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Old 19th August 2012, 01:05 AM   #41
AJT is offline AJT  Philippines
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1k grid stoppers for the 6N1p....
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Old 19th August 2012, 01:21 AM   #42
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Originally Posted by Tony View Post
1k grid stoppers for the 6N1p....
Can't hurt I suppose. I also left out protection diode(s) for the LTP when the amp is just turning on, I'm not sure which type I'd use and how.

It would also be pretty easy (I think) to run some GNFB to the 6AG5 cathode but I wasn't sure if it's OK to take it from the 16 ohm secondary, since it's now also the 6L6GC's cathode..?

Last edited by sorenj07; 19th August 2012 at 01:23 AM.
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Old 19th August 2012, 01:25 AM   #43
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now you have options....i'd say 16 ohms, but if you encounter motor-boating, then 0ohms.....you might want to consider O'Schading as well...
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Old 19th August 2012, 02:01 AM   #44
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Originally Posted by Tony View Post
now you have options....i'd say 16 ohms, but if you encounter motor-boating, then 0ohms.....you might want to consider O'Schading as well...
By Schade you mean plate-to-plate feedback?
Schade Feedback

I take it that the rest of the circuit looks ok? I wonder if anyone has suggestions for a CCS that will provide 1.4mA with a 170V supply. I'm not so experienced with the sand..
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Old 19th August 2012, 02:50 AM   #45
dgta is offline dgta  United States
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I don't see any point to the 700V. 400V is plenty B+ to supply swing to a pair of 6L6GC, particularly since you have -80V available for the tail.

700V requires a bit more attention to detail. Wire insulation, trace spacing if using a pcb, etc. Don't forget to up the rating on those 630V caps. And check your voltage ratings. Some 6N1P tubes have a Va max of 250V.
http://frank.pocnet.net/sheets/164/6/6N1Pspec.pdf
You're exceeding that in normal operation, never mind what happens at startup, etc.
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Old 19th August 2012, 03:00 AM   #46
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yes, commercial amp designers do not do that....but having such a big headroom means that distortion will be low, penalty is cost wrt to additional psu, but hey this is diy.......

tubes can take more plate voltage as long as it is in cut-off, what is is more important imho is the plate dissipation ratings, in this case around 700mW still way below 2.5 watts...
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Old 19th August 2012, 03:04 AM   #47
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Originally Posted by sorenj07 View Post
By Schade you mean plate-to-plate feedback?
Schade Feedback

I take it that the rest of the circuit looks ok? I wonder if anyone has suggestions for a CCS that will provide 1.4mA with a 170V supply. I'm not so experienced with the sand..
yes...

there are mosfets that can work into 1000volts, IXTP01N100D, just 2 resistors and you have a CCS...cost is cheap too, around $0.55 at Mouser....
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Old 19th August 2012, 03:53 PM   #48
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Originally Posted by Tony View Post
IXTP01N100D, just 2 resistors and you have a CCS...
Sorry, I didn't see any curves on that datasheet, do you know how I could figure out the resistor values? And would they be the same layout as with the 10M45 from the first schematic?
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Old 19th August 2012, 05:55 PM   #49
ilimzn is offline ilimzn  Croatia
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Originally Posted by sorenj07 View Post
Thanks to everyone for all their input. I finally got a new schematic up. Have yet to figure out exact resistor values, and a proper CCS implementation, but the bare bones are there. For the bias supply I was thinking 150K from the wipers to ground, a 25K trimmer, and 33K for the other four.

There should be ample swing from the LTP this time, on the order of +/- 200V. The 700V D+ is derived from a separate CLC supply from the Hammond 282X, 500-0-500V. Turns out a CCS is hardly necessary on the first stage, which is operating at 4v g-k by the way.

SIGNAL
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6AG5 Input
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6N1P LTP
Click the image to open in full size.
There are a couple of things to check.

1) The 700V supply should really not be necessary but in any case it's going to provide LOTS of swing, under a few conditions that I will get to later on. Firstly, the current required for the front end is not high so it may be very impractical to use a CLC supply. In fact, a simple RCRC migt be better since you actually have too much voltage. The low current will require a high calue inductor, no need to go to the expense.

2) Carefull with the cold start voltages! Look at specs for maximum voltage at cut-off or cold state. You may have problems with arc-over not only in the tube bu on the socket, due to pins being close. You WILL need to clean the solder joints from flux residue as this likes to provide an arcing part once it absorbs some moisture. In particula 300V is the specs of the 6N1P but this gives a DC voltage.

3) You might want to supply the whole front end with the 700V supply through a series of RCRC filters which would suitable reduce the supply voltage of the stages. This will give you good gain for the first stage as well as linearity, and lots of swing with the second stage.

4) there absolutely MUST be a protection diode between the grid and cathode of the 6N1P because at cold start you will have ~700V G! to Cathode aand this the 6N1P surely will not withstand.

5) You have an error on your 6N1P curves. If the supply is ~700V and the cathode is at 176V, thus the load line m8st be drawn for 700 - 176V = 524V or so, not 400. I would actually go for some 650V or so after RCRC filtering, so a 475 or so volts long load line. Draw the load line for a suitable plate resistor, then select an operating point for best swing, read off the current, and use a tail CCS of twice that current (for both triodes in the LTP).

6) Keep in mind the gain of the first stage is lower than your load ine predicts since you have an unbypassed cathode resistor.

7) With resistors from the bias supply to bias pot wipers, you actually don't need the resistors from the lefthand side of the pots to the bias supply. You can connect the lefthand side to the wiper.

All that being said, look at the 6N1P loadline. If you select a operating point such that the bias on the 6N1P is -4V, your maximum swing from the first stage is twice that, 8Vpp. This is wgy youa ctually CAN get decent swing from the 6N1P even from the 450V supply, because it's fairly easy to get 8Vpp swing from the first stage even at a low plate voltage from the first stage. A lower first stage plate voltage automatically gives you that much more headroom for the second stage.

Last edited by ilimzn; 19th August 2012 at 06:01 PM.
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Old 19th August 2012, 11:53 PM   #50
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Quote:
Originally Posted by sorenj07 View Post
Sorry, I didn't see any curves on that datasheet, do you know how I could figure out the resistor values? And would they be the same layout as with the 10M45 from the first schematic?
yes they are basically the same....
there was a groupbuy sometime ago, lots of info here: IXYS IXTP01N100D for non-USA citizens Groupbuy
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