http://tdsl.duncanamps.com/pdf/805.pdf
pretty powerful, but in SE you will need to run it with a positive bias on the grid.. hence the grid will draw current.. a fair bit too.
pretty powerful, but in SE you will need to run it with a positive bias on the grid.. hence the grid will draw current.. a fair bit too.
SHould be a big amp, power should be more than a 211 / 845 as it is an A2 tube. From the picture looks like a 845 with a top cap. Should probably go with an input stage coupled to cathode follower/or driver with transformer coupled to the 805 so that its grid requirements are fulfill.
If you willing to spare some, i don't mind to take them and try something out with these. I interested in transmitter class tubes now days.
I haven't look at the data sheet for gain requirements but i think
i would go with
input 6gk5
driver el34 (cathode follower ) cap coupled to the input
805 direct coupled from the driver
Well thats what i plan to do with my GM70, except i use a pentode in the input
There always something about transmitter class that thrills me. When they light up it is a sight to be seen.
If you willing to spare some, i don't mind to take them and try something out with these. I interested in transmitter class tubes now days.
I haven't look at the data sheet for gain requirements but i think
i would go with
input 6gk5
driver el34 (cathode follower ) cap coupled to the input
805 direct coupled from the driver
Well thats what i plan to do with my GM70, except i use a pentode in the input
There always something about transmitter class that thrills me. When they light up it is a sight to be seen.
> i can get ten 805 triode valves NOS at €10 each
Expensive at half the price.
Look at the graph:
In "good audio", we normally do not want to cross the grid zero-volt line (in red). At negative voltage, the grid is an infinite impedance; at positive voltage the grid is a diode with about 1KΩ resistance.
If you try to draw a load-line that does not cross the grid-zero line, about the best you can do is 1,500V B+, 20mA bias, 40K load, 800V swing, 8 watts out at large (10%-15%) distortion. That's an awful high B+ and load impedance and filament power for 8 dirty watts. With stock output transformers the power will be more like 2 watts, and even more distortion.
If we cross the zero-grid line, that transition from infinite to low-Z will cause the driver to distort. Because 1K is a low load for tubes, distortion is large. It can be managable with a step-down driver transformer. But usually the distortion curve rises quickly at low-mid power, then drops as we get far into Class 2 operation, before finally rising. That up-down-up distortion curve is un-natural.
Also note that the Plate Resistance of th 805 tube is MUCH higher than any of the Triodes normally used for audio. Rp is 10K or more, while all practical loadlines for this tube are under 10K. It will have low damping and high distortion compared to the triodes born and bred for "good" audio amplifiers. Yes, the datasheet shows an audio application, but it is a "monster power" commercial application, where quantity is more important than system cost or quality.
Many good A2 amplifiers have been built. But it is MUCH harder than building an A1 amplifier. By the time you over-build the driver to give ample drive power and grid current, you probably could have used that expense to buy a tube that is easier to drive and will run well in A1 mode.
If you must use this tube as class A2: look at 600V B+, 100mA bias, 5K load. That works out to about 25 watts RMS. The grid bias is about +20V, the grid swing will need to be about 50V peak, with about 50mA on positive peaks (zero on negative peaks). Up to about 5 watts, it will stay in Class A2 mode, constantly sucking grid current, so the driver load is low but fairly stable. A choke-coupled 6L6 as Triode could drive it. You could direct-couple if you mess with a negative supply (maybe -50V 50mA) pull-down. Note that the driver is dissipating over 30 watts, none of which gets to the speaker. Brett's transformer-coupled driver wastes less power, but needs a special transformer.
You could double the B+ and load impedance for 50 Watts "SET" Class A2, but that will be a very expensive custom transformer.
Expensive at half the price.
Look at the graph:
An externally hosted image should be here but it was not working when we last tested it.
In "good audio", we normally do not want to cross the grid zero-volt line (in red). At negative voltage, the grid is an infinite impedance; at positive voltage the grid is a diode with about 1KΩ resistance.
If you try to draw a load-line that does not cross the grid-zero line, about the best you can do is 1,500V B+, 20mA bias, 40K load, 800V swing, 8 watts out at large (10%-15%) distortion. That's an awful high B+ and load impedance and filament power for 8 dirty watts. With stock output transformers the power will be more like 2 watts, and even more distortion.
If we cross the zero-grid line, that transition from infinite to low-Z will cause the driver to distort. Because 1K is a low load for tubes, distortion is large. It can be managable with a step-down driver transformer. But usually the distortion curve rises quickly at low-mid power, then drops as we get far into Class 2 operation, before finally rising. That up-down-up distortion curve is un-natural.
Also note that the Plate Resistance of th 805 tube is MUCH higher than any of the Triodes normally used for audio. Rp is 10K or more, while all practical loadlines for this tube are under 10K. It will have low damping and high distortion compared to the triodes born and bred for "good" audio amplifiers. Yes, the datasheet shows an audio application, but it is a "monster power" commercial application, where quantity is more important than system cost or quality.
Many good A2 amplifiers have been built. But it is MUCH harder than building an A1 amplifier. By the time you over-build the driver to give ample drive power and grid current, you probably could have used that expense to buy a tube that is easier to drive and will run well in A1 mode.
If you must use this tube as class A2: look at 600V B+, 100mA bias, 5K load. That works out to about 25 watts RMS. The grid bias is about +20V, the grid swing will need to be about 50V peak, with about 50mA on positive peaks (zero on negative peaks). Up to about 5 watts, it will stay in Class A2 mode, constantly sucking grid current, so the driver load is low but fairly stable. A choke-coupled 6L6 as Triode could drive it. You could direct-couple if you mess with a negative supply (maybe -50V 50mA) pull-down. Note that the driver is dissipating over 30 watts, none of which gets to the speaker. Brett's transformer-coupled driver wastes less power, but needs a special transformer.
