Low voltage 12AU7 (19J6) -- re "starving" - 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 5th May 2011, 03:10 AM   #1
diyAudio Member
 
jackinnj's Avatar
 
Join Date: Apr 2002
Location: Llanddewi Brefi, NJ
Default Low voltage 12AU7 (19J6) -- re "starving"

when I built this the THD% using the 12AU7 model I have simmed the same as the one I breadboarded. I've used LED's on the cathode for a number of SY designs but here you are really constrained by low plate voltage:
Attached Images
File Type: gif Starving_1.GIF (7.0 KB, 597 views)
  Reply With Quote
Old 5th May 2011, 11:11 AM   #2
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
And your point is?

12AU7 and 6J6 are not the same, so THD simulation will not tell you much. Some of your distortion will be coming from the diode.
  Reply With Quote
Old 5th May 2011, 11:24 AM   #3
AJT is offline AJT  Philippines
diyAudio Moderator
 
AJT's Avatar
 
Join Date: May 2003
Location: Palatiw, Pasig City
Quote:
Some of your distortion will be coming from the diode
why? my 6H23 preamp so biased with a silicon diode sounds fantastic to my ears....
__________________
the best advertisement for a good audio design is the number of diy'ers wanting to build it after all the years....never the say so of so called gurus....
  Reply With Quote
Old 5th May 2011, 11:24 AM   #4
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
You probably won't have much distortion from the diode (that's something I've never been able to measure in a wide variety of circuits), but you may very well have unexpected distortion from grid current, which usually doesn't show up in sims. Many tubes of that class start showing significant grid current at Vgk < 1V or so.
__________________
And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
  Reply With Quote
Old 5th May 2011, 11:30 AM   #5
AJT is offline AJT  Philippines
diyAudio Moderator
 
AJT's Avatar
 
Join Date: May 2003
Location: Palatiw, Pasig City
I realise that with the forward biased diode, you are limited to an input voltage of 0.7 volts peak if you are to avoid getting the grid in positive region, i have used a pot in the input to pad the signal......
__________________
the best advertisement for a good audio design is the number of diy'ers wanting to build it after all the years....never the say so of so called gurus....
  Reply With Quote
Old 5th May 2011, 11:35 AM   #6
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Sorry, I misread the data. 6.44mV of signal across the diode is pk-pk, not pk or RMS. However, that would still give a few % of 2nd? (assuming a diode behaves something like a BJT Vbe? Vsig pk = THD 2nd % ?) Would be reduced because it is only a small part of the total signal. So 3.2mV gives 3.2% 2nd, but the diode signal is only 3.2% of the total so I make that about 0.1% 2nd due to the diode.

If the valve model includes grid current (some do, some don't) then you can check for distortion from this by adding some impedance to your signal source.
  Reply With Quote
Old 5th May 2011, 11:39 AM   #7
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by Tony
I realise that with the forward biased diode, you are limited to an input voltage of 0.7 volts peak if you are to avoid getting the grid in positive region
No, fortunately it is better than that. Most of the signal is developed across the grid-cathode so the diode only sees a fraction.

However, it is not the grid going positive you need to worry about. As SY says, grid current can start well before that. I seem to recall that the books warn that positive grid current can start anywhere between -1.5V and -0.5V, and the threshold varies not only from valve to valve (even of the same type) but can also change with ageing. I try never to have a grid more positive than -1V.
  Reply With Quote
Old 5th May 2011, 12:43 PM   #8
AJT is offline AJT  Philippines
diyAudio Moderator
 
AJT's Avatar
 
Join Date: May 2003
Location: Palatiw, Pasig City
i recall tubelab's experiments with the 845's those chines tubes seemed to pull grid currents even when not yet into positive grid.......
__________________
the best advertisement for a good audio design is the number of diy'ers wanting to build it after all the years....never the say so of so called gurus....
  Reply With Quote
Old 5th May 2011, 04:12 PM   #9
diyAudio Member
 
jackinnj's Avatar
 
Join Date: Apr 2002
Location: Llanddewi Brefi, NJ
OK, I straightened out the SIM by using the Leach models and setting the coefficients such that the VI curves look like the 19J6 datasheet.

With the diode and the sections of the 19J6 in parallel, the THD% was 0.31%, with a 2K resistor per the Starving Student schema it was 0.052%

We're constrained by the 48V supply. Even more constrained by 19J6 becoming unobtanium.
  Reply With Quote
Old 5th May 2011, 04:48 PM   #10
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
If you are very patient 19J6 appear on ebay from time to time, but who wants to wait a year before building an amp? You could consider UCC84 (or possibly UCC85), although these are not too common either. They are European 100mA heater valves, with E (6.3V) and P (300mA) versions too. The '85 is almost the same as a 12AT7, apart from the addition of a screen and different heater. The '84 was intended for cascode, so should be OK at low anode volts.
  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
low voltage "high gain" amp razorrick1293 Tubes / Valves 32 8th April 2011 04:45 AM
Advice Needed: "Flabby" Amp Low-End independentexile Tubes / Valves 2 23rd October 2010 04:27 AM
What makes an amplifier "bright", "warm", or "neutral"? JohnS Solid State 51 13th December 2009 07:42 PM
Linear voltage regulator: how to make good use of "sense" and "ground sense"? NeoY2k Analog Line Level 7 7th September 2008 12:35 AM
Headphone amp - low Z load and ... "echo" yagoolar Headphone Systems 12 31st March 2007 12:13 AM


New To Site? Need Help?

All times are GMT. The time now is 02:42 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