Common Grid design - 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 27th January 2015, 10:20 PM   #1
diyAudio Member
 
Join Date: Dec 2010
Location: Amesbury, MA
Default Common Grid design

I have now seen a couple of power amplifiers using a common grid configuration. or the input is the cathode of the power tube and not the control grid. The amp in question using 6L6GC's has 700v at the plates, 22v at the control grid, what gets me is the cathode has a power transistor connected to it, collector is tied directly to 6L6 cathode, emitter tied to ground through 3.9R resistor. base is biased from the high voltage supply and is fed by the phase inverter.

I was under the impression that because of the low input capacitance it was mainly used for RF.

So what is the advantage in audio? And are there any good links to the design process involved? I think my lack of terminology has hindered my searching.

I plan to draw up a schematic and posting it.
  Reply With Quote
Old 27th January 2015, 10:43 PM   #2
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 7
Quote:
Originally Posted by famousmockingbird View Post
I have now seen a couple of power amplifiers using a common grid configuration. or the input is the cathode of the power tube and not the control grid. The amp in question using 6L6GC's has 700v at the plates, 22v at the control grid, what gets me is the cathode has a power transistor connected to it, collector is tied directly to 6L6 cathode, emitter tied to ground through 3.9R resistor. base is biased from the high voltage supply and is fed by the phase inverter.

I was under the impression that because of the low input capacitance it was mainly used for RF.
Grounded grid is an RF topology, due to the absence of CMiller, and the Lo-Z input. Having a Lo-Z input is helpful if you're connecting a big linear to an existing low power transmitter that already is made to match the Zchar of a T-line. Otherwise, at frequencies in excess of 400MHz, GG is probably the only thing that'll work, but you have to trade off voltage gain since the inputs will need to be tapped down in order to match impedances. (Disk seal i.e. "light house" triodes and MICs excepted)

Quote:
So what is the advantage in audio? And are there any good links to the design process involved? I think my lack of terminology has hindered my searching.

I plan to draw up a schematic and posting it.
None that I know of, unless the GG is a second stage making up a cascode. You don't want Lo-Z inputs in audio circuits except if it's an unusual situation. GG audio finals present the same problems as Class *2: the Lo-Z that requires a substantial driver current, and the attendant problems of keeping Zsource down to reduce distortion. These days, transistors can take care of that for Class *2 than any old time solution.

This design looks like it's using GG finals as a novelty.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 27th January 2015, 11:05 PM   #3
cerrem is offline cerrem  United States
diyAudio Member
 
Join Date: Jun 2005
Location: San Diego, CA
Common Grid is used all the time in audio circuits...both Tube and Soild State design..
It just may not be apparent...
Just about any Diff Pair at input or Phase inverter is wired a Common Grid...
A diff pair with one terminal grounded as used in many Long Tail Phase Inverters....this is essentially two topologies...
A Cathodyne Phase inverter and a Grounded grid stage....
The cathode of the Cathodyne splitter provides the low Z to drive signal into the Cathode of the Grounded Grid amp... They share the same cathode load, which is essentailly common coupling.... The higher this Z the tigher the coupling approaches unity, since it is really a current divider....

Chris
  Reply With Quote
Old 27th January 2015, 11:48 PM   #4
banat is offline banat  Serbia
diyAudio Member
 
Join Date: Dec 2009
Location: near Belgrade
Quote:
Originally Posted by cerrem View Post
Common Grid is used all the time in audio circuits...both Tube and Soild State design..
It just may not be apparent...
Just about any Diff Pair at input or Phase inverter is wired a Common Grid...
A diff pair with one terminal grounded as used in many Long Tail Phase Inverters....this is essentially two topologies...
A Cathodyne Phase inverter and a Grounded grid stage....
The cathode of the Cathodyne splitter provides the low Z to drive signal into the Cathode of the Grounded Grid amp... They share the same cathode load, which is essentailly common coupling.... The higher this Z the tigher the coupling approaches unity, since it is really a current divider....

Chris
Agree on this ,
and next example for GG is simple cascode circuit , where lower triode anode directly drive cathode from upper (AC) Grounded Grid triode .
  Reply With Quote
Old 28th January 2015, 12:03 AM   #5
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
Quote:
This design looks like it's using GG finals as a novelty.
I have some simulations and built a grounded grid test amp right before packing up my lab to move. The results looked promising. I used a mosfet in the cathode circuit.

I don't want to go into the details now since I don't have a place to work right now, and it will be months before I do. There are some advantages to this type of drive especially when working with sweep tubes.

