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Old 10th February 2004, 02:56 PM   #1
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Default 12B4A PP schematic

Hi All,

I have a 6SN7-12B4A PP amp on the breadboard at the moment and I am thinking about committing it to a more permanent form with a second channel (wow, stereo). Would anyone care to suggest alternatives to the scheme attached before I take that step?

Thanks,
Michael
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Old 10th February 2004, 03:14 PM   #2
dhaen is offline dhaen  Europe
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Looks interesting What will you drive it with?

BTW, before any (other ) smart **** points it out, it looks as if you've omitted the grid leaks. Also, is the choke the only decoupling?
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Old 10th February 2004, 03:17 PM   #3
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I can't see any glaring mistakes...so

I would go right ahead and build the second channel and listen to it for a while....a real while like a month or two.

You can ofcourse change a thing or two...BUT I like your schematic a lot (and choice of tubes). Simple PP with a transformer doing the splitting early on..Wonderfully simple for a PP.

You can always apply the suggestions that may pop up later! Like a CCS or VR shunt reg PSU, grid stoppers...bla, bla bla. or WHATEVER...but they will remain suggestions..

Right now you have a gem in the making as far as I am concerned...

I am as always curious what people will suggest though.

Cheers,
Bas
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Old 10th February 2004, 03:22 PM   #4
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Default Drive and grid leaks

Drive is a worthy question. I don't know yet. On the breadboard I am taking my musical input from the headphone jacks of an old Sony onebox amp/cd. May be a problem driving it with a CD player? Suggestions welcomed.

I did omit the gridleaks on the 12B4A, they are 470k.

Thanks,
Michael
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Old 10th February 2004, 03:30 PM   #5
dhaen is offline dhaen  Europe
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Default Driving it...

You'll need something that will be happy with that 500ohm (very low) input impedance. Of course a headphone stage will do, or there are some valve preamps with very low output Z (but don't ask me to name one )
Another drive thought is a semiconductor one , my favourite line driver chip, the DRV134 / SSM2142. If you fed line level in, the differential outputs would drive the input transformer with no problem. These chips sound totally transparent to me, and require no external components apart from 2 decoupling caps.
Of course it'd be much more fun with valves
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Old 10th February 2004, 03:41 PM   #6
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Default Thanks Bas

I have had it on the breadboard for about a month now with daily listening and changing. I think, stress think, that this is my favorite operating setup.

The power supply is still to be considered, I am running it from an old Heath tube regulated supply at the moment. I am seriously thinking about VR tubes to regulate the 6SN7 supply and I have been meaning to setup a negative supply so I could test larger Rk for better differential balance and common mode rejection.

Also to be decided is the best volume control scheme. I have tried a 1Mohm variable resistor between the two ends of the input transformer and that seems it may work. Still need to do a little work in that area though. Suggestions welcomed.

Originally this was designed and planned as the frontend for another output tube, but the guy hasn't finalized his order yet. In listening to it I liked it enough that I think I will try it on it's own for myself. As a driver I tested it with a 100 ohm load on the secondary of the opt to reflect back ~50k and the distortion is quite low, I think, with a voltage swing of 400V pk-pk.
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Old 10th February 2004, 04:38 PM   #7
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Default input impedance

John,

A chip line driver is an idea, though if I had it's differential output I wouldn't need the transformer to split phase, correct?

Another omisson from the schematic, I have a 500 ohm resistor in series with the primary of the input transformer to tame some square wave ringing. Also, the transformer is rated as 500R:50k for single ended use, what does it reflect back when used as PP with a 47k (or larger) load on each side?

I know the breadboard loads a 500 ohm output signal generator but in a pretty frequency independant way. My Pioneer DV-563A didn't seem to have any problem driving a 500 ohm input passive line level filter when I inadvertantly designed the lowpass for the subwoofer filter with that impedance. Do you think it would have a problem with this amp?

Thanks,
Michael
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Old 10th February 2004, 05:00 PM   #8
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Where's the driver's PS bypass cap? Sure, it's balanced, but gee, it'd be 20 times more effective at keeping signal balance if you put the choke in the 6SN7's cathode circuit. As-is, you'll get nastified distortion for small imbalances!

Speaking of balance, the cathode bypass on the 12B4s is unnecessary if they are class A. (If class AB, you'll need a fixed bias supply instead. And I would recommend AB, given the low power handling of this tube.)

Tim
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Old 10th February 2004, 05:44 PM   #9
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Default PS cap and cathode choke

Hi Tim,

By PS cap do you mean a cap between the 75H choke and the plate resistors? I tried a 25uF polypro/oil there, both as connected to star ground and to the cathode of the 6SN7 above the Rk. Sort of empirical design method. Anyway, 2nd harmonic voltage increased from 16mV to 56mV and 28mV respectively with 8.2V output into a 16 ohm load. Third harmonic voltage increased from 46mV to 173mV and 116mV. Other orders were less affected.

Another schematic omission- I have a 50R pot between Rk and the cathodes on the 6SN7. Balancing it has a large effect on second harmonic. I tried without the 75H choke and with a 15H choke in the cathode load of the 6SN7. Both led to higher distortion, although I may have made mistakes in the implementation. I thought a choke in the cathode should be very effective? I also tried the 15H choke in the cathode load of the 12B4. Same result.

I thought I was running Class A. I get something around 4W before distortion starts climbing. When I inadvertantly connected the 55uF cap to the star ground while rearranging the circuit I had a 300% increase in distortion levels. It took me awhile to figure out what the heck was wrong with my setup.

Here is a distortion profile of the current setup. I don't really know if this is "good" or not, but it is fun to measure while trying to think of other circuit changes to try.

Michael
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Old 10th February 2004, 06:00 PM   #10
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Michael,

Even with a line driver chip that gives differential outputs, you'll still need the transformer. It's that that gives you lots of voltage gain.

I'm not clear about the transformer's spec. But from what I gather, the I/P Z won't be much more than 1K, which is still very low.
There's nothing wrong with that IMO, you just need to drive it properly.
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