User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read Search

 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
 18th January 2011, 06:21 PM #2 Wavebourn   Designer & Technologist diyAudio Member     Join Date: Sep 2006 Location: Pleasant Hill, CA Output AC current flows through the load (the next stage input that is driven by output of this one). __________________ The Devil is not so terrible as his math model is!
 18th January 2011, 06:23 PM #3 Frank Berry   diyAudio Member     Join Date: Sep 2003 Location: Midland, Michigan Under idle conditions with no audio, the DCR of the OPT/Choke is the plate load. The impedance will change in the presence of an audio signal. As for CCS loads, the current doesn't swing, the voltage swings. This means that the impedance of the CCS swings. __________________ Frank
Wavebourn
Designer & Technologist
diyAudio Member

Join Date: Sep 2006
Location: Pleasant Hill, CA
Quote:
 Originally Posted by Frank Berry This means that the impedance of the CCS swings.
What?
__________________
The Devil is not so terrible as his math model is!

dsavitsk
diyAudio Member

Join Date: Jan 2005
Location: Hartford
Quote:
 Originally Posted by db! However, when it comes to an inductive/reactive load(OPT/choke), it's impedance varies with the frequency; so how is it expected to maintain a certain impedance in the absence of a signal/when the amp is idling? Aside from the DCR, it's really no more than a wire (at idle); so how does this keep the plate loaded?
Think of the inherent impedance due to reactance as being a ceiling. The reflected impedance then sets the actual impedance. So, imagine you have a transformer with 40H of inductance and a 25:1 winding ratio. This would translate to a 625:1 impedance ratio which is 5K:8 and 5K of impedance at 20Hz (impedance = 2*pi*Frequency*Inductance). If you load it with an 8 ohm load, at less than 20Hz the reactance is lower than the reflected impedance, so you won't be able to actually reflect 5K which means you'll lose low bass. But, at 20Hz and above, there is enough inductance that you can reflect to 5K, so the tube sees a 5K load. In the case of a plate loading choke, it is only the inductance that matters, so above a certain frequency, the load is very high.

Quote:
 Since we are on the topic of plate loading, I thought I'd also ask about CCS loads. For the purposes of discussion, say a CCS allows 10mA on the plate of a tube; how does this allow for current to swing when a signal is applied?
The CCS sets the DC operating point. As signal raises and lowers the grid, the tube wants to draw more or less current. This adjustment is made through the load (the next stage, or the parafeed transformer.)

Last edited by dsavitsk; 18th January 2011 at 06:31 PM.

kevinkr
diyAudio Moderator

Join Date: Sep 2004
Location: Boston, Massachusetts
Quote:
 Originally Posted by Frank Berry As for CCS loads, the current doesn't swing, the voltage swings. This means that the impedance of the CCS swings.
The CCS impedance doesn't change. The internal resistance of the device connected to the CCS is doing the changing. Think of the tube (triode usually) as a voltage controlled resistor (or more accurately transconductance) with a constant current through it. Change the resistance and the voltage dropped across it changes.
__________________
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine

Last edited by kevinkr; 18th January 2011 at 06:41 PM.

wakibaki
Banned

Join Date: Jan 2008
Quote:
 Originally Posted by db! However, when it comes to an inductive/reactive load(OPT/choke), it's impedance varies with the frequency; so how is it expected to maintain a certain impedance in the absence of a signal/when the amp is idling? Aside from the DCR, it's really no more than a wire (at idle); so how does this keep the plate loaded?
It's true that the impedance of a choke varies with frequency, but chokes are rarely employed as loads in broadband amplifiers, they are more likely to be found in a narrowband, RF, amplifier.

At idle (DC) the current in the active device is controlled by the biassing, not the load. The load impedance is of significance only at AC (when there is a signal), the resistive component of a loudspeaker driver is multiplied back to the plate through the transformer turns ratio.

