Calculating expected output power.
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 9th September 2005, 11:42 AM #1 Alastair E   diyAudio Member     Join Date: Feb 2004 Location: South Wales Calculating expected output power. Is it possible to calculate the approximate output power of a P-P amp, with 811A's as O/P valves, set to run with idle around 75mA, (AB1 ish) at 400 odd volts into 6.6K A-A load Tx, while using MOSFET drive? What is the ideal Output Tx Impedance for this tube, under voltages of around 400v Va?--Ive seen designs on the 'Net with as low as 2K and as high as 7K anode loads, with voltages around 4-500v Va, and cant find the plate curves for it. (Not that Im much good at reading them anyway!) Im looking to designing OP TX, but need to have some kind of figure to base its sizing on and a more definate anode load figure.--The current breadboard is using an 'El Cheapo' Danbury Maplin 20W Tx with suprisingly good results, sound wise--Ive done no proper testing apart from listening at the mo.!--Sounds like a lot more than 20W at full o/p, before the Tx saturates and distortion is apparent. Would I be right in saying that as the Max Anode current of the 811A is 175 mA, and as Ive got P-P, Ive got 200 mA 'Usable Current' at 400V--Around 80w Input, not including bias, to the output stage. Assuming a close to perfect output Tx, and not taking into consideration anode dissipation would be exceeded at full output and all things being equal, could I expect around 50W output?--Or am I completely off-it this morning! __________________ Das Beste Oder Nichts
 9th September 2005, 04:59 PM #2 cerrem   diyAudio Member     Join Date: Jun 2005 Location: San Diego, CA Here is a link to the PLATE CURVES...... http://www.triodeel.com/811a_p6.gif Due to the nature of these curves.... It is not your normal TRIODE design procedure.... It is "almost" like PENTODE operation....... The plate load you choose here is really all about the PEAK current your able to tolerate.... The lower the plate load the higher the AC peak current....and the higher the drive signal needed.... I will crunch the numbers and get back to you... Chris
 9th September 2005, 06:56 PM #3 cerrem   diyAudio Member     Join Date: Jun 2005 Location: San Diego, CA WIth 400V at the plates.... and a 6.6K plate load... You would drive up to +50V on the 811-A grids...with about 70mA of grid current...which roughly looks like driving at input resistance as low as 714 ohms..... so figure you need a follower to drive this with pretty low output Z..... This would be a power Output of roughly 38.4 Watts.... Figure in Class AB2 you would bias above 0V...figure +20V would give you 60mA each valve.... Chris
 9th September 2005, 08:45 PM #4 cerrem   diyAudio Member     Join Date: Jun 2005 Location: San Diego, CA OK... You have 400V and a 4K Plate-to-Plate load..... You make 57 Watts of power output.... Drive signal to +67 volts AC at the grid... Grid current will be about 100mA ..... This will result in a 670 ohm input grid resistance at it's lowest...so you need to appropriately drive this.... I would drive this with a 50 ohm source minimum.... which is a 7% voltage loss...which constitutes a -.63dB loss at full steam... Figure you can bias at +20v which is 60mA per valve..... You need to establish what B+ you intend on using.... In this case you have round up and say 60W on the output transformer at 4K ...this will result in a AC voltage of 490V AC across the full winding.... For the transformer...Figure you want 13,000 Gauss at full power output at 20HZ .... That would work out to be a E-I 150 lamination...square core being 1.5" stack.... SO that would be 3260 Turns.... Using M6 core material, which is grain oriented...... Figure to use #29 or #30 AWG primary wire for about 900 to 1000 CM/Amp... I leave it to you to figure the secondary turns and winding geometries to best fit your application....
 11th September 2005, 12:42 AM #6 PRR   diyAudio Member     Join Date: Jun 2003 Location: Maine USA > using an 'El Cheapo' Danbury Maplin 20W Tx with suprisingly good results, sound wise OTOH, if you already built it this way, then I say drop on any good 8KCT 50W transformer. But... > more than 20W at full o/p, before the Tx saturates and distortion is apparent. A "20W" transformer is 20W only at a specific low frequency and distortion. It will take more power at higher frequencies. If it is good for 20W at 50Hz, it will do 80W at 100Hz at the same THD. Unless you are specifically fuzzing-up on bass tones, you are likely not bending the iron, you are running out of drive or just asking for more than 811 can do at 400V. And in ear-only tests, 20W and 40W are "the same".
