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Old 4th December 2011, 08:11 PM   #521
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Well, The 100K currently on the grid of the PI doesnt limit the HF response as its good to 200KHz....-3dB

What were you thinking? Why delete it?
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Old 4th December 2011, 08:23 PM   #522
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Not worried about HF rollof.

More to see what it does to response of the amp.

If its not needed for stopping oscillation and the amp is stable the CC will be traded for a MF.
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Old 4th December 2011, 08:50 PM   #523
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Think I know why.....

Steady-State measurements aint gonna tell the power of this amp....

Because, The By-Pass caps in the cathode-circuits of O/P tubes charge up under these conditions, upsetting the bias....

Doesn't happen till much higher percieved powers playing Music--After all, WHO listens to Sine-Waves!
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Old 5th December 2011, 02:11 AM   #524
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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....?
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Old 5th December 2011, 03:02 AM   #525
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Alastair E View Post
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.
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Old 5th December 2011, 05:18 PM   #526
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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!
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Old 5th December 2011, 06:58 PM   #527
M Gregg is offline M Gregg  United Kingdom
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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

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Old 5th December 2011, 08:59 PM   #528
M Gregg is offline M Gregg  United Kingdom
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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...

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Old 6th December 2011, 03:55 PM   #529
M Gregg is offline M Gregg  United Kingdom
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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...

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Old 6th December 2011, 06:24 PM   #530
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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....
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