Hey-Hey!!!,
The grid circuit resistance is likely arrived at with a bit of statistical research. It isn't a big deal, as a simple grid choke sounds better....
A grid inductor with a small fraction of 30k as DCR will yeild a lot of inductance.
Even 2A3's recommend a coil instead of a resistor. Lots of designs seem to ignore the spec. Examples of 7591 and 7868 with nearly half a Meg come to mind. EL34's with 270k and fixed bias are used on one of the most widely sold amps of all time. Massaging the split load circuit, and dropping to 100k is a simple improvement too.
On the 813, run U-L at ~600V, I am going to try 3k2 a-a load from a Citation II output. It is still class A with ~150 mA of plate current. It does seem a bit low...but ears will tell.
cheers,
Douglas
The grid circuit resistance is likely arrived at with a bit of statistical research. It isn't a big deal, as a simple grid choke sounds better....
A grid inductor with a small fraction of 30k as DCR will yeild a lot of inductance. Even 2A3's recommend a coil instead of a resistor. Lots of designs seem to ignore the spec. Examples of 7591 and 7868 with nearly half a Meg come to mind. EL34's with 270k and fixed bias are used on one of the most widely sold amps of all time. Massaging the split load circuit, and dropping to 100k is a simple improvement too.
On the 813, run U-L at ~600V, I am going to try 3k2 a-a load from a Citation II output. It is still class A with ~150 mA of plate current. It does seem a bit low...but ears will tell.
cheers,
Douglas
Re: Baird
Yes... sadly, today it appears that stupidity is desirable, because if you are stupid enough to touch it, and by some miracle survive, you are likely to benefit from a lawsuit. pparently uncrupulous lawyers have managed to arganize a civilisation where evolution is prevented from weeding out the fools.7N7 said:
955s
Thanks Sy.
The attached file is only 20kB.
I know it's off-topic, but the results were quite interesting.
I don't have my test book here with me, but my usual procedure was to follow up the valves on test with a cathode follower, to drive the 30k pad I used.
Otherwise I would configure the test valve into a mu-follower with something like a 6J5 on top.
Ah; it will not let me attach an Excel file!
Next tip please? (I do not have Winzip)
Paul
Thanks Sy.
The attached file is only 20kB.
I know it's off-topic, but the results were quite interesting.
I don't have my test book here with me, but my usual procedure was to follow up the valves on test with a cathode follower, to drive the 30k pad I used.
Otherwise I would configure the test valve into a mu-follower with something like a 6J5 on top.
Ah; it will not let me attach an Excel file!
Next tip please? (I do not have Winzip)
Paul
Re: Re: Baird
So true.
My girlfriend tells me that when she was a child, she climbed a tree, fell off and broke her arm. In view of this she imagines that the European Union will probably quite soon enact a law abolishing trees...
As a chap once wrote in an English newspaper:
"They won't be happy until we are all strapped into beds and being fed via tubes."
Paul
ilimzn said:
So true.
My girlfriend tells me that when she was a child, she climbed a tree, fell off and broke her arm. In view of this she imagines that the European Union will probably quite soon enact a law abolishing trees...
As a chap once wrote in an English newspaper:
"They won't be happy until we are all strapped into beds and being fed via tubes."
Paul
Aaaaarrrggghhh
WHOOPS!!
There is a small error in the files: The THD column has been formatted incorrectly - the cells should be formatted in percentage, so THD on the first row for example is 0.146%
All other figures are OK
I thought they looked jolly good!!
Thanks Sy, for posting them for me.
Paul
WHOOPS!!
There is a small error in the files: The THD column has been formatted incorrectly - the cells should be formatted in percentage, so THD on the first row for example is 0.146%
All other figures are OK
I thought they looked jolly good!!
Thanks Sy, for posting them for me.
Paul
Boise, I am using a 6SN7 preamp and a 6SN7 phase splitter with cap coupling. This driver works very well in my set up but it is not optimum since the 6SN7's don't quite have the peak voltage rating that is required for full power. My 813's use cathode bias. You know that the 813's are very easy to drive compared to triodes so that exotic driver stage is not needed as much. Good luck. I really like listening to my amp. The only cutting edge technology I am using is to use switch mode power supplies for the filament, screen and plate supplies. Do you have a torroid winder or are you going to wind several thousand wire turns by hand? I have some large cut cores that I considered using and they would be simpler than a torroid to wind. Sparky
Re 813 pp
Sparky, Thanks for the reply. Ok I have looked at 2 options, one is a very simple test set up keeping the power down with a 12AT7 and the other uses 2x 6BQ5 triode strapped. I am using a modified pc psu to give me 10V at 10A for the 813s. Because I didn't want the supply floating I decided to bias the grids with a variable supply. The heaters will be wired such that one valve heater is reversed, in this way every so often the valves can be swapped over. As for the Toroid, I have already wound it in a way that is a bit different to the normal methods, more on this when it has been tested.
