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 5th December 2011, 02:11 AM #521 Alastair E   diyAudio Member     Join Date: Feb 2004 Location: South Wales According to various sites on the web,-- RMS Wattage is the Product of RMS Volts and RMS Curent.... Therefore, 11V, RMS value 7.778 And current, 1.375, RMS value, 0.972 Equals, -- 7.76 RMS Watts.... So--Where did I go wrong....? __________________ Das Beste Oder Nichts
kevinkr
diyAudio Moderator

Join Date: Sep 2004
Location: Boston, Massachusetts
Quote:
 Originally Posted by Alastair E According to various sites on the web,-- RMS Wattage is the Product of RMS Volts and RMS Curent.... Therefore, 11V, RMS value 7.778 And current, 1.375, RMS value, 0.972 Equals, -- 7.76 RMS Watts.... So--Where did I go wrong....?
Is that V peak (Vpk) or V peak to peak (Vpp), you have not specified which.. You would normally use Vpk, and you can calculate rms power directly as Vpk^2/2RL or Vrms^2/RL if you have a good audio voltmeter. (Note the round off error if you convert Vpk to Vrms - it is significant) You can also use vpp just divide by half and then use the first equation..

More than likely the driver stage cannot quite make up for the voltage losses in the output stage which are quite significant recollecting from my 6C33 OTL debacle..

IIRC the source impedance of a 6C33 pair in a futterman configuration without feedback is in the vicinity of 12 ohms.. Not sure if the reverse futterman is better or worse than this.

I'd see how much more swing you can extract out of the first stage, and how much additional swing you can get in the second without compromising the sonic qualities you like. I'd run the 6SL7 at half the available supply and play some games with the concertina to maximize voltage swing while trying to keep the current above 6mA. (I like 9 - 10mA for the 6SN7)

FWIW: Mine with all of its design issues still managed to sound really good, noting that my six 6C33 "200W" design only achieved 25W as my driver stages could not deliver the required voltages and current.. It was my epic fail, and given this one works reliably and sounds good you are most of the way there.

One other comment I would make is that in characterizing and matching 6C33 for my amps I saw some very strange behavior when I only ran half the tube. (So long ago I do not remember what, so take it as anecdotal.) Obviously for matching I ran one section at a time and calculated the transconductance based on several different grid voltages and measured plate current at the operating plate voltage. Unloaded there was a measurable difference in linearity and source impedance obviously doubles. I always ran all filaments and just removed pairs when less power was needed.
__________________
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine

