Old amp hum repair

[Feagil]

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.

 

[trobbins]

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

 

[KatieandDad]

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".

 

[payitforwardeddie]

Could be as simple as HT capacitors need replacing. Is it definitely 60Hz or could it be 120Hz (full wave rectification)?

 

[Feagil]

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.

 

[KatieandDad]

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.

 

[KatieandDad]

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.

 

[KatieandDad]

Is that a choke I see on the bottom of the chassis? That could also be short circuit which would cause the hum.

 

[DF96]

As it is a UK design, you might get some help here. Or try this Vortexion site.

 

[Feagil]

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.

 

[Alexontherocks]

The caps may very well pass the DMM test and fail the "under load" test.

I would suggest replacing them (which would be required anyway given the age of the amplifier). I think the PP wiring (I suspect it is PP) will allow for a quick replacement.

from there you can start further investigation.

I'd also suggest retracing the circuitry and creating a schematic file for the others to look at.

Cheers.

 

[DF96]

You said that the hum increases as you raise the volume. This tells us that the source of the hum is before the volume control. So what is before the volume control?

 

[Brian Oshman]

Probably just bad tubes.

 

[DF96]

Yes, heater-cathode leakage in any stage before the volume pot is a likely cause.

 

[Feagil]

Volume pot appears to be straight after the audio in jack. The noise might be ground related?

Can this sort of problem be caused by impedances not being matched? This is still a fuzzy subject for me so bear with me if this is completely off. Can it be not expecting a line level input but rather a mic. The pair of EF40 pre-amp valves definitely suggest a mic as an input.

I am busy trying to find a good cap of about the same size and voltage rating. Most of the big caps are 40uF. If i put 1000uF across their poles will I cause any major trouble or should I try find a cap that matches the existing ones exactly?

The best part about all this is I am already learning a lot. Thanks.

 

[Ronsonic]

Those caps also have a voltage rating. The voltages in that amp are almost certainly higher than what your 1,000uF is rated for. They may well blow up injuring you and/or the amp. Look, it is almost 60 years old. The guys who built it expected the caps to last ten years. Be prepared to just replace the damn things.

First noise test, short the inputs and then turn up the volume. If it is not noisy then the problem is with how you are trying to connect it. If the pot really is the first thing signal hits, this is probably an issue.

 

[Alexontherocks]

2 cents of advice.... replace the caps. If you find 20uf at the correct voltage rating just parallel them.

Electrolytes have very short lives in tubes and class a amps. The heat just kills them

Inviato dal mio GT-I5800 usando Tapatalk

 

[KatieandDad]

You can't parallel a new cap with a defective cap, it wont work. You have to remove the connections to the old cap first.

 

[Alexontherocks]

I told the poster to parallel twp 20uf new caps if he can't gind a single 40uf new one

Inviato dal mio GT-I5800 usando Tapatalk

 

[DF96]

It would be unusual to put a volume pot straight on a microphone input, as it would create too much noise. Impedance matching is unlikely to be an issue, as impedance matching is generally avoided in audio.

If the circuit starts with a volume pot, and hum gets worse as the volume is raised, then two causes are likely:
- hum on the input signal, not the amp's fault at all
- hum fed to the first grid circuit, with the impedance seen at the pot wiper varying the amount of hum fed in - poor heater wiring might cause this, or a large 'audiophile' input coupling cap having too much stray capacitance to a nearby hum source.