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Old 22nd June 2008, 07:51 PM   #1
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Default 100mA heater tubes for phono

OK, here is an obscure one for you. I am planning a 6EM7 based integrated amp with phono. I wanted to try the old trick of putting the phono tube heaters in the output tube cathode circuit but the likely operating point for the output tubes is 50mA each = 100mA total. My initial choice of tube was the 12AY7 but it requires 150mA.

I had thought to use a bleeder resistor to supply the remaining current from B+ but that would be a whopper of a resistor and would be pushing the current limits of my transformer.

Well in flash of inspiration I decided to turn it around and see if there were appropriate phono tubes that required either 100 or 50mA heater current. Using Duncan's search capability I came up with a list of suspects which I have narrowed down (eliminated things like remote cutoff pentodes and power tubes etc.) to the following possibilities.

18FY6 Triode Dbl Diode u=100 $4.95
18GE6 Triode Dbl Diode u=70 $3.00
18GD6 Pentode $9.95

All have enough gain to be considered a possibility. Is anyone familiar enough with these tubes (or other possibilities that I missed) to opine as to their suitability for phono preamp service. Factors such as noise and microphonics are of course important.

mike
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Old 22nd June 2008, 10:11 PM   #2
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Mike,

Aside from the fact that the "Cheap Charlie" method of obtaining heater DC STINKS, you are overlooking the fact that 4 triodes are needed to complete a stereo phono section.

Building a 300 mA. regulated 12 VDC supply that supports a pair of 12AX7s is easy. Voltage double, using Schottky diodes, the O/P of a 2 A./6.3 VAC winding. Follow with a 7812 3 terminal regulator.
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Old 22nd June 2008, 10:26 PM   #3
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Thanks Eli;

I hadn't forgotten that four triodes were needed but I have plenty of real estate on my chassis and I need to drop 150V on the cathodes of the output tubes anyway.

I would be interested in what stinks about that approach. Being SE the DC would be pretty clean and the audio signal would be smaller than the AC on a normal heater supply wouldn't it? Heaters would be at higher potential than cathode.

Not being argumentative, really want to take advantage of your insight.
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Old 23rd June 2008, 12:32 AM   #4
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20EZ7: 100 mA equivalent of 12AX7 or 6EU7. Not plentiful, but cheap when you can find 'em... UCC85 is even harder to find - think of it as a 26AT7.

I'd use separate cathode resistors for the rest of the bias, as it will be self-adjusting to a degree. Common cathode resistors need matched tubes.
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Old 25th June 2008, 09:54 PM   #5
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Thanks for the information. I think I may build the amp without phono for now using big power resistors to drop the 150V at the cathodes. I can then build the phono amp separately and/or add it later if I want to experiment with that. This gives flexibility and allows me to get the most important stuff going quicker.

mike
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