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Old 6th June 2009, 10:50 PM   #1
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Default Opinions gratefully recieved.

I have added a 12AU7 front end to my MOSFET amp design.

Becasue it needs at least 100v to work properly I have used plus and minus 60 volts to power the valve.

This means the valve zero volts reference becomes -60 volts with respect to the rest of the circuit. Will this cause hum ?

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Old 6th June 2009, 11:39 PM   #2
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I have cracked it.
I have just decoupled to ground on +60v and -60v to make the circuit relative to ground.
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Old 7th June 2009, 01:29 AM   #3
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Make sure you are not exceeding the voltage heater-cathode spec on your tube. Not all tubes of the same "designation" are the same in this regard, and sometimes the spec is different for + and -. The split supply will make filtering far more critical, since you have to filter what the tube calls ground as well as the B+.
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Old 7th June 2009, 01:43 AM   #4
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Default I could be a bit thick, but

I don't get the topology of your 12au7. Whats going on here?
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Old 7th June 2009, 01:54 AM   #5
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The heater is 60 volts above the cathode.
According to the spec the cathode is indirectly heated.

Having said that its pretty easy to change it to make the heater around the same volts as the cathode.

The circuit has a constant current source for an common cathode amplifier. That seems to be quite a common theme. I am told it gives a lower impedance than a simple resistor.
It was commonly used in TV circuits.
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Old 7th June 2009, 02:16 AM   #6
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The RCA specs for 6SN7 heater-cathode voltage is +/- 200V peak, +/- 100V average (continuous). So you are quite safe.

The CCS (constant current source) actually gives a higher impedance that a resistor. Main thing is that it provides a more linear operation, as with a resistor the current through it decreases as the voltage drop becomes smaller. With a CCS the current remains more constant. (A low tube like the 6SN7 is not very constant CS even with the cathode resistor feedback, but quite better than a resistor.)
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Old 7th June 2009, 02:29 AM   #7
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My current circuit diagram with heater supply close to -60v.
Also shows decoupling from +60 volts and -60 volts.

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Old 7th June 2009, 05:03 AM   #8
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Default ...mmmm thought it was a ccs...

but couldn't for the life of me see the point of doing one with an 'au7 and at that low a voltage. Am I missing something?
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Old 7th June 2009, 05:22 AM   #9
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Default sorry - just realised my post reads a little arrogant!

I'm a relative newbie so I'm looking to clarify my understanding here too - no disrespect intended at all!
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Old 7th June 2009, 12:57 PM   #10
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Default Re: ...mmmm thought it was a ccs...

Quote:
Originally posted by aardvarkash10
but couldn't for the life of me see the point of doing one with an 'au7 and at that low a voltage. Am I missing something?

The reason +/- 60 volts is used is because that is what is already available.

The data sheet on the 12AU7 says you can go down to 100 volts B+.

120 volts is way more than what is needed to get 16 volts AC out of the 12AU7.

The 12AU7 doesnt need to have a lot of gain anyway as the MOSFET part can have as much as I want.
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