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Old 10th May 2013, 05:10 PM   #1
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Default Help me understand Edcor wiring?

Hey folks. I have a question or two that will reveal my ignorance quickly. Usually I leave tube wiring projects to my betters, but I am too curious not to poke around. Whatís a 400 volt shock or two?

I have a Magnavox 9302 with Edcor trannies. I'm about to change the speaker taps from 8 to 4 ohms in order to experiment with sound (my speakers are 6 ohms nominal). It has been suggested to me that, as an alternative, remove the 16 ohm taps (green wire) currently connected to the feedback resistor and replace it with the 4 Ohm taps (orange wire).

Two questions:

1. What is the purpose of having the 16 ohm taps connected to the feedback resistors?

2. If I change the speaker taps, can the green wires remain where they are?
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Old 10th May 2013, 06:48 PM   #2
DF96 is offline DF96  England
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Quote:
Originally Posted by Futureman
What’s a 400 volt shock or two?
Some people don't make it to the second shock. Be careful!

The feedback resistor needs to connect to one of the taps. Two options:
1. leave it on one tap, as you change speaker impedance to other taps.
2. put the FB resistor on the same tap as the speaker, but then change the resistor when you change the tap.
Option 1 is simpler, and easier for the non-technical user. Option 2 may be theoretically better for a technical user.
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Old 10th May 2013, 07:04 PM   #3
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Option 1 it is, then!

If I understand you, the 16 ohm tap (green wire) can stay where it is when I change speaker impedance, correct?

Here is what is motivating my question: the 6 ohm speakers are a recent acquisition, and I find that the mids and highs are slightly more muted than before (when using 8 ohm speakers). I was going to change speaker impedance, but changing the taps going to the FB was suggested as an alternative in order to get more gain. Not sure which I'll try.

I have a new preamp coming my way, so I may wait and here things things sound before exposing myself to high voltage.
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Old 11th May 2013, 02:18 PM   #4
12E1 is offline 12E1  United Kingdom
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One other tip...

Do not be tempted to change speaker connections to the different transformer taps while the amplifier is powered up, and do not run the amplifier without a load connected. If there is no load on the output transformer while the amplifier is running, it is possible for excessively high voltages to appear on the input side which can cause permanent damage to the transformer.
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Old 11th May 2013, 03:50 PM   #5
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Quote:
Originally Posted by 12E1 View Post
One other tip...

Do not be tempted to change speaker connections to the different transformer taps while the amplifier is powered up, and do not run the amplifier without a load connected. If there is no load on the output transformer while the amplifier is running, it is possible for excessively high voltages to appear on the input side which can cause permanent damage to the transformer.
Absolutely. I am not very good with or knowledgeable about tube gear, but I can discharge capacitors and avoid touching high voltages. I have recently learned that the 16 Ohm taps connected to the feedback resistor produce a cleaner sound, so I will likely leave them where they are. Still trying to decide whether to change speaker impedance.
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