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Old 28th Jun 2020, 6:53 pm   #1
bobbyd123
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Default Linear L50 Mains transformer Fault

Hell I hope someone can help
I have a Linear L50 Amp I am trying to restore
I had one of these when I was a teenager
Problem is one primary tapping is open circuit on the mains transformer
The other two when connected via my lamp limiter they are a short circuit Big bright light! I have found on line what might be a suitable replacement
MULLARD 5-20 MONO
410V-0-410 @ 180MA (Not sure if I will get the right HT)
6.3 @ 4amps centre tapped
6.3 @2.5 amps
5v @ 3amps
I have thought about having a rewind as a possible
Any suggestions I Would be very gratefull
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Old 28th Jun 2020, 8:28 pm   #2
Ed_Dinning
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Default Re: Linear L50 Mains transformer Fault

Hi Bobby, the Mullard 5-20 is only a 20w amp. the model number of the linear implies it is 50w, so may be a good bit more powerful.
I'm sure some of the other guys on this forum will have details of the transformer needed and you may get lucky and find one. If not it can easily be rewound

Ed
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Old 28th Jun 2020, 9:09 pm   #3
bobbyd123
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Default Re: Linear L50 Mains transformer Fault

Thanks Ed
I do hope someone on his site can help
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Old 29th Jun 2020, 1:22 pm   #4
bikerhifinut
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Default Re: Linear L50 Mains transformer Fault

If you can't get a replacement transformer at an economical price (Brace yourself here, big Valve amp mains transformers come at eye watering prices these days, certainly if You want something decent). But you do need the specs of the original amp, hopefully one of our resident experts will have the info to hand.
Ed D is one of our resident transformer experts and I am sure he can sort the original out for an economically sensible price.
A.
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Old 29th Jun 2020, 2:16 pm   #5
bobbyd123
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Default Re: Linear L50 Mains transformer Fault

Thanks bikerhifi
I no this job is going to be expensive
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Old 29th Jun 2020, 2:46 pm   #6
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Default Re: Linear L50 Mains transformer Fault

These were very cheap amps when new (I bought one for PA use - that's what they were meant for and cost me 19Gns. That's 19/19/00 in old money) You couldn't expect any quality components for that price, but they worked well enough for a time.
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Old 29th Jun 2020, 4:20 pm   #7
bobbyd123
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Default Re: Linear L50 Mains transformer Fault

What would happen If I fitted a smaller transformer just to get it working
Recapping and out of spec resistors etc
This would give me time to source the correct one or a rewind
I understand that the power would be less but would there be any damage If not turned up full I have no speakers that could handle it anyway
Thanks any way.....
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Old 29th Jun 2020, 4:39 pm   #8
bikerhifinut
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Default Re: Linear L50 Mains transformer Fault

Quote:
Originally Posted by bobbyd123 View Post
What would happen If I fitted a smaller transformer just to get it working
Recapping and out of spec resistors etc
This would give me time to source the correct one or a rewind
I understand that the power would be less but would there be any damage If not turned up full I have no speakers that could handle it anyway
Thanks any way.....
NO NO NO!

The amp will have a quiescent current draw thats very close to its full output assuming a standard class A/B design. Add to that the steady draw from the Valve heaters.
It sounds like you have a limited understanding of how the circuit works, or indeed basic valve theory and basic safety when dealing with high voltages.
I'd strongly advise you do some fundamental studying. You'll get plenty of friendly help on here and no question is too daft or basic.

Whats the valve line up for starters?

Andy.
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Old 29th Jun 2020, 5:27 pm   #9
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Default Re: Linear L50 Mains transformer Fault

This might help

http://www.chambonino.com/work/linear/lin5.html

Looks like a pair of EL34's

Cheers

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Old 29th Jun 2020, 6:30 pm   #10
chriswood1900
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Default Re: Linear L50 Mains transformer Fault

If you want to know more about the Linear company this site which hosts the diagram has a lot of other useful info including a rebuild of the similair 30W unit. http://www.chambonino.com/linwork.html
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Old 29th Jun 2020, 9:36 pm   #11
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Default Re: Linear L50 Mains transformer Fault

You could fit a mains transformer from a Linear Conchord, the 30W version of your amp. Your HT would then be around 430V instead of the 500V of the L50 and the output only a negligible 2dB lower.

