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Old 1st April 2012, 10:09 AM   #1
Feagil is offline Feagil  South Africa
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Default Old amp hum repair

Hi everyone

I am new in the world of tube audio, built a few small headphone amps, but now I have a slightly bigger and scarier fish to fry. I have an old amp given to me by a professor, it was built in 54 and looks like it was designed as a PA system. It has a pair of EF40 and a pair 5881 which as I understand are mil-spec 6L6 tubes. The output transformer has notches to set the output impedance, from 10 ohms to about 80. The problem is it has a very distinct 60Hz hum on its output. I want to use it as a guitar amp since it is a single channel amp.

I can either try and pull it apart and salvage the transformers or repair it as it stands. I was thinking one place to start would be a SS rectifier for the filaments. Would that help? and how would I ensure that the heater bias is correct, can I simply wire in a rectifier and connect it back on the points where the AC filaments currently are?

Any ideas are welcome.

Thanks for the help and sorry for the long post.
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Old 1st April 2012, 11:29 AM   #2
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A photo or two and any indication of manufacturer or part manufacturers, and why you think it was built in 1954. Best to understand the beast first :-)
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Old 1st April 2012, 11:54 AM   #3
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A "Distinct" hum to me would indicate a fault. I would try to rectify the fault before you start to "play" with the circuit.

Don't cure the symptoms, cure the cause - as true in medicine as it is in Hi-Fi DIY.

There are some pretty awful PA amps out there from all over the millenium, but its unusual to find one that had a "Distinct Hum".
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Old 1st April 2012, 12:57 PM   #4
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Could be as simple as HT capacitors need replacing. Is it definitely 60Hz or could it be 120Hz (full wave rectification)?
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Old 1st April 2012, 12:59 PM   #5
Feagil is offline Feagil  South Africa
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The amp is a Vortexion Ltd. Wimbledon C1955. I know its from 1954 because the technician left a little paper note bolted to the inside with the date on it. As you can see from the pics the transformers are pretty big. I live in South Africa so I dont know if that was manufactured here by a small workshop or weather it comes from England. The hum I am pretty sure is the mains somehow coming through into the audio line. It does increase as I raise the volume. The may help in the diagnosis (to reference a previous metaphor). I heard that AC on filaments can introduce the hum which is why I thought of rectifying the AC line but as you say at some point it must have worked without loud hum so there must be a fault. Could obe of the valves be blown? I heard that PP (i assume its push-pull because of the pair of 5881) can hum if the system becomes unbalanced.

To clarify, the hum is very loud. louder than the music that comes out when i feed it a line level input.

Again thank you all for the help.
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Old 1st April 2012, 01:09 PM   #6
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Hum that loud is DEFINITELY a fault.

Don't worry too much about the aesthetics of the repair at this moment in time. Just as payitforwardeddie said, just sub in some replacement HT caps.

In fact sub in all electrolytics. It doesn't appear that quality is an issue here.
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Old 1st April 2012, 01:10 PM   #7
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In time you can either source HT caps that will fit and look good or just empty the duff tins and fill them with modern equivalents.
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Old 1st April 2012, 01:13 PM   #8
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Is that a choke I see on the bottom of the chassis? That could also be short circuit which would cause the hum.
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Old 1st April 2012, 03:03 PM   #9
DF96 is offline DF96  England
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As it is a UK design, you might get some help here. Or try this Vortexion site.
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Old 1st April 2012, 03:44 PM   #10
Feagil is offline Feagil  South Africa
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I have tried the caps in situ with a multimeter and they seem to be ok. Should i try connect a new cap in parallel with each one and see if the hum suddenly stops? That seems a tad scary with the HT right there.

I think the choke is the giant "transformer" on the top of the amp in the middle. How would i check weather its shorted out?

Quality isnt an issue right now, maybe later I can worry about improving it. Also as a guitar amp for a VERY amatuer guitarist (more amatuer at guitar than valve amps) it wont matter much.

Would it be difficult to determine the transformers characterics and design an amp with those valves from scratch?

Thanks again everyone.
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