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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: 25 Aug 2009, 15:39 
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All I have to say is...... YAAAAAAAAAAAAAY!!!! :sing: :mrgreen:

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PostPosted: 25 Aug 2009, 18:23 
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Hi Alan,
On the heaters, is it really a 7805? 5V for the heaters seems low.
Cheers

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PostPosted: 25 Aug 2009, 21:25 
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Hey Gio, I use 5V on 6.3 volt tubes all the time with no problems, it would also lengthen tube life I think, I think I may use two 7805 regulators because both tubes together will draw 900MA at 6.3V which cuts close to the 1Amp limit of the regulator, plus that is a constant current.

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PostPosted: 26 Aug 2009, 01:46 
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You may also shorten tube life with too low heater voltage. I read in AudioXpress (I think) about experiments on improving sonic quality by lowering Vheater slightly, think this was used on a 2A3, but lowering the voltage from 6,3V to 5V ... Computer says nooo

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PostPosted: 26 Aug 2009, 18:03 
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I agree with using two regulators, but for the heater voltage, I try stay within +0 / -5% of the specified voltage.

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PostPosted: 28 Aug 2009, 10:43 
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I guess my best bet would be to use a LM317 and set the voltage to 6 or 6.2, then I only need one regulator because the LM317 is rated for 1.5 Amps.

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PostPosted: 28 Aug 2009, 12:18 
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Hi Alan,
You should be able to use a 7806 more readily than a LM317 which may have a higher dropout voltage than a 7806 (depends on current draw).

FWIW, this 5687 Tube Preamplifier purposely uses "starved" heaters - regulated 5.75VDC instead of 6.3VAC.

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PostPosted: 28 Aug 2009, 19:28 
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Well, I am stuck with the LM317 because I have all ready made the filament supply board, I am nearing the big test of this preamp, just need to finish the final stage with the 6SN7, I still don't know what kind of values I should use, the only value I have changed so far is the cathode resistor from 5.6K to 2.2K, will this help give more gain do you think?

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PostPosted: 29 Aug 2009, 12:29 
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I just descovered the reason why the preamp was running so hot! If anyone else has this problem and they are running 120VAC than you may want to check your HVAC winding!

Using my simple test jig I wired the primary (Brown+Yellow and black, as was in the preamp) and I got 405VAC!!!!! :hot: For the filament winding I got 19.5VAC!!! :hot:

By doing the simple calculation AC voltage times 1.4 I come out with 567V and 27.3V!!!!!! :hot: Mind you, this is off load voltage but it is still insane and this preamp cirtianly shows the kind of stress these insane voltages were giving it, and at 80-90 digrees C at a room temperature of 20 THAT IS HOT!!!!!! :hot: If my uncle was using this preamp during the few days that we had 30-35 outside the regulators would have easily peaked thier 125 degrees C limit and failed, most likely in a very spectacular way. :)

To fix this problem some testing and measuring is required to set the right primary voltage.

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PostPosted: 29 Aug 2009, 13:55 
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You really need to apply a load to get a reliable estimate of the output voltage. Even a small load should drop those down quite a bit.

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