Kinda Sorta Audio Equipment.

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Kinda Sorta Audio Equipment.

Post by NashvilleRad on Tue Apr 16, 2013 1:35 pm

Similar to my post about friends asking you to do stuff: "hey, you work on this kind of stuff ... can you make it so I can plug my iPod into this old radio?" ... which I decided not to do, by the way ...

Another friend wanted me to check out their so-called 'boutique' amp to see if it needed any work.

So I did.

It's a Laurel Electronics Co. Model 960, and while my friend is chatting to me about cool tube amps, the only thing I was thinking was ... this thing doesn't have a power transformer -- it's rectified straight off the line in, and of course, it's a non-polarized plug to boot. It's got a 35W4 rectifier, 50C5 (power amp, presumably), and a 12AU6 (preamp, presumably).

I'm not a big guitar amp type (and I don't want to co-opt this forum with guitar stuff - plenty of other dedicated sites for that) so I'm looking at this as a power supply/transformer challenge, and in the very least -- change the plug and definitely the capacitors to make the amp a little safer.

I've only just started sketching a schematic, but here's a quick pic:



A total of 6 resistors, 3 fixed caps, and a three-section catalytic of 60/40/20.

If I have the time, I'm thinking isolate the power and re-work the rectification, if needed.

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Re: Kinda Sorta Audio Equipment.

Post by Wildcat445 on Tue Apr 16, 2013 4:52 pm

I would change the caps (and perhaps the power cord plug) to assure safety and call it a day. That little amp is basically the audio section of an AA5 and has the same amount of audio power, approximately one watt. Making changes to the design of the amp, even if successful, then would yield a hack job with only one watt of audio power. The tubes in that amp will not work with a power transformer.


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WC

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Re: Kinda Sorta Audio Equipment.

Post by NashvilleRad on Tue Apr 16, 2013 8:27 pm

Wildcat445,

Your AA5 comment - that kind of rang a bell when I saw the 50C5.

And agreed, I won't waste time thinking to 'mod' this into some sort of hack job ... too much other stuff on the bench anyway.

Although .. I may ruminate on ideas to try and isolate the line-in just to help offer to make something safer - looks like if something is plugged in, it's part of the circuit (!)

Plus, they're a good friend, so there's that pressure too.

Thanks again.

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Re: Kinda Sorta Audio Equipment.

Post by N7ZAL on Tue Apr 16, 2013 8:31 pm

The tubes in that amp will not work with a power transformer.

Not sure what you mean by that?
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Re: Kinda Sorta Audio Equipment.

Post by Wildcat445 on Tue Apr 16, 2013 9:21 pm

The 50C5 and 35W4 tubes used in that amp are what I was referring to. I do not know of a power transformer available that has taps for 50 or 35 volts. Those tubes were designed for, and used in, equipment where the tube filaments were wired in series. Perhaps a line isolation cap could be used to somewhat isolate the line. Those things have been known to leak and create the same problem they were intended to eliminate. I would personally just make sure all the caps are in good shape, use a polarized power cord plug and leave well enough alone.

I also have had "good friends" who have tried to get me into messes. Seems I recall an old saw "A friend in need......." something to that effect?

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WC

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Re: Kinda Sorta Audio Equipment.

Post by N7ZAL on Tue Apr 16, 2013 9:25 pm

I do not know of a power transformer available that has taps for 50 or 35 volts.

Thanks. I was thinking you were referring to a technical problem in doing it.
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Re: Kinda Sorta Audio Equipment.

Post by Bill Cahill on Tue Apr 16, 2013 10:17 pm

This amp was originally an AA5 radio, by the looks of it. Missing the needed power resitor for the fillaments.
Should be 160 ohms, five watts.
Personally, I don't think it's worth messing with.
It's not safe, and, won't be able to drive an ipod, guitar, or, anything like that.
My best advide for a friend, nicely turn her down stating it won't work for what she wants it for, is low wattage, and, is dangerous to use.

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Re: Kinda Sorta Audio Equipment.

Post by Wildcat445 on Wed Apr 17, 2013 4:58 pm

With all due respect, Bill, I would not use the word "dangerous". The word that comes to mind is "futile." As in why mess with it in the first place. It appears to be factory made, so replacing caps and servicing it properly should eliminate any imminent danger. I have messed with stuff "not worth messing with", and have received sometimes hours of entertainment. I have even learned something from that experience. I fix sets that I bought for parts sometimes simply for the experience that it provides. And if I mess with something "out of my pay grade" and the outcome is not successful (a church organ comes to mind), I have lost nothing. Besides, any guitar amp, or potential guitar amp, that ends up in the junk or as parts for a radio is a bonus with me as well.

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WC

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Re: Kinda Sorta Audio Equipment.

Post by Bill Cahill on Wed Apr 17, 2013 5:04 pm

Yes, but, this wasn't a guitar amp. It was a simple radio.
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Re: Kinda Sorta Audio Equipment.

Post by Doug Burskey on Fri Apr 19, 2013 10:07 am

An isolation transformer 120v pri. 120v sec. would isolate that amp from the AC power line. It would not have to be a large one, good for 1 amp or so.I built a stereo tube amp for use with my PC nothing fancy circut wise single ended output,and a power transformer.Just plug it in to the headphone jack.Works quite well I might add.Of corse some know it all tried to tell me "that would NEVER work on a PC"

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Re: Kinda Sorta Audio Equipment.

Post by N7ZAL on Fri Apr 19, 2013 1:07 pm

An isolation transformer 120v pri. 120v sec. would isolate that amp from the AC power line.

That was my thought also.
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Re: Kinda Sorta Audio Equipment.

Post by Renaissance Man on Thu Dec 12, 2013 3:15 pm

use one of these
Triad N-68x

They run very warm but that is normal.
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Re: Kinda Sorta Audio Equipment.

Post by Dr. Radio on Thu Dec 12, 2013 11:24 pm

I'll also vouch for the realllllllllly good idea of adding an isolation transformer. Kinda scary thinking the original design consideration would put you possibly at "hot" level right at your fingertips..
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Re: Kinda Sorta Audio Equipment.

Post by codefox on Tue Oct 07, 2014 12:10 pm

Agree, use an isolation transformer and you don't have to worry about a polarized plug or mis-wired wall socket. Still, not a very valuable item, but safer.

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Re: Kinda Sorta Audio Equipment.

Post by 35Z5 on Tue Oct 07, 2014 1:14 pm

There may already be a B+/ground iso-xformer in it, that second transformer is there for some reason...
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Re: Kinda Sorta Audio Equipment.

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