2 stage 2A3 SET - why does the 6n6p sim so well? - diyAudio
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Old 5th July 2012, 04:16 PM   #1
regal is offline regal  United States
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Default 2 stage 2A3 SET - why does the 6n6p sim so well?

I've been toying with a 2 stage low power 6n6p-6s4s SET. At first glance the 6n6p would be dismissed for not having enough gain. But at the same Vrms across the output load it is shows a significant lower distortion according to spice.

I mean for a 2 stage common cathode driven 6S4S SET, the CCS-c loaded 6n6p beats anything I can find. I take it is some sort of harmonic cancellation. This is with the 6n6p swinging about 45Vpk.

The Spice FFT's of other tubes like 6c45p, 417A, triode D3A, look ugly higher 2h and more higher order at the same power output.

Could anyone try this simple simulation with a 6n6p drving a 2a3 or 6s4s. I have Coleman regs so leaving the heat out of the sim.

The biggest variable besides the input tube is the OPT which I don't have data for (3k:38).

Last edited by regal; 5th July 2012 at 04:24 PM.
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Old 5th July 2012, 04:19 PM   #2
kevinkr is offline kevinkr  United States
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Should provide the exact model you are using for the 6N6P which is incidentally noted as being quite linear.. You can just copy as text and paste here.
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Old 5th July 2012, 04:41 PM   #3
regal is offline regal  United States
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Thanks,

its from Slegels website:

Quote:
subckt 6N6P 1 2 3
+ params: mu=18.8 ex=1.666 kg1=810 kp=85.5 kvb=600 rgi=2000 vct=.02
+ ccg=4.4p cgp=1.7p ccp=1.85p
e1 7 0 value=
+{v(1,3)/kp*log(1+exp(kp*(1/mu+v(5,3)/sqrt(kvb+v(1,3)*v(1,3)))))}
re1 7 0 1g
g1 1 3 value= {(pwr(v(7),ex)+pwrs(v(7),ex))/kg1}
rcp 1 3 1g
c1 2 3 {ccg}
c2 1 2 {cgp}
c3 1 3 {ccp}
r1 5 6 {rgi}
v1 5 2 {vct}
d3 6 3 dx
.model dx d(is=1n rs=1 cjo=1pf tt=1n)
.ends
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Old 5th July 2012, 09:01 PM   #4
mogliaa is offline mogliaa  United Kingdom
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Hi Regal,
I did a lot of THD measures recently and 6n6 showed to be not that linear as a driver am afraid. You better want to go for 6e5p, 6c45 or 4p1l, those are good drivers :-)
Cheers,
Ale
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Old 6th July 2012, 12:20 AM   #5
regal is offline regal  United States
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Quote:
Originally Posted by mogliaa View Post
Hi Regal,
I did a lot of THD measures recently and 6n6 showed to be not that linear as a driver am afraid. You better want to go for 6e5p, 6c45 or 4p1l, those are good drivers :-)
Cheers,
Ale

Thanks I do appreciate you work especially for line line-level apps, the website it super.

but the testing was on its own, harmonic cancelation not included, also different type of load, just 10 vrms, etc.
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Old 6th July 2012, 01:33 AM   #6
Bigun is offline Bigun  Canada
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some thoughts on tubes here:

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Old 6th July 2012, 07:04 AM   #7
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by regal View Post
Thanks I do appreciate you work especially for line line-level apps, the website it super.

but the testing was on its own, harmonic cancelation not included, also different type of load, just 10 vrms, etc.
Thank you.
Testing was done to show how linear the valve was using a CCS as anode load and by driving it at +22.22dBu to maximise the evidence of the valve's non-linearities. Of course circuit design can optimise the performance of the valve itself as you clearly say, but tests performed show an easy way of comparisons between valves.
My suggestion was only on the basis of similar valves which are more linear and, particularly 6e5p which is not expensive and performs brilliantly as a driver, check out there and you will find many designs using it. I'm not telling you not to use 6n6p...
I tested a bunch of 26 6n6p (i was planning to use them in an OTL headamp as per Lars suggestion) and found parameters variance to be large amongst them...

Need to check Morgan Jones' chapter on H2 cancellation, but had the impression that it was not that easy to implement in practice?
Cheers,
Ale
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Old 6th July 2012, 07:22 AM   #8
regal is offline regal  United States
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Quote:
Originally Posted by mogliaa View Post
Need to check Morgan Jones' chapter on H2 cancellation, but had the impression that it was not that easy to implement in practice?
Cheers,
Ale
Well thats what I am getting at if believing LT spice we want more 2H from the gain stage for the 2A3/6S4S because the opposite phase cancels some of the distortion on the output. And for some reason 6n6p seems to be a great match according to spice. Could be a fluke, just don't know.

In other words an extreme example would be to drive the 2A3 with 44Vpk from a .00000001 % 2H bench supply would give more distortion om the output than a tube which matched its distortion profile.

See what I am getting at? The two stages both invert.

Last edited by regal; 6th July 2012 at 07:51 AM.
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