Troublesome AC-50

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Troublesome AC-50

Postby paulasgeorge » Thu Jul 30, 2009 12:18 pm

Hi everyone. I'm new here and am posting this topic per the "suggestions for topics" post.

This is probably an easy one for Don, and maybe others on this board.

I have a mid-sixties (JMI) AC-50 head that is giving me fits. It has had this problem for a very long time, but has gotten
worse as of late. I would be playing and the amp would sound great, then suddenly the volume would drop off significantly
and the sound would distort. This would happen only once at almost every show, but all I would do is go back and jiggle
the guitar cord in the amp and the great sound would return. It got a lot worse recently, where I may have to do the jiggle
thing several times. I spoke to an amp guy at a local music dealer and he said to first try changing the power tubes. I did it
and the amp worked great for a few months. As of late, it seems the problem has returned about as bad as before.
However, I have also had some rehearsals where the amp didn't act up at all. Another thing I noticed, I can tap on the
volume or treble (brilliant channel) pot and get the problem to occur. I always use the brilliant channel, by the way.
In the past, I would have the same problem even with the normal channel, but that doesn't seem to be the case anymore.

I will note that the amp did take a pretty good fall (which is why it stays home now) a few years back, but in subsequent
uses, the amp didn't act up at all. And as I said, the amp had this problem long before the fall. I'll also note that the
amp still has all of the original Mullard pre-amp tubes.

I have had the amp apart several times looking for loose connections or anything else obvious , but never found anything.
I guess that doesn't mean that there isn't something loose somewhere. I have also used other speaker cabinets, different
guitar chords, different speaker chords and all of the other obvious troubleshooting methods and the problem would still
occur.

Any suggestions? As a last resort, I guess I can take it to that tech I mentioned before, but I'd like to take a whack at it
myself, or at least have a little more knowledge of what the problem may be before I take it to anyone. The thought of
the amp leaving my house any length of time is not a happy one.

This is one amazing amp when it's working properly. especially playing through my repro AC-50 cabinet loaded with two
original silver Celestions and an original Midax horn.

Thanks in advance,
Paul
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Re: Troublesome AC-50

Postby Paladin » Thu Jul 30, 2009 12:48 pm

I would have it checked out thoroughly by a really good tech, and if it hasn't been recapped, get it done. It could still be a tube, socket, or even a cold or loose solder joint.

If you are close enough to Don, or don't mind shipping it, it would be well worth it to have 'the toneman' give it a good going over. He is the man!

AC50s are one of the best amps ever made. Is yours a small box or large box rig?
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Re: Troublesome AC-50

Postby paulasgeorge » Thu Jul 30, 2009 1:09 pm

Thanks for the quick reply. I was thinking a cap job as well, something I really shouldn't do myself.

I think Don is on the west coast and I'm near Chicago. The thought of shipping the amp isn't really something
I'd want to do, although I trust Don implicitly. I just don't trust the shippers.

It's a large box and is a solid state rectified amplifier. I know a lot of people prefer the tube rectified units, but I
can't say I've ever heard one in person, so I can't compare. I actually have two AC-50 rigs that are nearly identical.
I have to agree, this is best sounding amplifier I've ever heard.
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Re: Troublesome AC-50

Postby Don Butler » Thu Jul 30, 2009 2:49 pm

Change the filter cans & caps, look for a cracked or way out of tolerance resistor. Check the value of each resistor. If it's over 10% off then replace it. Vox used only Carbon Comps because Derek Underdown thought they sounded the best out of anything. You can use metal oxide or ceramics only in the voltage circuits. Those amps have a bias adjustment for each output tube that needs to be set across the 47 ohm resistor to ground and reads 2.2 volts. Make sure all the screws, nuts and bolts are tight. Use some DeOxit D-5 in each pot and spray some on a Q-Tip and clean the jacks with it. If you have any problems with it after that then have your tech call me. BTW, I would re-heat every solder connection in the amp since it took a fall. A solder joint can look fine but be broken.
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Re: Troublesome AC-50

Postby paulasgeorge » Thu Jul 30, 2009 3:31 pm

Thanks Don.

Glad to see that you didn't mention anything too scary.

I was looking at troubleshooting stuff on line yesterday and reheating solder joints was also mentioned. It seems like a lot
but I read that it really doesn't take that long.

Will there be any difficulty acquiring the parts you mentioned? How about the contact cleaner? Is that available at a Radio
Shack type store?

And I realize this is a difficult question to answer but any ideas why the amp worked perfectly for a few months after I replaced
the power tubes? The amp was never moved afterward where something could have been jostled loose again.
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Re: Troublesome AC-50

Postby bassbob » Thu Jul 30, 2009 5:42 pm

I took my fender out of the chasis and shipped it to Don (over insured) for around $60 each way. I wouldn't hesitate to do it again.


Welcome to the forum. :D
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Re: Troublesome AC-50

Postby Greggman » Thu Jul 30, 2009 10:18 pm

bassbob wrote:I took my fender out of the chasis and shipped it to Don (over insured) for around $60 each way. I wouldn't hesitate to do it again.


Welcome to the forum. :D

Welcome...
Smart man! He is the best ,period.
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Re: Troublesome AC-50

Postby bassbob » Fri Jul 31, 2009 5:41 am

paulasgeorge wrote:Thanks Don.

Glad to see that you didn't mention anything too scary.

I was looking at troubleshooting stuff on line yesterday and reheating solder joints was also mentioned. It seems like a lot
but I read that it really doesn't take that long.

Will there be any difficulty acquiring the parts you mentioned? How about the contact cleaner? Is that available at a Radio
Shack type store?

