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Old 15th May 2012, 05:35 AM   #1
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Default Most odd grid choke behaviour.

I have a most puzzling phenomenon I hope someone can explain.

The amp is a fixed bias SE300B, driven by triode-connected CCS loaded E180F, as per the Audioroundtable thread. Re: 300B SE Project, Part 9 - E180F, E280F, D3a pentodes - triode connected
Click the image to open in full size. I hope others can open these images, I cannot. There is no other way of downloading an image it appears.

Initially, not having grid chokes I departed a little from the schematic--mu output from CCS into 0.22uF coupling cap into 100K grid R. Perfectly satisfactory performance, on bench and sonically,driving just an Edgarhorn.

But the urge to try a grid choke was spurred on by the 640H eBay item, and good reports thereof, elsewhere on this forum.

However,their installation [plus 1uF coupler] had dramatic and unexpected impact on HF response, with rolloff beginning 3dB down at 10K,-6dB at 20K. Previous figures, -3dB 53K,-6dB at 100K. Checking with scope probe at E180F plate, all as before,from mu out,as above. Inserting 91K below the choke made little difference.

Restoring the cap/resistor restored the HF, and a decent 1K square wave....
Maybe the choke size is insufficient,[the original being 1700H], though others seem happy with it.

Does anyone have a simple explanation of this puzzle? It's got me beat.

Last edited by jackomancy; 15th May 2012 at 05:46 AM.
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Old 15th May 2012, 06:11 AM   #2
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The problem is likely stray capacitance of the grid choke that's bypassing the high frequencies. Increasing its inductance will improve low frequency roll-off.

I'd say t'hellwiddit. A 100K resistor works well, and it's not nearly so expensive as a really good grid choke.
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Old 15th May 2012, 11:27 AM   #3
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Thank you.
Wondered if taking the signal from the mu output is the problem, but can't see why. You're saying an 8000H choke wouldn't help? By the way, LF rolloff doesn't seem to be a problem. This mod seemed logical, after "purifying" filament supplies with a Coleman kit.....
Thanks again.
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Old 15th May 2012, 01:05 PM   #4
DF96 is offline DF96  England
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To wind a high inductance choke with low capacitance is not easy. However, you said you also tried a 91k resistor in series (below?) the choke and that didn't help. If the resistor went from choke to ground and made little difference this suggests that the choke, or something else, has high capacitance to ground or that the previous stage has high output resistance. Did you try adding the 91k 'above' the choke too? Does the 1uF coupling cap have high stray capacitance to ground? Did you accidentally disconnect the E180F triode connection?
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Old 15th May 2012, 01:37 PM   #5
MelB is offline MelB  Canada
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I used those same grid chokes in both my Parallel 300B amps and my GM70 amps with no problems at all. Testing my GM70 at 18k it is starting to drop off but I was blaming that on the Hammond output transformer. I guess it could be the grid choke??? I am using .47uF coupling caps only. I would think you could go even lower with no problem. Trying knocking that down I would be very interested in what happens. How much current is running through your E180F?
Also I emailed the guy that is winding them as I was concerned I might "burn one out" going full drive on my GM70. He says there is no chance of that but I am probably saturating them for sure with anything over 1 mA. In listening tests I do not hear any ill effects at very very loud levels. And oh yes they do make a huge difference in the sound. You can apply all the theory you want to that vs. a resistor but the results with these things are quite remarkable. I would go so far as to say best upgrade I've ever done.
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Old 15th May 2012, 02:05 PM   #6
MelB is offline MelB  Canada
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I should probably add the guy did say they are meant for small signal tubes and not large triodes.
I originally got these things as I'm using fixed Bias in my parallel 300B amps so was using them as a way to lessen the chance of a tube running away (lower grid resistance) I built these amps 8 years ago and have run through two sets of 300B's. My experience with the 300B's is by the time they are beginning to run away they are done anyhow. There is nothing left in there filaments. By the time one of them starts to run away testing the group of four tubes on my Hickok tester where I drop the filament voltage from 5 to 4 volts and watch the needle will drop like a rock. A new tube dropping the voltage from 5 to 4 volts the needle does not move.
So using them to stabilize your bias is really moot. If that is what you are using them for? I would guesstimate I am getting about 4000 hours out of my 300B's (Chinese) and 4 out of 4 aged pretty much the same two times now.
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Old 15th May 2012, 10:50 PM   #7
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Default Most odd grid choke behaviour

Quote:
Originally Posted by DF96 View Post
To wind a high inductance choke with low capacitance is not easy. However, you said you also tried a 91k resistor in series (below?) the choke and that didn't help. If the resistor went from choke to ground and made little difference this suggests that the choke, or something else, has high capacitance to ground or that the previous stage has high output resistance. Did you try adding the 91k 'above' the choke too? Does the 1uF coupling cap have high stray capacitance to ground? Did you accidentally disconnect the E180F triode connection?
Thanks for the assistance....

No, did not try 91k above choke--maybe I should. Thought choke had to be close to the grid....

If I could download a photo of the cap's location, I would, but it appears DIYAudio needs an URL, and all my images are on the PC,not cyberspace. Best described as very much suspended in free air....the chokes are attached to chassis wall 50-60mm from the pin,means a long grid stopper--9.1k Welwyn--and even longer space for cap to bridge from CCS to choke. Don't know if that answers your question re stray capacitance to ground...

No, no accidental disconnections.
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Old 15th May 2012, 11:33 PM   #8
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Default Most odd grid choke behaviour

Quote:
Originally Posted by MelB View Post
I used those same grid chokes in both my Parallel 300B amps and my GM70 amps with no problems at all. Testing my GM70 at 18k it is starting to drop off but I was blaming that on the Hammond output transformer. I guess it could be the grid choke??? I am using .47uF coupling caps only. I would think you could go even lower with no problem. Trying knocking that down I would be very interested in what happens. How much current is running through your E180F?
In listening tests I do not hear any ill effects at very very loud levels. And oh yes they do make a huge difference in the sound. You can apply all the theory you want to that vs. a resistor but the results with these things are quite remarkable. I would go so far as to say best upgrade I've ever done.
Thanks for your assistance. Did wonder how your frequency testing came out...

As per the circuit design, the E180F's pass exactly 11mA. But I take the output from the CCS' mu point, for lowest output impedance, rather than the E180F plate. Maybe the choke doesn't like that. Your GM70 amp I note the output is from the 7027's plate.Your 0.47uF coupler is quite small, cf 4.7uF for a 1700H L in the original circuit, yet your bass is unaffected. One would have predicted some rolloff, but apparently not in practice.

In my application, only driving mid-range, it could be argued one is being unduly fussy, but I figure one should always go for max extension possible, for best fidelity, besides may one day be asked to drive a full ranger....

Cheers and thanks again.
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Old 16th May 2012, 01:01 AM   #9
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Be sure to keep in mind that you have a series resonant circuit of about 6 Hz. Q is hard to say without more info.

All good fortune,
Chris
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Old 16th May 2012, 09:43 AM   #10
DF96 is offline DF96  England
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We need to see a circuit, especially around the CCS and the 'mu point'. I suspect that for some reason your 'mu point' has much higher impedance than you assume.
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