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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 30 Jul 2015, 08:49 
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Thanks Bruce. I could read some values, bias etc and may get an idea if the tube is working in the green zone. Sounds OK just a little softer and warmer. But I would expect that from a Uy.

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PostPosted: 30 Jul 2015, 22:33 
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I thought he sound a little too mellow with the U in. Reason is the impedance of the first stage drops to 7K from 44k. There is a 0.01uf cap to ground from the first stage via a 37k resistor, part of the EQ network. This network is out of whack now. So the Xs have to go back.

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PostPosted: 30 Jul 2015, 22:56 
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Currently trying the JJ Xs back in the phono preamp and the Us in the preamp. The preamp is two stage with feed back loop from stage two back to one. This may alter the effectiveness of that loop but not as critical as values in the phono stage.

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PostPosted: 23 Feb 2018, 20:20 
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gofar99 wrote:
Hi Mark, Yes the 12AU7s will drop in a circuit wired for 12AX7s and often work. The problem I have found is that the components are not optimal for 12AU7s. The different anode impedance, current ratings and Mu will often cause the 12AU7 to operate outside the best linear and low distortion range. If you can check the distortion level you may find you are OK. Once in a while the values for parts match up. I have played around with this and found the distortion to more than double and the head room to be seriously reduced and usually asymmetrical. Swapping like this is often done in guitar amps...12AX7, 12AT7, 12AU7, 5751 etc will all swap and since they want distortion it works fine. For hi-fi though I fond it not to usually be a good thing.

Good listening
Bruce

Just re-read Bruce's comment here. I have gone back to an MM cart. Current have a newish Garrott K2 but have a Garrott K3 in the post. The Garrott carts. are Ozzie design and made. The difference between the K1 to K3 is stylus profile only, same motor throughout. The K3 has a Shibata stylus. The carts are cheap at RRP AU$400 for the K3 I only paid $350 posted. That is a cheap cart. Luckily they don't sound cheap. Very natural earthy sound with thier patented "Dynamic Coil" design. The Garrotts have far more expensive carts than this one but I have enjoyed the K2 so much I just want to retain that sound so hope the K3 will do that.

Back to this thread. Because now I'm not using a MC stage ahead of my tube MM preamp the output was a little low with the 12AU7s in. So now I have gone back to 12AX7s for more gain. And as Bruce has pointed out possible a better match for the cct. Certainly sounds good. I have the JJ tubes back in but lately I have been swapping all my small tubes out for Psvane "T" series MKII gold pins. I'm finding everything I put them in sounds bloody fabulous.

In my latest headphone build I bought Psvane 12AU7 "T" series MKIIs for Sanctorum. The tube in before it was itself very good so only a marginal gain for another $45 but I know the tubes are good. Also in my 6L6 SE UL wine box power amp, stained base, I used Psvane U7s in that and felt an improvement in overall tone balance.

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PostPosted: 30 Jun 2018, 08:03 
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Another K318 build. This time in a project steel and Al box. External PS from another build.
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Projects: "Grace" - Psvane 6SN7-SE Globe preamp | | "VoXUno" - Single driver MarkAudio 12P speaker | | "Lagoon Take 2" - Single stage tube preamp | Website: retro-thermionic


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