And I realize this is a difficult question to answer but any ideas why the amp worked perfectly for a few months after I replaced
the power tubes? The amp was never moved afterward where something could have been jostled loose again.



Radio Shack carries some other contact cleaner. They don't carry the Caig De-Oxit, which is supposedly the best. I bought the radio shack crap and it worked fine. Around $12 a can.
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Re: Troublesome AC-50

Postby paulasgeorge » Fri Jul 31, 2009 7:51 am

Well, since Don offered his help if my local tech can't fix the problem, I think I'll still take it to him. With Don as a back-up
consultant, I don't think I have much to worry about. I'm just a terrible procrastinator, obviously.

And yet another constant in this whole saga, whenever I take the amp apart and look around inside, whether I do anything or
not, like pull out the preamp tubes or whatever, the amp will always function perfectly the very first time I turn it on afterward.
I had it apart Wednesday night and did nothing really, then put it back together and set it atop the trolley. When I fired it up
last night it sounded awesome and didn't make one unintentional sound. I think I let it warm up for about 30 minutes. I played
my Rocky through it for about an hour or so without incident. Strange, I think.

Thanks everyone!
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Re: Troublesome AC-50

Postby Epidrake » Fri Jul 31, 2009 10:02 am

Glad it worked out!
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Re: Troublesome AC-50

Postby paulasgeorge » Fri Oct 09, 2009 10:14 am

Don Butler wrote:Change the filter cans & caps, look for a cracked or way out of tolerance resistor. Check the value of each resistor. If it's over 10% off then replace it. Vox used only Carbon Comps because Derek Underdown thought they sounded the best out of anything. You can use metal oxide or ceramics only in the voltage circuits. Those amps have a bias adjustment for each output tube that needs to be set across the 47 ohm resistor to ground and reads 2.2 volts. Make sure all the screws, nuts and bolts are tight. Use some DeOxit D-5 in each pot and spray some on a Q-Tip and clean the jacks with it. If you have any problems with it after that then have your tech call me. BTW, I would re-heat every solder connection in the amp since it took a fall. A solder joint can look fine but be broken.


Don, what would you estimate the above would cost.
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Re: Troublesome AC-50

Postby Don Butler » Wed Oct 14, 2009 3:49 pm

Hi! sorry for not getting back to you sooner.. Been busy with the show and family in town..
Anyway, You'd be looking about $150. w/parts included + return shipping
Please let me know if you'd like to send it out. contact me at: 661.259.4544 or email: de1b@earthlink.net
Thanks!!
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Re: Troublesome AC-50

Postby paulasgeorge » Sat Oct 17, 2009 12:31 pm

Thanks again Don.

As it turns out again, I fired the amp up during the week to learn the Munster's Theme, and it acted up as usual. But, for some reason I disconnected my Boss reverb pedal, don't know why I did, and BAMMO! The amp worked like a champ! Couldn't believe the lovely tones. I'm almost positive I've tried going direct before and still had trouble, but maybe that was before I swapped the power tubes. I can't remember.

I'll probably fire it up again today cause I want to A/B my '65 Ricky 12 against my '87 V64 12. Can't believe I haven't done that yet.

So, stand by.

If you check your in-box, I PM'd you before I posted. I did find a place called British Audio which is about an hour from my house. I'd expect with a name like that, they should be experts. But your price is amazing and I'd just have to talk myself into shipping the amp to you if I do end up needing your services.

Thanks again,
Paul

EDIT: Just got done about an hour ago. The amp worked perfectly without the reverb pedal being plugged in. Hopefully it was just the short cord I was using and not the pedal. I paid a lot for that pedal a long time ago, used for like $120. I really like that pedal too.
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Re: Troublesome AC-50

Postby Don Butler » Wed Oct 21, 2009 3:26 pm

Paul; Sounds good! Glad it turned out to be the reverb pedal. I'm not fond of any Boss pedals. There are much better ones for reverb than that one. The TC Electronics Nova Reverb is about the most analouge sounding reverb pedal I've heard/used. There's a couple that are actually mini spring reverbs in a foot pedal as well (although the neames escape me at the moment..)
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Re: Troublesome AC-50

Postby paulasgeorge » Tue Nov 03, 2009 4:20 pm

Thanks again for all your help/advice, Don.

I really haven't determined if the problem is in the reverb pedal or the short cord to the amp yet. As long as the amp is working, I'm a bit
ecstatic about that right now. I've used the amp three more times since and no problems still.

In the near future, I'll try the pedal and cord combo in a different amp to see if I can narrow down the problem. I really don't mind the Boss pedal at all. I've gotten used to it over the years. When I bought it (used-many years back) I don't recall too many options for reverb effects that were in pedal form out there, all I remember seeing were rackmounts.

Anyhow, I'm also getting used to not having reverb at all, so if it turns out to be a bad pedal, it may not be a great loss. I did check out the TC Electronics ones on-line that you suggested and feature-wise, the Nova looks similar. I'll have to find one at a place that I can hear it first hand. $249.00 is hardly big money, but I'd like to hear it in person first rather than get one on-line unheard. Not that I don't trust your judgment, Don. It's amazing how subjective all this music gear stuff can be from one musician to the next. For example, I'm hardly a guitar virtuoso, but I find my cheap, Chinese made Casino to be a wonderful playing/sounding guitar, while many others think they are basically junk when compared to guitars twice the cost or more. I can't imagine what a way more superior instrument would feel like. Even my son's Chinese Casino plays amazingly. Maybe my lack of virtuosity makes me less fussy.

Peace,
Paul
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