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PostPosted: 02 Apr 2016, 06:36 
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Joined: 18 Aug 2015, 07:38
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I am in the process of getting all the parts together to build this pre. On the heater filaments, do you tie pins 4 and 5 together and use the center pin 9? Am I correct in assuming this being it is a 12V filament tube. Also I am impatient and thinking of ordering an Antec transformer. Have anyone used this one?

http://www.antekinc.com/as-05t200-50va- ... ansformer/

Is there a big difference in the sound quality using the JJ ECC802S long plates vs the gold pin ones. These are about 1/2 the price.


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PostPosted: 02 Apr 2016, 07:52 
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Joined: 26 Sep 2015, 13:59
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Well, I'm listeneing to it as I write!

I'm using a EZ80 recifier tube and thermal delay tube on the HT. I have a single Pi filter arrangement for smoothing using two 47uF caps and a 25H choke. I have had to use a seperate transformer for the amp valves LT, my main transformer has a common CT for both HT and LT making heater voltage elevation impossible (I think), a necessity for this circuit. The biggest probelm has been getting the voltage right. I eventually managed to get LT voltage within spec at about 12.4 volts. HT voltage has been much more difficult, like trying to pin down a consistently moving target! Currently anode voltage is about 210v until I get some more big resistors to put it right. It's been a very interesting build.

How does it sound? Well, I'm afraid I have 1uF electrolytics (sorry!) on my output as my Russian PIO caps have not yet arrived and I couldn't wait to listen. All looks good on the scope, and the sound does not dissapoint. It sounds somewhat softer and less dynamic than my other forewatt build. Soundstage is as impressive as ever and there is no hint of edge to the sound. Generally I would describe it as laid back, gentle, warm and relaxing.

This truly is a wonderful design with a fantastic sound. It also seems pretty flexible and tolerant too. I really can't praise it enough.

One question if I may. Does the output cap need to be 1uF? I have a wonderful pair of quality 0.22uF Jensen PIO caps, would they be too small? Is it to do with frequency roll off?

Thanks

Mark


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PostPosted: 02 Apr 2016, 11:30 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, The 12AU7 (and all the 12A_7s, ECC82s, ECC802s, ECC81s, ECC83s .... and similar tubes) can use either 6 or 12 volts for heater power. For 12 volt operation you use pins 4 and 5 (leaving 9 disconnected). For 6 volt operation you tie pins 4 and 5 together and use pin 9 as the other end.

I use a 1uf capacitor on the output so that the preamp can drive fairly low impedance power amps. If you use ones with high impedance inputs (say over 50K) then you can use smaller capacitors. With 50K you can use as low as 0.33uf and still have great bass response. At 100K a 0.22uf would be my choice. A 1uf is suitable down to about 10K and has minimal reduction down to 5K input impedance. Below that you start to run into the effective output impedance of the tube stage.

Good listening
Bruce

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PostPosted: 02 Apr 2016, 11:38 
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Thanks Bruce, so I can wire the heaters either way. And the Antec transformer should work as well?


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PostPosted: 02 Apr 2016, 11:51 
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Location: Arizona, USA
Hi Yes the Antek ought to work fine. I have a 1T200 here and it is rather large so take into consideration the size of it during the build.

Good listening
Bruce

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PostPosted: 02 Apr 2016, 14:44 
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Thanks Bruce, that's great. Not really sure how to measure input impedance of my power amps so I guess I'll stick to 1uF cap.

On another matter can you advise on snubbing the heaters? Since I have AC on the heaters I guess I will need two 100nF caps each going to ground. Does this ground need to be the CT of the heater transformer? This is, of course, elevated to ~85v and would mean running a seperate heater ground wire from the transformer. Or can I use the power ground which is a 'true' ground and at ~-85v with reference to the heaters? Also is there any value in adding a humbucker pot across the heater supply wires and the CT in order to balance their potentials? I have read about this but never seen it in any designs on this site.

Thanks again for your great advice.

Mark


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PostPosted: 02 Apr 2016, 20:36 
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Location: Arizona, USA
Hi, The snubbers if you use 12 volts connect directly between pins 4 and 5 right at the tube socket. One capacitor for each tube. They do not need to be rated over 50 volts as they only see 12. If you use 6 volts on the heaters then they go between the junction of pins 4 and 5 to pin 9.

Good listening
Bruce

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PostPosted: 03 Apr 2016, 06:34 
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I ordered the Antek transformer. I like Edcor transformers both PS and OPT but I do not like the wait. I have 4 tube amps using Edcor OPT and they sound fine to me but I have not tried other brands. Basically an OPT is nothing more than wire wrapped around a iron core. Others may have a different sound some may conceive as better sounding but much can be said about swapping tubes. If they are quality made they all should sound good but maybe different. Wire wrapped around an iron core no debating that.


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PostPosted: 14 Apr 2016, 06:30 
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I finished my build last night. I was going to stop at 6:00 PM after finishing the PS section but the audio section is so simple I spent another 2 hours and finished the pre. I went with a Wima 1.5uf MKP cap on the output. I thought I had read somewhere that this circuit inverts the signal and possible the output should be reversed. I built it just as the schematic and to me the sound is just fine as is. I built it point to point but with the parts Bruce supplies in the kit it is a very good deal for the price he is asking with a much better looking preamplifier than mine with a big 12" X 9" baking pan. Building point to point with the Antek transformer every bit of the 12" X 9" was needed. I am after the sound not looks so the outside appearance does not matter to me. It fired up with no problems the first try and after setting the B+ voltage and checking the input output DC and AC for voltages I installed it in my system. For all intents and purposes it is quite. I can hear a touch of hiss with my ear against the tweeter with my 103db horns but nothing from a few feet away. It could be contributed to the tubes I am using. Lovely sounding with my new Firstwatt M2 build. Very short listening period last night with more to follow. I went with a 1.2K resistor after the 1.5uf cap going to the output. Many thanks Bruce for making this possible.


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PostPosted: 14 Apr 2016, 14:59 
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Joined: 18 Aug 2015, 07:38
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I do have some questions. The thread has become rather long and I would like to know the latest improvements to make. From memory do I need to change the cathode resistors from 470R to 820R and raise the B+ to 250VDC? Also do I need to reverse the output if the circuit does invert as I thought I read somewhere? Info would be appreciated.


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