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Andre Jute[_2_] Andre Jute[_2_] is offline
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Default All the best in 2012A.


A positively thermionic New Year to all RATs.

Andre Jute
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patrick-turner patrick-turner is offline
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Default All the best in 2012A.

On Sunday, 6 January 2013 05:08:30 UTC+11, Andre Jute wrote:
A positively thermionic New Year to all RATs. Andre Jute


Fetch me my heart pills, for we have a visit by Andre.

All the best for 2013.

Meanwhile, as time allows, I continue to solder and cycle my way towards ETA of 2042, with help of a good Hubot, and yet more invented medical tricks if allowed by Unkel Fate, and the passing traffic does not squash me to a pulp while I am cycling about in a state of pleasantly transposed conciousness.

In a previous post I mentioned using a single EL34 in pentode mode to power 2 complementary pair source followers in class A, 2SK176, 2SJ56. I've now got bugs ironed and produced an ULTRA SIMPLESTICA amp with following character,
Noise 0.5mV, DC offset 10mVdc, Rout, class A, 0.07 ohms for DF = 85 with RL = 6 ohms, THD 0.1% at 10Vrms, BW = 4Hz to 300kHz, unconditionally stable at any power and with any RL type of value or no RL at all. Square wave = excellent, with low ringing with pure C loads. Amp makes 15W max, 6 ohms, Pdd at idle = 32W total using +/- 16Vdc rails at 1A each. Excellent overload and high tube voltages are excluded with gate-source zener diode clamps and with use of 2uF cap coupling to each gate with 560r series gate R.
At gross overload, caps develop negative charge like a tube amp to cause mosfets to be become biased for class C. But for normal use, bias is self regulating with 0.47ohms between each source and Vo.

Prototype amp has 2 chassis, with Vdc applied to EL34 heater. +/- 16Vdc rails have CLC with 10,000uF - 10mH - 15,000uF for Vr 4 mV. B+ = 350Vdc, well smoothed. Ia = 25mA and is delivered from B+ to anode via 40H plus 2k7, with Ea at 265V. So RLdc is extremely high value. Ek = 20Vdc, and cathode bias network = 660 ohms plus 470k. Screen is fed 4mA via 27k with Eg2 at 250Vdc, and screen is bypassed to Vo using 8uF el-cap.

EL34 is tricked into thinking its a triode with low THD. OP stage gain = 0.9, so if 9Vrms appears at output, 10Vrms is needed at gates and provided by EL34 anode. Screen signal voltage = Vo = 9Vrms, slightly less than anode voltage. Therefore the EL34 has gain just slightly higher than triode µ of 9. In fact EL34 gain averages 10.
So EL34 is acting similarly to a triode strapped tube where all anode voltage is applied as FB to the screen. In this case the mosfet OP stage is included in the simple loop and because RLdc plus gate bias resistance totals about 45k, then the gain of screen is screen gm x ( Ra // RL ) which in this case is about 14x. Hence any THD produced by mosfet stage is reduced by factor of about 0.07, and source follower Rout without FB of 1.3 ohms is reduced to 0.07 ohms.

I know Andre likes simple amps, well, here is such a one, just 3 devices in total, and yet measuring better than just about any other 15W amp with only 3 devices. Screen FB was used in 1955 in tube amps to avoid trying to apply GNFB from OPT sec to cathode of an input tube. It can be applied to an input pentode if you want, but here in my case I have JUST ONE TUBE that does everything by producing high current low Z drive to mosfet gates with high C, and while accepting FB in a simple easy loop that is similar to simple triode strapping.

I have to complete web page about it all and will post a notice when its ready for ppl to view.

Included will be another version using EL34 as pure triode strapped, and a paralleled 6CG7 normal input tube with FB applied conventionally. This way the very slight THD of 0.1% per 10Vrms of EL34 in triode PLUS the 0.4% of mosfet OP stage is ALL reduced by whatever factor I want; if Vin is allowed to be about 1.5Vrms, then THD should be 0.05% at 10Vrms output with say a 6 ohm load.

For those wanting higher power, then use 4 mosfets each dissipating 20W, and use +/- 25Vdc rails from 18.5Vac-0-18.5Vac PT winding. Expect extremely fine sound. PO max would be 35W, class A, with load of 6.8 ohms. In such a case, each mosfet has RL = 27 ohms, very easy for them, so in follower mode the gain reduction is high, and THD even lower than I am getting now in this present experiment.

In 1990s there was a maker in West Oz, Micrex, who made a commercial hybrid amp with mosfet output and one EL34. But they had bjts used for a differential input stage and mosfet OP stage was rated for 90W in low bias class Ab.. I once was asked to bring an 8585 amp ( with 4 x 6550 in class A in each channel ) to the owner of the Micrex and allow a comparison. I sat in silence for 3 hours while the owner changed CDs and amps. Some good Maggies with ribbon HF were used. Piano was especially good.
But after 3 hours, the guy says, "Well, your pure tube amp is vastly superior to the Micrex". I left him with a quote and returned home with my amp. A week later he said he wanted to get a full tube amp, and for more then what I quoted, but from his regular guy who had fixed and modded his other gear. OK, I was used, but a point was sort of proved. My present amp with class A mosfets and not a single damn bjt seen anywhere may sound as well as a fully well done tubed amp.

Patrick Turner.
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