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Old 20th March 2003, 10:04 PM   #1
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Lightbulb OTL 6L6 headphone amp-idea+need help

tube schem

Ok, I have this much (the drawing) and these sets of values:

Set1
P1- 250k
R1-275k
R2- 1k
R3- 680
R4- 100k
R5- 100k
R6- 330-680 1W
R7- 10k
R8- 1k
C1- 50uF 25V
C2- 50uF, 250V
C3- 50uF, 250V
C4- 10-20uF, 250+V
add coupling cap between 6L6+6SL7, after R5

Set1
P1- ? (100k maybe?)
R1- 1M
R2- 10k
R3- about 1k
R4- 200k
R5- not used (actually harms amp by shorting 6SL7?)
R6- 3k-5k 10W
R7- ? don't know
R8- 100
C1- ?
C2- not used
C3- ?
C4- 220-470uF 250+V

Of these sets of values, which are most likely to work best? Should I mix them? How?

Power Supply

Click the image to open in full size.

C1- 560uF, 250V
C2- 470uF, 250V
C3- 270uF, 250V
D1-D4- 1N4007
L1- 2H, 100mA, 175-ohm, 300V (Hammond 154M)
R1- not figured yet
T1- 40VA, 125-0-125@100mA, 6.3V@2A heater (Hammond 269AX)

B+ is 150VDC.

What do you think about these? Is the PS adequate? Which values should I use for the amp? Headphones are 300-ohm.

Thanks
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Old 20th March 2003, 10:14 PM   #2
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Default Headphone Amp.

Hi,

If going for the big output tubes, why not use a 6AS7G or its relatives?

It will give you plenty of power, low Zo and it is a real triode, to boot.
Moreover there are plenty of those around at about 10 $ a piece thanks to the military.

Drive it with decent current like 1/2 of a ECC99 and you're in business ...

Cheers,
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Old 21st March 2003, 01:24 AM   #3
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Oh, how quickly we forget

He has a whack of 6L6s to burn...

Here's a better schematic, plus my interpretation of it.
The values in the linked schematic look okay, except for that resistor to ground. It will indeed load the preamp tube, DC as well as AC; a coupling cap would help a lot.
Also, the lack of a coupling cap will put a lot of DC on the 6L6's grid, biasing it much higher than 0Vg would put it with a 680 ohm cathode resistor. My schematic allows a choice; take the first shown value for direct coupled, second for cap coupled.
Note that I haven't graphed the operating points, and the 5k 10W is a total SWAG.

Tim
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Old 21st March 2003, 02:26 AM   #4
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Tresspasser,

Nice and simple, but you may want to make a voltage divider with about 100 volts and attach it to one side of the heater circuit, otherwise you may get some unwanted hum due to some rectification effect between the heater and the cathode.

fdegrove,

Curious. Though I have never tried it nor read anything about it... but why would it make a difference what tube you use for a cathode follower as long as the current capability is enough?

I would think that they would all sound the same.

Gabe
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Old 21st March 2003, 09:16 AM   #5
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Default Re: Headphone Amp.

Quote:
Originally posted by fdegrove
and it is a real triode, to boot.
The anode voltage versus current curves of a triode and a beam tetrode are way different of course, but in this case we have the load connected to the cathode. With a pentode, beam tetrode and to a lesser extent a triode, the anode appears as a current sink and the cathode as a voltage source to whatever you are driving. Imagine the situation where you hold the grid at a constant voltage and gradually pull the cathode down further and further. Plot the anode current increase as you do this and there you have the difference between a "real" triode and a beam tetrode in this instance. That's what really matters here, not the difference in the plate curves. Not *entirely* sure but I imagine the main difference would be the steepness of the transconductance slope, i.e. how many milliamps increase per volt of cathode-grid voltage change. It could be that a 6L6 might be better than a triode as a cathode follower.

P.S. Something I read at tubecad the other day - If you use a tetrode or pentode as a follower and *don't* want it to be a psuedo-triode, (provided the scrren is fed with a resistor!) run a bypass cap from the screen grid to cathode so the screen to cathode voltage remains constant even though the cathode is jumping up and down.
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Old 21st March 2003, 01:44 PM   #6
Joel is offline Joel  United States
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Trespasser,

I was just thinking about the same type of circuit a few weeks back! How funny.
But, I was planning on using a choke as the cathode load for the 6L6 cathode follower. The key would be to use one that has a DC voltage drop across it equal to your necessary bias point.

If you use resistors for the load with so much current flow, you get too much heat in the chassis for my tastes.
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Old 21st March 2003, 11:30 PM   #7
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Default SAME, SAME BUT DIFFERENT.

Hi,

Quote:
Curious. Though I have never tried it nor read anything about it... but why would it make a difference what tube you use for a cathode follower as long as the current capability is enough?
Why wouldn't you hear differences between tubes whether they're in AF or CF mode?
After all it's not because the signal is taken off the anode or cathode that all of a sudden they're all going to sound the same.

Then again not all AFs are nor sound the same and surely this applies to CFs too...

Cheers,
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Old 22nd March 2003, 01:18 AM   #8
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fdegrove,

Well... Just for the sake of argument, cathode followers generally will have a gain of something like 0.9, give or take a few millivolts. What have others called it? 100% NFB? Whereas at the anode, not connected as a unity gain follower (since I didn't mention that) will have some positive gain. Depending on the tube, some more gain than others. The characteristics of the tube then will be more or less influential, depending on the gain, on the sound.

But with 100% or more NFB, there should be no influence at all... I guess. Hence why I wonder why a triode will sound different, not necessarily better, than a tetrode or pentode.

Still curious.

Gabe
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Old 22nd March 2003, 10:40 AM   #9
EC8010 is offline EC8010  United Kingdom
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Default Two birds with one stone

Joel's idea of using a choke to load the output valve is good. This would allow the grid of the output valve to be DC coupled to the input valve, and lose that grid-leak resistor that was loading the first stage down.
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Old 22nd March 2003, 04:35 PM   #10
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...Huh?

If you like +20V grid bias, I suppose...

Tim
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