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Old 9th April 2009, 07:11 PM   #11
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The mosfet is used to dynamically adjust the plate voltage in step with the audio. The zener diodes on the gate of the fet set the plate to cathode voltage across the output tube. The voltage is chosen based on the characteristics of the output tube. I set the voltage to a point where the output tube can pass the needed peak current with good linearity. This is fairly low with the right tube. A 6LW6 can crank out 1 AMP of current with 100 volts across it. The 6336A and the 6AS7 can run at 100 volts at lower current. Keeping the voltage constant across the output tube greatly lowers the distortion. Keeping the voltage low reduces the dissipation, although it just moves it to the mosfet. In this design most of the voltage is dropped across the mosfet (about 300 volts) which results in the need for a big heat sink. A smaller amp would have more reasonable dissipation requirements. You don't need to use a mosfet either, the upper element in my augmented cathode follower amp was the other half of the 6336A tube. The schematic was somewhere in the included links.

The mosfet can be replaced by a digitally controlled SMPS to eliminate the dissipation in the upper element. I built one of those too.
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Old 9th April 2009, 07:36 PM   #12
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There is something squirrelly about that schematic...
Looks like the "output Triodes" have been dumbed
down to mere diodes, and the MOSFETs are doing
all the source follower work.

It is kinda blurry, I mighta misread something...
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Old 9th April 2009, 08:14 PM   #13
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What is cathode, emitter, - follower?
A diode linearized by feedback by voltage. In both cases.
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Old 9th April 2009, 08:41 PM   #14
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Its all a question of how you look at things. The triode is still doing its triode thing (as much as a cathode follower is doing a triode thing) just in a more linear manor. I suppose the proof would be, if you substituted a diode would it behave the same ? Anyone care to simulate.

Tubelabs - what do you feel would be the minimum the Mosfet would need to be happy. Would it be the 120V of anticipated drive (plus a bit), or could it pull it off with just a few tens of volts. The whole design is inefficient enough as is, without loading things down with more inefficiency.

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Old 9th April 2009, 08:56 PM   #15
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Quote:
Originally posted by Shoog
Its all a question of how you look at things. The triode is still doing its triode thing (as much as a cathode follower is doing a triode thing) just in a more linear manor. I suppose the proof would be, if you substituted a diode would it behave the same ? Anyone care to simulate.
A triode is a controlled by a grid voltage diode.
It is useful to remember when thinking of curves and distortions. Particularly, neither emitter follower nor cathode follower is a magical device with zero distortions and zero output resistance; it is a linearized by feedback diode (either solid state diode, or a vacuum one).
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Old 9th April 2009, 10:01 PM   #16
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Had you considered a cathode-follower driver, direct-coupled to the output tubes?

Your bias would then be set up in the grid circuit of the driver tube, and your voltage amplifier would have an easier time with the input grid capacitance.

The amp should be good for about 10 watts total, depending on the plate voltage.

You're going to need more than a simple gain stage to make the voltage swing. Either you will need a very high power supply voltage, two stages of gain, or a high-gain voltage amp, either pentode or cascode.

I favor the cascode, if set up right they can be really good performers.
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Old 9th April 2009, 10:20 PM   #17
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Quote:
Your bias would then be set up in the grid circuit of the driver tube, and your voltage amplifier would have an easier time with the input grid capacitance.
I don't really want to go direct coupled, and I don't think the grids of the 6080 will be that difficult a load.



Quote:
You're going to need more than a simple gain stage to make the voltage swing. Either you will need a very high power supply voltage, two stages of gain, or a high-gain voltage amp, either pentode or cascode.
Yes i was scratching my head on that one. A CCS loaded LTP using ECC81 might just hack it - but isn't really very appealing. I have had success with using a 6AU6 LTP in my current Tabor clone - with a gain of about 60x it should do the trick with a 2v input. This can be achieved with a plate current of about 5mA and leaves plenty of input headroom. Of course the secret is to drive both the inputs - which requires an input transformer or balanced drive. Simple transistor screen regulation should keep things singing.

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Old 9th April 2009, 10:39 PM   #18
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The issue that you have if you don't use a direct-coupled driver is that the grids of the 6080 will need some fairly low values in order for the bias divider network to maintain control. This will be in direct conflict with the size of the coupling caps- the bigger you make them, the less transparent the amp.

Using the direct-coupled approach has no downside against that, however to do it right you will need a bi-polar power supply. That would not be hard to set up though, and would also give you good options should you want to do a CCS and LTP voltage amplifier. The advantage is the amp will have instantaneous overload recovery and you can keep the coupling caps a lot smaller. That makes them cheaper and more transparent at the same time.

I've found that a good 2-stage CCS (all-tube) is the key to getting good performance in the LTP.
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Old 10th April 2009, 12:15 PM   #19
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Quote:
Originally posted by Shoog



Yes i was scratching my head on that one. A CCS loaded LTP using ECC81 might just hack it - but isn't really very appealing. I have had success with using a 6AU6 LTP in my current Tabor clone - with a gain of about 60x it should do the trick with a 2v input. This can be achieved with a plate current of about 5mA and leaves plenty of input headroom. Of course the secret is to drive both the inputs - which requires an input transformer or balanced drive. Simple transistor screen regulation should keep things singing.

Shoog

hey Shoog,
I have a pair of amps running LTP pentodes, and a pair running a triode/MOSFET cascode( MOSFET on 'top'). I have some issues with the screen current; the cascode does not have this 'feature'. Finding a pentode with reasonable g2/plate current seems to be the trick here. I am using the 6AC7 instead of the 6AU6. Both implementations have the same sort of horizontal plate curves( and spacing )...and the g2 current seems to work out OK...
cheers,
Douglas
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Old 10th April 2009, 03:28 PM   #20
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Quote:
hey Shoog, I have a pair of amps running LTP pentodes, and a pair running a triode/MOSFET cascode( MOSFET on 'top'). I have some issues with the screen current; the cascode does not have this 'feature'. Finding a pentode with reasonable g2/plate current seems to be the trick here. I am using the 6AC7 instead of the 6AU6. Both implementations have the same sort of horizontal plate curves( and spacing )...and the g2 current seems to work out OK... cheers, Douglas
The 6AU6 and the 6AC7 look very similar, though the 6AU6 looks to have slightly more even line spacing. Essentially they look almost like drop ins for each other.
In my current amp I have the screens referenced to a VR150 which works well, but introduces a bit of cross talk between channels. If I could be bothered I would add a small transistor buffer or an extra VR150 for the other channel.
I was think of using a simple transistor referenced to a trimable resistive voltage divider to set the screens on this one. If I pass enough standing current through the resistor chain it should present a nearly constant voltage. How are you dealing with your screens??

Shoog
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