Coupling cap value with grid chokes: any new experiences or thoughts?

[Deke609]

I've ordered some supermalloy grid chokes (7,000H) to try in the Kaiju rebuild.  Doing a search on the BH forum turned up some interesting threads, including the following:

Doc B. discussing his use of grid chokes in an SR45 amp: http://forum.bottlehead.com/index.php/topic,127.0.html

Grainger inquiring about adding grid chokes to a Paramour, including a question about coupling cap value: http://forum.bottlehead.com/index.php?topic=132.msg635#msg635

Mcandmar noting that when he added grid chokes to a 6A3 amp, the bass was off with the 0.1uF coupling cap but fixed by going to a 0.22uF cap: http://forum.bottlehead.com/index.php?topic=9398.msg88024#msg88024

Anyone have any new thoughts or experiences with grid chokes and changing the value of the coupling cap?

cheers and many thanks, Derek

 

[Paul Birkeland]

7000H should be OK with the stock coupling cap value. 

 

[Deke609]

Many thanks PB.

A follow-up question: In the stock Kaiju, can the grid of the 300B ever be driven positive? 

I've just assumed that the amp is class A1 and keeps the grid negative at all times, but I don't know this for sure. I ask b/c the Silk grid chokes I'm getting cannot tolerate any DC current. I know that the coupling cap does DC blocking duty, so I'm ok from the driver end. But I don't know whether grid current has a DC component -- I *think* not b/c I imagine even in a class A2 amp the grid is only driven positive at the peaks, and so varies with AC signal. But WDIK -- better to ask people who understand these things.  ;D

IF there is any interest from the BH crew in getting some measurement data, I am happy to do some basic testing -- e.g., frequency response taken at grid of 300B comparing grid resistor to grid choke, or different values of coupling cap (although I'm not sure whether putting the scope in parallel with the grid choke would affect things). I have a signal generator and a scope. Otherwise, I will just go by ear.

cheers and thanks, Derek

 

[Paul Birkeland]

A follow-up question: In the stock Kaiju, can the grid of the 300B ever be driven positive? 

You could try, but you'd have to run the amp really hard and there would be a ton of distortion.

I ask b/c the Silk grid chokes I'm getting cannot tolerate any DC current.

They will tolerate a little grid current on high power transients.

I *think* not b/c I imagine even in a class A2 amp the grid is only driven positive at the peaks, and so varies with AC signal. But WDIK -- better to ask people who understand these things.  ;D

You can bias an amp in A2 where the grid draws current at idle.  We don't have any products using a design like this (yet at least), and it's rather uncommon.

IF there is any interest from the BH crew in getting some measurement data, I am happy to do some basic testing -- e.g., frequency response taken at grid of 300B comparing grid resistor to grid choke, or different values of coupling cap (although I'm not sure whether putting the scope in parallel with the grid choke would affect things). I have a signal generator and a scope.

Most scopes have a very high input impedance and very low input capacitance, so the presence of the scope across the grid choke isn't going to throw things off noticeably.  It would be interesting to see how pronounced the grid choke resonance is and whether that's audible when you listen to the amp.

 

[Deke609]

Many thanks again, PB.

They will tolerate a little grid current on high power transients.

Would that grid current be DC or AC? - I'm still assuming AC and that this would be fine for the grid choke - but I'm not sure. If not -- if it's DC -- is there a way of preventing this? I doubt it b/c I can't see a benign way of capping signal voltage at a certain max (other than turning down the volume a lot). As I understand it, to avoid saturation (maxing out the tube's current capability), you can bias the tube super low, but then you get cut-off and no current -- pretty much the inverse of what you're trying to prevent with saturation. So I guess that's just the way it is?

It would be interesting to see how pronounced the grid choke resonance is and whether that's audible when you listen to the amp.

Would that resonance show itself as ringing?  I'm guessing you mean the coupling cap and grid choke form a resonant circuit. I just used an online LC resonant frequency calculator for 7000H and different coupling cap values: 6Hz @ 0,1uF; 4Hz @ 0,22uF; and 2.7Hz @ 0,47uF.

cheers and thanks, Derek

 

[Paul Birkeland]

When we talk about grid current in the context of an amp that's otherwise A1, it's presumed that we are discussing AC, as it's only present on one end of the swing, so the behavior is far from DC, but it is only in one direction.  This current will charge up the coupling cap a bit and create blocking distortion that's unpleasant.

You're attempting to avoid something that isn't a problem.  It's not like the choke explodes if the 300B draws grid current briefly; incidentally this is the point of grid chokes btw...

Yes, there is a resonant circuit, but I wonder if it would even show up in a response curve.

 

[Deke609]

Thanks PB.

When we talk about grid current, it's presumed that we are discussing DC, as it's only present on one end of the swing, so the behavior is far from DC.

[Edit: PB edited his post to clarify that it is an AC current, so I am crossing out my response/question b/c it is confused and confusing] Was that last "DC" supposed to be "AC"?  I assume so. So it's a varying DC current.  I keep slipping into thinking that any varying voltage/current is AC, forgetting that DC or direct current is uni-directional current.  A helpful reminder, thanks (assuming I interpreted you correctly).

