settings. Early OD is one of the signs of too hot biasing.


The silverface Bassman does not have a traditional bias-adjustment circuit. It's actually a bias *balance* arrangement.

Arjay

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Post subject: Re: '68-'69 Fender Bassman 50, 215 cab
Posted: Mon Apr 08, 2013 5:29 pm
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Hey guys, thanks for all the input, this is really helping out.

So, a few things I forgot to mention:

I don't think the amp had even really been touched since the 70's, the tubes in it were OLD.

When I had the new power tubes put in I had it re-biased. I trust the guy, but there is a chance he amped it up a bit too much you think? I have no way of checking myself, so I can't say until I take it into someone.

Someone mentioned something about having the caps and resistors replaced since they are probably the original parts inside of it right now. Do you think this could be the problem based on what I've explained?

If not, what other sort of maintenance issue does this sound like?
Does anyone have a similar experience with a problem like mine?

I apologize if I am sounding a little ignorant, I am just not getting into tube amplifiers, and this is my first one. Up until now I have only owned Solid State. I am trying my best to learn about these things, and I wasn't getting far on my own, so I figured I should ask some people who know a thing or two.

Thanks!


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Post subject: Re: '68-'69 Fender Bassman 50, 215 cab
Posted: Mon Apr 08, 2013 5:53 pm
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" The silverface Bassman does not have a traditional bias-adjustment circuit. It's actually a bias *balance* arrangement. "


Yeah, I knew that. :D

Maybe, check the bias anyhow. Maybe someone altered the dropping resistor in the bias supply. Like Jerry's amp. His new EH 6L6GC had a bias of over 70mA each, prior to redoing the bias supply.


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Post subject: Re: '68-'69 Fender Bassman 50, 215 cab
Posted: Mon Apr 08, 2013 6:42 pm
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The bias supply should be checked, as well as the idle current at each of the power-tube sockets. Assuming all is well there, the AB165-type balance system is quite forgiving and a sonically-pleasing setting is usually quite easy to achieve.

Arjay

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