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ECC82 pre with cathode follower
ECC82 pre with cathode follower
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Old 22nd November 2019, 04:19 PM   #21
disco is offline disco  Netherlands
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Received a dozen of Fairchild FQPF8N60C today. LTspice has no model for this FET but I read here one could be produced from the datasheet. Has anyone experience with this job? The sheet doesn't hold all info for completion, educated guesses are welcome.
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File Type: jpg FQPF8N60C-data.JPG (78.2 KB, 183 views)
File Type: jpg FQPF8N60C-short1.JPG (95.4 KB, 183 views)
File Type: jpg FQPF8N60C-short2.JPG (128.0 KB, 181 views)
File Type: jpg FQPF8N60C-short3.JPG (92.5 KB, 179 views)
File Type: jpg mosfet model.JPG (25.6 KB, 181 views)
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Last edited by disco; 22nd November 2019 at 04:47 PM.
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Old 22nd November 2019, 05:23 PM   #22
Vintage Audio Projec is offline Vintage Audio Projec
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Ever tried of bootstrapping the gain stage?

Split the anode resistor in two equal resistances and couple the output of the CF to the midpoint of these resistances with a capacitor..
The voltage over the resistance will be constant so the current will also be constant.
As a result gain will be very close to the Mu of the tube and distortion will be lower. The anode load behaves as a very high resistance.

As a CF I would use the ECC88 or similar at around 10mA (see the datasheet as it is a recommended bias point).

I recently experimented with an adjustable gain stage using the two triodes of a 12ax7 followed by a CCa (luxury ecc88 sold way overpriced but it was the only ecc88 like tube I had around)

First stage used the normal grounded cathode amplifier using Ra 220k and Rk 1k5 with a cap in parallel and 15k in series to ground for feedback.

Second stage was with two 100k resistors for Ra and Rk was just 1k5 with a cap in parallel. The grid was capacitor coupled to the output of teh first stage.

The third stage was the ECC88 (CCa) as a CF direct coupled to the output of the second stage and Rk was 10k to ground (used a 5W resistor for that one)
The output of the CF was coupled with a cap to the second stage middle of the anode resistors (bootstrap).
Then the output of the CF was coupled with a resistance to the feedback input at the kathode of the first stage so you could adjust gain.
Supply voltage used was 300v

this worked very nice actually. In a load of 10k I was able to get around 45vrms. And just as an experiment I tried a 1k load and got around 7vrms.
Also the output impedance is very low. changing from no load to 1k load almost didn't change the output voltage (as long as it didn't clip of course....).
Got to start a topic around this gain stage soon.
I simulated it first also and simulating the output impedance in Ltspice it was only 7ohm or something. Maybe this is optimistic but it's low enough

What really impressed me is how good the bootstrapping can work and it's almost never seen... The psrr is alot better
It's just an idea...

The 6c45 as CF is also a good choice. it's even better then the ecc88 but for driving just the input of a SS amplifier it wouldn't make much difference I think

Last edited by Vintage Audio Projec; 22nd November 2019 at 05:29 PM.
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Old 22nd November 2019, 07:54 PM   #23
disco is offline disco  Netherlands
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I haven't and perhaps I'll try one day as seven ohm output impedance is impressive for a tube built. High PSRR is practical (hence the MOSFETS as I need two PSUs) but three stages and high GNF does not appeal to me as such, although it would undoubtedly measure excellent. What I'm after here is "the sound of the good ol' 82" and the urge to use 6c45. I'll keep an eye open for your thread to come.
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Old 22nd November 2019, 09:32 PM   #24
indra1 is offline indra1  Indonesia
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Quote:
Originally Posted by disco View Post
... LTspice has no model for this FET but I read here one could be produced from the datasheet...
But OnSemi has. Try Here

Last edited by indra1; 22nd November 2019 at 09:38 PM.
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Old 22nd November 2019, 10:42 PM   #25
disco is offline disco  Netherlands
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Thanks for the tip Indra, it's location is here: https://www.onsemi.com/pub/Collateral/FQPF8N60C.lib If someone would be looking for a tutorial, Simon Bramble did a nice job.
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Old 22nd November 2019, 11:05 PM   #26
disco is offline disco  Netherlands
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did a quicky with this shunt regulator but what's happening below 2KHz, is that because of the FET capacitance?
Attached Images
File Type: jpg ouch.JPG (50.6 KB, 33 views)
File Type: jpg outch.JPG (38.3 KB, 81 views)
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File Type: zip 8N60CshuntV1.zip (8.1 KB, 2 views)
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Last edited by disco; 22nd November 2019 at 11:07 PM.
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Old 23rd November 2019, 06:20 AM   #27
refference is offline refference  Brazil
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Quote:
Originally Posted by nigelwright7557 View Post
I would have just used one ECC82 instead of two different valves.
I am a big fan of the ECC82.
Me too !!! As a u follower , 12AU7 is amazing !!!
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Old 23rd November 2019, 08:03 AM   #28
artosalo is offline artosalo  Finland
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Quote:
what's happening below 2KHz
I found two errors in simulation and schematic:

- R7 too small, the lower NFET can not conduct
- number of simulation points were 100m (=0.1), should be bigger, 100 for example
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Old 23rd November 2019, 09:44 AM   #29
jan.didden is offline jan.didden  Europe
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R7 can go, just cut it out.

Also check the voltage across the zeners, see if they really are conducting. If not, C2 and a non-conducting D1 might explain it.

Jan
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Old 23rd November 2019, 04:31 PM   #30
disco is offline disco  Netherlands
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Loosing R7 helps 35dB @100Hz
C2 seems unrelated though.
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File Type: jpg Rout-2a.JPG (34.4 KB, 55 views)
File Type: jpg withoutR7.JPG (36.1 KB, 27 views)
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