Now remember your paralleling the input tube, so your load doesn't need to be as high as a single. 
I'd go with a 43k plate load resistor, 450v B+ (giving the plate roughly 190v) and ma at roughly 6mills.
Follow that up with 1 more 6sn7 (or other dual triode) as cf for both channels (raising the heater), should be all set.
I would agree though, no feedback and UL or Pentode may have your output impeadance just a tad high, but try it first.
I'd go with a 43k plate load resistor, 450v B+ (giving the plate roughly 190v) and ma at roughly 6mills.
Follow that up with 1 more 6sn7 (or other dual triode) as cf for both channels (raising the heater), should be all set.
I would agree though, no feedback and UL or Pentode may have your output impeadance just a tad high, but try it first.
Sounds about right. Those look like measured voltages. 8 volts on your 1150 ohm cathode resistor is very close to 7 ma. That current across 30k is ~209, so 192 volts plate to ground. Your B+ is 900 volts. The max driver plate resistor you could use is (900-192)/0.007=101.4 kohms. With 60k on the 813 grid the AC load seen by the driver goes from 20K to 37.6K, which should buy a nice decrease in front end distortion without changing the tube's operating points. Pick a comfortable spot in the middle somewhere. You should double check the plate curves to make sure it still provides the required swing however. If that's a borrowed design there's also the possiblity the driver's distortion was tuned to cancel the 813. FYI.
With the 813 at 900V/100mA, Vg is going to be somewhere near 80V. A 6SN7 doesn't have enough gain.
This tube also sounds it's best with a driver capable of supplying some heft: a trioded pentode driven by the 6SN7 perhaps. Grid current can also be a bit of an issue if you drive them within a couple of volts of Vg=0 so account for that in the design.
Because I ran mine PP into horns, I didn't nedd the power usually only using a few watts max, so i found one of the 12HL7/12HG7 pentodes to be excellent as the gain/driver.
Edit: above applies for triode. Why bother with UL?
This tube also sounds it's best with a driver capable of supplying some heft: a trioded pentode driven by the 6SN7 perhaps. Grid current can also be a bit of an issue if you drive them within a couple of volts of Vg=0 so account for that in the design.
Because I ran mine PP into horns, I didn't nedd the power usually only using a few watts max, so i found one of the 12HL7/12HG7 pentodes to be excellent as the gain/driver.
Edit: above applies for triode. Why bother with UL?
Now I've never built an 813 amp, someone mentioned there easy to drive, but if there anything near the
requirements of say what a 300B needs to drive it, you may find my original 300B circuit somewhat useful?
Since I put this together it's now an E80CC driver to the 12BH7 to the 300B. It works nicely with plenty of drive.
Just some food for thought.
You could run your parallel 6sn7 front like you have with a dual triode backend like the one in this scheme for drive.
Just throwing it out there.
requirements of say what a 300B needs to drive it, you may find my original 300B circuit somewhat useful?
Since I put this together it's now an E80CC driver to the 12BH7 to the 300B. It works nicely with plenty of drive.
Just some food for thought.
You could run your parallel 6sn7 front like you have with a dual triode backend like the one in this scheme for drive.
Just throwing it out there.
Attachments
Gents,
Thanks a BUNCH for all the help. This is indeed a bastardized design of one borrowed...
I as well thought that the 813 was quite easy to drive; seeing as the origional design used a single 6SN7 to drive.
I am quite curious as to how it would be using the paralleled 6SN7...I suppose I could always add a 6L6 or something in triode mode in between if I don't have enough drive ? Actually, maybe an EL34 would be more linear. Whatcha guys think about this ? Or maybe I will use Kegger's design completely and use the 300B to drive the 813
So I'll bring the plate load up to 43k and bump up the B+ for the front end....
Thanks again for all of your time,
Steve
Thanks a BUNCH for all the help. This is indeed a bastardized design of one borrowed...
I as well thought that the 813 was quite easy to drive; seeing as the origional design used a single 6SN7 to drive.
I am quite curious as to how it would be using the paralleled 6SN7...I suppose I could always add a 6L6 or something in triode mode in between if I don't have enough drive ? Actually, maybe an EL34 would be more linear. Whatcha guys think about this ? Or maybe I will use Kegger's design completely and use the 300B to drive the 813
So I'll bring the plate load up to 43k and bump up the B+ for the front end....
Thanks again for all of your time,
Steve
By looking at the biasing on the 813 I'd say you need some drive.
You could use my basic 300B schem (your parts/voltages) and sub your 813 in for the 300B, you don't need the 300B to.
Your Parallel 6SN7 with 450v and 43k pate load to a 12BH7 (Like the scheme) to your 813 output section should be fine.
You could use my basic 300B schem (your parts/voltages) and sub your 813 in for the 300B, you don't need the 300B to.
Your Parallel 6SN7 with 450v and 43k pate load to a 12BH7 (Like the scheme) to your 813 output section should be fine.
Steven-H said:Gents,
Thanks a BUNCH for all the help. This is indeed a bastardized design of one borrowed...
I as well thought that the 813 was quite easy to drive; seeing as the origional design used a single 6SN7 to drive.
I am quite curious as to how it would be using the paralleled 6SN7...I suppose I could always add a 6L6 or something in triode mode in between if I don't have enough drive ? Actually, maybe an EL34 would be more linear. Whatcha guys think about this ? Or maybe I will use Kegger's design completely and use the 300B to drive the 813
So I'll bring the plate load up to 43k and bump up the B+ for the front end....