It's not like the choke explodes if the 300B draws grid current briefly;

Hah! I was more concerned about saturating the core. I don't understand the physics of these things at all, and I don;t know how long it takes for a saturated core to recover (in this case a supermalloy core). 

incidentally this is the point of grid chokes btw...

Huh - you mean to deal with grid current?  I was only thinking of grid resistors and chokes as loading the plate of the preceding tube (in parallel with whatever the plate load is), and providing a ground reference for the signal/grid. 

.... AND the new grid chokes just arrived as I was typing this! Exciting! They're tiny, so they shouldn't be too difficult to fit.

Will report back later.

cheers, Derek

 

[Paul Birkeland]

A grid choke provides a low DCR path to ground to discharge the coupling cap far more quickly if you encounter blocking distortion.

 

[Deke609]

Thanks for that. I've never heard of capacitor blocking distortion - I will read up on it.

cheers, Derek

 

[Paul Birkeland]

Don't study too hard, you are using headphones after all...

 

[Deke609]

Meaning, "You're using headphones, so it should be easier to hear" or "You're only using headphones, so who cares"?

 

[Paul Birkeland]

You are very unlikely to use the full output power of the amplifier when using headphones, so you're very unlikely to encounter blocking distortion.

 

[Deke609]

Two hours of listening with the grid chokes and I am very pleased. I put this mod on par with going from stock interstage and parafeed caps to the Vcaps (which cost a lot more). And way ahead of replacing the 'lytic cathode resistor bypass with a big film cap, and all the various other "tweaks" I've made. That said -- I suspect it's all additive and that the grid chokes let me hear more fully the benefit of the previous changes.

Listening impressions: Added clarity and "naturalness" across the board - sounds both clearer and better (more toe-tapping, head-nodding enjoyable). I'm hearing extension on on both ends of the spectrum -- bass seems to go lower and highs higher. The grid chokes even cleared up some glare on the top end that I thought was just built into certain recordings. And the clarity and hit of the bass! Sweet Marie! Of course, to my ears, YMMV, etc.

Good stuff!

No hum or other bad things. Although, funnily, I thought I had some persistent hum when first listening to Stevie Ray Vaughan... but it's the hum of his amp. Prior to the grid chokes, I'd hear his amp in certain passages. But now I can make it out in pretty much every song all the time.  Neat!

In case anyone is interested in exploring doing the same, the grid chokes are "Silk" made by SAC Thailand. I bought them from diyhifisupply [no affiliation] - b/c the price was slightly better than buying direct.

cheers, Derek

 

[2wo]

I have used grid chokes in a few of my amps, I really like the sense of dynamics they add...John

 

[Deke609]

And here's a pic of them installed. They're quite tiny.  And helpfully labelled "grid choke" so they shouldn't be too hard to spot.

I increased the interstage coupling to 0.22 uF just to see. The higher capacitance seems to have shifted the focus away from the highs and onto the mids.  Not sure how I feel about this yet. I've kind of gotten used to what to my ears is the more treble-focused sound of the Kaiju. So it sounds a bit better in the mids, but a bit worse in the highs -- it's lost a bit of "sparkle."  Happily, those caps are easily swapped out, so I can always go back, or split the difference and go with 0,15uF cap.

cheers, Derek

 

[Deke609]

I couldn't take the 0.22uF - it just sounded too unbalanced. With more hours of listening, I realized that the amp wasn't so much more mid-focused, but rather tilted towards the low end with a corresponding loss at the high end. And it sounded slightly "slower". Put the 0.1uF caps back in and everything is nicely balanced again and back to the "usual speed".

I was trying to to get a sense of the "speed" issue and wondered whether it had something to do with time constants and transient recovery that PJ has posted about in the past, but I couldn't find a simple (or indeed any) LC equivalent to the R*C formula. And ended up going down a rabbit hole about critical damping, bandwidth and Q-factor which really hurt my head. So I still have no clue about what's going on.  But the amp sounds good 

cheers, Derek

 

[Paul Joppa]

There is a resonance (IIRC, you've posted it earlier) - 6Hz for 7000 H/0.1uF. That resonance is damped by the grid resistor if you have one in parallel with the grid choke. The height of that resonance relative  to the midband is the Q. The impedance for unity damping is (IIRC) equal to the reactance of either the choke or the cap; in this case about 250K.

You would learn some things (and so would we all!) if you compared with and without the parallel grid resistor, and compared a 0.047uF or 0.033uF capacitor.

 

[Deke609]

Many thanks PJ.

There is a resonance (IIRC, you've posted it earlier) - 6Hz for 7000 H/0.1uF. That resonance is damped by the grid resistor if you have one in parallel with the grid choke. The height of that resonance relative  to the midband is the Q. The impedance for unity damping is (IIRC) equal to the reactance of either the choke or the cap; in this case about 250K.

