Low gain, low current, low impedance stage

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Low gain, low current, low impedance stage

Postby DeathRex » Mon Jun 15, 2015 6:18 pm

I'm hoping to build a PPP 6GB5 amp. The amplifier is pretty much a Poseidon with a LM334 in the phase splitter (they have them at mouser again), using a 6N1P and a 6N2P in front of it, inplace of the 12AX7. It also has Morgan Jones tone controls with a 6EJ7, and a 6N23P in front for a buffer. So 6N23P - 6EJ7 - 6N2P - 6N1P - 6GB5. So nothing has gain until the 6N2P, and I was wondering if I could configure the 6N23P to give a little gain 2 or 3 times and get a low impedance into the tone controls. I'm thinking the middle of the bass and treble controls of a Baxandall type are equivalent to ground, so the impedance of the stage would be about 33K to ground.
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Re: Low gain, low current, low impedance stage

Postby EWBrown » Mon Jun 15, 2015 7:19 pm

I'd use the 6N23 in a voltage amp - cathode follower stage, the gain of the voltage amp can be set by the ratio of the plate and unbypassed cathode resistors.
The unbypassed cathode resistor in the gain stage introduces some negative feedback, to reduce the gain and lower its output impedance.
Cathode followers have a low output impedance, so it should be able to easily drive the tone stack.
A 6CG7, 12BH7 or equivalent might be a better choice, as it has about half the mu of a 6N23.

HTH


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Re: Low gain, low current, low impedance stage

Postby DeathRex » Tue Jun 16, 2015 9:13 am

Problem is I only have one place for a tube for both channels. So I suppose I'll start with just a buffer and see how it goes. I still haven't ordered the transformers, so it will be a while. I suppose I could adjust the gain with the output's NFB going to the 6N2P. I was also thinking about doing some Schadey stuff around the outputs.
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Re: Low gain, low current, low impedance stage

Postby LW1DSE » Mon Apr 18, 2016 10:05 am

EWBrown wrote:I
The unbypassed cathode resistor in the gain stage introduces some negative feedback, to reduce the gain and lower its output impedance.


This sentence is not true. As you let the cathode resistor unbypased, then it (The resistor) appears reflected in the anode circuit multiplied by (µ + 1) times plus the plate resistance. So, the output impedance is bigger using NFB than if you don't use it. This is because the NFB topology is current/series.
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