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Old 9th August 2012, 06:56 PM   #1
hkoetz is offline hkoetz  Netherlands
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Default EF86 + 4*EL84PSE

Can I use an EF86 in pethode mode to drive 4 EL84 PSE. I think I need the penthode gain, as I plan to use cathode feedback from the OPT secondary.
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Old 9th August 2012, 07:31 PM   #2
DF96 is offline DF96  England
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You have not given anywhere near enough information for your question to be answered. Show us your draft circuit, and expected signal levels etc.
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Old 10th August 2012, 12:31 AM   #3
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short answer is "possibly"

Obvious question is "why feedback from the OPT when you have all the elements in place for a Schade setup?"
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Old 10th August 2012, 06:20 AM   #4
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EF86 is a not a very good candidate as Schade driver. More Gm and current is a must. A depletion MOSFET will also work better than most pentodes. But the basic Schade idea is good.

Cathode feedback might work with a triodestrapped EF86, triodestrapped EL84s and something like a 1.3k:8ohm OPT as the feedback will be very, very low.

But as DF96 says more data is needed.

Last edited by revintage; 10th August 2012 at 06:31 AM.
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Old 10th August 2012, 06:37 AM   #5
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Quote:
Originally Posted by hkoetz View Post
...I plan to use cathode feedback from the OPT secondary.
Does this mean that the OPT secondary will be connected between the cathodes of the output tubes and ground ?

Or do you plan to feed the voltage from the secondary to the cathode of EF86 ?
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Old 10th August 2012, 07:13 AM   #6
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About typical cathodefeedback (to the output tubes cathodes) it will introduce some DC over the speaker. Will typically be 20-50mV with 160mA Ia.

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Or do you plan to feed the voltage from the secondary to the cathode of EF86 ?
Didnīt see that one.
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Old 10th August 2012, 04:42 PM   #7
hkoetz is offline hkoetz  Netherlands
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Default EF86 + 4*EL84PSE

Quote:
You have not given anywhere near enough information for your question to be answered. Show us your draft circuit, and expected signal levels etc.
I was thinking in terms of output impedance of the EF86 in penthode mode, which is, depending on the anode resistor 100 - 200k. 4 EL84 PSE with 1 M grid resistor each, may present to low input impedance.

Quote:
Obvious question is "why feedback from the OPT when you have all the elements in place for a Schade setup?"
What would be a Schade set-up?

Quote:
Does this mean that the OPT secondary will be connected between the cathodes of the output tubes and ground ?
Yes

Thank you all for your reactions sofar.
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Old 10th August 2012, 04:57 PM   #8
jane is offline jane  Norway
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Quote:
Originally Posted by revintage View Post
EF86 is a not a very good candidate as Schade driver. More Gm and current is a must.
Who is talking about Schade feedback?
He mensions cathode feedback, and EL84 is a good candidate for this kind of feedback (especially in triode mode).
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Old 10th August 2012, 07:05 PM   #9
DF96 is offline DF96  England
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Quote:
Originally Posted by hkoetz
I was thinking in terms of output impedance of the EF86 in penthode mode, which is, depending on the anode resistor 100 - 200k. 4 EL84 PSE with 1 M grid resistor each, may present to low input impedance.
Pentodes are used to working into a lower impedance than their own anode impedance; unlike triodes they don't need a high impedance load for linearity. However, there is a potential problem of Miller capacitance. The EL84 has low feedback capacitance itself, but the base and wiring will add a little and then you multiply by 4 and the output stage voltage gain. You could find that it works fine at LF but loses gain at HF. You need to estimate the capacitance which the EF86 anode will see.

The so-called Schade connection involves turning the output stage into an anode follower (analogous to an op-amp inverting configuration). This will reduce the effect of capacitance so may be worth exploring.
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