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 4th November 2012, 04:17 AM   #311
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
Oops didn't search enough:

http://www.ebay.com/itm/LME49810-200...item27c5ff0561


less than eight with shipping

I plan on using one for my tracer if I ever get back to it.

Last edited by TheGimp; 4th November 2012 at 04:19 AM.
  Reply With Quote
Old 4th November 2012, 04:33 AM   #312
diyAudio Member
 
jazbo8's Avatar
 
Join Date: Jan 2011
Location: In Transient
Quote:
Originally Posted by TheGimp View Post
Oops didn't search enough:

LME49810 200V HiFi Audio Power Amplifier Driver Stage Qty 1 | eBay


less than eight with shipping

I plan on using one for my tracer if I ever get back to it.
The LME49810 is about $4/ea. here in Shanghai, I am going to the surplus store to see if there is a suitable transformer first, if not, I will go with some back-to-back filament transformers that I have - looks like I won't have any room left in my so-called "chassis".

Jaz
  Reply With Quote
Old 4th November 2012, 06:14 PM   #313
mogliaa is offline mogliaa  United Kingdom
diyAudio Member
 
mogliaa's Avatar
 
Join Date: Dec 2009
Location: London
Quote:
Originally Posted by jazbo8 View Post
Ale,

I tried your values just now, and the result was the same... Vg=0 gets distorted when the grid current begins to flow. No problem with the small signal triodes since they don't draw much grid current (at least the ones I tried).

Also, even with 15V on the source, it still does not pull above 0, so no way to trace Vg>0, I'm still not sure why that would be the case, as the sim showed that it should be able to pull above 0.

With about 250V on the plates, here are some shots of the grid drive:

With 5881 (see the bow on the Vg=0 step):
https://dl.dropbox.com/u/1326040/IMG_1669_sm.jpg

with 12AX7 (clean Vg=0):
https://dl.dropbox.com/u/1326040/IMG_1674_sm.jpg

Thanks,
Jaz
Hi Jaz,
Well, not sure what bias voltages you have on the FETs. Are u using same power supplies, 5 instead of 15V will make an impact and you will need to change the resistor values.
Looking at your first step is clearly sign of grid current. What grid stopper are u using? I have a switch to select from 0, 100 and 1k resistor I think. I do play with it when this is the case...
Ale
__________________
"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)
http://www.bartola.co.uk/valves/
  Reply With Quote
Old 5th November 2012, 02:25 AM   #314
diyAudio Member
 
jazbo8's Avatar
 
Join Date: Jan 2011
Location: In Transient
Quote:
Originally Posted by mogliaa View Post
Hi Jaz,
Well, not sure what bias voltages you have on the FETs. Are u using same power supplies, 5 instead of 15V will make an impact and you will need to change the resistor values.
Looking at your first step is clearly sign of grid current. What grid stopper are u using? I have a switch to select from 0, 100 and 1k resistor I think. I do play with it when this is the case...
Ale
Ale,

Everything was the same as your drawing. The grid stopper was 1K, tried 10K as well - same result. As I already made a mess of the breadboard, I will re-do it again properly and see if the wiring/layout makes a difference.

Jaz
  Reply With Quote
Old 5th November 2012, 03:27 AM   #315
diyAudio Member
 
Join Date: Jan 2008
Location: Southern California
Jaz, did you loose a post? I got an email with a post that is not showing here.
  Reply With Quote
Old 5th November 2012, 06:46 AM   #316
mogliaa is offline mogliaa  United Kingdom
diyAudio Member
 
mogliaa's Avatar
 
Join Date: Dec 2009
Location: London
Me too!
Ale
__________________
"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)
http://www.bartola.co.uk/valves/
  Reply With Quote
Old 5th November 2012, 07:12 AM   #317
diyAudio Member
 
jazbo8's Avatar
 
Join Date: Jan 2011
Location: In Transient
Quote:
Originally Posted by mogliaa View Post
Me too!
Ale
Yes, I deleted it. I thought I had found a solution to the grid drive (pulling above zero), but it turned out to be short-lived. This was what happened, I did some tests with an IRF840 as the source follower and put +100V on its drain, -100V on its source via a 22K resistor. The waveform looked great without the TUT, when I plug it in and switched on the heater, then the top step got squashed again, anyway, now I can get a solid Vg=0 but not much above that, I don't understand what's happening, need to go back to the textbooks to see how the heater voltage affect the grid, why is it acting like a clamp on the positive grid voltage? Will report back once I get a handle on it, or if you already know the answer, kindly share it with me.

Thanks,
Jaz
  Reply With Quote
Old 5th November 2012, 07:42 AM   #318
mogliaa is offline mogliaa  United Kingdom
diyAudio Member
 
mogliaa's Avatar
 
Join Date: Dec 2009
Location: London
I think what you're experiencing is a lot of grid current. When near 0V or above you will get current flowing through the grid and this creating a voltage drop across the grid resistor which will turn off the FET depending on the bias used.
I will try to remove the grid resistor - just ensure you have some protection on the drain of the output FET and see what you get.
Ale
__________________
"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)
http://www.bartola.co.uk/valves/
  Reply With Quote
Old 5th November 2012, 08:56 AM   #319
Yvesm is online now Yvesm  France
diyAudio Member
 
Yvesm's Avatar
 
Join Date: Jul 2004
Location: Ardeche
Hi Jaz,

Simply because the grid/cathode space become a forward biased diode when a positive voltage is applied to the grid.
The grid becomes a small anode and "sink" some so called "grid current".

You may choose to build a beefy grid voltage generator with the risk to blow up the grid of a tinny tube.

The same problem will rise up when wou'll start playing with tetrodes and pentodes !

Yves.

Last edited by Yvesm; 5th November 2012 at 09:00 AM.
  Reply With Quote
Old 5th November 2012, 09:54 AM   #320
diyAudio Member
 
jazbo8's Avatar
 
Join Date: Jan 2011
Location: In Transient
Hi Yves,

I will let all it sink in. In the meantime, also found this:
Grid stoppers with AB2 which is relevant to the discussion, I think... It appears that I went overboard with the drain voltage and using a large NMOS like IRF840 probably isn't a good idea either (well that was what I had on hand), so I will try a smaller source follower at lower voltage and move the grid stop resistor closer to the gate (too far now with long wires). Getting there, slowly but surely...

Thanks,
Jaz
  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
DIY Curve Tracer for PC locky_z Solid State 444 6th April 2014 02:52 PM
B-H Curve Tracer gpapag Equipment & Tools 9 16th January 2011 08:22 PM
curve tracer barclaycon Equipment & Tools 6 11th February 2010 08:10 PM
Wanted Diy curve tracer! amc32 Car Audio 1 19th October 2009 05:52 AM
DIY tube testing Curve Tracer whitelabrat Tubes / Valves 5 24th October 2007 09:11 PM


New To Site? Need Help?

All times are GMT. The time now is 09:08 PM.


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