You could double the B+ and load impedance for 50 Watts "SET" Class A2, but that will be a very expensive custom transformer.
805 design (and 838)
There is an approved 805 amp concept from Nobu Shishido, this is for sure a class A2 (positive grid) design.
I have actually also a 805 Project in a planning stage, using a similar-to-WE102 tube in the line, a 46 as a driver, interstage coupled to the 805. Operating point of the 805 is 800V Ua at 125mA Idc and +25V Ug. Not very surprising, it is very close to the Shishido design. With a voltage swing of +/-35V you can get approx. 25W Po. Output transformer has 5k impedance (but you can go down to 3k to 3.5k).
BTW the 805 ist closest to the 838, both are variable µ triodes.
A friend of mine from Italy had build a 838 class A2 amp, using for line/driver a 6EA7 and a Lundahl IT to couple to the 838.
Regards
Ralf
There is an approved 805 amp concept from Nobu Shishido, this is for sure a class A2 (positive grid) design.
I have actually also a 805 Project in a planning stage, using a similar-to-WE102 tube in the line, a 46 as a driver, interstage coupled to the 805. Operating point of the 805 is 800V Ua at 125mA Idc and +25V Ug. Not very surprising, it is very close to the Shishido design. With a voltage swing of +/-35V you can get approx. 25W Po. Output transformer has 5k impedance (but you can go down to 3k to 3.5k).
BTW the 805 ist closest to the 838, both are variable µ triodes.
A friend of mine from Italy had build a 838 class A2 amp, using for line/driver a 6EA7 and a Lundahl IT to couple to the 838.
Regards
Ralf
Tell me ´bout it, I finished the last of my two 811A SE monoblocks two hours ago. Pure A2 driven by EL86 cathode followers direct-coupled to the grids. It´s a bloody mess, but I would´ve been happy if it wasn´t for the mechanical buzz from the amps.
Seems to come from the rectifiers for the driver stage (PY 82), which is weird...
GM 70 . This one ? http://www.nhcbrenners.hu/ It seems good to me , but no component values in it .
TugaTweaker said:
That was in Glass Audio in 2000 or 2001.
Seems waaaaaay too complicated for a relatively easy to drive (for a bright emitter) tube.
I played with the GM70 a lot over the last year, with a lot of different drivers, incl 300Bs, and the best driver I found was a 12HG7A in pentode. Made a very clean two stage amp and the driver tubes cost $US 0.70 each.
GM70 datasheet
Take a look at these for some of the inspiration I found to do this
Similar 813 amp
Pentode test data
Legacy amp
Lots of people won't try this because they don't "beleive" in pentodes (I was one of them) but for the cost of the tubes involved, and there are others applicable such as the EL84, SV83 and 6AQ5, C3m etc, I'd strongly suggest giving it a try.
grataku said:
I found it online somewhere, but don't recall where exactly. I collect these sorts of things when I find them. Seeing as it's a WAVAC, I'd say the design was by Nobu Shisheido, and I'd suggest his book on Transmitter Tube Amplifiers.
I'm sure there are other sources, but try here , ninth item from top.
Shishido design
Yes, the mentioned Wavac 805 is a Nobu Shishido Design. A lot of Shishido's Amp have the same features: Greinacher (voltage doubler) Power Supply, Interstage couplung with an "inverted" transformer, Class A2, overall feedback, and a feedback winding on the output transformer.
There are similar Shishido amp's with 801A, 811A, 826, 838.
Also there is a commercial amp with the 805 from Antique Soundlab, who is clearly based on Shishido:
http://www.divertech.com/explorer805dt.html
Regards
Ralf
Yes, the mentioned Wavac 805 is a Nobu Shishido Design. A lot of Shishido's Amp have the same features: Greinacher (voltage doubler) Power Supply, Interstage couplung with an "inverted" transformer, Class A2, overall feedback, and a feedback winding on the output transformer.
There are similar Shishido amp's with 801A, 811A, 826, 838.
Also there is a commercial amp with the 805 from Antique Soundlab, who is clearly based on Shishido:
http://www.divertech.com/explorer805dt.html
Regards
Ralf
ASL vs. Shishido
I didn' talked about the amps, I only referred to the basic layout of the ASL schematic, which has a lot of the Shishido design: 12AX7 - 6L6 - IT - 805. Operating points are a bit hotter: 833V Ua, 140mA Idc and +28V Ug. Also it is using the Greinacher voltage doubler in the Power Supply.
Regards
Ralf
I didn' talked about the amps, I only referred to the basic layout of the ASL schematic, which has a lot of the Shishido design: 12AX7 - 6L6 - IT - 805. Operating points are a bit hotter: 833V Ua, 140mA Idc and +28V Ug. Also it is using the Greinacher voltage doubler in the Power Supply.
Regards
Ralf
Brett
i am also going with a pentode front stage but would be a 3 stage
6688 ----el34 cathode follower ---gm70
might go with a 2 stage but power would be lower (can it sustain A2 operation?). Probably try the fame c3m or the sv83
what's the specs for 12HG7A rp,mu ?
would look at pete millet's side if he has it
i am also going with a pentode front stage but would be a 3 stage
6688 ----el34 cathode follower ---gm70
might go with a 2 stage but power would be lower (can it sustain A2 operation?). Probably try the fame c3m or the sv83
what's the specs for 12HG7A rp,mu ?
would look at pete millet's side if he has it
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