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.
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!
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!
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
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
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
(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:
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....
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....
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.
Alastair E said:
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
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....
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....
Looking forward to your progress.
My components should be here by the weekend...
Regards
M. Gregg
Just some thoughts..
Regards capacitor types and this is not audiophile types just observations from experiments.
If you want bass drive on the auto bias amp then use PET "yes polyester" caps as the output coupling caps Vishay 1813 type 400V work well..
I mean you have to turn it down to protect the speaker cones..then again its only 3 watt..LOL
Orange drop 716 give a better sound stage ..the trade off is less power in the bass you could say easy listening..values are the same 470nF
The orange drop 715 is OK but it has steel leadouts so less treble a bit more veiled..
All just for fun...
Regards
M. Gregg
Regards capacitor types and this is not audiophile types just observations from experiments.
If you want bass drive on the auto bias amp then use PET "yes polyester" caps as the output coupling caps Vishay 1813 type 400V work well..
I mean you have to turn it down to protect the speaker cones..then again its only 3 watt..LOL
Orange drop 716 give a better sound stage ..the trade off is less power in the bass you could say easy listening..values are the same 470nF
The orange drop 715 is OK but it has steel leadouts so less treble a bit more veiled..
All just for fun...
Regards
M. Gregg
Last edited:
Excellent Plan!
Some sort of clamp say, a high power zener on the cathodes of O/P pair, set to conduct just above the normal bias voltage would help maybe...
Any other ideas..?
Looking at Alastair's circuit in post #10 , I see that there is a big waste of power over the 150 Ω cathode resistors , because of the voltage drop in there , and when the volume goes on , this loses is getting bigger . And if we want to get more power from the output tubes we have to remove those resistors and use fixed bias . And after that we have to specify the capabilities of the PI stage , this is my opinion .
Components arrived today..
I have fitted the 270 ohm gate stoppers on first stage ccs..
47K now on each side of the PI...
I have used a 10K as grid stopper on PI..
Took about half an hour..
Sound has changed....more detail in the highs...
just breaking in at the moment ..then some scope readings..
Next step to look at the 6c33c cathode resistors...
Regards
M. Gregg
I have fitted the 270 ohm gate stoppers on first stage ccs..
47K now on each side of the PI...
I have used a 10K as grid stopper on PI..
Took about half an hour..
Sound has changed....more detail in the highs...
just breaking in at the moment ..then some scope readings..
Next step to look at the 6c33c cathode resistors...
Regards
M. Gregg
Ive made some Mosfet CVS for the cathodes of O/P bottles.
Still running the PL509 and nothing else changed, the 11V P-P Maximum before clip has increased to 13V P-P. I carefully set this up for a Perfectly clean and nice sine-wave, before measuring.
--The MOSFETS got Nice and Hot, during the 2-3 mins I had it at full-chunter!
Not a Vast improvement in power.
Checked the PI, and looks like I'm running out of steam in that stage, Currently it manages about 65V P-P on each of its outputs, further increasing drive exhibits that flat-top and bottom nipple-distortion as illustrated on 'The Valve Wizard' website the Grid-Resistor is now 100K instead of the 470K I originally had there to prevent this.
For the 6SN7, we need more +B to get much more outta it I think, My +B to the PI is only 320V.
Any suggestions welcome for this issue, but we would prefer to keep the 6SN7....
So, Partial success. For further power increase with my current PL509 pair I need to get more drive from the PI as well as probably more +-B rails to O/P stage.
The drive and cathode-resistor/mosfet mods would carry over to any O/P triode used, and particularly the Cathode resistor/mosfet mod should show benefits for higher current triodes like the 6C33C
Still running the PL509 and nothing else changed, the 11V P-P Maximum before clip has increased to 13V P-P. I carefully set this up for a Perfectly clean and nice sine-wave, before measuring.
--The MOSFETS got Nice and Hot, during the 2-3 mins I had it at full-chunter!
Not a Vast improvement in power.
Checked the PI, and looks like I'm running out of steam in that stage, Currently it manages about 65V P-P on each of its outputs, further increasing drive exhibits that flat-top and bottom nipple-distortion as illustrated on 'The Valve Wizard' website the Grid-Resistor is now 100K instead of the 470K I originally had there to prevent this.
For the 6SN7, we need more +B to get much more outta it I think, My +B to the PI is only 320V.
Any suggestions welcome for this issue, but we would prefer to keep the 6SN7....
So, Partial success. For further power increase with my current PL509 pair I need to get more drive from the PI as well as probably more +-B rails to O/P stage.
The drive and cathode-resistor/mosfet mods would carry over to any O/P triode used, and particularly the Cathode resistor/mosfet mod should show benefits for higher current triodes like the 6C33C
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.