Posted by Damir [ 213.202.99.207 ] on March 09, 2006 at 14:19:29:
These high-gm pentodes have the same base and similar specs. D3a and E280F are very similar looking – the same inside construction and height, «Siemens» produced, E180F is shorter. Those are SQ 10000h , gold pin tubes, and in this case it actually means something – OP is stable, they are close to the specs, the sound is beautiful. 
When we connect screen grid g2 to the anode ("through" g2-"stopper" resistor R9 ) we get high-quality triode – high µ, high gm, low rp, very linear – Fig. 1 :
The same active load as before, similar OP. I measured A=µ about 50,5 with E180F, with E280F – 58, and D3a – 74.
E180F and D3a have about the same Ua=200V, E280F a little less, about 150V in this OP (11mA CCS, Rk=226 Ohms). All three tubes have rp in this OP about 3kOhms or a little bit more.
Input capacitance is the problem here – we have rel. large amplification, our Cag ~3pF, and Miller capacitance is large, especially in D3a case – about 220pF. We must use a lower value input pot than "standard" 100k, about 10k, 25k max. is still adequate.
Sound test: E180F has the softest and smoothest sound, E280F is drier, and D3a "in between", clean, a little dark top end, a little sharp, but not too much. But, all three have a good sound, IMO – in class "above" (say similar) 6C45Pi. E180F has great bass, detailed mids, punch, maybe highs aren`t too enhanced, but it is clean, balanced, smooth, dynamic, direct and natural – my overall favorite. It has the "right" amplification (A=50,5 times) – input sensitivity is 1Vrms for max. power. I tried a few more dB "in the red" (a little bit of "class A2"), and low output impedance of the driver and grid choke helped that distortion and compression almost aren`t noticeable.
D3a is also good, has the larger amplification (A=74), it means input sensitivity of about 1Vp=0,7Vrms for full output power – large "reserve" is possible.
E280F is also usable, but it`s a little rarer. One of the main criterions for the driver tube choice is "common", not expensive tube. Little E180F NOS "Philips" is all of that, and I`ll use it (with D3a as a backup, "plug in" compatibility) as a driver in my amp.
I tried resistive load too, just swapped CCS with 27k/5W resistor – circuit "pulls" about 10mA, and I measured A=66 times with D3a and A=45 times with E180F. Sound is a little softer and rounder, but very good, too.
Usual precautions about oscillations (stopper resistors close to the pins) are necessary. I used cathode bypass cap, it seems to me that the sound is somehow "fuller and smoother" with it.
Use high-quality, high-temp. electrolytic, soldered on the one cathode pin and the driver ground, short wires.
CONCLUSION: E180F triode connected driver "has it all" - from the "right" amplification (~50) to the "right" sound, it is quality NOS tube that isn`t hard to find or expensive. D3a is very close, and with a higher amplification factor.
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