If you need assistance, please send an email to forum at 4hv dot org. To ensure your email is not marked as spam, please include the phrase "4hv help" in the subject line. You can also find assistance via IRC, at irc.shadowworld.net, room #hvcomm.
Support 4hv.org!
Donate:
4hv.org is hosted on a dedicated server. Unfortunately, this server costs and we rely on the help of site members to keep 4hv.org running. Please consider donating. We will place your name on the thanks list and you'll be helping to keep 4hv.org alive and free for everyone. Members whose names appear in red bold have donated recently. Green bold denotes those who have recently donated to keep the server carbon neutral.
Special Thanks To:
Aaron Holmes
Aaron Wheeler
Adam Horden
Alan Scrimgeour
Andre
Andrew Haynes
Anonymous000
asabase
Austin Weil
barney
Barry
Bert Hickman
Bill Kukowski
Blitzorn
Brandon Paradelas
Bruce Bowling
BubeeMike
Byong Park
Cesiumsponge
Chris F.
Chris Hooper
Corey Worthington
Derek Woodroffe
Dalus
Dan Strother
Daniel Davis
Daniel Uhrenholt
datasheetarchive
Dave Billington
Dave Marshall
David F.
Dennis Rogers
drelectrix
Dr. John Gudenas
Dr. Spark
E.TexasTesla
eastvoltresearch
Eirik Taylor
Erik Dyakov
Erlend^SE
Finn Hammer
Firebug24k
GalliumMan
Gary Peterson
George Slade
GhostNull
Gordon Mcknight
Graham Armitage
Grant
GreySoul
Henry H
IamSmooth
In memory of Leo Powning
Jacob Cash
James Howells
James Pawson
Jeff Greenfield
Jeff Thomas
Jesse Frost
Jim Mitchell
jlr134
Joe Mastroianni
John Forcina
John Oberg
John Willcutt
Jon Newcomb
klugesmith
Leslie Wright
Lutz Hoffman
Mads Barnkob
Martin King
Mats Karlsson
Matt Gibson
Matthew Guidry
mbd
Michael D'Angelo
Mikkel
mileswaldron
mister_rf
Neil Foster
Nick de Smith
Nick Soroka
nicklenorp
Nik
Norman Stanley
Patrick Coleman
Paul Brodie
Paul Jordan
Paul Montgomery
Ped
Peter Krogen
Peter Terren
PhilGood
Richard Feldman
Robert Bush
Royce Bailey
Scott Fusare
Scott Newman
smiffy
Stella
Steven Busic
Steve Conner
Steve Jones
Steve Ward
Sulaiman
Thomas Coyle
Thomas A. Wallace
Thomas W
Timo
Torch
Ulf Jonsson
vasil
Vaxian
vladi mazzilli
wastehl
Weston
William Kim
William N.
William Stehl
Wesley Venis
The aforementioned have contributed financially to the continuing triumph of 4hv.org. They are deserving of my most heartfelt thanks.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Is there any way to make Steve Wards 811A vttc circuit work with a mosfet? Without any circuit changes? I would be greatful if someone could drop me a line. Someone on this forum said something about just changing the negative bias into positive or something. I dont know. But surly it is possible considering the vacuum tubes and mosfets are both electronic 'valves'.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
A MOSFET is just a functional analogy to a Triode tube, they are not drop in replaceable. For one thing, the grid voltage requirements are entirely different for a triode and a MOSFET. Stick a MOSFET in place of that 811A with Steve's circuit, which if i remember correctly used a grid leak biasing network to turn the tube on/off, and you'll blow that MOSFET to smithereens!!!
Registered Member #952
Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388
Arcstarter wrote ...
Is there any way to make Steve Wards 811A vttc circuit work with a mosfet? Without any circuit changes? I would be greatful if someone could drop me a line. Someone on this forum said something about just changing the negative bias into positive or something. I dont know. But surly it is possible considering the vacuum tubes and mosfets are both electronic 'valves'.
There was something like that on the old forum: They got a IRFP460 working straight from a feedback coil!
Registered Member #1406
Joined: Thu Mar 20 2008, 12:12PM
Location: Deepest darkest Wales, UK
Posts: 27
Aha - I was wondering if the replace-triode-with-MOSFET technique familiar to me from audio stuff would be applicable for a coil.
Lots of info about this substitution (for audio use) at Runoffgroove.com in general and here specifically. J201's are very popular in that application; I'm going to experiment with similar techniques for high power using possibly 2SK899's, which I have a bunch of.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I've played around with JFET and valve preamps too. The valve-to-JFET substitution works (sort of) because these are small-signal Class-A circuits.
Since the 811A runs off something like 2kV, the plate voltage goes to at least 2 times, maybe pi times the supply voltage, and nobody makes 6kV MOSFETs, you are doomed to fail unless you lower the voltage, which is a circuit change. After a few more circuit changes, you should end up with something not unlike Steve Ward's Class-E micro SSTC.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
I just mean use the same basic circuit meaning a royer oscillator or what ever it is called. I know 2kv is way above operatic characteristics of any mosfet. I just heard someone say it could happen if you change some stuff up. Spark had the right forum. Read it and mabey you will see what they mean..I am a little stupid when it comes to substituting parts..
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Arcstarter wrote ...
I know 2kv is way above operatic characteristics of any mosfet.
They can sing?!
Seriously though, FETs are like tubes in the same way that a Geo metro and a monster truck are both cars. They need slightly different conditions to show their best. FETs nowadays are designed for switched mode power supplies and can be a bit unpredictable in these simple self-oscillating circuits. They can suffer from thermal runaway, parasitic oscillations, etc, whereas tubes tend to "just work".
I'm currently working on a Fender Bassman-style preamp made of BF245A JFETs cascoded with high-voltage BJTs. I'd never seen the runoffgroove stuff before, and I didn't know about using a resistor on the source to change the square-law characteristic to something more "tubey". It actually seems to work! :)
BTW, I always thought FETs behave more like pentodes...
Registered Member #1406
Joined: Thu Mar 20 2008, 12:12PM
Location: Deepest darkest Wales, UK
Posts: 27
Steve Conner wrote ...
I'm currently working on a Fender Bassman-style preamp made of BF245A JFETs cascoded with high-voltage BJTs. I'd never seen the runoffgroove stuff before, and I didn't know about using a resistor on the source to change the square-law characteristic to something more "tubey". It actually seems to work! :)
- Apologies to OP for straying off topic, but check out the Dr. Boogey pedal. It's built from the Mesa Boogie Dual Rec head schem, which is almost exactly copied with the tubes replaced with JFETs and appropriately biased.
It seems like it shouldn't work; yet sounds closer to the real thing than any other analogue/digital/software emulation I've heard.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Ooo, awesome! I must try building that.
In a pathetic attempt to stay on topic, note that it runs off 9v, not 250v like the original did! :P
Also, JFETs are depletion-mode devices, so they can be biased the same as tubes. MOSFETs are enhancement mode: they don't conduct until the gate is brought more positive than the source. So the "cathode bias" trick won't work directly.
This site is powered by e107, which is released under the GNU GPL License. All work on this site, except where otherwise noted, is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. By submitting any information to this site, you agree that anything submitted will be so licensed. Please read our Disclaimer and Policies page for information on your rights and responsibilities regarding this site.
Just a thought about sstc and vttc.
