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Old 28th May 2012, 11:17 PM   #1
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Default stupid question about heater wire to ground

On my Wurlitzer 6420's the one heater wire goes to ground. I assume it was a common practice in the 40's when it came out. Maybe so maybe not. Anyway, I am adding a 12.6 volt transformer to run some different tubes in my new front end stage. The transformer I am adding will only run the VA and the phase splitter tubes.

Would it be a good idea to re- wire the 6.3 heater circuit to the output tubes so that one side of the transformer isn't grounded or does it matter?
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Old 28th May 2012, 11:23 PM   #2
7N7 is offline 7N7  United Kingdom
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Originally Posted by Original Burnedfingers View Post
On my Wurlitzer 6420's the one heater wire goes to ground. I assume it was a common practice in the 40's when it came out. Maybe so maybe not. Anyway, I am adding a 12.6 volt transformer to run some different tubes in my new front end stage. The transformer I am adding will only run the VA and the phase splitter tubes.

Would it be a good idea to re- wire the 6.3 heater circuit to the output tubes so that one side of the transformer isn't grounded or does it matter?
Correct procedure is to use (assuming AC operation) a centre-tapped transformer with the centre tap going to 0V or better still being returned to a DC voltage of say 25V. If you do not have a centre=tapped transformer, then you can derive a centre tap using a couple of small resistors connected across the AC output with their join going to 0V - or to a DC supply. This DC potential runs no current of course, it just serves to turn off the "diode" that exists in the valve between the heater and the cathode,

Paul
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Old 28th May 2012, 11:33 PM   #3
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Ok,

The 6.3 has no center tap so I will make a CT connection and take it to ground. Running no DC on it as it is running to the 6L6GC's or what ever I throw into it for output tubes. Running a 1634 as VA with a 1634 long tail phase splitter. Haven't finished putting the amp all together yet but I am close.

Why did they ground one side of the 6.3 ? Easier to make?
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Old 28th May 2012, 11:37 PM   #4
7N7 is offline 7N7  United Kingdom
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Originally Posted by Original Burnedfingers View Post
Ok,

The 6.3 has no center tap so I will make a CT connection and take it to ground. Running no DC on it as it is running to the 6L6GC's or what ever I throw into it for output tubes. Running a 1634 as VA with a 1634 long tail phase splitter. Haven't finished putting the amp all together yet but I am close.

Why did they ground one side of the 6.3 ? Easier to make?
The DC would make no difference to your output valves; RCA recommended this technique for reducing noise originating in the heater circuitry. I used this in all my amplifiers.

As for why they did it, I have seen the technique used in cheaper amplifiers - It saved running an extra wire ) and therefore production costs) and the hell with the noise!
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Old 29th May 2012, 10:14 AM   #5
DF96 is offline DF96  England
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Early equipment, especially radios, just used one heater wire with the chassis used as the return. Then people started worrying about hum, so to reduce magnetic induction they used a twisted pair of heater wires but with one side grounded. Then they thought about electrostatic induction (i.e. capacitance) so they used voltages balanced about earth, using either a secondary CT or a pair of resistors. Then they used a humdinger pot to get a better balance. Then they raised the heater DC reference above ground. Then they used DC heaters.

Heaters need a DC reference. In many cases ground is good enough. If you do it some other way then you have to ensure that the AC impedance is low enough, otherwise you might reduce hum but increase noise. Grounding one side of the heater is cheaper than grounding the CT, and is often good enough.
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Old 29th May 2012, 10:40 AM   #6
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Well the original front end has been replaced with two tubes both 1634'2. Why? I have 50 of them and because basically they are a 6sl7 with the cathodes tied together which is what I do when I use a long tail 6SL7 phase splitter. The voltage amp stage I paralled up. They require a 12.6v source for heater voltage so I need to add a transformer to the circuit. Thus the stupid heater questions. So do I add 1/4 of the 1634's B+ supply to the heaters for those two tubes?

Last edited by Original Burnedfingers; 29th May 2012 at 10:43 AM.
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