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6DJ8 (ECC88) Tube Headphone Amp Project
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4161
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Author:  laurie54 [ 23 Dec 2017, 14:51 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

If there is ever a need to discharge a capacitor, do so but use a resistor in series with the short. Connect each end of the shorting jumper to the terminals and leave for a few seconds before removing. For these amps 10K at 2-5w would work. A dead short on a cap can cause damage between the conductive fields and the dielectric layer if too many electrons pass from one conductor to the second conductor at one specific area. A hole will form in the dielectric substance causing a short between the conductive layers resulting in a good cap turning bad.

Author:  gofar99 [ 23 Dec 2017, 15:39 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Hi, Thanks for the addition, Yes correct, I have a cold and was not thinking quite as clearly as normal (some say ...what's new ?:eek: )

Use the resistor, if you make a dedicated thing for this purpose the resistor in the middle of the wire can be insulated with a piece of heat shrink.

Good listening
Bruce

Author:  Beardy [ 23 Dec 2017, 18:39 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Hi Bruce,

What B+ would you expect to see without the tubes plugged in?

Minimum quantity for the boards were three sets. So if you can PM me your address I will send you a set. They seem to check out as faithfully copying your design.
cheers
Beardy

Author:  gofar99 [ 23 Dec 2017, 20:09 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Hi, If you used the same transformer, rectifiers and filters about 280. (1.4 times the RMS voltage of the winding)

Good listening
Bruce

Author:  Beardy [ 23 Dec 2017, 21:23 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Yes, I use the same transformer and values of resistors and caps. The local supply voltage measures 119+/- and I see 300V with no load
thanks
Beardy

Author:  gofar99 [ 24 Dec 2017, 16:26 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Hi, Allowing for tolerances it is in the right range.

Good listening
Bruce

Author:  Beardy [ 24 Dec 2017, 16:56 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Hi Bruce,

What would be a good resistor value to use a dummy load instead of the tube. I could then get real numbers without cooking things...
Cheers

Author:  gofar99 [ 25 Dec 2017, 16:12 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Hi, My first question is why do you want to do this? Just finish the build and let the tubes be the load. However...I calculate that a resistor of about 3000 ohms would be needed. I would use a 20 watt one. Be aware it will get quite hot.

Good listening
Bruce

Author:  Beardy [ 25 Dec 2017, 17:53 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

OK I was just trying to avoid burning up the tubes unnecessarily.

So I should see about 150V across the tube when loaded....?

Festive tidings to you and yours,
Beardy.

Author:  Beardy [ 26 Dec 2017, 22:06 ]
Post subject:  Re: 6DJ8 (ECC88) Tube Headphone Amp Project

Hi Bruce,

I measured voltage across Pin 1 & 8
Tube 1 started out at ~280V, after 1 min this dropped to ~178V and after 2 mins ~170V and then after 3 mins to around 167V
Tube 2 started out at 280V, after 1 min, this dropped to ~170V after 2 mins and then after 3 mins to around 168V
seems to be asymptotic and would settle somewhere in the >160V range.
cheers
Beardy

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