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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: 08 Aug 2009, 14:26 
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Hi Alan, so what are you planning to keep from the original preamp?

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PostPosted: 08 Aug 2009, 19:29 
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The phono stage and the hybrid design idea, and all the original parts that go with it. Other than that, the preamp will look the same on the outside and will seem to function in the same way except that the "standby" relay will be used to enable HV supply rather than just cutting off the output, which in my opinion does nothing to lengthen tube life. :)

The entire power supply will be reworked and redesigned and I was thinking of having a low power amplifier tube as the last stage for the fun of it, but it all depends on current requirements, I don't want to exceed the 90MADC of the transformer, but that probably is not an issue. :?

I would like to use an EL84 set at 30MA bias or so (makes a total of 60MA, giving another 30MA for the regulated HV supply and phono stage). I really don't know how well this will work, I know the IRF740A will be at the output but not sure what kind of mess I may end up with, all I know is that if I get the right sweet spot of this output design this will be an amazing preamp, that could potentially make for a good headphone amp or maybe even a small class "A" power amp in a later project to come. :)

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PostPosted: 08 Aug 2009, 22:32 
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Hi Alan, A little unusual, but that is no reason not to try it. It certainly can work, but seems like monstrous overkill. If the 12AX7 isn't up to the job for you, I believe I would redo the circuit values for a 12AU7 or ECC802S. It would require minimal changes and provide 5 to 10 times the amount of drive. Regardless, the specs on the transformer you listed are plenty sufficient. 640 at 90ma is, if you do the usual PS (320-0-320) would equate to 320 at 180ma. The choke might be a bit small though. Running chokes close to ratings generally reduces their inductance so there is no telling exactly what you will have. What might be another consideration is the amount of heater current the EL84s need. Most preamp transformers are a bit on the low side for heater power as the typical tubes use relatively small amounts. The EL84 uses about .75 amps so a pair added to the circuit might be more than is available. Otherwise, I see no particular reason that what you want to do won't work. Keep us posted on the outcome.

Good listening
Bruce

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PostPosted: 09 Aug 2009, 13:31 
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Yup, it is very unusual idea indeed, I just thoutght it might be fun to try, and I have taken into account the filament current and I plan to put the EL84 filaments in series so that I can run them at 12V off a 7812 regulator on a massive heatsink so cooling it at 750MA shouldn't be problem but I may just go for a 6SN7GT and save the EL84's for single ended ahybrid amplifier using a couple of 2SK643's I found in my salveged components pile along with a 225VA 30VCT Hammond turoid that has been collecting dust for yeeeeeears! ;)

So I shall first off build the phono stage and test that then I will work on the final stage. :wizard:

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PostPosted: 18 Aug 2009, 12:02 
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Yay!! :mrgreen: it finally works! The most important part of this project is finished, now I just need to build the last stage of the preamp and the power supply.

Sadly my $20 HK6100 test amp no longer works, for some reason the right channel just died, but atleast I know my phono stage will work and even off 50V! amazing!!!

I will post some pictures of my birds nest soon :D

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PostPosted: 29 Aug 2009, 11:34 
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Now that I have the power supply problem solved, I need to work on the final stage. There are many changes I have decided on, Bruce suggested I use a 12AU7, I am going to use a 6SN7GT (I like octal tubes more) but I am not really sure what to use as a cathode resistor, the original is 5.6K and on average it is 2.2K, or maybe an LED.

Are there other big changes I should make? Like what is the use of the 47K and 1K resistors, I am using an IRF740A instead of the MPSU10, I also have a capacitor voltage rating problem, whoever built this preamp was a cheap ass and had 200V rated film caps throughout the entire preamp and this includes the 5uF output coupling cap, I am guessing if my supply is putting out 200V I have to replace them with higher voltage caps don't I? Or will they possibly survive for the time being, otherwise I am stuck with cheap electrolytics. :(

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PostPosted: 30 Aug 2009, 22:15 
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Hi The earlier guy was a cheapskate for sure. I like 6SN7s as well. Actually, I like the sound of 6/12SL7s better, plus they have a bit more gain. They make great SRPPs. With the amount of changes you are doing, I believe I would run a few sets of numbers through a program like Tube Cad to get some starting point. An alternative is to look in the RCA tube manual for some typical values.

Good listening
Bruce

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PostPosted: 31 Aug 2009, 11:31 
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Darn, I wish I had Tube Cad, all I can do is guess on some values and try them, RED LED for cathode bias (just for fun) and then I have no clue hat to change for the FET, I don't think I can reduce the plate resistor because that might not be good for the FET's Gate, not really sure, I am just going to try whatever and just maybe I will learn something good.

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PostPosted: 23 Sep 2009, 16:00 
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I officially have this preamp project working and completed! yay!!! :mrgreen:

It sounds so stunning listening to LP's with it for the first time, everything is so clear and the gain difference from what it was is tremendous! I am very pleased with the result of this project but it took so long to make al the circuits happy and to get the right voltages, for what I need.

Much thanks goes to tombethe for helping me find and create a suitable power supply and also to Gio who is always a useful companion when working on such a long project.

This means that now it is time for some rewarding pictures of my semi creation.....


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PostPosted: 24 Sep 2009, 16:13 
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I am going to take some time out this weekend and create a web page file that I can put up on the web in the future when I figure out how to.

I will post the "full" set of schematics for other people to try out this preamp, the phono stage is said to be a class "C" design.

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