I think you are conflating gain and headroom.
We are going off the subject here, though. ;^)
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I think you are conflating gain and headroom.
I really don't believe I am. However, the larger resistor will yield both more gain -- before clipping -- and more headroom to the stage.
As the OP of this thread, I disagree. I did agree with you when you went "off topic" (your words, not mine) and down the 2.5% difference in resistance rabbit hole. Charge away, Martin, we're totally on topic and I think there is good information to be had from this dialog.
Maybe we can drill that down after you’ve had a chance to read the reference I provided. Deal?martin manning wrote: ↑Thu Aug 29, 2019 5:07 pm Gain is the slope of the transfer curve (dVa/dVg), is it not?
What I mean by off topic is we are now discussing the sound of changes made to the circuit vs. the sound of new resistors vs. old ones.
To recap, I will list again my original claims that a larger plate load resistor results in:martin manning wrote: ↑Thu Aug 29, 2019 6:13 pm Hold on... here is gain (specifically small signal gain like we were discussing above):
-mu Ra/(Ra + ra)
This is defined at the quiescent point, and it has nothing to do with headroom or clipping.
Hmmm, not quite, let me summarize again. The LCA0414 Dralorics are carbon film resistors and replaced almost everything (not counting the power supply resistors, but counting the bias feed resistors mounted on the preamp board), except: (a) V1 grid stoppers (at least for now I still have old NTE metal films there); (b) Power amp feedback loop 4.7K resistor (at least for now, I still have an old NTE metal film there); and (c) V1 and V2 plate and cathode resistors. The old plate resistors were Dale MFs, the old cathodes KOA Speer MFs; all were replaced with Roderstein MF resistors. Hope this makes it clear now.
Hmmm, not quite, let me summarize again. The LCA0414 Dralorics are carbon film resistors and replaced almost everything (not counting the power supply resistors, but counting the bias feed resistors mounted on the preamp board), except: (a) V1 grid stoppers (at least for now I still have old NTE metal films there); (b) Power amp feedback loop 4.7K resistor (at least for now, I still have an old NTE metal film there); and (c) V1 and V2 plate and cathode resistors. The old plate resistors were Dale MFs, the old cathodes KOA Speer MFs; all were replaced with Roderstein MF resistors. Hope this makes it clear now.
Gil
I sincerely don’t have the knowledge to answer with authority. Others that have built and tweaked to death more amps than I will probably be able to chime in and weigh in on this. As I said in the beginning, building to a given schematic will most likely result in a good, perhaps great sounding amp. It’s the extra few % points that are a very personal decision. Different iron goes a long ways, for example. And I have built identical amps that used the exact same everything and one always sounded better than the other. Furthermore, some amps will sound good using a given circuit, and still sound good if you try something else in them. Others don’t behave in the same way, in my limited experience. My low plate amp always sounded good, no matter what. Not perfect though. I’m looking forward to giving it a makeover and see where it ends up. The changes in my 102 were well worth the effort. I hope to be able to say the same after I’m done with the low plate amp. Wish me luck!
When using a larger plate resistor( like 220k or 150k) you automatically are in curved part of the load line of a 12ax7, so the upper half is different from the bottom half of the audiowave, which is harmonic distortion. Or do you draw your loadline different?" To recap, I will list again my original claims that a larger plate load resistor results in:
(b) Less harmonic distortion at the output (taken off the plate) before clipping occurs. CHECK (well, unless you have some place you can point me to that suggests otherwise)
The answer is right here.jelle wrote: ↑Thu Aug 29, 2019 4:40 pm RDH4 describes a shift in the harmonic content when changing plate resistor from 2x internal plate resistance from mostly 2nd harmonics to 3rd and 5th harmonics mostly when going to a plate resistor equal of 4-5x internal plate resistance of a tube. So the harmonic content shifts when doing that, and the result is very audible in these overdrive amps.
PS: yes that is the Radiotron Designers Handbook, 4th ed.
jelle
Ok, here is the list of what I have in stock and can provide you all with. Please email me through www.welagen.comjelle wrote: ↑Mon Aug 26, 2019 12:36 pm I have gotten a few messages from people about these resistors. They can be found here: https://www.ebay.com/sch/lampje*2007/m. ... ksid=p3692
When time allows, I will make a list of what I have available and post it here, but for now all is listen on ebay as well, so people can see it there. I apologize for the pricing, these were pulled from the NATO repositories for me and imported.
Also, please do not PM me here, but email through my website. Happy soldering!
Jelle