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Old 31st March 2003, 09:12 AM   #1
Jay is offline Jay  Indonesia
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Question RCA 12AT7 or Mullard CV4024?

For a pre-amp (hooked up to an Aleph; each gain stage uses paralleled triode of a single 12AT7), which one is better, RCA 12AT7s or Mullard CV4024?

With current exchange rate, the Mullard costs about USD35. Haven't check for the RCA price.
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Old 31st March 2003, 10:05 AM   #2
SHiFTY is offline SHiFTY  New Zealand
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CV4024 are military, so they will probably be a little more shock resistant. Whether this is useful in your preamp is another matter.

Just use whatever... Build it and see. You don't always have to buy the most expensive tubes out there, and 12AT7 are cheap cheap cheap. You could get heaps of different pairs for that price, even on eBay.

In general I find with ECCxx the bigger the plates the better the sound
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Old 31st March 2003, 10:15 AM   #3
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Default 12AT7 or CV4024?

6H30pi.

Why do you need a tube with a u = 70 in a preamp?
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Old 31st March 2003, 11:25 AM   #4
Jay is offline Jay  Indonesia
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<B>ShiFtY</B>,

The CV4024 are more rugged. They are recommended (not talking about price-to-quality ratio here) almost at all position in a pre-amp. Mullard is another “recommended” brand name. The RCA is another preferred brand version for 12AT7 (with AW suffixes?).

So, the military versions are shock resistant? Hehehe I thought that they have been manufactured with great precision and useful life in order that no rocket are going to the wrong country.

And ShiFtY,

I cannot just build it and see (I want to build the best pre-amp I could find, but I’m not wealthy, and I don’t have a credit card). I need 7 12AT7 for the preamp (US$245). I already bought NAD C320 (another US$245) to house my pre-amp. I believe that price tag doesn’t lie, except for NOS, where the tag stands for scarcity.

ECC81 is the European version if I’m not mistaken. May be it’s not really a big plate issue, but another brand or manufacturing issue?

<B>Brett</B>,

What do you mean with a u = 70? I did an internet search for 6H30pi, and I found what I think is a real high end pre-amp.

I have done a market survey for tubes and PCB availability to build the best pre-amp. I hoped I could find a transformer-coupled in order to drive my MOSFET Aleph amplifier, but I couldn’t find one.

So, if 6H30pi is really good, I need to see if I could find the tube and the schema, and workout existing PCBs. Brett, please tell me about good tube pre-amp? I choose this one because it uses 3 gain-stages (each with paralleled triode) so I think the output impedance will be suitable for a 2-stage SE MOSFET. The circuit is found in a US$4500 pre-amp.
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Old 31st March 2003, 11:52 AM   #5
SHiFTY is offline SHiFTY  New Zealand
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Quote:
I need 7 12AT7 for the preamp (US$245).
That is a HUGE amount of money, I bet you can get 7 decent NOS 12AT7/ECC81 for far, far less than that.

Here is an ebay auction for 50 various US made ones, with a buy it now price of US$65. Just do a search for 12AT7 and there are many, many more.

Seriously, don't get ripped off by scamsters... Old tubes aren't worth that much (unless they are 300B i guess)
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Old 31st March 2003, 12:10 PM   #6
Jay is offline Jay  Indonesia
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<B>ShiFtY</B>,

I don't live in the USA, and I don't have a credit card

The price was for Mullard cv4024, the best version I could find. The idea is I want to build the best pre-amp available, not necessarily 12AT7. But I should have access to the tubes, working schema, PCB (I cannot build the PCB from scratch!), as well as other components.
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Old 31st March 2003, 12:13 PM   #7
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Quote:
Originally posted by Jay
[B&lt;B&gt;Brett&lt;/B&gt;,

What do you mean with a u = 70? [/B]
u, or mu is the amplification factor of the tube. I made a typo before, the 12AT7 has a u=60, so depending on how you set it up, it could have a gain of up to 60. Most CDPs have an output of 2Vrms, and most amps need about 1-2Vrms to drive them to full power, so a LOT of gain is not neccessary.