Boiss
Sparky, Thanks for the reply. Ok I have looked at 2 options, one is a very simple test set up keeping the power down with a 12AT7 and the other uses 2x 6BQ5 triode strapped. I am using a modified pc psu to give me 10V at 10A for the 813s. Because I didn't want the supply floating I decided to bias the grids with a variable supply. The heaters will be wired such that one valve heater is reversed, in this way every so often the valves can be swapped over. As for the Toroid, I have already wound it in a way that is a bit different to the normal methods, more on this when it has been tested.
Boiss
Re: Re 813 pp
I ran my 813s at around 850V and 90mA each. AT this point RCA 813s needed bias of about -80V. This suggests that you need to have a driver stage that can swing at least 200v pk-pk - better to have more. Then there is the capacitance of the 813 as a triode to consider. This means plenty of "cojones" from the driver stage. I eventually used a 6SN7 diff pair on a solid state CCS, DC coupled to a 6BX7 that sat on a EL822 pentode sink. This certainly could swing the voltage and was beefy enough to drive the 813s.
[edit] I would not be inclined to use ECC81/12AT7 as I do not consider it to be nearly as good as lower-mu valves such as 6SN7 - if you need more voltage gain then try 12AY7 with a cathode follower.
If I were doing it again I would look into a cathode follower set-up with +ve and -ve rails and DC couple to the 813s.
7N7
boiss said:
I ran my 813s at around 850V and 90mA each. AT this point RCA 813s needed bias of about -80V. This suggests that you need to have a driver stage that can swing at least 200v pk-pk - better to have more. Then there is the capacitance of the 813 as a triode to consider. This means plenty of "cojones" from the driver stage. I eventually used a 6SN7 diff pair on a solid state CCS, DC coupled to a 6BX7 that sat on a EL822 pentode sink. This certainly could swing the voltage and was beefy enough to drive the 813s.
[edit] I would not be inclined to use ECC81/12AT7 as I do not consider it to be nearly as good as lower-mu valves such as 6SN7 - if you need more voltage gain then try 12AY7 with a cathode follower.
If I were doing it again I would look into a cathode follower set-up with +ve and -ve rails and DC couple to the 813s.
7N7
813 pp
Thanks 7N7,
My main psu is borrowed from an old linear amp and appears to give about 950V at around 400mA so from everything I've seen with Ia around 80-90mA that should be Ok. I know that the 12AT7 is not adequate but for some simple tests will be Ok and for the moment everything will be mounted on a wooden base. The O/P transformer has been wound to give 40% taps for UL operation and the secondary offers a choice of 2 ratios. One thing that came to mind was that in almost all UL connected O/P stages that I have seen the g2 is taken directly to the 40% tap, putting the HT on to g2, wouldn't this be better with a capacitor bypassed dropper resistor? I used multisim to get some sort of idea on the phase splitter and only tried it with valves that I have to hand.
The reason for swapping the 813s over every now and again is because of the difference in potential across the heater and any effect it might have re emission from the heater long term (probably not worth worrying about).
My main concern is my O/P trans and running some tests because I can't shell out big bucks for the trans, so if it doesn't work I can stop there and wait untill I can afford the transformers.
Boiss
Thanks 7N7,
My main psu is borrowed from an old linear amp and appears to give about 950V at around 400mA so from everything I've seen with Ia around 80-90mA that should be Ok. I know that the 12AT7 is not adequate but for some simple tests will be Ok and for the moment everything will be mounted on a wooden base. The O/P transformer has been wound to give 40% taps for UL operation and the secondary offers a choice of 2 ratios. One thing that came to mind was that in almost all UL connected O/P stages that I have seen the g2 is taken directly to the 40% tap, putting the HT on to g2, wouldn't this be better with a capacitor bypassed dropper resistor? I used multisim to get some sort of idea on the phase splitter and only tried it with valves that I have to hand.
The reason for swapping the 813s over every now and again is because of the difference in potential across the heater and any effect it might have re emission from the heater long term (probably not worth worrying about).
My main concern is my O/P trans and running some tests because I can't shell out big bucks for the trans, so if it doesn't work I can stop there and wait untill I can afford the transformers.
Boiss
Re: 813 pp
If you run 813s UL at 900V you will pass huge current. I only have experience running them triode-strapped - they make a super triode.
As for the filaments, probably the best solution is DC - with a current-regulated supply - this ensures a slow start so the valves will last much longer.
Best of luck
7N7
If you run 813s UL at 900V you will pass huge current. I only have experience running them triode-strapped - they make a super triode.
As for the filaments, probably the best solution is DC - with a current-regulated supply - this ensures a slow start so the valves will last much longer.
Best of luck
7N7
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