I am a relatively new tube amp DIYer. Built two spud amps, a 6BM8, and a 6LU8.
Now, I'd like to try a single ended EL84. I understand that there are at least three output modes, pentode, triode strapped, and ultralinear. I haveno interest in triode mode.
My understanding is that pentode mode would have highest power, and highest distortion. That is mitigated through global negative feedback which reduces distortion, but also decreases output. Ultralinear, reduces distortion,likely better than gnf, but doesn't it also reduce output power?
My goal is to drive the el84s to getthe highest listenable output, basically, headroom. I normallylistento my 6lu8 at a modest volume, and likely similar volume for the el84s. My quest would be not to have the lowest distortion at max output, but low(ish) distortion at listening level, butwiththe mostheadroom I can get.
Now, I know gnf is typically defined in dB negative feedback, but I do not have the knowledge to understand the relative significance of say 15db at a 3 watt output. Especially compared to say any power sacrifice going UL.
There are ma y schematics outthere using both either UL. or gnf.
Help me understand the benefits of each approach.
Tia,
Roger
Now, I'd like to try a single ended EL84. I understand that there are at least three output modes, pentode, triode strapped, and ultralinear. I haveno interest in triode mode.
My understanding is that pentode mode would have highest power, and highest distortion. That is mitigated through global negative feedback which reduces distortion, but also decreases output. Ultralinear, reduces distortion,likely better than gnf, but doesn't it also reduce output power?
My goal is to drive the el84s to getthe highest listenable output, basically, headroom. I normallylistento my 6lu8 at a modest volume, and likely similar volume for the el84s. My quest would be not to have the lowest distortion at max output, but low(ish) distortion at listening level, butwiththe mostheadroom I can get.
Now, I know gnf is typically defined in dB negative feedback, but I do not have the knowledge to understand the relative significance of say 15db at a 3 watt output. Especially compared to say any power sacrifice going UL.
There are ma y schematics outthere using both either UL. or gnf.
Help me understand the benefits of each approach.
Tia,
Roger
UL and GNFB are two different hammers. UL, or more generically screen tapping, lowers distortion up to a certain point with minimal loss in output gain(power), per the UL specification for tap %. GNFB begins to lower gain as it's applied but continues to lower distortion and gain more as more is applied but also has the affect of flattening the FR. It also can cancel noise and other forms of signal issues produced in the amp that are not in the music source signal. Pentode amps give enough power to accomodate power reduction in trade for cleaner SQ and better FR.
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You can think of triode and pentode modes as two extremes, with UL lying somewhere in the middle depending on the tapping point. For example if the tapping point is right at the anode end of the primary winding, you have triode mode: if it’s at the HT centre of the primary, you have pentode mode. Anywhere in between is UL mode, with the benefits depending on exactly where. Hafler & Keroes identified 20% by inductance, or 43% by turns ratio, as an optimum.
It is true that gnfb lowers the gain but that does not mean it lowers output power!
If you plan for gnfb you typically start out with higher gain than necessary so you can turn some of it into distortion-reducing gnfb.
In any case, you turn up the volume to get the output level you want.
Jan
If you plan for gnfb you typically start out with higher gain than necessary so you can turn some of it into distortion-reducing gnfb.
In any case, you turn up the volume to get the output level you want.
Jan
Well its more that poor damping factor effects the frequency response of a speaker system partially the bass response. So its quite audible, and in my opinion the bigger factor. UL, triode mode as well as NFB all increase damping factor.
Frequency response it very audible, but sometimes forgivable. Sometimes the errors caused by non-zero amplifier output impedance are even desirable with a particular loudspeaker, but that can't be counted on as a general case. We need to remember to keep the amplifier's output impedance as low as possible, if only to make it portable between speakers. For conventional speakers, less than an Ohm, fersure.
All good fortune,
Chris
All good fortune,
Chris
okay, and lastly, the two amps I built were UL, with plate feedback. My understanding of this s that it is a form of negative feedback, but because t is not global, it is less effective in reducing noise but does reduce distortion.
If I may summarize, GNF is effective in reducing both noise and distortion, but reduces the gain of the input stage, but not the power stage. UL reduces distortion with minor effect on output (power, gain?). So, *** long as the input stage has sufficient gain to overcome the loss from the GNF, the highest power can still be achieved by the output pentode. I have looked at several schematics for el84 amps, but have dismissed most of the ones without UL mode. Was that a mistake?
Roger
If I may summarize, GNF is effective in reducing both noise and distortion, but reduces the gain of the input stage, but not the power stage. UL reduces distortion with minor effect on output (power, gain?). So, *** long as the input stage has sufficient gain to overcome the loss from the GNF, the highest power can still be achieved by the output pentode. I have looked at several schematics for el84 amps, but have dismissed most of the ones without UL mode. Was that a mistake?
Roger
Right, about the output stage power available. The GNFB will make the input sensitivity lower so you'd need more preamp drive to get the volume/power up vs. without or lesser amounts.
As far as dismissing a pure pentode design, I'd say yes that's a mistake. One of the chief designers from Dynaco published an EL-84 circuit in Radio Electronics and left out the UL connection from their own OPT saying it had too little affect to use in place of a common B+ screen feed. Pentode Magnavox amps support that conclusion, too.
As far as dismissing a pure pentode design, I'd say yes that's a mistake. One of the chief designers from Dynaco published an EL-84 circuit in Radio Electronics and left out the UL connection from their own OPT saying it had too little affect to use in place of a common B+ screen feed. Pentode Magnavox amps support that conclusion, too.
Gnfb subtracts part of the output signal from the input signal thus lowering sensitivity. It does not change the gain structure inside the amp. In fact it does not change anything in the amp itself.
Jan
Right. It often is sobering to realize that the amp itself always works with its open loop gain and phase!
Jan
Jan