Hello,
I am considering to use Siemens F2a tube in pentode mode as SE output. However all schematics and load lines that I could find online use F2a triode strapped (which is perfectly fine, but this build I want to experiment with pentode). I could not find Shindo Cortese schematic either.
According to the datasheet suggested pentode operation in Class A is Ua=Ug2=250V with 60 Ohm cathode resistor yielding ~6.5V bias and 2.2 kOhm load.
Load line therefore looks like this
I am a bit puzzled with the signal voltage swing of just 13Vpp, what am I supposed to use in a driver stage?
Continuing the "Euro pentode" theme, classic C3m and C3g tubes have amplification factor of 19 and 41 respectively. However my source DAC can deliver up to 6Vpp itself, gain is too high.
Do I use a step-down interstage? Do I use C3m/C3g as cathode follower instead and use a step-up interstage?
Any ideas of a viable C3m/C3g - F2a pentode SE amplifier are welcome.
Thanks.
I am considering to use Siemens F2a tube in pentode mode as SE output. However all schematics and load lines that I could find online use F2a triode strapped (which is perfectly fine, but this build I want to experiment with pentode). I could not find Shindo Cortese schematic either.
According to the datasheet suggested pentode operation in Class A is Ua=Ug2=250V with 60 Ohm cathode resistor yielding ~6.5V bias and 2.2 kOhm load.
Load line therefore looks like this
I am a bit puzzled with the signal voltage swing of just 13Vpp, what am I supposed to use in a driver stage?
Continuing the "Euro pentode" theme, classic C3m and C3g tubes have amplification factor of 19 and 41 respectively. However my source DAC can deliver up to 6Vpp itself, gain is too high.
Do I use a step-down interstage? Do I use C3m/C3g as cathode follower instead and use a step-up interstage?
Any ideas of a viable C3m/C3g - F2a pentode SE amplifier are welcome.
Thanks.
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I would suggest you look at ideas for KT88 SE, it's the closest tube to the F2a and there are lots of schematics available. But Pentode mode is likely going to lead you to guitar amplifiers.
The Cortese uses feedback from the plate so it's not truly in Pentode mode either.
I tried a spud amp in Pentode mode ("Yukon Gold") but didn't like it and in end prefer triode wired. Pentode mode is good for heavy metal since your ears are going to bleed anyway.
You may have to tread your own path.
The Cortese uses feedback from the plate so it's not truly in Pentode mode either.
I tried a spud amp in Pentode mode ("Yukon Gold") but didn't like it and in end prefer triode wired. Pentode mode is good for heavy metal since your ears are going to bleed anyway.
You may have to tread your own path.
Thanks a lot @Bigun, this is probably the reason. I was more looking towards HiFi amp rather than guitar amp so perhaps triode-wired makes more sense here. Curves are so much nicer.
Do you think at least C3m as pentode driver would work fine? Or shall I go all-triode in all stages for maximum linearity?
Thanks for suggestion @lcsaba, I'll check out the schematics.
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There's no way you can correlate reviews of the Cortese to the use of Pentode mode of operation - at least I hope it would be obvious that reviews are often suspect and depend greatly on all system aspects including source, speakers and environment.
The Cortese has feedback (as mentioned) from the plate so there's no pure pentode mode here, the feedback will change the plate impedance and hence it's interaction with the OPT and speaker and in operation it's more like an Ultra-Linea which is a mode of operation that sits between pure Pentode and triode-wired. The effective plate curves of the F2a in the Cortese are hence not even available to be inspected in any simple way from the datasheet. Feedback makes the circuit, to some extent, less depending on the individual tube characteristics too and in particular makes it easier to match the gain of the L and R channels.
Having said this, it doesn't mean that a tetrode driven pentode isn't something special - I've no direct experience but have also read some good things about pentodes driving 2A3 and 300B.
The Cortese has feedback (as mentioned) from the plate so there's no pure pentode mode here, the feedback will change the plate impedance and hence it's interaction with the OPT and speaker and in operation it's more like an Ultra-Linea which is a mode of operation that sits between pure Pentode and triode-wired. The effective plate curves of the F2a in the Cortese are hence not even available to be inspected in any simple way from the datasheet. Feedback makes the circuit, to some extent, less depending on the individual tube characteristics too and in particular makes it easier to match the gain of the L and R channels.