As regards the CCS, this is easiest thought of in terms of the limiting conditions. When the valve is saturated (fully ON) all the current flows from the CCS through the valve. When the valve is cut off (fully OFF) all the current flows from the CCS into the load (which is connected at the junction of the CCS and the plate).

w

It's important to realise that there's a certain amount of operator overloading taking place. Sometimes load means the device that gets the power in the end, and sometimes it means whatever arrangement of components is in the anode circuit.

Last edited by wakibaki; 18th January 2011 at 07:11 PM.

DF96
diyAudio Member

Join Date: May 2007
Quote:
 Under idle conditions with no audio, the DCR of the OPT/Choke is the plate load. The impedance will change in the presence of an audio signal.
No. The DC resistance stays as it is for 0Hz 'signals'. The inductive reactance is set by frequency, not signal amplitude. A 10H choke has an impedance of 6283ohms at 100Hz, whether or not there is actually any 100Hz signal present.

A choke and OPT are not really the same thing, although there are some similarities. If you think they are the same you will get confused. If you think they are completely different you will also get confused!

I think your problem is that you associate impedance with signal. Impedance is unaffected by signal. It tells you what would happen if a signal is present, but with the signal gone the impedance is still there.

 18th January 2011, 07:29 PM #9 db!   diyAudio Member   Join Date: Nov 2009 Location: Ontario Thanks for the replies! The plate choke idea is for parafeed, I am still exploring my options for a SE amp. I have to admit, I'm still a bit confused; maybe I'm looking at it from the wrong way... I tend to think of biasing in terms of a load line. You choose a plate load, apply a negative voltage to the grid(that corresponds to one of the tube curves) and that should set the idle current. If at idle, the DCR of an OPT was the load, would this not cause the current to shoot up?
 18th January 2011, 07:49 PM #10 dsavitsk   diyAudio Member     Join Date: Jan 2005 Location: Hartford with a resistor load, AC and DC load lines are the same. With a choke load, the load line is the choke in parallel with the actual load but this does not correspond to DC conditions which are set by B+ and bias. So yes, if you would use say a 300V supply with a resistor load to get 150V on the plate, then with a choke load you are going to want a ~150V supply. This is explained in the link I posted above. __________________ http://www.ecpaudio.com :: http://diy.ecpaudio.com

 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 Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Site     Site Announcements     Forum Problems Amplifiers     Solid State     Pass Labs     Tubes / Valves     Chip Amps     Class D     Power Supplies     Headphone Systems Source & Line     Analogue Source     Analog Line Level     Digital Source     Digital Line Level     PC Based Loudspeakers     Multi-Way     Full Range     Subwoofers     Planars & Exotics Live Sound     PA Systems     Instruments and Amps Design & Build     Parts     Equipment & Tools     Construction Tips     Software Tools General Interest     Car Audio     diyAudio.com Articles     Music     Everything Else Member Areas     Introductions     The Lounge     Clubs & Events     In Memoriam The Moving Image Commercial Sector     Swap Meet     Group Buys     The diyAudio Store     Vendor Forums         Vendor's Bazaar         Sonic Craft         Apex Jr         Audio Sector         Acoustic Fun         Chipamp         DIY HiFi Supply         Elekit         Elektor         Mains Cables R Us         Parts Connexion         Planet 10 hifi         Quanghao Audio Design         Siliconray Online Electronics Store         Tubelab     Manufacturers         AKSA         Audio Poutine         Musicaltech         Aussie Amplifiers         CSS         exaDevices         Feastrex         GedLee         Head 'n' HiFi - Walter         Heatsink USA         miniDSP         SITO Audio         Twin Audio         Twisted Pear         Wild Burro Audio

 Similar Threads Thread Thread Starter Forum Replies Last Post jmillerdoc Tubes / Valves 5 9th March 2010 10:14 PM Nevod Tubes / Valves 12 12th August 2008 06:52 PM Josephjcole Full Range 1 15th November 2005 10:27 PM valveitude Tubes / Valves 19 22nd July 2005 11:00 PM jwells777 Parts 1 11th May 2003 08:40 AM

 New To Site? Need Help?

All times are GMT. The time now is 02:10 PM.