Go Back   Home > Forums > Live Sound > Instruments and Amps

Instruments and Amps Everything that makes music, Especially including instrument amps.

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 13th January 2013, 02:42 AM   #1
diyAudio Member
 
Join Date: May 2010
Location: was Chicago IL, now Long Beach CA
Default Grid choke

Can someone please explain the purpose and practical application of large-value grid chokes on a parallel push-pull output stage in a tube guitar amp.
  Reply With Quote
Old 13th January 2013, 08:48 AM   #2
Bone is offline Bone  United Kingdom
diyAudio Member
 
Join Date: Jun 2004
Location: Dorset, UK
Can you show us a circuit schematic, or at least tell us which amplifier?
  Reply With Quote
Old 13th January 2013, 09:16 AM   #3
es345 is offline es345  Germany
diyAudio Member
 
Join Date: Jan 2012
Location: Ulm
A schematic really would help.

Myself I have used a choke in my last Bassamp I have designed (PP 6xGU50). Here are the reason for that case:
- minimize the voltage reduction caused by the filtering
- avoid screen grid modulation caused by the ripple on the supply by using CLC filtering. Without that filtering the AC ripple at the screen grid has caused amplitude modulation at the anode when driving the powerstage near clipping.
  Reply With Quote
Old 14th January 2013, 10:40 PM   #4
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
Do you mean a large iron core choke between the B+ node for the power tube plates and the B+ node for the power tube screens? If so, it is simply part of the B+ filter circuit. It offers more filtration than a simple resistor.

The way your question is worded, though, makes it sound like individual chokes for each grid. ANd speaking only for myself, I tend to reserve the word "grid" for the control grid, leaving the screen grid to be called just the screen. SO if you are not just refering to the power supply filter, please provide a link to an example.
  Reply With Quote
Old 14th January 2013, 11:20 PM   #5
diyAudio Member
 
Join Date: May 2010
Location: was Chicago IL, now Long Beach CA
I ran into this a few places talking aobut hi-fi, not guitar amps. They made it sound like using a "grid choke" instead of a "grid resistor" (definitely control GRID) was a huge upgrade in fidelity and made a big difference. This is not B+ or B++ or to screens. I first ran into the term while shopping for other chokes.
  Reply With Quote
Old 14th January 2013, 11:22 PM   #6
diyAudio Member
 
Join Date: May 2010
Location: was Chicago IL, now Long Beach CA
LOL if I had a schematic my quistions would probalby not be so general. They way it was thrown around I assumed it was a common term...kind of a grid stopper using a choke maybe?
  Reply With Quote
Old 14th January 2013, 11:56 PM   #7
diyAudio Member
 
Loudthud's Avatar
 
Join Date: Nov 2007
Location: Near Dallas Texas USA
With 2 or 3 tubes in parallel, the allowable grid leak resistor can get pretty low and hard to drive. The choke gives a low DC resistance but a higher impedance in the audio band. Many times you'll see transformer drive as the next step and in class AB2 amps.
  Reply With Quote
Old 16th January 2013, 03:59 AM   #8
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
What Loudthud said.

Gingertubes "essay"

As you drive more current thru' an output tube then grid 1 current increases, this grid current develops a voltage across Rg1 which SUBTRACTS from the bias voltage. Thus you get more anode + Screen current and therefore more grid current which develops more voltage across Rg1 etc. If Rg1 is too high then thermal run away occurs untill the tube "melts". Thus each tube will have a max Rg1 spec. This spec depends upon whether cathode bias or fixed bias is used. With cathode (auto) bias this opposes the bias shift caused by the voltage developed across Rg1 from grid current. With fixed bias that does not happen and that is why max Rg1 values for Fixed bias are smaller than for cathode bias.
With 2 tubes in parallel and with fixed bias then the max Rg1 is typically 50K even thou' most manufacturers use 220K or so.
That is why guitar amps blow up about 10 times as frequently as tube HiFi Amps where proper attention to such things is more likely (but not guaranteed). If you have 10K grid stops then that leaves just 40K for the grid leak back to 0V via the bias supply.
The common 12AX7 phase splitter would have a hernia trying to drive that so it is not done.

If you look at some old tube data sheets, take KT88 as an example, you will see that in fixed bias at 42W Anode + screen dissipation then max Rg1 is 100K. If you take the disspation down to less than 35W then Rg1 can be increased to 220K. This is the maximum value to prevent thermal run away of your output tube.
So for your average guitar amp with max Rg1 RECOMMENDATIONS violated/ignored then biasing so that tubes idle at around 70% of max dissipation makes good sense from both a reliablity point of view as well as tube longevity. I would use 80% of max dissipation as the absolute limit.
UNLESS
You use a good beefy driver (NOT a 12AX7) with low value Rg1
OR
A grid choke. The grid choke gives you lots of AC Impedance so as to not load down the phase splitter / driver BUT very low DC resistance so that max Rg1 values are not exceeded, that would allow you to bias the output tubes right up to the 100% dissipation limit without the large risk of thermal run away.

For HiFi Amps I use a cathode follower driver, direct coupled to the output tube grid with an active current source as the cathode follower load instead of a resistor. You would not believe how that affects the sound, very strong dynamic sound with the characteristic of an amp with 3 or 4 times as much power and dead quiet background, no hiss (except for what is coming in from the preamp). It sounds like this becaudse we have taken firm control of the output tube grid1, it is no longer "flapping in the thermal breeze". That would add another 30 or 40 bucks to the manufacturing cost so of-course no one does it. No reason why DIY'ers should'nt do it though.
Hope this "rave" is of some value to somebody.
Cheers,
Ian

Last edited by gingertube; 16th January 2013 at 04:18 AM.
  Reply With Quote
Old 16th January 2013, 08:09 AM   #9
diyAudio Member
 
Join Date: May 2010
Location: was Chicago IL, now Long Beach CA
Man this is really extremely interesting reading.

Anybody got some relevant schematics to look at?

In the guitar amp I'm fooling around with at the moment, (see attached schematic) with 6 6L6 output tubes, how would such a "grid choke" conversion and conversion to bias pots (instead of "output balance" pot) ideally be done? Replace each of the two 33K resistors with an 8,000H choke and a pot? Or rework the circuit with a choke and pot for each output tube?

Maybe my Sound City project with its 6 EL34s might be a candidate too, though I think maybe the EL34s might be easier to drive than the 6L6s.
Attached Files
File Type: pdf Super Twin without reverb Fender 180W Amp 012380.pdf (234.9 KB, 25 views)
  Reply With Quote
Old 16th January 2013, 09:42 PM   #10
diyAudio Member
 
Join Date: May 2010
Location: was Chicago IL, now Long Beach CA
See attached I stole from auidiokarma.org post
Attached Images
File Type: jpg Grid-Choke-1.jpg (44.9 KB, 128 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
grid stopper and grid choke on pentode input YouAgain Tubes / Valves 3 12th February 2012 09:32 PM
26H choke good enough for a grid or anode choke? speakerfritz Tubes / Valves 7 2nd December 2011 06:05 PM
Issues with grid choke phrarod Tubes / Valves 15 23rd August 2009 02:19 PM
Grid choke as I/V? dsavitsk Tubes / Valves 1 8th April 2008 06:44 PM
change to grid choke? Joel Tubes / Valves 2 5th December 2002 03:20 PM


New To Site? Need Help?

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