Common Cathode 6922 output impedance? - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 28th May 2004, 12:02 AM   #11
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
"Quantify" is the word you're looking for. Anyway, you can see the effect of Crss by using f3 = 1/(2piRC), where R is the source impedance of the driver stage and C is the reverse transfer capacitance.

Depending on what you're trying to drive, grataku's point may be valid- for example, an IRF820 (which is one I use a lot) has only 10 pf drain-to-gate C. With 20Kohm driving it, the f3 is somewhere in the AM radio band.
__________________
The more you pay for it, the less inclined you are to doubt it.- George Smiley
  Reply With Quote
Old 28th May 2004, 01:06 AM   #12
grataku is offline grataku  United States
diyAudio Member
 
Join Date: Dec 2000
Location: -
Joebob,
I looked at the model for mosfets, it seems to me that the in the follower both Cgs and Cgd are in series so I am not seeing them low pass filter but rather as a source of local feedback.
In the reference I have given the author makes a point about the mosfet being a better follower than the tube (vide infra). I wouldn't get into a glass vs. sand dispute if it was a BJT but when if we talk mosfet the sound is soo sweet and the output impedance soo low...

SY
sorry for taking the thread off topic but what I was planning to do with my not-so-original idea was a driver stage for some big triode. I have a pair of 300BXLS and I would like to drive them a little bit differently. Have you ever done such thing?
  Reply With Quote
Old 28th May 2004, 01:21 AM   #13
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Cool THE CRITICS EYE....

Hi,

Quote:
I wouldn't get into a glass vs. sand dispute if it was a BJT but when if we talk mosfet the sound is soo sweet and the output impedance soo low...
If you look at low Zout as THE GOAL per se then I think you're missing the bigger picture.

In the context of tubes you'll usually find yourself driving high enough impedances with mostly stray capacitance and Miller effects the worry about in the Zout context and than some...

What's often more important is the voltage swing and current capability of the driver stage so you don't run into slew limiting.

I'm well aware high voltage FETs do exist and both tubes and FETs can live symbiotically...just trying to point out that bigger DHTs such as the 300B like to be kicked by the driver stage.
After all that's the one you'll be listeneng to at the end of the day.

Cheers,
__________________
Frank
  Reply With Quote
Old 28th May 2004, 02:26 AM   #14
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
sorry for taking the thread off topic but what I was planning to do with my not-so-original idea was a driver stage for some big triode. I have a pair of 300BXLS and I would like to drive them a little bit differently. Have you ever done such thing?
With 300Bs? No. But they're just triodes, albeit expensive ones. Frank's on point- you just need to swing enough volts to drive 'em and enough current to overcome the input capacitance. That's it- you don't need a stage that will swing a kilovolt at half an amp! It's an easier load than the sweep tubes I'm driving, and I'm not doing anything exotic other than the MOSFET source followers.
__________________
The more you pay for it, the less inclined you are to doubt it.- George Smiley
  Reply With Quote
Old 28th May 2004, 04:02 AM   #15
grataku is offline grataku  United States
diyAudio Member
 
Join Date: Dec 2000
Location: -
The price of the tubes is a very important point, since I don't know what tomorrow holds and I may not be able to swing another 300 bucks for a new pair of KR300B I would like to build a driver stage with enough nuts to drive bigger and hopefully cheaper tubes.

SY if you're at liberty, what are you driving exactly with your mosfet follower?

Anyway, fun discussion. The tube forum is quickly becoming my favorite
  Reply With Quote
Old 28th May 2004, 04:23 AM   #16
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
SY if you're at liberty, what are you driving exactly with your mosfet follower?
6LF6. When my last 6LF6 bites the dust, I'll get around to whatever 509/519 is around.

Quote:
The tube forum is quickly becoming my favorite
Mine, too. Love tubes. Always loved tubes.
__________________
The more you pay for it, the less inclined you are to doubt it.- George Smiley
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Common Source versus Common Drain output stages alaskanaudio Solid State 36 1st June 2014 02:35 AM
Common Cathode Resistor D0Hbert Tubes / Valves 1 4th March 2005 11:42 AM
cmrr of common cathode metebalci Tubes / Valves 14 21st August 2004 07:25 PM
Help with common-cathode circuit. JoeBob Tubes / Valves 48 1st March 2003 10:57 PM
hybrid common cathode amplifier? mwmitchell Tubes / Valves 19 16th January 2003 01:51 PM


New To Site? Need Help?

All times are GMT. The time now is 03:49 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2