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 Post subject: 1926 Metrodyne Single Dial schematic.
PostPosted: Sep Tue 15, 2020 2:08 am 
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Location: Kokomo, Indiana 46902
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I found one in a news bulletin from a Historical Radio Society.


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 Post subject: Re: 1926 Metrodyne Single Dial schematic.
PostPosted: Sep Tue 15, 2020 10:05 pm 
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Location: Keystone Heights, FL, USA 32656
Missing a line from “speaker “ to plate of bottom 2nd AF plate.

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 Post subject: Re: 1926 Metrodyne Single Dial schematic.
PostPosted: Sep Wed 16, 2020 12:43 am 
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Where’s B-???


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 Post subject: Re: 1926 Metrodyne Single Dial schematic.
PostPosted: Sep Wed 16, 2020 1:03 am 
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Groundhog74 wrote:
Where’s B-???

B- is connected to C+/A-/GND

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 Post subject: Re: 1926 Metrodyne Single Dial schematic.
PostPosted: Sep Wed 16, 2020 1:05 am 
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Missing B+90V.

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 Post subject: Re: 1926 Metrodyne Single Dial schematic.
PostPosted: Sep Wed 16, 2020 1:05 am 
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Missing B+90V.

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 Post subject: Re: 1926 Metrodyne Single Dial schematic.
PostPosted: Sep Wed 16, 2020 1:21 am 
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The radio has clearly marked connections for A+, A-, C+/GND, C-, B+45, B+135 and AERIAL.
No other connections available.


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 Post subject: Re: 1926 Metrodyne Single Dial schematic.
PostPosted: Sep Wed 16, 2020 2:44 am 
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A C- of 9 volts will keep the RF stages in good order with the 135 volts...

The C+/A- is the connection for B-

The schematic although cleanly drawn has at least three errors, the one mentioned in the text, the one found by Jeff in the plate of the lower last audio tube and the lack of identification of the B-.

I strongly suggest following line by line the circuit in the radio and append the schematic as need.

FWIW the parallel last audio may have too much plate current for "some" speakers, consider either an output transformer or an output choke and a coupling cap... Chas

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 Post subject: Re: 1926 Metrodyne Single Dial schematic.
PostPosted: Sep Wed 16, 2020 4:02 pm 
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I have a Freed-Eisemann NR7 that has the parallel 2nd audio section like this. Using 2 01A tubes it works fine with a Peerless speaker.

FE says if using 01A tubes use both sockets; if using a '71 tube, one tube is sufficient in the 2nd audio.

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 Post subject: Re: 1926 Metrodyne Single Dial schematic.
PostPosted: Sep Wed 16, 2020 4:55 pm 
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jrehkopf wrote:
I have a Freed-Eisemann NR7 that has the parallel 2nd audio section like this. Using 2 01A tubes it works fine with a Peerless speaker.
FE says if using 01A tubes use both sockets; if using a '71 tube, one tube is sufficient in the 2nd audio.
Thanks Jeff. I do not have a battery set with a parallel output, I could only speculate that there could be excessive current. I suppose that could depend on the speaker too? chas

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 Post subject: Re: 1926 Metrodyne Single Dial schematic.
PostPosted: Sep Wed 16, 2020 11:12 pm 
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Joined: May Sat 30, 2015 11:49 pm
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Location: Keystone Heights, FL, USA 32656
I am sure it would depend on the speaker. By this time (1926) they would be pushing the magnetic cone-type speakers, not horns, I would think.

I am suspicious of the schematic around the 3rd RF tuner and grid voltage at A-. Seems if the 1st and 2nd RF grids are at C- that the third RF should be, too. Never seen the RF tubes run at 135 volts before.

Maybe check by hooking up a 9 volt battery to C- and testing for -9 volts at the grids of all tubes except the detector.

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 Post subject: Re: 1926 Metrodyne Single Dial schematic.
PostPosted: Sep Thu 17, 2020 11:06 pm 
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Location: Kokomo, Indiana 46902
Subbed in two audios due to both the ones in the set being bad. they are replacements anyway.
Works very well using hook up as on the back A+ 6V, B+ 45 and 135, C+ 4.5. B- going to A- or GND C+, seemed to make no difference either way.


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 Post subject: Re: 1926 Metrodyne Single Dial schematic.
PostPosted: Sep Thu 17, 2020 11:41 pm 
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Running the C backwards will increase the plate current and has the potential to shorten the useful life of the tubes subjected to positive bias. You may not hear a difference, it should be distortion but their your tubes will know. Try a measurement of the total plate current in the B+135...:shock: Chas

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 Post subject: Re: 1926 Metrodyne Single Dial schematic.
PostPosted: Sep Sun 27, 2020 11:33 pm 
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Joined: May Sat 30, 2015 11:49 pm
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Location: Keystone Heights, FL, USA 32656
Groundhog74 wrote:
Subbed in two audios due to both the ones in the set being bad. they are replacements anyway.
Works very well using hook up as on the back A+ 6V, B+ 45 and 135, C+ 4.5. B- going to A- or GND C+, seemed to make no difference either way.

Did you mean C- 4.5?

If you look at the schematic, A-and C+/gnd are connected, so B-to either one would make no difference.

Can you please measure between C+ and each of the RF tube grids and report results?

You might try C- at 9 volts to note any difference.

Thanks.

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