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Old 14th November 2006, 12:25 AM   #101
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ok, i'm posting first off like that guy at the beginning of Monty Python and the Holy Grail - "not dead yet!" this project, that is. basically the only thing keeping me from building this sucker is my laziness at having to wake up early to go to this metal shop in Dorchester to have a suitable aluminum plate punched for me, 20x14x1/8". one other thing is these second thoughts I keep having.

i've been looking over the schematic and i notice a standard grounded-cathode gain stage between the 6SL7 phase splitter and the 6L6 finals. i was wondering if it would be OK to nix that gain section, seeing as most CD players etc. already have higher voltage output, and i like the idea of a separate linestage (i.e. i already have one). does this sound like an OK idea?

one more idea: the biasing of the 6L6 finals seems pretty basic, one resistor. if i wanted to really nerd it up, would it be worth it to substitute each tube's cathode resistor with a lower value, and a smallish little trimpot, to allow fine-tuning of the tubes' bias current? or, if i'm going to go to the hassle of modding the schematic, should I just figure out how to make a CCS? i figured out what current the schematic set to run through the resistor, i have it written down somewhere.
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Old 14th November 2006, 01:15 AM   #102
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On nixing the stage:

How much gain do you really have after feedback?
6SN7s have lower output impedance than 6SL7s.

Doing it two stage I think youd be looking at a redesign. Maybe 6N1Ps as splitter/driver?
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Old 14th November 2006, 02:00 AM   #103
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Quote:
Originally posted by Tweeker
How much gain do you really have after feedback?
idk

Quote:
Doing it two stage I think youd be looking at a redesign. Maybe 6N1Ps as splitter/driver?
It's a posibility, seeing as I have a pair of those lying around. The problem is, I already have two pairs of matched-section Sovtek 6SN7GT's and 6SL7GT's and the sockets to go with them . I don't suppose I could just use a 6SN7 to drive the 6L6's, if they have lower output impedance?

my main question is: how do i figure out where my 6L6's are biased? i have the plate curves for the 6L6, i know that the cathode resistance is 700 ohms per tube, and the voltage at the centertap is 385 ideally, and that the load through the transformer is 3300 ohms per tube. when i apply this to a tube curve, i can't find a value along the curve that spits out 700 ohms for the cathode resistor. what am i doing wrong? does the grid stopper have something to do with it?
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Old 14th November 2006, 03:29 AM   #104
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Where did you find 6L6 UL curves at? Grid stopper has little bearing on it.

Quote:
...and that the load through the transformer is 3300 ohms per tube...
Load varies with operating point, only in class B would the load be 3.3K. Use the plate to plate load of 6.6K.
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Old 14th November 2006, 03:53 AM   #105
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so when i'm trying to design a class AB amplifier, looking at a load line, I should probably use 8k instead of 4K for an 8K a-a OPT? that might necessitate some changes to an amp I've designed
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Old 14th November 2006, 04:22 AM   #106
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Er, just to confuse you more, youd use 2k.

Check the loadline series.
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Old 14th November 2006, 12:28 PM   #107
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OK, reading now ( http://members.aol.com/sbench102/composit.html ). i'll also browse Valve Amplifiers to get more of an idea of wth is going on . thanks!
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Old 14th November 2006, 05:56 PM   #108
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Quote:
Originally posted by sorenj07

It's a posibility, seeing as I have a pair of those lying around. The problem is, I already have two pairs of matched-section Sovtek 6SN7GT's and 6SL7GT's and the sockets to go with them . I don't suppose I could just use a 6SN7 to drive the 6L6's, if they have lower output impedance?
Here's another crazy thought... with some extra 6SN7's you could just wire it up as a Williamson amplifier instead, using the Hafler/Keroes output stage -- this has been done many times, and sounds verrry good. This would give you much more reasonable (lower) gain, and likely lower distortion. There are many great examples of that circuit online (Acrosound catalog, for example). Your power supply / OPT would be perfect.

Save the 6SL7's for a phono preamp, perhaps?
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Old 14th November 2006, 09:52 PM   #109
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sup jon. about the williamson, that's an idea and a half i'll definitely take a look, i think i have the article where Hafler and Keroes hijack the Williamson driver/splitter stage lying around somewhere. didn't make the williamson crowd too happy, iirc. Of course, it means picking up new biasing junk, but that's peanuts compared to a more convenient amp. besides, that quad of 6SN7's would be put to good use

here's my power supply, for future reference

PT: Hevi-Duty E550, 125V -> 500V (4x), 1.14A
secondary winding resistance: ~5.1 ohms

Diodes: 2x6AY3 Fullwave
Choke: 193M
Cap: 400uF 500V
Output Voltage: ~390VDC, current, 260mA (this can increase to 300mA with minimal loss)

I have two 500uF 500V cans lying around, but PSUDII says that the current draw from using both paralleled would blow the 6AY3's. I'm considering installing both, and only wiring one for use until I pick up some sturdier 6DN3's, at which point I will have a beast PSU of 1000uF 500V

anyway, reading the Williamson circuit. also, checking out Jones' section on push pull
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Old 14th November 2006, 10:25 PM   #110
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Click the image to open in full size.

that's the williamson. i'm good for everything except the 450V plate voltage, which i had worried about. i wonder what to do..
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