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PostPosted: 19 Jan 2013, 16:26 
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Location: Arizona, USA
Hi, An alternative is any small push-pull output transformer with the right value for the primary and something that will match your phones on the secondary. You can also play the impedance game with them. Find one with the right number of turns ratios and use it. The precise number is not all that important, but the ratio in the ones used is 8.2 to 1. Remember that the actual impedance is the square of the turns ratio times the load impedance. (of if you rather ...divide the primary impedance by the square of the turns ratio). A 600 ohm to 8 ohm would be a pretty good match (ratio of 8.6).

Good listening
Bruce

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PostPosted: 20 Jan 2013, 16:10 
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Thanks for the info Bruce. I'll have a look about on eBay. I've found 10:1 already but would rather get closer to 8:1


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PostPosted: 11 Feb 2013, 17:48 
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Joined: 11 Feb 2013, 17:20
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Location: CA, USA
I was looking for amp like this for long time, but have a question:
Looking at the schematic, the tube current is 12.5mA per side. The max power for 6DJ8 is 1.8W per triode. Tube will be operated close or even above max rating, at B+ 135-150V. Maybe tubes like 6CG7/6FQ7 will be a better choice?
Not sure how long 6Dj8 is going to last.


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PostPosted: 11 Feb 2013, 23:47 
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Hi,

A 6CG/FQ7 is a different animal entirely. A fraction of the transconductance and curves that woul make an ideal Ra-a about 25K.

:2c:

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PostPosted: 12 Feb 2013, 14:11 
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Hi, The 6DJ8s seem to hang in there fine. If you want to have a little more comfort factor then the B+ can drop to about 120 without any changes.

Good listening
Bruce

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PostPosted: 12 Feb 2013, 21:31 
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andrew11 wrote:
Looking at the schematic, the tube current is 12.5mA per side. The max power for 6DJ8 is 1.8W per triode. Tube will be operated close or even above max rating, at B+ 135-150V. Maybe tubes like 6CG7/6FQ7 will be a better choice?
Not sure how long 6Dj8 is going to last.

A B+ of 150V will be fine when you consider the losses that occur across the primary of the audio output transformer.

Cheers

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PostPosted: 15 Jun 2013, 05:12 
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Congrats Bruce. This amp sounds wonderful. I am very very pleased with how it preforms, I do have one thing happening that may just be my lack of experience with tube amps. When i first turn on the power only one heater in each tube lights. It takes about 40 seconds for that heater to dim from yellow to orange at which time the second heater lights from cold to same orange color as the other one. Is this normal? As soon as both are equal brightness the sound is heavenly and with no source the amp is dead quiet. No hiss or rush even, just silence and that is at 2:30 AM when even the mice are asleep. I don't have words to convey just how pleased i am with this amp. The tubes i am using are the new JJ E88CC and i have put the power supply in a separate cabinet for isolation. Thank you so much for this one.


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PostPosted: 15 Jun 2013, 14:16 
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Hi, I'm glad you like the amp. It was a sort of fun project to build. I wanted to see how simple a headphone amp could get and still have excellent performance. I'm not sure why it would seem that one filament would warm up much sooner than the other as both triodes are supposed to be identical. I have seen stranger things though and would not worry about it. Both sides may actually be warming up just fine and because of the construction of the tube it may not be obvious that it is doing so. I have a number of tubes around here that don't show any exterior glow when operating and they work just fine. Manufacturing tolerances and variations may account for the way they look.

Good listening
Bruce

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PostPosted: 15 Jun 2013, 14:22 
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laurie54 wrote:
When i first turn on the power only one heater in each tube lights. It takes about 40 seconds for that heater to dim from yellow to orange at which time the second heater lights from cold to same orange color as the other one.
I have a matched set of JJ 6V6s that do exactly the same thing. I just assumed that one had a filament which was slightly heavier and as such it took a while for the resistance to rise to the point where the current settled down. It has always seemed to work fine. I wouldn't worry about it.

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PostPosted: 15 Jun 2013, 22:07 
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Can you use the 6n1p instead of the 6dj8? They are "supposed" to be similar. Would the 6n23 be a better choice?


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