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single Lundahl LL1670 grid choke for Push-Pull?

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Posted on November 30, 2015 at 08:13:23
rongon
Audiophile

Posts: 27
Joined: July 2, 2009
 I have a general question about the Lundahl LL1670 grid choke. I noticed that it's center tapped. Does that mean only one is needed for a push-pull pair of tubes? If so, then each end of the choke would go to a grid, while the center tap would go to ground. Is the LL1670 meant to be used this way?

LL1670 data sheet:
http://www.lundahl.se/wp-content/uploads/datasheets/1670.pdf

Thanks...

 

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RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on November 30, 2015 at 08:33:49
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 It should work fine. With 540hy of end to end inductance expect each signal to see 1/2 that value for calculations.

dave

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on November 30, 2015 at 08:43:15
rongon
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Posts: 27
Joined: July 2, 2009
 Thanks Dave!

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on November 30, 2015 at 09:16:10
rongon
Audiophile

Posts: 27
Joined: July 2, 2009



When using grid chokes on a push-pull pair of output tubes (like triode-wired 6L6, for instance), what does the driving (series) resistance need to be? I threw the grid chokes in a quick sim, and got a horrendous, high-Q peak at 10Hz. In the screenshot, the green trace is the 4 ohm tap on the OPT, the blue trace is grid of U3 (6L6).

This is with a 5687 cathodyne using 10k plate/cathode load R's, 13.5mA Ia. I varied the value of interstage caps from low to way high, which only moves the frequency of the peak up or down, but not the amplitude or Q.

Is that because of the high DCR of the choke? That's 2k65 on either side of the center tap.

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on November 30, 2015 at 11:11:12
Tre'
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Posts: 17302
Location: So. Cal.
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 The higher the DCR, the lower the amplitude and Q of the peak.

Also the peak should be lower in amplitude and frequency as you increase the value of the coupling cap.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on November 30, 2015 at 11:13:49
rongon
Audiophile

Posts: 27
Joined: July 2, 2009
 > The higher the DCR, the lower the amplitude and Q of the peak.

I wonder... Should I try a resistor in series with each 'output' of the choke? In other words, a series resistor between the outer end of each winding to the grid of each output tube. That would be instead of loading each winding with a parallel resistor (33k).

Thoughts?

Ron

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on November 30, 2015 at 11:39:28
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 I would do neither.

To me a parallel resistor defeats the purpose of having the grid choke.

What happens if you use 4.7uf caps instead of 1uf caps?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on November 30, 2015 at 12:25:09
rongon
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Posts: 27
Joined: July 2, 2009
 I tried that, and it lowered the peak's amplitude just a bit, and lowered the frequency of the peak to down below 10Hz (moved the peak's center frequency down). It didn't seem to change the Q much at all (the shape of the peak stayed pretty much the same).

Weird. Could just be an anomaly of the model. I'll have to use alternate tube models to see if that makes a difference.

Aren't magnetics fun?

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on November 30, 2015 at 13:01:57
Tre'
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Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 Great fun! ???

If the frequency is below 10Hz then I would say it's out of your way, don't worry about it.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on November 30, 2015 at 15:33:19
Caucasian Blackplate
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Posts: 8313
Location: Seattle
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 Remember, everything is a tradeoff!

Yes, you will have that peak down there to deal with, but in the end you will have improved overload characteristics as you drive the grids of the output tubes to and past 0V (do you actually care about this?).

Another thing that folks like with a center tapped grid choke is that you can do phase inversion with the choke itself.

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on November 30, 2015 at 16:01:17
rongon
Audiophile

Posts: 27
Joined: July 2, 2009
 Well, maybe not out of my way, since I do play records ("vinyls" as the young 'uns call 'em). The cartridge/tonearm resonance can be right there in the 8 to 11 Hz area, so boosting it by a dozen dB would make the woofers pump quite a lot.

I tried plugging that choke after a long-tailed pair, and got the same resonance. So it's not the tubes used, or even the kind of phase splitter circuit used.

