HV Tube Preamp, is it heresy to consider a JFET or HV MOSFET first stage?

[Dpappas]

I’m building a preamp in a pedal, with a SMPS HV supply at about 300V for real tube performance.  I was shooting for three gain stages in total.  I’m working on the space and power requirements and I am leaning more and more toward using 1 tube and 1 J201 or LND150 transistor stage in front.  The first stage isnt’ going to be driven by more than humbuckers so I’m not interested in its distortion characteristics.  It’s just there for amplification.  Of course tubes have more mojo but I was going to use a parallel triode as the first stage but it’s a lot of work.  The remaining tube would provide two gain stages to get progressively overdriven.  So here are my questions.

1. Is this heresy?

2. I could do a J201 JFET stage at 12V as the first stage, or a LND150 MOSFET at 300V.  Any good reason to do the MOSFET in this circuit aside from convenience of one voltage source for the plates and drains?

[Merlin]

You could also do a hybrid stage... (this is not a cascode!)

[Dpappas]

You could also do a hybrid stage... (this is not a cascode!)

Merlin thanks for the suggestion, I’ll read into it.  Also, thank you for sharing your knowledge in your books and forums.  Your preamp book was indispensable when I was designing a parallel triode stage

[glass54]

Hi Dpappas,

is it heresy to consider a JFET or HV MOSFET first

Alexander Dumble did it 
Regards
Mirek

[kagliostro]

I think you already know this, but if you don't

http://www.hawestv.com/amp_projects/amp_solid_tube/fetpreamp_200v1.htm

Franco

[Dpappas]

I think you already know this, but if you don't

http://www.hawestv.com/amp_projects/amp_solid_tube/fetpreamp_200v1.htm

Franco

I’ve seen it but thank you!  I am curious how this circuit sounds as a first stage.  I’m going to just have to go ahead and build it and see

[Willabe]

David Lindley plugged his lap steel into the Jfet channel on his Dumble.

That says something. 

[Dpappas]

An update.  I put the whole thing together last night, using a LND150 as the first stage and then two standard triode stages.   It sounds amazing, usable gain across the range and a really full sound. 

BUT…

The SMPS I’m using (cheap $10 PCB from Amazon) is noisy, I’ll need to add some filtering to quiet the supply noise.  I also need to adjust the gain and output range (make a voltage divider with each bottom on the bottom leg).  It has too much gain right now, so I could also clip the source bypass capacitor on the LND150.  I’m going to add one of these to any amp or preamp I build from now on, I’m sold on the idea.

[kagliostro]

Your ears will be the better judge for the mod on lowering the gain you want to try

if I can suggest, give a try also to the Split Load Resistor mod (without any changes to the LND150 circuit)

I mean use 2 resistors in place of R3 100K and connect C1 to the junction of this new resistors, the sum of the value must be 100K, if you use two 50K resistors you'll obtain 50%of signal as output and if the resistors are one 30% and ore 70% (heaving the smaller connected to the LND150) you'll obtain 30% signal level at the output

The Split Load mod permit to your gain stage to work exactly as before but you have a lower level output

Franco

[Dpappas]

Your ears will be the better judge for the mod on lowering the gain you want to try

if I can suggest, give a try also to the Split Load Resistor mod (without any changes to the LND150 circuit)

I mean use 2 resistors in place of R3 100K and connect C1 to the junction of this new resistors, the sum of the value must be 100K, if you use two 50K resistors you'll obtain 50%of signal as output and if the resistors are one 30% and ore 70% (heaving the smaller connected to the LND150) you'll obtain 30% signal level at the output

The Split Load mod permit to your gain stage to work exactly as before but you have a lower level output

Franco

Franco,

Split load was on my list to try, I’m going to bump it up to the top. 

I made a new SMPS and the noise has dropped significantly.