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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Tue 13, 2018 9:22 pm 
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Location: Gainesville, Florida
check ripple across the filter with an AC meter ? more than 5VAC may be too much


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Wed 14, 2018 11:45 pm 
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Joined: Nov Fri 23, 2007 6:42 pm
Posts: 201
Location: Noblesville, IN 46060
Tom - This is an AC/DC set, pulling a tube kills the set.

tubeAMP - good call, there is nearly 11 VAC across the first filter

I really think it's just a poor design. The hum is at the plates of the output tubes which are feed directly from the rectifier with a 33 uF cap. A 47 uf cap makes little difference in the level of hum.


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Thu 15, 2018 1:12 am 
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What sort of rectifier? Tube, metal...... One hates to modify, but it may need to happen, or an alternate DC supply used, is the AC PSU wired correctly?

Marc


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Thu 15, 2018 2:21 am 
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Nick Rhode wrote:
Tom - This is an AC/DC set, pulling a tube kills the set.

tubeAMP - good call, there is nearly 11 VAC across the first filter

I really think it's just a poor design. The hum is at the plates of the output tubes which are feed directly from the rectifier with a 33 uF cap. A 47 uf cap makes little difference in the level of hum.

For a 6 volt tube in a series string, you can just put a jumper across the heater if you want to remove the tube.

It is not the ripple at the first filter that I was thinking about. It is the ripple at the second filter cap that would be important here. The choke will provide some filtering even if the cap after the choke is open. But a bad second cap would leave too much ripple for the audio circuits.

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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Thu 15, 2018 4:11 am 
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My last post got me to do more thinking. The filter choke could be bad. A shorted turn inside the choke could keep it from acting like a choke. That would cause too much ripple at the second filter cap.

In any case, the ripple at the cap after the choke needs to be almost zero.

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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Thu 15, 2018 8:40 pm 
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Location: Gainesville, Florida
you may want to try jumping another filter capacitor across the one with 11VAC and notice any changes


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Fri 16, 2018 1:31 pm 
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Joined: Nov Fri 23, 2007 6:42 pm
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Location: Noblesville, IN 46060
Thanks for all the suggestions. I tried them all and here's the report:

The ripple at the second filter is only .1 VAC. The cap is new but I also tried a new substitute. The first filter cap is good and I've also swapped it with other new filter caps of various values.

The filter choke measures nearly 400 ohms, the schematic calls for a 200 ohm choke. The choke appears original but could have been replaced. The only extra choke I have is 50 ohms. I tried it and still have the same level of hum.

Taking the 6J5 out of the circuit does not effect the hum, still there. The rectifier is a glass tube.

Maybe I should eliminate the filter choke and use a voltage divider with an extra stage of filtering.


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Fri 16, 2018 8:50 pm 
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I just grabbed a copy of the schematic so that I can see exactly what the circuit looks like.

You could keep the choke and just add a resistor and cap to make a third stage of filtering. But the problem might not be power supply ripple

It looks like the power supply ripple should show up on both output tube grids, in which case it should cancel out. If you can, check for AC on the plate of the 6J5 and see if it is different from that .1 volts AC that you measured on the B+. I am wondering if there could be some coupling from the AC line or some of the filament wiring into the 6J5 plate or the output tube grid. Could the coupling cap be very close to some of the filament wiring? Could there be some solder flux that has become conductive bridging either the plate of the 6J5 or the grid of the output tube to some of the filament wiring?

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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sat 17, 2018 4:54 pm 
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Joined: Nov Fri 23, 2007 6:42 pm
Posts: 201
Location: Noblesville, IN 46060
Tom,

You were right in your suspicion that a third filter stage wouldn't help. It makes no difference.

I can't find any evidence of coupling with the AC line or filament leads anywhere in the set or unintended connections between the filaments and grids. The AC at the 6J5 plate is roughly the same as across the 2nd filter. I swapped the 6J5G for a metal 6J5 and that reduced the hum to the point that it is not objectionable when tuned to a station. It's still louder than normal when not tuned to a station but I'm convinced it's working as designed.

I think I am at the end of the road with this radio and thank all of you who helped along the way.

Cheers,

Nick


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 Post subject: Re: Allied E-10807 filter hum head scratcher
PostPosted: Feb Sat 17, 2018 6:29 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4790
Location: Gainesville, Florida
tube shields of course. oscillation, diminished volume. typical


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