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Old 10th April 2003, 01:52 PM   #1
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Default Comments on 71a PP Pre-amp

I want to build a PP pre-amp. Thinking about using 71a tubes in a push-pull arrangement.

Input phase-splitter tranny and output transformer.

Want to use 150v voltage regulator tubes and MV rectifiers.

I want a total voltage gain of 8-10.

Does anyone see any problems with this??

Thanks!!!
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Old 10th April 2003, 02:00 PM   #2
Joel is offline Joel  United States
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Default Re: Comments on 71a PP Pre-amp

Quote:
Originally posted by RichardJones
I want a total voltage gain of 8-10...
Does anyone see any problems with this??
I do. Seriously, what's the point of such a low gain pre-amp? If you love your vinyl, then you'll need a lot more gain than that.... and if you use CD's exclusively, you won't need any of that gain.
What is this going to be used for exactly?

Secondly, 71-A's are not cheap, require dc heaters (pain in the butt), and can be fairly microphonic. I love them, but only in power amp duty, with high signal levels.

Joel
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Old 10th April 2003, 02:35 PM   #3
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Default well....

Because I want to drive a SE MOSFET Class A amp that has a voltage gain of 1. SO.... I need the higher voltage gain.

Total voltage gain of 1.

PP will minimize any hum.

With a input tranny will be high level signals... 5 or so volts input.

plus the fact I allready have 2 matched globe 71a's.

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Old 10th April 2003, 02:38 PM   #4
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Default PP

Hi,

Quote:
plus the fact I allready have 2 matched globe 71a's.
In that case you're still short of another pair of tubes...unless you're going for mono.

Cheers,
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Old 10th April 2003, 02:58 PM   #5
Joel is offline Joel  United States
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Default Re: well....

Quote:
Originally posted by RichardJones
...plus the fact I allready have 2 matched globe 71a's.
Good! Then forget the pre-amp idea, and build yourself a 71-A single-ended amp. You'll throw the mosfet amp in the garbage the next day, and never look back.

Let me know if you'd like some ideas/schematics. I've done a lot of work with this tube.

Joel
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Old 10th April 2003, 02:58 PM   #6
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Default mono?

Nope stereo...

Yeah I realize that I need 2 more tubes... that is no problem.

Other than that... what is wrong with my approach?

I love the sound of 71a's and I love the sound of my 6B4G PP more than my old 6B4G SE amp.

it makes for a simple PS anyway.
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Old 10th April 2003, 03:09 PM   #7
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ok.... also my 6B4G amp has an input stage biased at 4v. (ECC99).

So... I need at least 4v input to my amp.

Still want to do a 71a PP pre-amp. with a 6K:1K output tranny, what input step up phase splitter should I use?

Could get away with total gain of 5 or around there.

power supply is LC stage with a 150v regulator tube and say MV rectifier tubes.

Thanks!!!

p.s. I am going to ignore microphonics for now.
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Old 10th April 2003, 03:28 PM   #8
Joel is offline Joel  United States
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Ok, so basically you want to use four 71-A's, two presumably well made transformer phase inverters, and two high bandwidth output transformers in order to get a gain of 8 to your power amp that was poorly designed to begin with, since it has an input sensitivity of a whopping 4V!

sheesh.

Richard, I'm all for lots of iron, DHT's, and experimentation - but this really seems fruitless.
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Old 10th April 2003, 03:32 PM   #9
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Default fruitless?

Well it may seem fruitless.... but it sounds damn good. and thats what counts!!!!!

I could get away with a gain of 5.

Whats wrong with lots of iron, MV rectifiers, 4 71a's... etc?????

sounds like fun to me!
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Old 10th April 2003, 03:40 PM   #10
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Default INPUT SENSITIVITY.

Hi,

Quote:
ok.... also my 6B4G amp has an input stage biased at 4v. (ECC99).
So -Vg = 4VDC.

Quote:
So... I need at least 4v input to my amp.
No, not necessarily.

It appears to me that you do not really know the input sensitivity of this amplifier.
While it could be true that it is going to need a 4 Volt input to drive it at full power, concluding that since the bias of the input tube is set at -4V you're going to need at least 4V to drive it is not correct.

Cheers,
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