The lowest cost approach might be to look out on eBay etc for a scrap Conchord, or the RSC A10 equivalent, with a working mains transformer. Probably cheaper than a rewind.

Martin
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Old 29th Jun 2020, 10:06 pm   #12
bobbyd123
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Default Re: Linear L50 Mains transformer Fault

Hartley
Many many thanks what a good Idea you made my day
I already have a conchord that's not working but the transformer is good) so tomorrow I will change them
Cheers
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Old 29th Jun 2020, 10:21 pm   #13
vidjoman
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Default Re: Linear L50 Mains transformer Fault

Make sure you change the coupling capacitors to the output valves BEFORE you power up. If leaky they will cause the valves to take excessive current and are likely to burn out transformers, either mains or output. It's worth checking all the other components as well.
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Old 30th Jun 2020, 11:49 am   #14
bobbyd123
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Default Re: Linear L50 Mains transformer Fault

Okay Yes I will change the coupling capacitors same as I do on my radios
As a matter of interest Can I Use the normal caps for my radio work
Seems on the internet a lot of the Guitar valve amp people say you must use special high quality audio capacitors for better tone
Thanks again..........
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Old 30th Jun 2020, 12:26 pm   #15
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Default Re: Linear L50 Mains transformer Fault

Depends on what caps you use for your radio repairs. If it's the standard yellow types rated at 630 volts etc., then yes you can. Don't believe the rubbish spouted by these guitar amplifier folk!

Why not just ask Ed Dinning to rewind your original transformer as in post #2? This would be better than spoiling another potentially good amplifier by removing its original transformer to repair this one.

It goes without saying that you should thoroughly investigate the cause of the original transformer failure before attempting to fit a replacement, as they don't just fail on their own! Obviously you should check the continuity of the output transformer before going to a lot of time and expense on a mains transformer.
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Old 30th Jun 2020, 1:22 pm   #16
bobbyd123
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Default Re: Linear L50 Mains transformer Fault

Okay yes I do use the yellow 630 volt caps
I have had a look as to why the transformer has blown
Nothing I can see at the moment except the earth connection was onto a tag strip
fixed by a loose pop rivet no continuity (No protection for the primary against a short)
Is it possible that I can ask Mr Ed Dining Not sure how you message if this is okay
Thank you................
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Old 30th Jun 2020, 4:11 pm   #17
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Default Re: Linear L50 Mains transformer Fault

Easiest way to send a message is to click on Ed's name in post #2. That gives you the option to send him a private message (PM).

Martin
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Old 1st Jul 2020, 3:04 pm   #18
Techman
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Default Re: Linear L50 Mains transformer Fault

I would advise checking the transformer closely, it could be just a bad or no connection where the lead out wire comes out of the bobbin - I say this as mains transformers usually develop shorted turns, rather than just going o/c on a primary winding.

Edit: Forget that last comment, as re-reading your first post I see that the other windings do in fact have shorted turns - a job for Ed, I think.
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Old 1st Jul 2020, 3:39 pm   #19
bobbyd123
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Default Re: Linear L50 Mains transformer Fault

Okay update
Ed is going to sort out the power transformer Great
I have tested the continuity of the output transformer
All the windings seem okay (Fingers crossed) I have found one of the 50uf 50v electrolite bypass capacitors for one of the EL34 Output valves was not connected broken tail
I,m still trying to figure out why the TRANSFORMER MAY HAVE BLOWN
Cheers
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Old 1st Jul 2020, 3:47 pm   #20
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Default Re: Linear L50 Mains transformer Fault

Could be a faulty rectifier or shorted smoothing capacitor, or it could be leaking coupling capacitors to the output valves or a valve itself - you need to check it out thoroughly or you'll blow the newly wound mains transformer.
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