 11th September 2005, 09:53 PM #7 Alastair E   diyAudio Member     Join Date: Feb 2004 Location: South Wales Thanks for the info Great Info Cerrem! Im grateful for your input, and have started the transformer design based on your excellent info! Now, Mr PRR, Firstly, Im sorry, I have obviously irritated you, and I grant your knowlege is obviously superior to my own. I didnt have the luxury of being in the Industry when valves were the norm, When I started, the best you got was Hybrid sets, maybe with a restoration of a Vox AC30 occasionally! I Quote-- 'Ugh. Antique brutes. Shame we don't have anything better' What do you consider better?--The Boring more like! 300B anyone?? Maybe PX25, Equally Antique!! If I wanted to build a valve amp with something as boring as a 6L6, 807, EL34(Ugh, I would never use an EL34, Vile things!) 300B, or a million other well known and equally boring output valves, I would have done so, and followed the croud as many have done before, and Im sure with excellent results..............! However-- The idea was to make something slightly different out of the very cheap, available, and DIFFERENT 811A I assume when you were referring to Cost, It was component and not energy efficiency, If we were making amps for efficiency, we would be making Tripath based amps! (Been there, Done that!) You can get at least 10 811A's for the price of a second-hand well used balloon PX25!--I have two NOS specimens of these antquated beasts, which I would never ever dream of making a quality amplifier with! I would rather sell them to the 'easily brainwashed' and NOT be drawn into all that 'This valve sounds better than that one' crap About the 'never able to exceed the Max Anode dissipation rating with only 400V on the anode--Wrong! My Bias arrangement follows the rather excellent 'Power-Drive' devised by Tubelabs and is MOSFET based, (Maybe my use of silicon has been the source! of your irritation) the preceeding stages are Mu stage then simple phase-splitter (both Valve Based) During my testing, I was able easily to exceed the anode diss of the 811A, by winding the bias to end-stop, and applying 38V to its grid, I had 135mA Anode current per triode which at 400V is I believe 54W, some 9W over its maximum rating! The power-drive stage has its own dedicated +150v/-50v supplies, thus the issue of power wastage from the main +b is not an issue Conserning Output Transformers, Im well aware of the efficiency at different frequencies, as I have had quite some success designing and winding my own. Thus in my statement-- more than 20W at full o/p, before the Tx saturates and distortion is apparent' I meant that bass lines are distorted at high outputs, which must be exceeding the LF power pass of the Tx Im using, which is a cheap cruddy part! I assumed this would have been clear, evidently not!--The idea of this post was to assess the size of a trans. where this would not become a problem--I would rather other factors limit the the amplifier than the iron, which is dirt-cheap if you design and wind yourself, and gives a much greater sense of achievement! I will not post the schematic, as Im sure it would cause you even greater irritation................. So, in ending, I apologise if I upset/irritated you, it was never intended that I do this, with my very stupid questions, But Remember-- No One Knows Everything! __________________ Das Beste Oder Nichts
 12th September 2005, 12:19 AM #8 PRR   diyAudio Member     Join Date: Jun 2003 Location: Maine USA Well stated.
 12th September 2005, 11:54 AM #9 Alastair E   diyAudio Member     Join Date: Feb 2004 Location: South Wales PRR- Can we be friends now...........? __________________ Das Beste Oder Nichts
 12th September 2005, 12:00 PM #10 SY   On Hiatus     Join Date: Oct 2002 Location: Chicagoland Man, I must be the apex of insensitivity; I didn't pick up irritation at all. In fact, I thought PRR's response was a fine read (as usual) and quite an enlightening exegesis. There are some things in this world that just aren't great ideas, and there's nothing wrong with knocking them down early in the process. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."

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