I'm restoring this Emerson DP-332 that uses a dynamic coupled output amp. A 6AE5 driving a 25ac5:
http://www.nostalgiaair.org/PagesByModel/039/M0005039.pdf
The 25AC5 requires a plate load resistance/impedance of 10,000 ohms ....using a standard arrangement of a 50:1 audio output transformer feeding a 4 ohm PM speaker.
The radio has a nice large speaker & audio transformer that is mounted right into the cabinet and a cable connected by a 4 pin plug to the audio tube in the chassis.
I had removed the chassis to have easy access for re-capping leaving the speaker/transformer in the cabinet.
So I needed another PM speaker to use while testing.
So I grabbed a spare 4 ohm PM speaker to use while testing and I connected it to the output tube and turned the radio on again for evaluation.
It seemed to play quite nicely ...but it had rather low volume. Played for a 1/2hour or so that way as I started recapping and measuring voltages etc. ... I couldn't figure out why the vol was so low.. until it hit me... lol
That's when I suddenly realized that...
The boner I pulled was ...was that I had connected the 4 ohm voice coil DIRECTLY to the output tube plate and B+ WITHOUT an audio output transfomer!!
Just a 4 ohm ( voice coil) as a plate load instead of the 10,000 ohms as expected!!
So why did it work so well with no disastrous side effects and only have a low volume??... lol
Why didn't it draw too much current and either blow the voice coil or damage the output tube?
Why did this work as well as it did?
Would it have worked the same way with a 50L6 or such?
As you found using a speaker directly without output transformer will give sound but low volume. I have a speaker with switch I use for testing. The switch connects wires through an output transformer or directly to the speaker. Sometimes I have it in the wrong position and still hear radio.
The tube won't draw too much current as there is another spec., plate resistance. Tube plate resistance limits current.
This will work with almost any output tube. Don't expect you would hear much from 3Q5's or other battery type tubes.
Norm
:I made an interesting error that I'm curious about.
:
:I'm restoring this Emerson DP-332 that uses a dynamic coupled output amp. A 6AE5 driving a 25ac5:
:http://www.nostalgiaair.org/PagesByModel/039/M0005039.pdf
:
:The 25AC5 requires a plate load resistance/impedance of 10,000 ohms ....using a standard arrangement of a 50:1 audio output transformer feeding a 4 ohm PM speaker.
:
:The radio has a nice large speaker & audio transformer that is mounted right into the cabinet and a cable connected by a 4 pin plug to the audio tube in the chassis.
:
:I had removed the chassis to have easy access for re-capping leaving the speaker/transformer in the cabinet.
:
:So I needed another PM speaker to use while testing.
:
:So I grabbed a spare 4 ohm PM speaker to use while testing and I connected it to the output tube and turned the radio on again for evaluation.
:
:It seemed to play quite nicely ...but it had rather low volume. Played for a 1/2hour or so that way as I started recapping and measuring voltages etc. ... I couldn't figure out why the vol was so low.. until it hit me... lol
:
: That's when I suddenly realized that...
:
:The boner I pulled was ...was that I had connected the 4 ohm voice coil DIRECTLY to the output tube plate and B+ WITHOUT an audio output transfomer!!
:
:Just a 4 ohm ( voice coil) as a plate load instead of the 10,000 ohms as expected!!
:
:So why did it work so well with no disastrous side effects and only have a low volume??... lol
:
:Why didn't it draw too much current and either blow the voice coil or damage the output tube?
:
:Why did this work as well as it did?
:
:Would it have worked the same way with a 50L6 or such?
:
:
:
:
:
:
:
:
:
: As you found using a speaker directly without output transformer will give sound but low volume. I have a speaker with switch I use for testing. The switch connects wires through an output transformer or directly to the speaker. Sometimes I have it in the wrong position and still hear radio.
:
: The tube won't draw too much current as there is another spec., plate resistance. Tube plate resistance limits current.
:
: This will work with almost any output tube. Don't expect you would hear much from 3Q5's or other battery type tubes.
:
:Norm
:
::I made an interesting error that I'm curious about.
::
::I'm restoring this Emerson DP-332 that uses a dynamic coupled output amp. A 6AE5 driving a 25ac5:
::http://www.nostalgiaair.org/PagesByModel/039/M0005039.pdf
::
::The 25AC5 requires a plate load resistance/impedance of 10,000 ohms ....using a standard arrangement of a 50:1 audio output transformer feeding a 4 ohm PM speaker.
::
::The radio has a nice large speaker & audio transformer that is mounted right into the cabinet and a cable connected by a 4 pin plug to the audio tube in the chassis.
::
::I had removed the chassis to have easy access for re-capping leaving the speaker/transformer in the cabinet.
::
::So I needed another PM speaker to use while testing.
::
::So I grabbed a spare 4 ohm PM speaker to use while testing and I connected it to the output tube and turned the radio on again for evaluation.
::
::It seemed to play quite nicely ...but it had rather low volume. Played for a 1/2hour or so that way as I started recapping and measuring voltages etc. ... I couldn't figure out why the vol was so low.. until it hit me... lol
::
:: That's when I suddenly realized that...
::
::The boner I pulled was ...was that I had connected the 4 ohm voice coil DIRECTLY to the output tube plate and B+ WITHOUT an audio output transfomer!!
::
::Just a 4 ohm ( voice coil) as a plate load instead of the 10,000 ohms as expected!!
::
::So why did it work so well with no disastrous side effects and only have a low volume??... lol
::
::Why didn't it draw too much current and either blow the voice coil or damage the output tube?
::
::Why did this work as well as it did?
::
::Would it have worked the same way with a 50L6 or such?
::
RCW
An output transformer is just a matching device, when you match the output Z of the tube to the input Z of the speaker you get the maximum transfer of power.
Not using a transformer, or using the wrong transformer, just results in low volumn, no damage to the avarage amplifier will occur. (In high power amps this will not be neccessarly true)
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ok... So I guess I had always kind-of been thinking of the output transformer ..sort-of.. like the "pull-up" current limiting resistor that is commonly used on the collector of a transistor. ( employed such that if the transistor is ever "turned on" hard ( zero ohms from collector to base) it would short to B- and that collector resistor would act to limit that base-to-collector current)
And so I guess I always thought (wrongly) that the transformer acted as a current limiting resistor of sorts...
But Norm has reminded me now of the fact that the tube has an inherent "plate resistance" that would act to limit the current when the tube is saturated.
...so can see now, as I think about more, that the transformer only has "reactance"/resistance to the audio frequencies but no "resistance" per se at all to the DC B+.