If these simulations are accurate, I think I'd use a grid choke in a single-ended circuit, but not in a push-pull circuit.

--

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on November 30, 2015 at 16:06:46
rongon
Audiophile

Posts: 27
Joined: July 2, 2009
 > Another thing that folks like with a center tapped grid choke is that
> you can do phase inversion with the choke itself.

How is that done? Like in the second stage's grid, in this circuit?
http://www.electra-print.com/docs/el34lowimd.pdf

--

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on November 30, 2015 at 16:33:46
rongon
Audiophile

Posts: 27
Joined: July 2, 2009
 I found the original Electra-Print circuit, but I can't find any specs on the PSA-2 (N or D versions) such as DC resistance of the coil, inductance, etc. Does anybody have that info?

--

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on November 30, 2015 at 17:21:49
coolhand
Audiophile

Posts: 537
Joined: June 5, 2006
 I use a LL-1670 in a DHT PP amp in much the same manner, and also compared it to the Silk GC-1(its inductance is more than 7,000 Henry at 12Hz while having very low DCR of just 1.3K ohm.)

Although the specs suggest potentially more ideal characteristics for its typical application, I preferred the resulting sonics of the LL-1670... you will certainly need to experiment with the coupling cap and as Tre mentioned, adding a series resistor defeats the point of the exercise.

In my circuit I was able to further tune the low end response by virtue of interaction with the cathode bypass caps, which I ended configuring to ultra path.
After numerous listening tests I was able to achieve truly outstanding low end performance to my preferences, via tuning of the collective interactions.

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on December 1, 2015 at 08:21:53
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 the grid choke does the same thing in a single ended circuit. Typically the response in spice is much worse than you ever see in practice and typically a 50-100K resistor across the choke will damp the resonance nicely. This resistor is really easy to put in and out of circuit while listening and then your ears can decide.

The actual frequency where it happens is purely a function of the values of L and C and everything else just works into the Q (how well it is damped)

dave

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on December 1, 2015 at 09:41:35
rongon
Audiophile

Posts: 27
Joined: July 2, 2009
 Ah HA!

Yeah, I found that 33k in parallel with each 270H winding made everything perfectly copacetic in LTspice.

So it is expected/accepted practice to place damping resistors across the choke. Just like damping ('tuning') a transformer. I had to do exactly that with the Edcor 15kCT:15kCT transformers I was playing with. I found that a 10k resistor across each secondary winding sounded OK. 22k end-to-end also worked fine. Tune to taste...

I'm not sure why putting a parallel resistor 'defeats the purpose' of using a grid choke. Seems to me that most commercial magnetics need damping in real world use.

Good learning experience, this...

 

I hope someone does answer your question..., posted on December 1, 2015 at 11:11:50
Lew
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Posts: 10911
Location: Bethesda, Maryland
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 Why WOULD putting a resistor in parallel defeat the purpose of the choke? For one thing, it would limit the net grid resistance to 33K max, but is that a bad thing?

 

RE: I hope someone does answer your question..., posted on December 1, 2015 at 11:47:47
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 I thought the point(s) of a grid choke was having a low DCR to ground from the grid (he would still have that) and a high AC impedance to ground from the grid so the driver stage is loaded with a high impedance for less distortion (he won't have that anymore).

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: I hope someone does answer your question..., posted on December 2, 2015 at 10:33:20
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
 Based on my quick look at the proposed schematic before posting last time, if the proposed 33K resistance is in parallel with the grid choke, then it does limit the max grid to ground resistance to 33K, but how does this affect the "load" on the driver stage? (This is probably a dumb question, but I think of the load as the impedance between the plate and B+.)

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on December 2, 2015 at 13:42:59
rongon
Audiophile

Posts: 27
Joined: July 2, 2009
 My understanding is that a grid choke presents a high impedance to AC (audio signal) while providing a low resistance to DC (grid current). But I am far from expert, or even well informed on this kind of stuff.