 5th December 2011, 05:18 PM #523 Alastair E   diyAudio Member     Join Date: Feb 2004 Location: South Wales Ah, Hi Kevin, I value input from a top guy such as yourself, Thanks.... Maths was never my strong-point! seems to me rather an odd way to reference power though its the accepted 'norm' First, take the P-p value, divide by two then take approx a little less than 2/3 (the rms of whats left) and Thats the RMS Watts in Audio...(Which seems to be approx half the RMS Watts in electrical engineering...) My measurements were in P-P value. But Must add, Its the Loudest 3.5W ish Ive ever heard. I was running it today, I live on the third-floor of an apartment-block, and my partner came in said could be heard plain at bottom of stairs, and that I should turn it down for fear neighbour complaints, and this was through two closed doors! My speakers are hardly what could be called 'efficient' either! --I dunno,-Maybe its the effect I described earlier...... Concerning the drive to the O/P pair, I have available 60V P-P to each grid, before onset of clip. Maybe a different PI tube or a higher +B to the PI stage would increase this, although I dont think this is the limiting factor. --The cathodes of the PL509 I'm experimenting with are approx 23V, so the 60V P-P should be enough to drive them, although with the design as is, I would not be able to go positive-grid, as I'm driving direct from the PI.... You are certainly right, The 6C33 does have some strange habits all its own, Thats for sure! __________________ Das Beste Oder Nichts
 5th December 2011, 06:58 PM #524 M Gregg   Account disabled at member's request   Join Date: Jun 2010 Location: UK You know I have been thinking about this.. I had thoughts of trying fixed bias etc etc Then I had a thought..I had 8 EL34's running fixed bias some time ago and was forever turning it down....it only sounded at its best when on the verge of to loud.. I have the amp running as I type this and if I turn it up I have to shout to be understood...Why do I need it louder? Still sounds great....unplug the tubes plug back in thats it... Why do I need something louder ?? Just a reflective thought.. I guess thats my take on it...If I want louder I will get 100db speakers these are only 4 ohm..the cones are nearly leaving the cab..YMMV Regards M. Gregg
 5th December 2011, 08:59 PM #525 M Gregg   Account disabled at member's request   Join Date: Jun 2010 Location: UK Alastair, As discussed.. I will do the PI mods and check the sine wave/square wave is clear. Also replace the gate stopper on 1st stage FET with 270 Ohm carbon.. Update the coupling caps to the 6c33c's. Just waiting for components on order... Regards M. Gregg
 6th December 2011, 03:55 PM #526 M Gregg   Account disabled at member's request   Join Date: Jun 2010 Location: UK It would be interesting, (I get the impression some people want me to try fixed bias...LOL) To hear peoples ideas of improvements and mods... Lets keep looking at ways forward.. Lets keep talking OTL's what would you do? Just find the OTL subject quite interesting... Regards M. Gregg Last edited by M Gregg; 6th December 2011 at 03:59 PM.
 6th December 2011, 06:24 PM #527 Alastair E   diyAudio Member     Join Date: Feb 2004 Location: South Wales Well, Fixed-Bias wouldnt be that difficult. For the Upper bottle, a pot-divider to the neg rail could be used, setting the grid-voltage to give say, 200mA through the cathode-resistor which could now be a 1 ohm, the cathode-caps deleted. --This bias supply could be well de-coupled to avoid any interactions from the neg rail, then straight to the grid-resistor which no longer returns to Gnd. For the Lower bottle, a separate single bias supply, say of Minus 75V, referenced to the neg rail could be used, and again the bias set to give 200mA through a 1 ohm in its cathode... Doing so, and we are back to the more usual Inv. Futterman design, we lose the 'fiddle-free' self-biassing on the O/P pair--But we do gain higher power possibilities. It means also, an additional PSU rail referenced to -B of say, Minus 75V, adjustable for correct current through the tube.... __________________ Das Beste Oder Nichts
cnpope
diyAudio Member

Join Date: Aug 2009
Location: College Station, TX
Quote:
 Originally Posted by Alastair E Well, Fixed-Bias wouldnt be that difficult. [...] Doing so, and we are back to the more usual Inv. Futterman design, we lose the 'fiddle-free' self-biassing on the O/P pair--But we do gain higher power possibilities. It means also, an additional PSU rail referenced to -B of say, Minus 75V, adjustable for correct current through the tube....
The output stage would then be very like the Hans Beijner design, and that is giving me almost exactly 25W rms into 8 ohms at clipping.

Quote:
 Originally Posted by Alastair E [...] Maths was never my strong-point! seems to me rather an odd way to reference power though its the accepted 'norm' First, take the P-p value, divide by two then take approx a little less than 2/3 (the rms of whats left) and Thats the RMS Watts in Audio...(Which seems to be approx half the RMS Watts in electrical engineering...) [...]
V_{rms} = V_{peak}/sqrt(2) for a sinewave, and V_{peak}=V_{peak-to-peak}/2. I would think electrical engineers must use this too. The heat dumped into the resistor R is precisely V_{rms}**2/R. Reading off V_{peak-to-peak} is convenient when looking at a trace on a scope, but the voltage between the two terminals of the load never gets greater than one half of V_{peak-to-peak} at any time. And the extra sqrt(2) denominator takes care of the time averaging of the oscillatory voltage. (Literally, the square root of the time average of the voltage-squared.)

Chris

 6th December 2011, 08:13 PM #529 Alastair E   diyAudio Member     Join Date: Feb 2004 Location: South Wales Hi Chris, Yup--Quite right. My original idea for this OTL was not the persuit of Power, but for ease of build and a fair performance. --For reliable High Power, We DONT build OTL's! The power being only 3.5W does however surprise me greatly, Certainly Sounds like at least 20W! --Be that as it may, the O/P stage is easily adaptable to Fixed-Bias, the rest of the amp should drive it without issues. Hans Beijner's design is nice and isn't Hugely different from this one.... __________________ Das Beste Oder Nichts
 6th December 2011, 08:42 PM #530 flatheadmurre   diyAudio Member   Join Date: Mar 2011 Location: Jarna What attracted me first with this design was the lack of adjusting and fiddling. Alistair lets try to get the most out of it without fixed bias.

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