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In Reply to: RE: OK, Great ........................ posted by Cut-Throat on March 27, 2021 at 08:12:38
Mike, on the Magnequest forum here on the AA, used to host the paper by
Voltsecond but it's not there anymore.All output transformers have a limited amount of inductance. Single ended output transformers need to have a gap so the idle current for the output tube doesn't saturate the core. When a gap is placed in the core of a transformer the inductance goes way down. The inductance can be brought back up by winding more wire around the core but there are limits. Too much wire and you will began to lose highs because of the winding capacitance. The end result of all the this is that SET transformers do not have a ton of inductance. The lack of inductance means that the reactance of that inductance will be small for the lower frequencies and that is what causes the elliptical load line.
If you allow the lower frequencies to reach the output transformer the load line becomes elliptical instead of the straight load line we think of when we're dealing with a plate resistor. An elliptical load line causes the tube to operate in a less linear part of the operating curve for part of the wave form.
The part that most people miss (and the part that Voltsecond points out) is that once there are low frequencies present in the output transformer all the frequencies have to follow that elliptical load line, not just the bass frequencies. If you can keep the low frequencies out of the transformer the load line will be much more like a resistive load line and there will be less distortion.
Even if you don't follow what I am trying to explain or if you don't believe me, it costs very little to try this and it is totally reversible.
After reading Voltsecond's paper, I placed caps in front of my SET 300b amps driving my midrange and tweeter and was shocked at the improvement.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 03/27/21Follow Ups:
I agree with the main premise and suggestions.
I'd just like to add that the inductance and capacitance (and it affect on high-frequency performance) seems often overstated. A good winder using suitable materials and techniques should be able to achieve sufficient inductance and high frequency extension for most common SE output requirements. A 5k:8 ohm, 35mA transformer ain't that tough.
And I'm not suggesting a "perfect" transformer can be wound, or that there is only one right way etc... that easing of inductance for a different set of compromises is not desirable for some tastes in some systems.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
I don't know of any gapped output transformers that have enough inductance to prevent an elliptical load line in the presents of low bass.Let me explain a little more. I don't play bass using SET transformers for three reasons. One, not enough power, 2 too low a damping factor and 3 the subject at hand.
I have seen sims that show the elliptical load lines for a "normal" SET output transformer at low frequencies and it's not pretty. I have also seen sims of the load line when the bass is limited to 200Hz for the same "normal" SET output transformer. With the bass frequencies limited the load line almost looks like a resistive load line resulting is much less harmonic distortion.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 03/27/21
As I said, I agree with your point in principle and would do similar if biamping. I think biamping is great approach for the reasons you suggested.
My point was that the severity and significance of some of the compromises mentioned can be overstated, especially where quality output transformers are concerned, and IME don't preclude fine or enjoyable performance used full range in a suitably matched system.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
.
Have Fun and Enjoy the Music
"Still Working the Problem"
nt
I wanted to stay away from your 270Hz crossover point but make sure to limit the low bass as much as possible to your output transformer.
If you're 270Hz crossover is 6db then you could use a .00056uf cap instead and get rid of the passive 270Hz crossover part but that would leave your horn subject to any bumps that might happen in the amp so it would not be as safe.
Or, if your 270Hz crossover is 12db then you could change it to 6db and have 6db at 270Hz before the amp and 6db at 270Hz after the amp and still be safe-ish.
But let's not get too off track. Limit the low bass to the amp and enjoy the improvement.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Seems a bit hard to find for me..
Edits: 03/27/21
http://www.soniccraft.com/index.php/film-capacitors-rel-cap-rte-c-301_48
I'm having trouble too.
Why is the grid resistor 1 meg?
You could get two 1000pf (.001uf) and two 500pf (.0005uf)
Place one 1000pf in parallel with one 500pf. That would be one .0015uf cap and that will work.
Against the 1meg grid resistor it would make a 6db per octave high pass filter with a -3db point of 106Hz.
https://tubedepot.com/products/silver-mica-1000pf-500v?utm_source=google&utm_medium=cpc&adpos=&scid=scplpCP-SM-1000-500V&sc_intid=CP-SM-1000-500V&gclid=EAIaIQobChMIvYjtuKbR7wIVfyGtBh1UYgR_EAQYBCABEgJiAvD_BwE
https://tubedepot.com/products/silver-mica-500pf-500v?utm_source=google&utm_medium=cpc&adpos=&scid=scplpCP-SM-500-500V&sc_intid=CP-SM-500-500V&gclid=EAIaIQobChMIvYjtuKbR7wIVfyGtBh1UYgR_EAQYASABEgJ2uPD_BwE
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
What do you think of these?
https://www.ebay.com/itm/0015-uF-1500pf-630-volt-Polystyrene-Capacitor-NOS-QTY-20-ea-P510/192162051515?hash=item2cbdc035bb:g:fJQAAOSwE1xbyhgr
or
https://www.ebay.com/itm/2-NOS-Vintage-Pyramid-0015-uf-300v-PIO-Capacitors-Vitamin-Q-Oil-Treble-Bleed/122629572318?hash=item1c8d4ac6de:g:TMAAAOSwjC9ZgoWT
or
https://www.ebay.com/itm/2-NOS-Vintage-Fast-0015-uf-300v-PIO-Capacitors-Vitamin-Q-Oil-Treble-Bleed-Caps/112507170078?hash=item1a31f32d1e:g:21wAAOSw3IdZgoZL
Polystyrenes are pretty good sounding caps.
I don't like PIO caps anymore, I use to.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I have a 3535Pf (.003535uf) Russian silver mica cap against a 330k ohm grid resistor for a -3db point of 136Hz. My midrange speakers are about 3db down and falling at 200Hz all by themselves. I use an electronic (opamp based) low pass filter at 200Hz. in front of the SS amp I use to drive my JBL 2231s woofers.I find the Russian silver mica caps to be very transparent. I use them in my phono preamp also.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 03/27/21 03/27/21
https://www.hificollective.co.uk/search/site?title=.0015
The Audio Note Tin Foil Capacitor is not to bad.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"Why is the grid resistor 1 meg?"
I don't know, you'd have to ask Jeff Korneff :-)
It goes back to the Article in 'Glass Audio' to a Schematic from Eric Barbour, where Jeff Korneff got his initial design. Here is the Schematic from that Article.
It would be nice to use a Single Capacitor.
You can make that grid resistor just about any value you like.
I use the 330k on my first stage because it fits well and gives me the frequency I want with the sliver mica caps I had. And it's plenty high as to not load down my preamp.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Can't you get a capacitor close to crossover FQ and fine tune with the grid resistor value?
Wouldn't it be cheaper to swap out resistors than caps?
What formula do you use to calculate crossover FQ?
What can a 500 ohm output impedance preamp be comfortable with?
"Can't you get a capacitor close to crossover FQ and fine tune with the grid resistor value?"
That's what I do.
"What formula do you use to calculate crossover FQ?"
-3db point of the filter = 159155/(cap value in uf times resistance value)
"What can a 500 ohm output impedance preamp be comfortable with?"
Almost anything. The rule of thumb is 1:10 so it will drive 5k without trouble. Just don't forget about capacitance reactance at the high frequencies which is in parallel with the grid resistor value. That would include cable capacitance and the Miller capacitance of the tube being driven. Also the source needs to be able to supply the current that capacitance reactance will require.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
T
.
Have Fun and Enjoy the Music
"Still Working the Problem"
nt
Sometime this year... It's quick and easy to do.
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