FET JUNIOR (Fet version of Epi Valve Jr @ 45V)

[mac]

Some ago I quickly breadboarded a Fet version of the Epi Valve Jr @ 9V. As I expected it was not close to the real thing.
I know that a 9V batt can't do miracles.
Yesterday I finally built a 45V PS, high enough to fry some fets but ok for a pair of 2SK117, high Yfs: 15Ms, 50V, Low Vgsoff: 0.2V - 1.5V, low noise.
I guess that 5 x 9V batts can do the job.
Now I can get VJ tones at higher volume out of my LC30II.
I wish I had higher voltage fets but I'm pleased with the results.
One con is that it begins to distort sooner than the VJ. Maybe it needs more voltage and/or this fets are hotter than a 12AX7.
I measured plate voltages and they are near 2/3 of B+, so I set both drains at 30V. Fortunatelly a pair of 68K worked fine.
I added a BMP tone control just for fun.
Combining different bypass caps is a nice addition. A couple of 0.68uf make a nice cleanish treble booster.
Enjoy.



mac

[Toney]

Frequency central has me thinking and thinking of simple Fender circuits scaled for the Submini tubes ala Run off Groove and Fets.
I wish I had more knowledge of effective ways of scaling down actives and tubes in general.
I revisit newby-ness on this subject.
But I digress,  very nice work there 

[frequencycentral]

Neat idea mac! - but what about using high voltage mosfets such as IRF740 and an 80v charge pump to get even closer?

Frequency central has me thinking and thinking of simple Fender circuits scaled for the Submini tubes ala Run off Groove and Fets.

Strange how things go full circle - it was mac's 'Fet Phaser Idea' that inspired my submini tube Vintage Vibe.

[mac]

Neat idea mac! - but what about using high voltage mosfets such as IRF740 and an 80v charge pump to get even closer?

I have a lot of 220v to 110v transformers that can deliver 155vdc, more on tube territory. I'm going to buy some high voltage fets or mosfets and experiment. IMHO much of the difference between SS and tubes is the working voltage. The rest is the OT and the fet itself (bias topology, V(i) function).

Does mosfets distort like fets? If not, which is more tubey?

Frequency central has me thinking and thinking of simple Fender circuits scaled for the Submini tubes ala Run off Groove and Fets.

I think that ROG projects will sound a lot better at higher voltages.

Strange how things go full circle - it was mac's 'Fet Phaser Idea' that inspired my submini tube Vintage Vibe.

Thanks Rick. Your vintage tube is a fine project.

mac

[luix]

Hi!

I have the same idea of running JFET with "high" voltage, why use a 50V device with 9V??!!

I think you have to low the gain of the fet because tubes work with higher voltage so clipping accour at higher input voltage....

Also have to consider that input capacitance of a 12AX7 is 1/10 of 2SK117 so the resistor and capacitor is to be adapted... I found very usefull pspice simulator, of course the reality may be little different but it help a lot!!!

PS vintage vibe is great!!!

[mac]

I have some 2SK246, less gainy than 2SK117. But Vgsoff is higher, above a volt, which means that collector resistor will be lower. I'm going to try them tomorrow.

mac

[Toney]

I spent a little time going through this with the real schem at hand.
Have you tried adding the final stage and actually making a little amp from this?

Tell me, have you experience auditioning any of any other higher voltage FETs other than the 2sk117?

[mac]

Today I noticed something unusual, some kind of misbiasing sound similar to a little ringing. I suspect that the low Vgsoff of the 2sk117, source is at 0.3v, has somthing to do with it.
I replaced them with 2sk246, source at 1.1v, and the ringing was gone, but gain is far less than the real VJ.
I want to get 2sk373 or even higher.
And yes, I added a darlington pair BD237 and a small transformer I found in an old telephone, 16 ohm: 8 ohm. Promising results. I have to experiment a little more.

mac

[luix]

What is the voltage at the drain and the Vgs?

[mac]

2sk246:
rs=1k5, rd=22k, vs=1.1v, vd=30v

2sk117:
rs=1k5, rd=68k, vs=0.32v, vd=30v

mac

[luix]

30V is a bit high, why not the classic 1/2V+?

Idss of 2SK246 is from 1.2 to 14mA, you are running fet at ca. 733uA, is a bit low...

I don't know what is the Idss of your device but a better polarization is in this zone:



I think you  have to scale the gain because the V+ is lower than the V+ of a valve junior, 300V VS 45V, maybe the gain have to be 1/7 of the original valve junior...
Original Vj gain is At1*At2=60*60=1200 ca. so the total fet gain have to be 171....

The source resistor can be unbypassed, to reach the same frequency responce is necessary to modify the resistor/capacitor in the circuit...

Is not so simple to emulate a tube circuit with solid state device, you cannot replace tube with fet directly (in my opinion of course)...