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300B SE Project, Part 9 - E180F, E280F, D3a pentodes - triode connected
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.
Re: 300B SE Project, Part 9 - E180F, E280F, D3a pentodes - triode connected
Awesome, Damir. Just excellent. I wish I could listen to each of these circuits, maybe I'll be able to one day soon.
Thank you. I tried so many things these days, and learned a lot. When you like it, it`s not too hard to solder and unsolder few wires and listen to the changes...again and again... A few more experiments, then (finally) chassis build.
On the E180F driver schematic, the cathode resistor R6=226 ohms is noted like three (parallel) resistors of 680 Ohms on pins 1, 3, 8. This is not correct - E180F has only two cathode pins, 1 & 3. The pin 8 is actually g3 and shield. I actually used three 680 Ohms resistors on the pins 1, 3 and 6 - where pins 3 & 6 are connected together. I used "blank" pin 6 like soldering terminal, and forget about it - it`s better to draw a schematics based on datasheets, and not on (bad) memories... In fact, it is better and simpler just to use two cathode resistors of 453 ohms, connected on pins 1 & 3. In Part 11 we`ll see the complete and correct amp schematic + wiring diagrams and tube bases. Here`s the corrected E180F driver schematic.
Dear Damir (and everybody else)..... can anybody tell me if it will be possible for the E180F to drive 2 2A3 tubes in parallel.... i know that the capacitance of the output tubes is a bit of a problem - maybe better to use E280F but still i want to know if it will be possible to drive PSE with 2A3's using E180F........ any opinions thanks in advance....