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LND150 tube emulator attempt.

Started by Steve Conner, February 04, 2011, 10:34:33 AM

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Steve Conner

OK, so following the recent discusion on FET-based tube emulators, I did some more work on my own ideas. Friday afternoon at work, and all that.  ::)

First I took the dual gate MOSFET cascode one and compared it against a Mullard ECC83. After some playing with the component values, they gave similar traces, but it was extremely temperature-sensitive. Someone had borrowed the heat gun, but there was no need for it: even breathing on the MOSFET caused about 10V shift in drain voltage. That made me think that the variation between different MOSFETs would be even worse.

So, I tried the LND150 in a circuit somewhere between the Trioderizer and the scalable diode. It worked surprisingly well. The clipped waveforms were as good, if not better than the dual gate circuit, and temperature stability seemed much better. Also, the component count is lower.

The major drawback is that it leaks DC out of its "grid". In conjunction with the usual 1M grid-leak resistor found in a tube circuit (well, 910k, as I ran out of 1Ms) this creates the positive voltage needed for the LND150 to have the right bias point. But tube circuits often use a volume control, or a different value to 1M, as the grid leak.

So, I'm working on that one. For now, this circuit would only be usable in a design that took the DC into account: it might not work if you just plugged it into a tube socket.

The scope shots show the LND150 circuit working alongside the Mullard ECC83, also set up with a 100k plate and 1.5k cathode resistor. Both are running off the same 250V supply. Top trace is the input signal, and the bottom two are the outputs from the ECC83 and LND150. I will leave you all to figure out which is which. :)

To paraphrase the Monty Python sketch, "Bring out your FETs!"

rowdy_riemer

Quote from: Steve Conner on February 04, 2011, 10:34:33 AM
The major drawback is that it leaks DC out of its "grid".

Do you think the gate insulator might be damaged?

Steve Conner

No. It does it on purpose, because I stuck some resistors from drain to gate.

See this article for why I put the resistors there: http://www.radiomuseum.org/forum/the_trioderizer_a_solid_state_triode.html

They also help the LND150 to bias up hotter than it otherwise would, especially since it needs resistance in its source to bring its gm down in line with a 12AX7.

KMG

At the first waveform can be seen that the FET cut-off "sharper" than the tube. I solved this problem by placing GE diodes in the source of FET.
There is no information in LND150 datasheet about gate-source protective zener. Negative half-wave may exceed maximum gate-source voltage specification.

J M Fahey


rowdy_riemer

Quote from: Steve Conner on February 04, 2011, 11:00:48 AM
No. It does it on purpose, because I stuck some resistors from drain to gate.

See this article for why I put the resistors there: http://www.radiomuseum.org/forum/the_trioderizer_a_solid_state_triode.html

They also help the LND150 to bias up hotter than it otherwise would, especially since it needs resistance in its source to bring its gm down in line with a 12AX7.

Duh!! I thought you were talking about leakage through the gate.  :duh

KMG

#6
Continue the story
1-st stage of JCM 800 (2k7/0.68u, 100k)
Left - 12AX7, right - LND150 (with different component values of course)







"See the cat? See the cradle?"


Schematic of test equipment.

J M Fahey

*Very* good  :tu: :tu: :tu: :tu: :tu:
Can you please add the driving voltage on each step?
Thanks.
PS: I would love to see what comes out of a 2nd stage, when the first is overdriving and providing all it can (200Vpp or thereabouts?)

PS:Just in case I forgot to say so earlier:  :tu: :tu: :tu: :tu: :tu:
Congratulations.

PS2: I guess it's already beyond the "attempt" stage, isn't it?  ;)

Kaz Kylheku

All that's left to say is: bottle it and sell it.  ;D


   
   
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http://www.kylheku.com/mp1http://www.kylheku.com/diy

KMG

QuoteCan you please add the driving voltage on each step?
I don`t remember values, but they was the same for both stages (tube & fet).
QuoteI would love to see what comes out of a 2nd stage, when the first is overdriving and providing all it can (200Vpp or thereabouts?)
Will be, wait some time.

joecool85

Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

KMG

#11
Actually the basic stage.


RG (with interstage circuits) and RP are original (from tube schematic).
Q1, Q2, ZD1 and R1 is Q3 gate protection, in the first stage is not used, just put in 20V zener at gate.
R2 determines the smoothness of "grid" limitation, R3 voltage at which it occurs.
R4 determines the gain at low frequencies, R5 - gain above the cutoff frequency, C2 - the cutoff frequency.
C1 simulates the Miller effect  (0-100pF, depending on the gain of the original stage and presence of capacitor in the cathode).
VBIAS for JCM800 got the same for all stages "-2.75V"
JCM800 using LND150

Reamp sample
http://milas.spb.ru/~kmg/files/projects/jcm800mv/fet/lnd150/s/8-Southern%20LP.mp3

KMG

While testing I found that there is no need in additional gate protection, only high voltage diode in source chain.
So, the basic stage for lnd150 is the following

Comparative samples of "glass" JCM8000

And SS (last version) JCM8000

http://milas.spb.ru/~kmg/files/projects/jcm800mv/jcm800_fet_vs_tube.zip

Ilya-v

Bump.

KMG, this will revolutionize the guitar world.

Can you please elaborate on the final gain stage and what each component does?

Thank you.

KMG

There the full final design of LND150 JCM800 & answers to some questions
http://www.ssguitar.com/index.php?topic=2006.msg15734#msg15734