OK. Great. I was sort of on the right track then.  I did a calculation for critical damping last night -- twice, in fact -- and both times came up with slightly greater than 500K. But that was late at night -- I will redo that. Maybe I was using the wrong formula.

I will dig back into this. Your pointing out the magnitude:midband Q relationship is helpful and gives me some orientation points for further study.

  You would learn some things (and so would we all!) if you compared with and without the parallel grid resistor, and compared a 0.047uF or 0.033uF capacitor.

Exciting! I will do this. I'll need to get some new caps and break them in to do a good test.  I think I'll go with 0.47uF [edit - forgot a decimal place] 0.047uF. Adding them to my existing 0.22uF and baseline of 0.1uF, this will let me test 2X and 1/2X baseline. I don't know if there's any particular empirical value in this approach, but the symmetry appeals to me.  ;D

I'll figure out a test setup for switching caps and toggling the parallel damping resistor in/out.  Mostly b/c I don't want to repeatedly lift the amp -- too heavy! 

cheers and thanks, Derek

 

[Deke609]

@PJ - I finally have time to experiment with this, but I could really use some guidance about what to experiment with and measure.

I have the 0,047 and 0,1 and 0,22 uF Vcaps (the 0,047 still need to be broken in - I will get that started either tonight or tomorrow).

I also picked up a handheld LCR meter that is good up to 10,000H, 100mF, and 100Mohms. And it will measure/calculate dissipation factor, Q, phase and ESR. Stated accuracy of 0,2%. Output signal is 0,6V. Test frequencies are 100, 120, 1K, 10K and 100K Hz (no sweep function unfortunately - meters with that functionality were well outside my price range).

I also have a scope and signal generator, and of course a DMM. I only mention all this so that you know what equipment I have at my disposal in case any of it influences your recommended tests.

The quoted specs for the grid chokes are 7,000H @ 12Hz and 1,300 ohms DCR. But I plan to measure both with the LCR to confirm.

There is an interesting thread about grid chokes on Audio Asylum: http://www.audioasylum.com/cgi/vt.mpl?f=tubediy&m=198414
Of particular interest (at least to me) is the first post by "Naz" about 7 posts down where he suggests tuning the circuit by varying both the cap value and damper resistor value. He also includes a Spice screenshot showing just that.  I am tempted to try doing a similar LTSpice sim for my three cap values -- perhaps even one that simulates a frequency sweep. At the same time, I'm wary of doing the sim for fear that it will lead to confirmation listening bias. At present, I have absolutely no idea or expectation about what would sound best, so maybe it's better that I go into this without any expectations whatsoever about what is going on under different test conditions.

I still need to order the damping resistors. Do you think it worthwhile to try different values as suggested by Naz? If so, what values do you suggest I try?

All of the above boils down to following question: what tests do you think would be interesting/informative to do?

many thanks, Derek

 

[Paul Joppa]

I agree that it would be less biased if you did the listening test first.

For a practical application, you'd like to know the frequency response, especially in the bass, so you could most easily just install the choke an the amp and drive the amp at various frequencies and levels - thus incorporating the driver source resistance and output tube Miller capacitance.

For Science, you'd want to know the parameters and how they vary. Here are some thoughts:

* The impedance magnitude is just the voltage across the choke divided by the current through it, so can easily be measured with the choke in place in the amp.

* The impedance of the choke will be inductive (rising with frequency) below resonance, and capacitive (falling with frequency) above resonance.

* Resonance will likely be in the 1kHz-4kHz range. It is easily identified by finding the minimum current as you vary the frequency with a constant voltage. This will also tell you the Q of the resonance.

* Above resonance, the impedance falls with frequency at 6dB/octave and is expected to be independent of signal level. Normally, the outer end of the winding is grounded; the innermost end has the full AC voltage, and has capacitance to the core as well as coil self-capacitance - so be sure to connect the core to the grounded end when testing. Here I am assuming it's a single-chamber bobbin, otherwise it gets complicated  :^)

* Below resonance, the coil is damped by the series DCR (independent of frequency or level), the eddy-current losses in the core (probably dependent on frequency but not level), and the magnetic hysteresis losses (most likely dependent on magnetic flux, which is proportional to voltage divided by frequency).

* The inductance is also a function of the flux level

* To sort these out, you'll need to do some modelling. The standard model would have a series resistance (the DCR) and two parallel resistances (eddy current and hysteresis).

* Without phase information, it's hard to separate the effects. One way is to assume the impedance has a fixed phase angle, meaning the impedance magnitude (after correction for the DCR) is proportional to frequency to some power between 0 and one; the exponent (and hence the phase angle) can be determined from the slope of impedance vs. frequency which will be between 0 and 6dB/octave. This gets tricky because you can't correct for DCR unless you know the exponent, which you can't determine until you've removed the DCR effect. Best bet that I've found is to use an iterative approach to find what fits best. A decent first approximation is to use the slope  from the region where the impedance is greater than the DCR but frequency is below resonance.

* If you do the above at a few different flux levels, you can get a sense of whether you need to separate out the hysteresis terms or not.