The question is whether or not the 33k in parallel with the 270H of the grid choke would cause a problem. Will it reduce the inductance 'seen' by the driver tube plate?

That 33k resistor will actually decrease the DC resistance from output tube grid to ground, which should not be a problem.

Ain't this fun?

--

 

RE: I hope someone does answer your question..., posted on December 2, 2015 at 14:52:05
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 The grid resistor of the stage driven is in parallel with the plate resistor and those two parallel resistances are seen by the driver tube as it's load.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on December 2, 2015 at 16:07:25
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 "Will it reduce the inductance 'seen' by the driver tube plate?"

Yes, but with an explanation.

The plate of the driver tube doesn't see the inductance of the grid choke, it sees the reactance of that inductance.

The reactance of 470Hy at 1kHz is 2.953 meg ohms.

The 33k ohm resistor is in parallel with that.

2.953 meg ohms in parallel with 33k ohms is 32.635k ohms.

32.635k ohms is what the driver tube's plate will see in the grid circuit of the output tube at 1Khz.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on December 2, 2015 at 19:12:09
rongon
Audiophile

Posts: 27
Joined: July 2, 2009
 Thanks for the explanation.

Well, that really is a buzz-killer. So we have yet another trade-off to consider.

If we leave the 270H without a parallel resistance to damp resonances, we have a potential subsonic problem (much like an underdamped cartridge/tonearm resonance, but on everything).

If we damp the resonance to flat response, we're loading the previous stage more heavily, increasing harmonic distortion and reducing gain. Fortunately, 32.6k ohms is still a reasonable load for something with low ra to work into, like a 5687 or a triode wired EL84. (Output Z will be ra || Rload, that being in parallel with 32.6k. So in the case of a 5687 with ra = 3k and a 15k Rload, we'd have Zout of 2.5k ohm working into 32.6k ohms. The choke with 33k damping resistance would still present >10X load resistance to the Zout of the 5687 with a 15k plate load resistor.)

If a tube with higher ra is required, then a higher parallel resistance might be acceptable, not damping the resonance completely, but perhaps enough so that it's not a major problem. Hopefully the parallel resistance can be made high enough that it doesn't load the driver tube too heavily. Something like 56k should work, still presenting a load that's well more than 5X the output resistance of a 6SN7. (ra of 9k || Rload of 33k = 7k, working into 55k. Not as good a situation as with the 5687, but still OK at about 8X the Zout of the 6SN7 with 33k plate load resistor.)

In both cases, cathode resistors will need to fully bypassed, so that ra is kept as low as possible.

How does that look to you?

--

PS - I see that your example used 470H for grid choke L. With 270H, the reactance at 1kHz will be 2.2M ohms instead of 2.9M ohms.

--

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on December 2, 2015 at 19:34:24
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 Looks like you have a good handle on all the issues.


Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: single Lundahl LL1670 grid choke for Push-Pull?, posted on December 2, 2015 at 19:35:42
rongon
Audiophile

Posts: 27
Joined: July 2, 2009
 Thank you for guiding me to a better understanding! Your input helped a lot.

 

Duh!, posted on December 3, 2015 at 05:59:11
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
 Of course. Thanks.

 

Just to complete the thought, posted on December 3, 2015 at 10:40:46
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002



If I load the plate of my 6sn7 driver tube with a CCS, the load line for that tube will not be totally horizontal.

The value of the grid resistor on the output tube will be the load for the 6sn7.

Using a 270k grid resistor gives a near horizontal load line for the 6sn7.

I get almost the full mu of the tube for the gain of the stage and the distortion will be much lower than a standard 33k plate resistor but there will still be some current change through the 6sn7 so the cathode will still need to be bypassed.

Any audible contribution of that bypass cap will be greatly minimized vs. a tube loaded with a normal sized plate resistor. In other words, the cathode bypass cap is not "in the signal path" nearly as much.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

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