Melody guitar amp has leakage current.

I'm servicing a low end Melody guitar amp. Guitar amps are something I do not normally service. It looks like it's from the 1960's. It has a speaker, three inputs, but only one volume control. There's a plug for a foot pedal and controls for volume, tone, speed and depth. Internally, there are five tubes and NO ac input transformer. It looks as if someone has replaced a few parts. I can't tell whether or not the circuit has been modified. The customer complaint is that it hums when idling. There is a small amount of background hum. I wouldn't exactly call it objectionable. The audio output is not too bad. In doing a basic leakage test with my Sencore PR57, the leakage current reads 550µA on one side. The other side is worse. The meter is pegged even before I push the leakage current button on the probe. When I press the leakage button on the probe, the speaker emits a loud hum and the meter pegs all the way.

Are these sorts of readings to be expected on an amp like this?

Thanks for your reply.

Reply to
David Farber
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"David Farber" bravely wrote to "All" (05 Jul 05 11:32:51) --- on the heady topic of "Melody guitar amp has leakage current."

DF> From: "David Farber" DF> Xref: aeinews sci.electronics.repair:52531

DF> I'm servicing a low end Melody guitar amp. Guitar amps are something I DF> do not normally service. It looks like it's from the 1960's. It has a DF> speaker, three inputs, but only one volume control. There's a plug for DF> a foot pedal and controls for volume, tone, speed and depth. DF> Internally, there are five tubes and NO ac input transformer. It looks DF> as if someone has replaced a few parts. I can't tell whether or not the DF> circuit has been modified. The customer complaint is that it hums when DF> idling. There is a small amount of background hum. I wouldn't exactly DF> call it objectionable. The audio output is not too bad. In doing a DF> basic leakage test with my Sencore PR57, the leakage current reads DF> 5505A on one side. The other side is worse. The meter is pegged even DF> before I push the leakage current button on the probe. When I press the DF> leakage button on the probe, the speaker emits a loud hum and the meter DF> pegs all the way. DF> Are these sorts of readings to be expected on an amp like this?

DF> Thanks for your reply.

Yes, that's about normal for those pieces of crap. The problem was the skimping on the transformer in the first place. You will notice they used a very high value bias resistor in parallel with a bypass cap to connect the 1st tube's cathode or input to the powerline. There will always be a leakage current though it will be least depending which way the AC plug is oriented in the wall socket. I came up with a humbucking circuit for those and it worked great but it means modifying by adding an extra transistor. At the same time my mod reduced the leakage current by 100 fold but it did require a 200 volt transistor to have the proper compliance. The whole humbucking idea was a study exercise for myself but the wisest thing would be to find a 1:1 line isolation transformer and use that. It costs more but the piece of mind not worrying about electrocution is priceless. Why did the CSA/UL ever approve that crap to be sold?! I suppose if it allowed the All American Five then it it had to allow that too since it was basically the same. What is the final in there a 50L6 or something like that?

A*s*i*m*o*v

... New computer? But I like my vacuum tubes... They keep me warm.

Reply to
Asimov

The all-american five didn't have any circuits that could be touched by the casual user.

Reply to
Ol' Duffer

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Thanks for the detailed electrical explanation.

The tubes are physically located in a straight line. The last two tubes, closest to the audio output transformer, are 50C5, and 35W4.

I'm going to return it to the client and tell him it's not safe to operate. It's not worth the expense to upgrade it.

Thanks for your reply.

--
David Farber
David Farber's Service Center
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Reply to
David Farber

the circuit, if like the all american 5, is directly connected ot the AC line and not a problem as long as no one can get to the circuit.

The audio output transofmer can provide isolation of the output.

Does it also have an input transformer?

Mark

Reply to
Mark

"Mark" bravely wrote to "All" (06 Jul 05 19:53:55) --- on the heady topic of "Re: Melody guitar amp has leakage current."

Ma> From: "Mark" Ma> Subject: Re: Melody guitar amp has leakage current. Ma> Organization:

formatting link
Ma> Xref: aeinews sci.electronics.repair:52654

Ma> the circuit, if like the all american 5, is directly connected ot the Ma> AC line and not a problem as long as no one can get to the circuit.

Ma> The audio output transofmer can provide isolation of the output.

Ma> Does it also have an input transformer?

Mark,

There is no input transformer. THe input tube circuit provides a chassis ground to the AC powerline via a large value resistor (i.e.

270K ohms) bypassed by a medium value capacitor (i.e. 0.047uF) which results in a tiny fixed leakage. The ground point is actually midway between the rectified plate supply and the powerline neutral. This formed a voltage divider so that the user would be at 60 VAC and DC. This is the kind of guitar amp musicians used to get electrocuted with when touching a microphone or lighting up the electric bass player's smoke. It wasn't just the All American Five that connected directly to the AC power line. There were also cheap "Line Operated Phonographs" which were even more common than these guitar amps.

A*s*i*m*o*v

... That was a fascinating period of time for electronics

Reply to
Asimov

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