I haven't seen the design you refer to, but I would be suspicious of any design that ran a 6L6GC or any of the common audio tubes with 700 volts on the plate. The plate is on pin 3 and the heater is adjacent on pin 2. 700 plate volts equates to about 1400 volts of peak audio and DC under ideal loading. Add some clipping and a reactive speaker load, and you will get an arc across the socket or inside the base of the tube. If the heater circuit is grounded, the fuse should blow. If not parts will fry, often the power or output transformer....This happens all the time in 420 volt guitar amps.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 28th January 2015, 12:06 AM   #6
diyAudio Member
 
Join Date: Dec 2010
Location: Amesbury, MA
Quote:
Originally Posted by banat View Post
and next example for GG is simple cascode circuit , where lower triode anode directly drive cathode from upper (AC) Grounded Grid triode .

Yes a cascode! I see it now thank you. Last time I saw a cascode it was for a preamp and both upper and lower sections were a triode, dual triode bottle 12AX7 I believe. The transistor and power tube threw me off.

Thanks for the replies I knew you guys would help me

Last edited by famousmockingbird; 28th January 2015 at 12:34 AM. Reason: typo
  Reply With Quote
Old 28th January 2015, 12:16 AM   #7
diyAudio Member
 
Join Date: Dec 2010
Location: Amesbury, MA
Quote:
Originally Posted by tubelab.com View Post
I have some simulations and built a grounded grid test amp right before packing up my lab to move. The results looked promising. I used a mosfet in the cathode circuit.

I don't want to go into the details now since I don't have a place to work right now, and it will be months before I do. There are some advantages to this type of drive especially when working with sweep tubes.

I haven't seen the design you refer to, but I would be suspicious of any design that ran a 6L6GC or any of the common audio tubes with 700 volts on the plate. The plate is on pin 3 and the heater is adjacent on pin 2. 700 plate volts equates to about 1400 volts of peak audio and DC under ideal loading. Add some clipping and a reactive speaker load, and you will get an arc across the socket or inside the base of the tube. If the heater circuit is grounded, the fuse should blow. If not parts will fry, often the power or output transformer....This happens all the time in 420 volt guitar amps.
So there seems to be some merit to the design, I am looking forward to hearing your results once you get a lab setup and do some more testing.

The 700v was a shock to see but I guess the amp has been running on the original tubes for quite some time, maybe it is never pushed hard.
  Reply With Quote
Old 28th January 2015, 12:36 AM   #8
banat is offline banat  Serbia
diyAudio Member
 
Join Date: Dec 2009
Location: near Belgrade
Quote:
Originally Posted by famousmockingbird View Post
Yes a cascode! I see it now thank you. Last time I saw a cascode it was for a preamp and both upper and lower sections were a triode, dual triode bottle 12AX7 I believe. The transistor and power tube through me off.

Thanks for the replies I knew you guys would help me
You are welcome !

BTW , here is one link to one interesting DIY article related to use of EL36 - TV sweep power tube in GG mode writen by Graham Dicker , it is GG - SE circuit but I think that implementation altogether with some practical details can be useful even for some future design of some GG - PP audio amp.

Best Regards !

EL36: Applications in Audio
  Reply With Quote
Old 28th January 2015, 01:41 AM   #9
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
Quote:
it is GG - SE circuit
That's the basic idea. The solid state stuff is a constant current source that is modulated by the audio signal. The tube on top handles the high voltage, in a manner similar to a cascoded CCS.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 28th January 2015, 11:23 AM   #10
banat is offline banat  Serbia
diyAudio Member
 
Join Date: Dec 2009
Location: near Belgrade
Quote:
Originally Posted by tubelab.com View Post
That's the basic idea. The solid state stuff is a constant current source that is modulated by the audio signal. The tube on top handles the high voltage, in a manner similar to a cascoded CCS.
You are right George !

But did you have any idea which type of lets say some single medium power NJFET type can be directly implemented on that place instead of that AF modulated BJT based CCS ?
  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 03:35 AM
common mode filter design mail2vasu2004 Power Supplies 2 18th March 2011 06:46 AM
common-grid I to V converter Radioman62 Digital Source 12 11th April 2008 07:15 AM
differential design, common mode noise rejection Sebastiaan Solid State 0 17th April 2006 09:16 PM
Any common cathode (grounded grid) schematics? leadbelly Tubes / Valves 2 24th February 2006 08:29 AM


New To Site? Need Help?

All times are GMT. The time now is 07:04 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright 1999-2015 diyAudio

Content Relevant URLs by vBSEO 3.3.2