Thanks again for all of your time,
Steve
You have to decide how much gain you need; what are you going to connect to the amplifier? If it's a CD player, then given that they deliver 2v rms maximum, then I should say that you will be marginal. With a c. 40k anode load you'll probably get A of about 15 from your 6SN7, so you will probably not have the gain you require. I would probably use a 6072 (µ = 44) and the 6SN7 as a cathode follower.
7N7
Someone posted this in another thread recently and it may have potential; at least the driver. The mu stage could easily be improved.
http://www.diyaudio.com/forums/attachment.php?s=&postid=1389374&stamp=1199289078
http://www.diyaudio.com/forums/attachment.php?s=&postid=1389374&stamp=1199289078
Gents,
Thanks again. Brent - that screen driver is a very cool idea. I can take B+ right off of my 900v supply and toss it at the EL36. Maybe tone the gain down a *little* and use 5751/12BH7 combo.
Im going to figure out what the Op points he has set there, and come up with what I would need for 900v B+ on the pentode.
One question, C1 and C2 - Am I reading it correctly that they are to be 1uF, and not 0.1uF ?
Thanks again,
Steve
Thanks again. Brent - that screen driver is a very cool idea. I can take B+ right off of my 900v supply and toss it at the EL36. Maybe tone the gain down a *little* and use 5751/12BH7 combo.
Im going to figure out what the Op points he has set there, and come up with what I would need for 900v B+ on the pentode.
One question, C1 and C2 - Am I reading it correctly that they are to be 1uF, and not 0.1uF ?
Thanks again,
Steve
With an active pre yah you can get away with less and push the driver tube harder, but for me if I'm building the
thing I'd rather go the right direction in the first place, with a nice low distortion high drive low impeadance front end
it should sound better as well, might as well give it it's best chance to shine, at least on a nice project like this one here.
If your bias is say -85v you'd need double that to drive it full, not saying you need to drive it "full" but get it up there a bit.
The bias on the 300B is quite high, look at some well done 300B circuits as well to see how there driver schemes workout.
thing I'd rather go the right direction in the first place, with a nice low distortion high drive low impeadance front end
it should sound better as well, might as well give it it's best chance to shine, at least on a nice project like this one here.
If your bias is say -85v you'd need double that to drive it full, not saying you need to drive it "full" but get it up there a bit.
The bias on the 300B is quite high, look at some well done 300B circuits as well to see how there driver schemes workout.
I think it looks like it has potential, but I've never done it so merely offered as a suggestion.Steven-H said:
Blech. I'd find a better combination, one that doesn't have a 12AX7 derivative in there. No use having a nice linear OP stage and driver if you bollox it up with the gain stage. Start at the OP stage and work backwards and figure levels/gains and then decide on what to use.Steven-H said:
I read it as 1uF.Steven-H said:
When I did the 813 PP, I used either the 12HL7/12HG7 or an ECC99 - IT - big driver (DHT's and trioded tets/pents) - IT - 813 trioded.
The latter was far better near full power, but I ultimately preferred the former.
Gents,
The KegMan kindly tried to SIM that pentode driver schematic - and the program he uses seems not to like the circuit. Has anyone built this before ?
Im going to spend some time looking at some other triode-strapped pentode front ends and see what I can come up with.
On a side note - I had ordered Morgan Jone's book (The $45 one...), and it seems that someone wanted it more than I; as it was stolen from my porch (along with a $500 miter saw...). So guess I'll never see that again.
-Steve
The KegMan kindly tried to SIM that pentode driver schematic - and the program he uses seems not to like the circuit. Has anyone built this before ?
Im going to spend some time looking at some other triode-strapped pentode front ends and see what I can come up with.
On a side note - I had ordered Morgan Jone's book (The $45 one...), and it seems that someone wanted it more than I; as it was stolen from my porch (along with a $500 miter saw...). So guess I'll never see that again.
-Steve
Gents,
Well I think I may have found a solution to my driver - Any thoughts on the TubeLab SE Driverboards ?
http://www.tubelab.com/
I am not sure if I like the idea of the onboard PSU; but I imagine it works OK. I need to do some more research; but the Pentode driver board might do the trick (300B's would look cool tho !).
Any thoughts ?
Thanks a bunch,
Steve
PS - Upon getting a police report; I contacted Amazon.com, and they are sending out a replacement FOC. Haven't heard anything on my saw yet though....
Well I think I may have found a solution to my driver - Any thoughts on the TubeLab SE Driverboards ?
http://www.tubelab.com/
I am not sure if I like the idea of the onboard PSU; but I imagine it works OK. I need to do some more research; but the Pentode driver board might do the trick (300B's would look cool tho !).
Any thoughts ?
Thanks a bunch,
Steve
PS - Upon getting a police report; I contacted Amazon.com, and they are sending out a replacement FOC. Haven't heard anything on my saw yet though....
hey-Hey!!!,
Look at something healthy for the pentode driver. Remember, when rigged as a voltage amplifier, it is going to be a *VERY* easy load. Fix g2 with an appropriate shunt regulator. Look at high-gm TV sweeps, perhaps the 6CB5A or 6CD6. They will all take the kV plate supply and if you source B+ from the screen tap you can do some interesting stuff feedback-wise. 6CB5A with a 10k plate load and a 75V g2 supplied with an 0A3 will net *LOTS* of gain...
cheers,
Douglas
Look at something healthy for the pentode driver. Remember, when rigged as a voltage amplifier, it is going to be a *VERY* easy load. Fix g2 with an appropriate shunt regulator. Look at high-gm TV sweeps, perhaps the 6CB5A or 6CD6. They will all take the kV plate supply and if you source B+ from the screen tap you can do some interesting stuff feedback-wise. 6CB5A with a 10k plate load and a 75V g2 supplied with an 0A3 will net *LOTS* of gain...
cheers,
Douglas
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