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Old 3rd July 2012, 09:47 AM   #31
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Hi formula 22!

If you accept the idea of changes, then maybe you could go right back to the beginning and start by defining what your actual needs are. How much gain do you need? What output resistance do you need? What is this typically going to be driving? Is the amp close or far away, so what length of leads? Will you need to receive or drive a balanced stage at all, i.e. is there an argument for it to be balanced? Might this be driven directly from a balanced DAC output so there will be something like 0.5v DC on each leg? Do you need a line preamp at all, or would this stage be better integrated into the amplifier you are using? Is there any compelling reason to use a ECC88 or could you use another kind of tube? You're kind of designing in a void here, so it's a bit hard to make useful recommendations. Maybe you could start by giving us the above information?

Andy
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Old 3rd July 2012, 05:28 PM   #32
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You have one of the finest cleanest sounding amplifiers out there - in the form of the F5. I say learn to appreciate these outstanding qualities and build a preamp which preserves them.
My first option would be one of Allen Wrights designs in the form of the FVP5, or one of the later incarnations. That would be one of the best sounding - purest designs available and relatively easy to build if you restrict yourself to the Line stage component and don't include the superregulator.

Shoog
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Old 3rd July 2012, 07:24 PM   #33
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Ah right - missed the F5 bit. Another alternative is the 26 preamp with a LL1660/5mA output transformer in 4.5:1. This is being used a lot and I can guarantee it sounds great! All details in the "#26 pre amp" thread.

7N7 has mentioned his own preamp circuit, which he hasn't published yet. It's extremely neutral and detailed and if he does publish it, it's worth taking a look at. It has a cathode follower so low output impedance.

andy
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Old 3rd July 2012, 08:09 PM   #34
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Quote:
Originally Posted by Shoog View Post
My first option would be one of Allen Wrights designs in the form of the FVP5, or one of the later incarnations. That would be one of the best sounding - purest designs available and relatively easy to build if you restrict yourself to the Line stage component and don't include the superregulator.

Shoog
FVP5 schematics have 6922 (E88CC) or 5687 tubes in the line stage.
A 5687 version will give enough voltage gain for an F5 (F5 needs a little over 4 VRMS for full power).
A 6922 version would give too much voltage gain as a stand alone line amp; remember the 6922 line stage section of the FVP5 was to amplify a relatively low gain phono stage.
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Old 3rd July 2012, 08:17 PM   #35
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Hi the heater supply is not the best . adjustable voltage regulator set at 12volts then each tube set with a constant current feed to each tube raised about 40 volt ablove ground will quiet the set way down. there is not a need I see for the input cap since the grid is a ground . There is no cap like no cap . 1000 dollar silver teflon caps only get chose to a straight piece of wire. But if you like the sound go with . The magic is in know where the line is between too little and too much .
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Old 4th July 2012, 02:24 AM   #36
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Quote:
Originally Posted by kevinkr View Post
In theory when RL is a lot less than the degenerated rp of the triode in question.. The RL in this design is already far too low for good linearity even without cathode degeneration. (The minimum RL for an ECC88 for acceptable linearity IMHO is in the range of 10 - 12K)

I've found there was an improvement in linearity in most cases with the addition of local cathode degeneration, but I don't deliberately load them below rp so I am not certain this would apply in a case where RL was significantly less than degenerated rp - probably a quick sim would indicate what to expect in this case. I've also found no reason to use degeneration for any reason other than to reduce stage gain..
Ok, I can buy that. In a pathological case the degeneration resistance would consume a significant part of available output swing, which would already be too low.

Maybe we just need to help move this design a little further from this danger zone. A good teachable moment as they say.

Thanks,
Chris
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Old 4th July 2012, 03:08 AM   #37
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Quote:
Originally Posted by Chris Hornbeck View Post
Ok, I can buy that. In a pathological case the degeneration resistance would consume a significant part of available output swing, which would already be too low.

Maybe we just need to help move this design a little further from this danger zone. A good teachable moment as they say.

Thanks,
Chris
Silk purse sows ear !

To quote an old Irish Proverb;

When the lost man asked for directions the reply was "I wouldn't start from here".

Shoog
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Old 4th July 2012, 05:02 AM   #38
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Part of this of-course is matching your preamp to the power amp to get the COMBINED sound you want.
I have a pre amp which has a SS front end to volume control to tube buffer. When I'm using a really clean SS power amp I take the preamp output from the tube buffer output for some additional mostly 2nd harmonic distortion. When using a tube power amp I take the preamp output directly off the volume control bypassing the tube buffer because the tube power amp has enough "warmth" of it's own.

Oh! the tube buffer is a ECC88 cathode follower thing with a phase inverted slave drone so that the total power supply draw is constant. Then for a bit more "warmth" substitute an ECC189 (6ES8) VARIABLE Mu tube instead. Is it HiFi? Who cares it's lovely.

With an F5 power Amp some "warmth" from the preamp is probably required to suit most of our tastes (we would not be on a tube amp forum if we were happy with that cold sterile and boring SS sound).

Cheers,
Ian
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Old 4th July 2012, 05:24 AM   #39
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Hi,
With 2k2 and 2k7 resistors in the anode Vta (1/2ECC88) is obtained by Gain of 18dB or 8x.
Output resistance preamplifier is abaut: Zout=140ohm
NOTE:
Otherwise, I wanted to create a preamplifier will have a gain of up to10x with the output impedance (Zout) to <1K (for F5-amplifier), where will be the second (2H) harmonic is about 1% and 3H <0.05%, for Vout = 1.5V preamp. There have 2 pieces ECC88 and 2 pieces PCC88. That is why I want to I create preamplifier with ECC88.
Thank you for your cooperation and cheers!

Last edited by formula 22; 4th July 2012 at 05:38 AM.
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Old 4th July 2012, 06:06 AM   #40
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Gingertube,

You indicate that a tube amp per definition is adding "warmth" to the sound. This is the case for most unbalanced amplifiers and can be good for lots of listeners.

I prefer the "wire with gain sound" and the balanced RTP3D preamplifier is the one for me.
I also tried the FVP5 and remember it as very good but it didn't fit my philosophie (balanced end to end).
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