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In Reply to: RE: Maximum Output posted by Tre' on January 27, 2015 at 13:24:53
Interesting that EML 45 data sheet says 3.9k for lower voltage/current operation too.
Doesn't specify for 2A3.
I assume this is all for 8ohm on 8ohm tap.
If you use 16ohm speaker, don't know what Denis uses, on 8ohm tap then that effectively doubles the load impedence seen and reduces power and distortion still further. Am I correct? Need help here.
OOI I use a 5K with 16ohm on 8ohm tap for my 45s. Maybe I should use a 3.9k next time around.
Cheers
Follow Ups:
Up tapping (16 ohms on 8 ohm tap) does indeed double the reflected impedance seen by the output tube.
Because the impedance has changed but the inductance of the primary has not, the -3db point, in Hz, of the bass response will double.
If, for instance, the -3db point (of the response of the transformer, not the system as a whole. There are many other things involved with that) would have been 20Hz with 8ohms on the 8ohm tap then it will be 40Hz with 16ohms on the 8ohm tap.
There's nothing magic about 3.9k ohms as a load.
If you know the plate resistance, the voltage across the tube and the idle current you want to run......the formula will tell you what a good load impedance, for that tube under those conditions, will be.
3.9k ohms just happens to be a good load for Dennis' 2a3 assuming he's running 250 volts, and 43ma. with a plate resistance of 800ohms as he seems to report.
What impedance would be good for your 45 the way you're running it? I don't know. I don't know the particulars of your output stage but 10k on a 45 is probably pretty good.
Once you do the math you can always go a little higher giving up some max power for even lower HD.
BTW, I also up tap my ElectraPrint PSSS output transformers.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Up tapping (16 ohms on 8 ohm tap) does indeed double the reflected impedance seen by the output tube.
Check
Because the impedance has changed but the inductance of the primary has not, the -3db point, in Hz, of the bass response will double.
This depends if you are using a triode or a pentode. From the simple model the -3dB point is where the inductive reactance is equal to the parallel combo of the Tube Rp and the reflected load so simply doubling the reflected load so keeping the tube Rp constant typically makes the push up in frequency much smaller than an octave for a triode but does approximately double it for a pentode
dave
".... From the simple model the -3dB point is where the inductive reactance is equal to the parallel combo of the Tube Rp and the reflected load ..."Thanks for that information. I stand corrected.
Dave, can you tell me what happens at the other end?
What effect on the high frequency -3db point does "up tapping" the output transformer have?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/28/15
What Dave gave you in the table was the "small signal" response. Now, look at the "large signal" response and see what happens when you (via creative tapsmanship) reflect 6400 ohms to the primary as opposed to it's design center of 3200 ohms.
MSL
Builder of MagneQuest & Peerless transformers since 1989
is there a table or formula I can look at?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
for an explanation of the differences btwn small
signal response and large signal response see:http://www.magnequest.com/tech_article2_voltsecond.htm
for further discussion of the effects of L vs "reflected impedance"
see:http://www.magnequest.com/tech2.htm
Builder of MagneQuest & Peerless transformers since 1989
Edits: 01/29/15
"Phase Shift Desired
Multiplier
5 degrees
11.43
10 degrees
5.67
15 degrees
3.73
30 degrees
1.73
45 degrees
1
Here is an example of how to use the above multipliers. Say you have a 5,000 ohm nominal primary impedance and you desire only 15 degrees of phase shift at 100 hertz.
So at 100 hertz we would want (by referencing the table above) an inductive impedance of 3.73 times 5,000 ohms.
Solve for the following;
18,650 equals 6.2834 times 100 times L
L is the unknown variable. In this case L must be 29.6814 henries."
So if the 5K is changed by up tapping to 10k the required inductance for a desired phase shift will double.
So for a -3db point (phase shift of 45 degrees) L = reflected impedance / (6.2834 * frequency). (added to my spreadsheet)
If I double the reflected impedance by up tapping the output transformer the -3db point goes up an octave.
3200:8 with 25.5Hy will do 20Hz at -3db. Connecting a 16 ohm speaker will reflect 6400 ohms and the -3db point will be 40Hz. To maintain 20Hz at -3db would require 51Hy.
