Years ago I designed built the "Noreaster" around Svetlana 572 tubes and later GM70s. That used the 8010 too. A great tube! Could drive those outputs to over 18 watts output. I hate to say it but I liked the Panasonic SQA30(?) (Lux) 6BQ5 amp better.
I found this on the web, and someone on YouTube posted their pentode-wired SE gk71 monoblocks. The sound seemed a little harsh but still good.
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You can try to simulate it with LTSpice. Not the real thing but should give you an idea.
Pentode mode has a high output impedance which results in a poor damping factor for the speaker. This may accentuate the self resonant frequency of the speaker. Its probably more dominant the the THD in terms of audibility. Global NFB will help here.
I like SE amps for their unmatched magical midrange. I like more PP too, but for totally different reasons.
From my experience EC8010 is a very neutral sounding tube.
R7 is the “Schade feedback” resistor.
R8 is part of the fixed bias device.
R8 is part of the fixed bias device.
The above amp using the 6E5P was mine. The 68k Schade resistor effectively changes the GU50 to having a triode character, in some ways better in terms of the GU50's Rp (ie. lower than triode mode). The comment above about poor damping factor is rubbish. Also the higher the transconductance of the driver (in this case the 6E5P) the better the "Schade" effect, hence me not using the usual EF86 suspect.
Hello,
Thanks for replies (and schematic) guys.
Shade feedback seems too complicated for me today.
At first the challenge is for me to build a simple amp who works the best as possible.
Then i will learn how shade feedback works.
I got no 6e5p but any near 10ma/v sharp cut-off pentodes in my boxes (here is the poor man's 300b amp , isn't it?)
Regards
Thanks for replies (and schematic) guys.
Shade feedback seems too complicated for me today.
At first the challenge is for me to build a simple amp who works the best as possible.
Then i will learn how shade feedback works.
I got no 6e5p but any near 10ma/v sharp cut-off pentodes in my boxes (here is the poor man's 300b amp , isn't it?)
Regards
Attachments
Hi, you miss an obvious fact in the circuit above your post. The output pentode has local ”Schade” feedback making the pentode get triodelike characteristics and also lower Zout. As a bonus you get pentode effiency. Note the driver also has local feedback. Search for Michael Koster and Schadeode. It uses a depletion MOSFET as driver that has close to pentode curves. If you sim a circuits like this, you usually get a typical triode overtone spektrum.
EDIT: Hi Ejam, wrote my post before reading yours, seems we totally agree 🙂.
This circuit is badly designed, with a wimpy driver unable to drive the low impedance network. It is also triode strapped which will not work in a Schade design.
My thoughts on this discussion:
a lot of effort was made on the gu50 to approach the 300b, but without really getting there.
Is it worth it?
In my opinion two 6l6 oides will do the job better than a single Gu-50.
A Gu-50 is a very good pentode, but remains a pentode.
What is the value of an SE amplifier with a GU 50 wired in pentode mode?
Is there anyone here who has tried it like this, please?
a lot of effort was made on the gu50 to approach the 300b, but without really getting there.
Is it worth it?
In my opinion two 6l6 oides will do the job better than a single Gu-50.
A Gu-50 is a very good pentode, but remains a pentode.
What is the value of an SE amplifier with a GU 50 wired in pentode mode?
Is there anyone here who has tried it like this, please?
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Yes I have one, on here somewhere, a modified 'Sweet Peach' cheap chinese amp.
It was triode, the sound sucked - mainly due to the recycled input caps, interestingly enough - I'd never realised caps could remove treble inline LOL, it was bizarre.
But I wanted more power, and to test an idea... no GNFB over the OPT.
In my (limited LOL) view Triode mode is simply a pentode with a bit of intrinsic feedback - i.e. gain is lower but output impedance is lower, distortion is lower: all that we'd expevct from local feedback.
But GNFB in a tube amp makes no sense to me, since Thorsten Loesch (in some article IIRC) pointed out that at the frequency extremes the OPT(ransformer) has lousy gain anyway, so all the GNF was hammering the midrange and ignoring the edges, and the 'phase margin' battle with this slow mass of wires, iron and parasitic capacitance made for endless 'fun' and sonic misery.
So my amp is a full pentode with gas discharge regulation for the screen grid (buffered through a MOSFET as the gas tube regulator had too high impedance for the GU50's demands) with a large amount of feedback from the GU50 anode to the driver tube.
There's probably a better way of arranging this feedback using only resistors, but I ended up with a fat film cap which seems to work fine.
So my OPT sits alone, chatting with the speakers, all feedback is local on the input tube, and hard between the GU50 and it's driver tube.
The sound is effortless. Of a far, far larger amplifier. Not noticeably struggling more than the solid state 150W/channel class A 45kg monster it replaced.
Open, sweet, detailed, musical: So I left it like that. Also the gas tubes are very pretty 🙂
So I can recommend a GU50 pentode SE with only feedback around the 2 tubes behind the OPT.
One thing I would caution against is using cheap sockets for the GU50. The tube may survive vertically, but stand it up for tweaking - so the GU50 is horizontal and they crack around the base - I lost 4 tubes that way. The Sweet peach sockets seemed Ok, but if buying your own, use the proper soviet dustbin sockets like the OP of this thread!
This GU50 SE with an alternative Schade driver to 6E5P.
https://www.bartola.co.uk/valves/2017/02/14/schade-se-amp-example/#more-6251
https://www.bartola.co.uk/valves/2017/02/14/schade-se-amp-example/#more-6251