Having said this, it doesn't mean that a tetrode driven pentode isn't something special - I've no direct experience but have also read some good things about pentodes driving 2A3 and 300B.
Some time ago there was a discussion here about driver tubes for 300B. Thorsten Loesch had the opinion that triode wiring of C3m is wasting a great driver pentode. Probably the same applies for F2a. I have no reason not to believe him. I'm sure Shindo would triode wire F2a if it sounded better.
The great thing is that you can build any configuration. I'm building a 300B amp with Siemens E280F triode wired (DRD), another one with C3m pentode (Legacy) and and F2a (tetrode) with EF804S driver. I want to see which has "something special".
The great thing is that you can build any configuration. I'm building a 300B amp with Siemens E280F triode wired (DRD), another one with C3m pentode (Legacy) and and F2a (tetrode) with EF804S driver. I want to see which has "something special".
It is mentioned, that the schematic is not the original Cortese schematic, but the outcome of the discussions (If my google translate made no mistake). I was not able to find even a backengineered schematic of Cortese so far.
I have read positive reviews of Cortese but also less-than-positive reviews suggesting that Shindo is more about style and vintage parts rather than quality of sound, don't know how much truth is in this statement.
Thanks @Bigun and @lcsaba, really like your suggestions - will experiment with different driver setup to see which one works best.
Thanks @Bigun and @lcsaba, really like your suggestions - will experiment with different driver setup to see which one works best.
I don't see any feedback (for example divided cathode resistor) in first stage.
https://www.hifido.co.jp/sold/16-03387-73436-00.html?LNG=E
Backengineering is even possible if you want to copy Cortese ... but IMHO better solutions are available than 10W/10%.
https://www.hifido.co.jp/sold/16-03387-73436-00.html?LNG=E
Backengineering is even possible if you want to copy Cortese ... but IMHO better solutions are available than 10W/10%.
No, I do not want to copy Cortese. I'm cloning the Siemens KlangFilm 204a, which has a different driver and local feedback. I know I can do better than 10W/10% even with tubes, not to mention VFET-s, which I'm also building with. I just want to taste vintage. Maybe it's something special, at least the "first watt".
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If you want to explore alternative approaches before going to the work bench, Igor Popovitch (in this book: https://www.fishpond.com.au/Books/A...--926Oyqa383myMdM-HRaCRFX7d_RqGRoCzoUQAvD_BwE) showed one version of the Cortese circuit without NFB. He then built with the F2a in triode mode with various driver arrangements using the 6SN7, 6CW5 and 6EM7, evaluating as he went. PM me if you can't find a copy.
i think shindo use 350 v in hv of cortes f2a(i never seen shido amp or preamp) . ken shindo used 6aw8 in cascode circuit .forget telefunken or other xpensive hifi tubes jan philips 6aw8just 10$ 😍 and prefect sound.
f2a in fix bias ...-10 to- 12 on g1. filimant of 6aw8 shuold be dc. in power suplay we can see ac transformer with out put winding center tab. full wave si rectifiers .....and ey88 in center of hv winding .shindo use this kind of tv recrectifiers for soft start . this 10000$ amp
Ths 10000$ amplifier will surprise you😱
f2a in fix bias ...-10 to- 12 on g1. filimant of 6aw8 shuold be dc. in power suplay we can see ac transformer with out put winding center tab. full wave si rectifiers .....and ey88 in center of hv winding .shindo use this kind of tv recrectifiers for soft start . this 10000$ amp
Ths 10000$ amplifier will surprise you😱
Before the revolution, the army of king and government of agencies used to buy tubes from Germany, England, and America. I was able to buy 70 of them 10 years .A large number of new box hi fi es were exported from Iran by tubes lovers.
f2a is german tetrode tube .. ..... life is morethan 10000 .
f2a is german tetrode tube .. ..... life is morethan 10000 .
With one name and one model, Shindo used different circuit .The latest version of Corts f2a is mu follower with 6aw8 in the input circuit .