Thanks Mike.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hi Tre:I didn't check your calculations or anything but I think
you got the picture.Just using a small signal model will make some crudballs
look like their ready for prom night...the tougher test.. and the more realistic... is the large
signal test.I found the following text on a Lundahl spec sheet...
to explain large signal vs small signal....FPL= large signal
FRL= small signalWe define Power Low Frequency Limit, FPL, as the frequency where wLP = RLOAD. (The reactive impedance of
the transformer equals the primary load impedance). At FPL, the output power is reduced to 50%.We define Response Low Frequency Limit, FRL as the frequency where a (small) output signal is reduced with
-1 dB due to finite primary inductance. FRL = w / p, if you solve w in
wLP = (RLOAD in parallell with RANODE).
In the FPL when the power is reduced by 3db the phase angle of the load impedance will be 45 degrees. Instead of your loadline being a straight line it's going to begin to look like a NFL football (i.e., ellipse). Your tube generated distortion is going to increase dramatically and that isn't even taking into account that the transformer borne distortion will also be going up at the same time.Plate curves assume a linear with frequency unchanging
pure resistance is loading the anode of the tube. If your instead loading the anode with a transformer... nothing could be further from
the truth. Now you must take into account the characteristics and functioning of the transformer... cause now we have reactive components in a complex relationship with "pure" resistances.Now the effective loadline can look more like a football or beachball.
And this elliptical loadline changes shape with frequency... so that your tube now has to drive an everchanging loadline....and it aint going to be pretty when having to do all that work.So instead of focusing on the "easier" small signal response... looking at and evaluating the large signal response (the decidely tougher test) will be much more predictive of how well your tube and circuit will perform.
MSL
Builder of MagneQuest & Peerless transformers since 1989
Edits: 01/29/15 01/29/15 01/29/15
"It does not matter if your speaker does not go down to 31.25 Hz or not. If you are sending low frequency information to the amplifier, the amplifier will make the plate voltage move. If the plate voltage moves, the tube must spend bias current to charge and discharge the primary inductance as well as the actual primary load impedance. If the plate voltage or current "clips" because of low frequency plate excursions, the sound will suffer. "
"With 27 H, the load line is nasty. With a 27 H primary if there is any low frequencies making it from the preamp to the output transformer, I would expect the sound to be a bit nasty too. "
The practical implications for me; I just placed the high pass filter cap back in series with the input to my SET amp that drives my midrange (and tweeter) that only plays to 200Hz.
Thanks again Mike.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
That's a very important point that Voltsec made--- cause a lot of people will initially say I don't care what my amp is doing down at 50 or 40 or 30 hertz... cause my speakers only go down to X... but as Voltsec points out by then the damage is done....Your response--- using a high pass filter--- it's generally regarded as a taboo approach in hi end circles--- but I think the approach has a lot of initial merit to it. Frank Van Alstine in the mid eighties started using a high pass filter on the input of his ST-70 mod/circuit cause (his reasoning) it did not allow the circuit to be swamped with frequencies which it could not handle.
Or spec and use a transformer with really, really good capabilities in this respect (i.e., large signal goodness).
And to bring this back to "creative tapsmanship"-- i.e., ratioing transformer impedances-- don't do it--- your not likely on the vast majority of transformers to have enough "surplus" L to keep the loadline from becoming a beachball. And your tube really won't be all that happy with that.
MSL
Builder of MagneQuest & Peerless transformers since 1989
Edits: 01/29/15 01/29/15 01/29/15 01/29/15
"Your response--- using a high pass filter--- it's generally regarded as a taboo approach in hi end circles--- but I think the approach has a lot of initial merit to it."If this was a full range amp I might regard it as taboo as well but this is a bi-amped system with a SS amp driving a pair of 15" JBL 2231s using an active 12db per octave low pass filter at 200Hz.
The JBL 2118J midrange speakers roll off on their own and are -3db at 200Hz.
I think keeping some of the LF out of the tube amp with a cap should help the issue that Voltsec points out and I believe this would be true even if my tube amp's OPT had "full" inductance.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/29/15
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