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cyclecamper 13th January 2013 02:42 AM

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.

Bone 13th January 2013 08:48 AM

Can you show us a circuit schematic, or at least tell us which amplifier?

es345 13th January 2013 09:16 AM

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.

Enzo 14th January 2013 10:40 PM

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.

cyclecamper 14th January 2013 11:20 PM

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.

cyclecamper 14th January 2013 11:22 PM

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?

Loudthud 14th January 2013 11:56 PM

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.

gingertube 16th January 2013 03:59 AM

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.
You use a good beefy driver (NOT a 12AX7) with low value Rg1
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.

cyclecamper 16th January 2013 08:09 AM

1 Attachment(s)
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.

cyclecamper 16th January 2013 09:42 PM

1 Attachment(s)
See attached I stole from post

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