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Old 25th March 2014, 11:53 PM   #1
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Default Valve heater question

Hi,
I'm build an MM phono pre-amp. It has alot of 60 hz hum.
The heaters are wired as shown in the diag. Is this part
of the noise problem?

Thanks
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Old 26th March 2014, 12:14 AM   #2
piano3 is offline piano3  United Kingdom
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I always make sure my heaters are ground referenced, otherwise the heater to cathode potential is not defined. I also probably wouldn't share the supply between rectifier and phono valves. AC is ok if your wiring is really good.
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Old 26th March 2014, 12:16 AM   #3
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Quote:
Originally Posted by brucetassin View Post
Is this part of the noise problem?
Probably "most" of your noise problem. Separate windings to heat the rectifier and DC for the signal tubes is pretty standard for a preamp AFAIK.

jeff
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Old 26th March 2014, 02:10 AM   #4
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I've used the 6CA4 before with filament voltage shared between the rectifier and other tubes as you show, with no hum, although not with a phono preamp.

You should try ground referencing or elevating the heaters as shown here:

The Valve Wizard

That may resolve your problem.

By the way, I used to live in Lawrenceville and have been to Loganville many times.

Last edited by scott17; 26th March 2014 at 02:22 AM.
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Old 26th March 2014, 03:05 AM   #5
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It is possible to have AC heating in a phono section, but the challenge is DAUNTING. The use of tubes with hum bucking, spiral wound heaters, is mandatory and, as has been mentioned, the heater supply must be biased off B+. Suitable tubes are the 6EU7 and the 7025. In current production, the Sovtek 12AX7LPS is a genuine 7025 equivalent.

Constructing a regulated 12 VDC supply for the phono section 'X7 heaters is easy enough. A "Rat Shack" 273-1366 filament trafo, a MBR20100CT twin Schottky diode, a big 25 WVDC 'lytic, and a 7812 3 terminal regulator are what you need.

BTW, regulating phono section B+ is a good idea. The little TO92 case LR8 is great for the job. Use 1 LR8 in each channel.
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Old 26th March 2014, 12:03 PM   #6
painted is offline painted  Luxembourg
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Another reason for hum might arrive from the grounding regime used.
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Old 26th March 2014, 12:21 PM   #7
DF96 is online now DF96  England
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If the valves share a heater winding then the rectifier must be first on the chain, not last as you show it. This reduces the AC current around the first stage. Use thick wire to the rectifier, then thinner wire from there so you can twist it more tightly.

You must have a DC reference for the heaters; what voltage it is is less important than the fact that it exists.

Show us a picture. AC heater wiring is one of the few areas of audio where a picture tells us more than a circuit diagram.
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Old 26th March 2014, 02:55 PM   #8
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Thanks all for your most valuable help.
I can see that I'm going to have to
rethink my approach to this project.
Bruce
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Old 26th March 2014, 04:47 PM   #9
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For phono I prefer dc heaters raised about 45volt above ground . This works based on RCA, and others data from the 1930s . At the time it was ac and raised voltage. I have seen an unused bias tap as a reference voltage be very quite . Care must be taken with heaters since the signal voltage is so vey small .
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Old 26th March 2014, 05:15 PM   #10
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Make a voltage doupler circuit from 6,3 VAC to 6,3 VDC with suitable filtering.
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