Quote:
I did an internet search for 6H30pi, and I found what I think is a real high end pre-amp.
Could we see a schematic or a link to it?

Quote:
I have done a market survey for tubes and PCB availability to build the best pre-amp. I hoped I could find a transformer-coupled in order to drive my MOSFET Aleph amplifier, but I couldn’t find one.
Why use a PCB? Adds to the expense, and there are so few parts in the main audio path it's just as easy to do it point to point. P to P also sounds a lot better in my experience.

Preamps with output transformer (WOT) are easy. Each channel is one tube one OPT, two or three resistors, and (maybe) a bypass cap, not including the power supply.
<img src=http://www.fortunecity.com/rivendell/xentar/1179/projects/euridice/Euridice.gif>
Euridice WOT preamp

Quote:
So, if 6H30pi is really good, I need to see if I could find the tube and the schema, and workout existing PCBs. Brett, please tell me about good tube pre-amp? I choose this one because it uses 3 gain-stages (each with paralleled triode) so I think the output impedance will be suitable for a 2-stage SE MOSFET. The circuit is found in a US$4500 pre-amp.
The 6H30pi is really good, with very low distortion and low plate resistance, and makes a very good choice for many linestage applications. But because the Alephs have quite a low input impedance, the Euridice will probably be a better choice.

I have no idea why you need a 3 stage linestage, that's just crap. Nor will you need a PCB.

For 6H30pi or 6C45 (Euridice) try DIY Hifi Supply in Hong King Brian is great to deal with.

If you really want me to draw up a 6H30pi linestage I will.

Cheers
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Old 31st March 2003, 12:29 PM   #8
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Quote:
Originally posted by Jay
[BI need 7 12AT7 for the preamp (US$245).[/B]
WHAT?

I missed this part. You DO NOT need 7 double triodes of any sort of a linestage. One single 6H30pi, or two 6C45 are all the tubes you need for a stereo amp.

12AT7s are not that good a tube, no matter who made them, and for $US245, you could possibly get the 6C45 <i>and</i> the OPTs.

Please post a schematic of the 12AT7 design you're looking at.
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Old 31st March 2003, 01:40 PM   #9
Jay is offline Jay  Indonesia
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<B>Brett</B>,

The Euridice was the one I have been looking for. But I couldn’t find the OT. It is a single ended single stage, wow!

The idea behind the choice was the output impedance. My assumption was that tubes are voltage amplifiers. The transformer will transform the voltage gain into current gain as needed by the MOSFET. The fact is I cannot find an OT for the pre-amp! The parallel arrangement and the gain stages are something I hope to replace the OT function.

Okay, the 12AT7 are 60mu. But aren’t 12AY and 12AU (lower gain tubes) common for a pre-amp? 12AX7 are 100mu (the high gain version), but I assumed that the higher the gain in a stage, the more the distortion.

<B>The 6H30pi schematic</B>

I only saw a high-end pre-amp using the tube, not a schematic.

<B>The 12AT7 schematic</B>

Unfortunately I have it on paper, and cannot transform to an electronic format (but later will draw it on computer). There’s nothing special with the circuit: 3 stages, paralleled triode, that’s all it is. 1uF input – gate stopper – 12AT7 – 0.5uF coupling – 12AT7 – 12AT7 – 2uF output – The resistor. And a global feedback (damn!) In the last stage, the output is taken from the cathode.

This will use the best component available, and the best regulation (for the plates as well as the filaments, especially for the first stage). What else could I do?

Well, seems that I need to postpone this project. A friend (a supplier) is in the process of importing parts for an OT-based pre-amp. I hope it is Euridice

Brett, if you don’t mind, I will need the drawing, please. It surprised me that you think 12AT7 families (including CV4024) are crap. I still cannot believe so. It's just a tube. And already a triode
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Old 31st March 2003, 01:49 PM   #10
robbie is offline robbie  United Kingdom
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Jay
what will you be driving with this pre-amp?
why did you buy the nad,if only for the case?
if you are usuing a cd player with 2v output ,have you concidered a passive pre?
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