12at7 LTP CCS - 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 22nd April 2008, 09:27 PM   #1
wicked1 is offline wicked1  United States
diyAudio Member
 
wicked1's Avatar
 
Join Date: Jan 2008
Location: Midwest
Default 12at7 LTP CCS

Hello,
I've been reading about what to set the current to on my CCS for a 12at7 LTP.
Everyone seems to agree on about 3ma, but is that 3ma per side, so set the CCS for 6ma, or is it 3ma between them?

thanks!
  Reply With Quote
Old 22nd April 2008, 09:54 PM   #2
diyAudio Member
 
Join Date: May 2004
Location: WI
Hi,

It's 3 mA per side, so 6mA through a common CCS. In Eli Duttman's El Cheapo, which uses a CCS for the 'AT7 LTP, some are running up to 3.5 mA per side. I can't hear a difference, so I keep mine at about 3.2 mA.

-- Jeff
  Reply With Quote
Old 22nd April 2008, 09:59 PM   #3
bigwill is offline bigwill  United Kingdom
diyAudio Member
 
Join Date: Dec 2004
Location: UK
Regarding this configuration of valves, how balanced is this used as a phase splitter? Given identical anode resistors, is the gain balance near perfect? Is it possible for it to NOT be perfect?
  Reply With Quote
Old 22nd April 2008, 10:04 PM   #4
wicked1 is offline wicked1  United States
diyAudio Member
 
wicked1's Avatar
 
Join Date: Jan 2008
Location: Midwest
Thanks, That's what I thought.

Unfortunately I realized it right after I placed my parts order.
  Reply With Quote
Old 22nd April 2008, 10:06 PM   #5
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
I'd use a bit less current, more like 2mA. Make the plate loads as large as practical and match 'em- the balance will be as good as your matching, assuming you use a decent CCS.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 22nd April 2008, 10:30 PM   #6
kmaier is offline kmaier  United States
diyAudio Member
 
kmaier's Avatar
 
Join Date: Jul 2007
The LTP inverter is quite accurate and has good balance, but it's not perfect. Adding a CCS at the bottom will not make it perfect either. As the CCS maintains a constant current flow, whatever one tube increases by, the other tube decreases by. This alone does not result in perfect balance. You can still have imbalance in the pair of triodes used so you should try and source a balanced twin triode (both sections match well). If you want to get the best performance, in addition, I would use a linear pot in the plate circuit as an AC balance control, best adjusted using a distortion analyzer.

Regards, KM
  Reply With Quote
Old 22nd April 2008, 10:40 PM   #7
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
You can still have imbalance in the pair of triodes used so you should try and source a balanced twin triode (both sections match well).
With a good CCS as the tail, the triodes should be able to be thoroughly mismatched and the balance will still be as perfect as the loads. Think about it: if the current IS constant (and even in a cheap bipolar cascode CCS, the current is VERY constant), any decrease in one plate current MUST have an equal and opposite response in the other, otherwise Kirchoff's Law is violated. There's no other place for the current to go as long as the stage is not overloaded.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 22nd April 2008, 11:22 PM   #8
Salas is offline Salas  Greece
diyAudio Chief Moderator
 
Salas's Avatar
 
Join Date: Oct 2002
Location: Athens-Greece
Quote:
Originally posted by kmaier
The LTP inverter is quite accurate and has good balance, but it's not perfect. Adding a CCS at the bottom will not make it perfect either. As the CCS maintains a constant current flow, whatever one tube increases by, the other tube decreases by. This alone does not result in perfect balance.
Regards, KM

Here is what I use in my amp. Although I could get 5% different DC voltage drops at each plate, for some AT7 tubes in my collection due to differences of gm in the triode pairs, the AC balance was always spot on for AC RMS measurements at the grids of the final stage tubes.
Attached Images
File Type: jpg screenshot4.jpg (48.4 KB, 429 views)
  Reply With Quote
Old 23rd April 2008, 12:06 AM   #9
kmaier is offline kmaier  United States
diyAudio Member
 
kmaier's Avatar
 
Join Date: Jul 2007
Yes, I should probably qualify my statement... if you have different gain tubes you can still get some nonlinear performance, not necessarily inversion balance. You might not see it on a scope but a good analyzer will show it. Also, you're probably doing something with the output of the LTP, either driving a pair of output tubes or pre-drivers. This also implies two devices operating 180 degrees out of phase, but it's very unlikely you will have a perfectly matched pair of outputs, as many are only matched to within 5%. As a result, having an AC balance control is a nice option, and adding DC balance to the output stage is also a nice option... you can actually null distortion and output noise to a considerable degree.

Also, the amount of quiescent current in the LTP should be adequate to drive it's intended load at the voltage swing and bandwidth required for your application (ie, slew rate calculation). In the past, I ran a 6FQ7 LTP with a CCS at 10ma. It was driving a pair of KT-88 in UL-PP and required P-P swings in excess of 150 volts. Bandwidth was flat beyond 30KHz.

Regards, KM
  Reply With Quote
Old 23rd April 2008, 12:29 AM   #10
Salas is offline Salas  Greece
diyAudio Chief Moderator
 
Salas's Avatar
 
Join Date: Oct 2002
Location: Athens-Greece
Quote:
Originally posted by kmaier
Yes, I should probably qualify my statement... if you have different gain tubes you can still get some nonlinear performance, not necessarily inversion balance. You might not see it on a scope but a good analyzer will show it. Also, you're probably doing something with the output of the LTP, either driving a pair of output tubes or pre-drivers. This also implies two devices operating 180 degrees out of phase, but it's very unlikely you will have a perfectly matched pair of outputs, as many are only matched to within 5%. As a result, having an AC balance control is a nice option, and adding DC balance to the output stage is also a nice option... you can actually null distortion and output noise to a considerable degree.

Also, the amount of quiescent current in the LTP should be adequate to drive it's intended load at the voltage swing and bandwidth required for your application (ie, slew rate calculation). In the past, I ran a 6FQ7 LTP with a CCS at 10ma. It was driving a pair of KT-88 in UL-PP and required P-P swings in excess of 150 volts. Bandwidth was flat beyond 30KHz.

Regards, KM

Very good additional points. Though it may be argued that some residual harmonic imbalance will be mainly of the second order variety, needed somewhat subjectively in a PP amp to counterbalance the predominating 3rd.
For instance by using that LTP I posted, utilizing the particular non linearities of 12AT7, and not taking measures for absolute harmonic balance of the KT88 output stage more than the actual performance that the PP OPTs could achieve, I got the attached distortion spectrum, where 2nd is stronger than 3rd at -72dB vs -80dB.

(test signal peak is at 100dB, 10dB/div).
Attached Images
File Type: gif screenshot3.gif (9.8 KB, 337 views)
  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
Ecc81 12at7 316a Swap Meet 0 30th May 2008 11:51 AM
6DJ8 or 12AT7 hags Tubes / Valves 4 22nd February 2007 12:16 PM
6V6 ->EL84 with 12AT7? zxx123 Tubes / Valves 8 5th April 2006 06:12 PM
Can I use 12AT7? Ahmad_tbp Tubes / Valves 6 25th January 2006 06:11 AM
RCA 12AT7 or Mullard CV4024? Jay Tubes / Valves 11 31st March 2003 02:30 PM


New To Site? Need Help?

All times are GMT. The time now is 04:11 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