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Old 19th July 2012, 11:58 PM   #1
slor is offline slor  United States
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Default Switchable Amp Output?

I'm working on a fairly cheap old Silvertone chord organ. The amp uses 2 x 12AX7 (one for vibrato) and a single 6AQ5A output tube.

Here's my question: I'd like to enable the amp to feed either a speaker (there's currently one installed but I'm removing it for space and other considerations) OR provide a line level signal out to a DI or what have you for live work. But not both at the same time.

Can I install a "make circuit" output jack so that the output transformer is fed the signal from the output tube ONLY when a jack is inserted? In other words, is lifting one leg of the output primary enough protection from damaging the output transformer?

Many thanks!
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Old 20th July 2012, 08:51 AM   #2
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by slor View Post
I'm working on a fairly cheap old Silvertone chord organ. The amp uses 2 x 12AX7 (one for vibrato) and a single 6AQ5A output tube.

Here's my question: I'd like to enable the amp to feed either a speaker (there's currently one installed but I'm removing it for space and other considerations) OR provide a line level signal out to a DI or what have you for live work. But not both at the same time.

Can I install a "make circuit" output jack so that the output transformer is fed the signal from the output tube ONLY when a jack is inserted? In other words, is lifting one leg of the output primary enough protection from damaging the output transformer?

Many thanks!
It depends what you want to achieve..

The sound of the organ through the tube amp is different if you take a preamp signal or the op from the OPtx...

You could just replace the speaker with dummy load and then put a volume control across it take that to your stage PA. You could isolate the connection so you have the dummy load across the Tx ...then a capacitor to the volume control.. track across the op from capacitor to the dummy load wiper out to signal. It helps if we could see a circuit..others would probably comment. Its worth a lash up and test see if you get any instability due to to any feedback connections on the op tx.
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Or disconnect the HT from the OP Tx that will shut down the OP stage. However Switching the HT is one thing doing it safely is another..

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M. Gregg
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Old 20th July 2012, 09:03 AM   #3
M Gregg is offline M Gregg  United Kingdom
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I would also say,

Take care where you take any signal out from..most coupling capacitors in the amp could have B+ on one side...

Thats why a circuit would be better than just ..guess work.

Of course you could just Mike up the organ..

Does it have a HP Jack?


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M. Gregg
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Last edited by M Gregg; 20th July 2012 at 09:07 AM.
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Old 20th July 2012, 10:18 AM   #4
DF96 is offline DF96  England
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Use a jack which switches to an internal resistor dummy load when nothing plugged in. This will keep the output transformer safe. Alternatively, use a Zobel/snubber network to damp any spikes. Then use a DI box which can provide a proper load and can handle a few watts input.
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Old 20th July 2012, 12:37 PM   #5
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In other words, is lifting one leg of the output primary enough protection from damaging the output transformer?
Lifting one leg of the OPT will protect it sinc there is no current flow. This however will usually burn up the output tube since there is screen voltage without plate voltage. This causes the screen to eat all of the electrons emitted by the cathode making it glow.
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Old 20th July 2012, 03:26 PM   #6
slor is offline slor  United States
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Thanks so much for the thoughts...after sleeping on it I woke up this morning and came to the same conclusion as DF96. Just make the jack switch between a dummy load and the output!

In addition, I believe I will wire up a "line out" somewhere earlier than the output tube.

Mic'ing the organ would be okay in the studio, but not live. Too much bleed, too many harried soundpeople!
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Old 20th July 2012, 06:29 PM   #7
12E1 is offline 12E1  United Kingdom
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Originally Posted by slor View Post
Thanks so much for the thoughts...after sleeping on it I woke up this morning and came to the same conclusion as DF96. Just make the jack switch between a dummy load and the output!
In general I'd advise not to switch the output when the unit is running. Leaving an open circuit on the output side while switching loads, even for a brief fraction of a second can damage the output transformer on the primary side if there is any signal being driven at the time. The safer way would be to switch the dummy load into the circuit before you disconnect the speaker (going one way) and to connect the speaker before disconnecting the dummy load (going the other).
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Old 20th July 2012, 06:53 PM   #8
DF96 is offline DF96  England
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Most switchable jacks will ensure make (external load) before break (internal load), but safest to switch off first.
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Old 21st July 2012, 05:52 AM   #9
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I advise not to use a switching circuit at all. You resistor load the output in parallel to the speaker jack. that away it doesn't matter if the speaker is connected.

then derive a balance connection off of feedback network or resistive load. then attenuate for proper signal level.
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