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Old 22nd March 2003, 05:52 PM   #11
EC8010 is offline EC8010  United Kingdom
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Default The shame!

Whatever was I thinking of? You are, of course, absolutely right.
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Old 23rd March 2003, 01:09 AM   #12
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Default CF.

Hi,

Quote:
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.
CF's are widely misunderstood, one good reason to opt for a triode is lower noise and better linearity.
A triode is more linear than a penthode for the triode has inherent feedback built in.

If you wish, penthodes can make for better CFs, in the case o the headphone amp I'd chose a triode such as the 6AS7, it has great current drive, low distortion, low Zo and is very linear as a CF.

Also, when talking CF there are half a dozen toplogies for those out there...

Cheers,
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Old 23rd March 2003, 01:48 AM   #13
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Default Re: CF.

Hi Frank,

Quote:
Originally posted by fdegrove
Hi,

CF's are widely misunderstood, one good reason to opt for a triode is lower noise and better linearity.
AFAIK, noise only matters 1. in high-sensitivity circuits, and also 2. at high frequencies. And noise is determined more by Gm.

Quote:
A triode is more linear than a penthode for the triode has inherent feedback built in.
Sigh. The internal NFB, though it does reduce distortion and Zo (manifested as Rp), is nonlinear, and as such a pentode has lower distortion than a similar triode when sufficient NFB (to equalize gain) is applied externally.

Quote:
If you wish, penthodes can make for better CFs, in the case o the headphone amp I'd chose a triode such as the 6AS7, it has great current drive, low distortion, low Zo and is very linear as a CF.
I agree that the 6AS7 is a great tube, but we don't know if he has any, nor do we know if he wants to buy any.

Quote:
Also, when talking CF there are half a dozen toplogies for those out there...
There are? When I hear CF, I instantly think "plate to +V, grid input, cathode output". not much simpler than that, and any other CF, White for example, has the qualifier to specify what it is ("White").

And remember. The first post in this thread says he has the topology, is going to use 6L6s for output, and is only asking for R/C values and if the power supply is sufficient.

Speaking of which -- I suppose someone should answer his questions!!
I'll have to graph the tube operating points, but otherwise:
The 269AX will do a good 330V 60mA as shown. (I'm listening to my 6L6GC SE right now (now playing: AC/DC - Highway To Hell) which uses it.) Technically, that's over the 125-0-125V 100mADC published figure (which translates to 250VCT 50mADC, meaning I'm 40% over the limit), but it still passes the 10 second finger test with aplomb.
Your filtering is very much more than adequate, though I'd like to see no more than 100uF for the first cap at the rectifier - reduce charging spikes, make the transformer run a little cooler.
The final RC is unnecessary IMHO, unless you need to drop a bit of voltage. (There's no reason the design can't use 330V.)

I'll get back to you on the values...

Tim
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Old 23rd March 2003, 01:52 AM   #14
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Default CF.

Hi,

Quote:
And remember. The first post in this thread says he has the topology, is going to use 6L6s for output, and is only asking for R/C values and if the power supply is sufficient.
Tim,

I was only replying to Gabe here, not the original thread poster as such.

Cheers penthode man,
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Old 24th March 2003, 01:54 AM   #15
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Tim, fdegrove, sch3matic, EC8010,

I thank you for all your input to my queries, and I am sure the original poster thanks us as well. I am sure he learned much from this.

Trespasser,

As for values...

For the power supply: I would think that with the bridge, the output voltage is 250 RMS, not 150, at 100 mA, not 50 as someone else said. With the 100F cap right after the bridge, the output voltage will be more like 353, but realistically, with load, 320 volts. Choke... you don't need much more than 1.5-3 Henries. The cap after... use as large as you can get. 100-300F. Otherwise you will hear the hum. The RC after I presume will power the preamp/driver tube.

Don't use a choke load for the cathode, as it will linearly boost bass and reduce highs. Chokes have a linear roll off. You want a linear flat response, not reduction in highs. So get a nice juicy 25 or higher watt resistor of about 330 ohms to idle at a reasonable current. This way you could opt to remove the grid resistor and cap and attach it directly to the plate of the first tube.. or not. You want about 15 volts at the cathode at idle with the capacitor and grid resistor.

If you opt for the direct couple approach (which may introduce hum), then the grid's being positive doesn't matter, because the cathode's voltage will be a certain voltage different than the grid to make up for the difference. As long as the grid voltage is negative with respect the cathode voltage it will be fine.

As for what to do with the screen grid... it doesn't seem to matter. The output is taken from the cathode. Screen grid functionality only affects the output at the plate (I know I will get scolded here). You may get a little more current gain (where the Gm, or transconductance comes in) but not much. It may only determine the resistor needed for the same voltage.

As for noise... again, it is only a factor where there is gain. We have none here, unless you call a gain of .8 or .9 "gain".

Stick with what you have there... except get rid of R5.

As for C4... for a headphones of about 20 to 30 ohms, you want at least a value of 270 F! Why??? unless you don't care for any music below 240 hertz, you ain't gettin no bass from the recommended value. (F=1/{2PiXcC}) So go for the isolated output tube, putting a capacitor and grid resistor at the grid of the 6L6. Then calculate for that 15 volts idle at the cathode so you can apend $1.25 for a 220 or even a 330 F cap at 35-50 volts.

Also, for power supply and other caps... 350 or higher volts. Remember, 125-0-125 translates to 250 center tap... which center tap you are not using. With a bridge you get 250 volts RMS, 353 volts peak with no load (250X1.414), more like 320 with load.

FWIW,
Gabe
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Old 25th March 2003, 12:46 PM   #16
Joel is offline Joel  United States
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Quote:
Originally posted by Gabevee
Don't use a choke load for the cathode, as it will linearly boost bass and reduce highs. Chokes have a linear roll off. You want a linear flat response, not reduction in highs.
Gabe, you are incorrect on this point. Use the formula 2*Pi*F*L and tell me how you arrive at the conclusion that bass will be boosted and highs will be ruduced?

The gain of the CF can only approach 1, which it will very soon when using any choke of reasonable size inductance. This increased gain will only increase with frequency, not decrease. And it can only ever increase up to the number one. If the "rolloff" you are referring to is from capacitance, this occurrs at frequencies many many times greater than audio.

As far as biasing, the DC drop across the choke (or resistor) in the cathode is what sets the op point.
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Old 25th March 2003, 01:26 PM   #17
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Joel,

Whoops! You're correct. I was speaking of my experience with chokes as bypass.

Thanks for pointing that out!

Gabe
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