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..?
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By Schade you mean plate-to-plate feedback?
http://www.diyaudio.com/forums/tubes-valves/177680-schade-feedback.html
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..
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.
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.
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...
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...
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....
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?
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.
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yes they are basically the same....
there was a groupbuy sometime ago, lots of info here: http://www.diyaudio.com/forums/group-buys/189345-ixys-ixtp01n100d-non-usa-citizens-groupbuy.html
Ok, this simplifies things a bit. I didn't know you don't have to subtract the voltage drop as a result of the plate resistor! This makes it a lot easier, and I'll probably re-draw the schematic keeping the original 460V supply. I don't want to risk arc-over on the noval tube socket just for superfluous headroom..
You mean cathode resistor?
Keep in mind the tube curves show voltage respective to cathode. It does often say, eg. on the X axis, Plate Voltage - in fact it's Cathode to Plate voltage. Same thing with grid voltage, it's actually Cathode to Grid voltage.
The basic construction of a load line assumes a common cathode connection, i.e. cathode is grounded, which is why we use said curves that imply cathode = 0V = ground.FOr that scenario, the maximum current through the circuit would be if there was 0V Vkp, which is why we put B+ divided by plate resistor as one point the load line goes through, on the Y axis. The maximum voltage in the circuit is the actual B+ which is why we use this as a second point the load line goes through, on the X axis.
Here you might notice that in a realistic circuit, there is a cathode resistor used to develop bias voltage, but because this is fairly small with respect to B+ is, we tend to disregard that the cathode is not a 0V when drawing the load line, unless we are designing for something like a low voltage tube.
However, in order to get a LTP to work well, either we need to have a common cathode resistor with a rather high voltage drop, or a CCS which also uses a minimum of voltage to even work. With a CCS, there is an extra feature, that the cathodes will follow what you do on the grids in trying toi keep the current constant (which is what a CCS does).
Because you are connecting your LTP directly to the plate of the previous stage, the grids will be at the same voltage as the plate of the previous stage. This means the cathodes will be a few V higher, and in this case these voltages are not at all small with respect to B+ so you MUST take them into account. So, in this case, you can't use B+ on the X axis of the tube curves for the LTP, but the actual maximum Vkp, which is for all intents and purposes B+ less Vp of the first stage.
From the looks of things, especially on Pete Millett's page,
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the 10M45 isn't unhappy at 5mA, so I think to keep things simple (and the parts list shorter) I'll just use those, since I have 4 already. 600 ohms should provide the 5mA current, yes?
Really glad to clear this up. I've made a final final (I promise) design that I'll post as soon as I've written everything out nicely. There should still be plenty of swing, probably +117V -160V using 47K plate loads on the 6N1P, with the previous stage at 100V a-k.
Current Source
the 10M45 isn't unhappy at 5mA, so I think to keep things simple (and the parts list shorter) I'll just use those, since I have 4 already. 600 ohms should provide the 5mA current, yes?
Really glad to clear this up. I've made a final final (I promise) design that I'll post as soon as I've written everything out nicely. There should still be plenty of swing, probably +117V -160V using 47K plate loads on the 6N1P, with the previous stage at 100V a-k.
newest iteration. neglected to draw 1K grid stoppers on the 6N1P inputs. also haven't sorted out the protection diode, everything else seems pretty solid.
An externally hosted image should be here but it was not working when we last tested it.
Looks good... protection diode between grid and cathode of top 6N1P, anode to grid, cathode to cathode. 1N4148 will do but you might want to look up something a bit more sophisticated, maybe BAS21 or something like this, as you want low capacitance.
I would also recomend a bleeder resistor between 463V supply and ground, just in case. Also, grid stoppers (1k for now) on 6L6GC. These may need to be made a bit higher, depending on how the amp will clipping (blocking distortion due to attempting to drive the grids of the 6L6 positive).
Finally, check your swing considering that you have 47k plate resistors for the 6N1P and 100k grid leaks for the 6L6, the swing will be determined by a dynamic load line through your operating point on the 47k lstatic load line, but for 47k in parallel with 100k, roughly 31k or so, which is signifficantly smaller than the 47k plate resistor.
I would also recomend a bleeder resistor between 463V supply and ground, just in case. Also, grid stoppers (1k for now) on 6L6GC. These may need to be made a bit higher, depending on how the amp will clipping (blocking distortion due to attempting to drive the grids of the 6L6 positive).
Finally, check your swing considering that you have 47k plate resistors for the 6N1P and 100k grid leaks for the 6L6, the swing will be determined by a dynamic load line through your operating point on the 47k lstatic load line, but for 47k in parallel with 100k, roughly 31k or so, which is signifficantly smaller than the 47k plate resistor.
Looks like the 1N4148 has only 4pF judging by this datasheet..?
http://www.nxp.com/documents/data_sheet/1N4148_1N4448.pdf
When you say "cathode to cathode", what does that mean? I get that the grid is always supposed to stay negative with respect to the cathode, and anode should always be positive with respect to the grid and cathode, but the last part stumped me..
Maybe 220K 5W or something?
Ugh.. can I just make the plate resistors 68K and the grid leaks 130K to average out 44.6K? Or is that cheating?
Thanks so much for all the help so far. Can't wait to get this project up and running, and post some in-progress build pictures
Ah, sorry for that.
Diode anode to tube grid
diode cathode to tube cathode
This will make the diode cinduct whenever the grid of the triode goes about 0.5V positive WRT cathode of the tube. In all normal conditions the diode will be reverse-biassed and will not conduct.
The problem with the capacitance is that it varies with the reverse voltage on the diode junction. Check that it is small at around -2V or so, if yes - 1N4148 will do. Don't have the diode data at hand so I'm leaving this to you
Yes... although I think up to 470k will do just fine too.
Probably... in either case the position of the actual dynamic load line will not be the same but you might wiggle around the values and get what you need. Incidentally, IIRC 100k is given as a minimum grid leak for a 6L6 with fixed bias... so that should be more or less fixed.
1N4148 it is. Capacitance stays under 1pF at all times.
Sounds good! I'm polishing some transformer bells now.
got 'er built. all is well except those 72K plate resistors on the 6N1P are dropping way too much voltage, I'm reading like 180V on the anode. looks like it's back to 47K as planned. that said, it doesn't distort audibly and there isn't any audible hum, so i'm happy so far
overall wiring
bias supply
outside
inevitable glow shot
overall wiring
An externally hosted image should be here but it was not working when we last tested it.
bias supply
An externally hosted image should be here but it was not working when we last tested it.
outside
An externally hosted image should be here but it was not working when we last tested it.
inevitable glow shot
An externally hosted image should be here but it was not working when we last tested it.
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