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Old 22nd August 2009, 04:32 PM   #1
phrarod is offline phrarod  United States
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Default Issues with grid choke

I replaced my 220K grid to ground resistors with grid chokes. 640 Henry with a listed 20 kOhm DCR, actual DCR 21.7K.

While I got less overall distortion I also lost some dynamics mainly impact and speed. I'm not sure what to do to get it back. I have an intermediate choke between the regulated supply and the amp board (this is a simple pre-amp) and I kept raising the henries on the choke which helped get some of the dynamics back but also made the bass more sloppy the higher I went. This is a choke with multiple taps so I can go from 10-30H in 5H steps.

Anyway. my concern is with the grid choke. Has anyone replacing resistors with a grid choke had a similar issue? If so what was the fix? I have a 1000uF cap on the bias.
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Old 22nd August 2009, 05:45 PM   #2
rdf is offline rdf  Canada
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While it's hard to be helpful without better info (schematic?) the place to start is the coupling cap ahead of the grid choke if AC coupled. Calculate the LC resonant frequency, better yet drop the values into LTspice and see what it's doing to the low end frequency response.
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Old 22nd August 2009, 05:55 PM   #3
phrarod is offline phrarod  United States
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Its very simple. I'm using on half of a tube which is transformer output so there's only bias resistor 220R and the bias cap which is 1000uF. Pretty simple. I tried running without a grid to ground choke but lost all dynamics. I worked hard on voicing the circuit and the voltage (193VDC) and resistors mentioned above give the best sonic performance.

These are the chokes I bought http://cgi.ebay.com/ws/eBayISAPI.dll...#ht_2423wt_884

Someone mentioned a good grid choke should have thousands of henries...

Silk has one but they're in Thailand.
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Old 22nd August 2009, 06:45 PM   #4
disco is offline disco  Netherlands
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Things happening here are:

1) loading the driver tube (noticable)
2) applying a high pass filter (also noticable)
3) supplying grid current to the power tube (of minor importance)

@ #1: the plate resistor is paralleled by the grid leak resistor. Usually Rleak is 10 times larger than Rplate to have only marginal influence on the load.
@ #2: not until 117Hz your 640H grid choke equals the same Z as the grid leak resistor of 470K ohm.

When the inductive reactance of the grid choke becomes a dominant factor to the loading of your driver, a phase angle starts to turn up its head. To counteract the effect one could decrease Rplate, if one has margin in the amount of amplification.
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Old 22nd August 2009, 07:18 PM   #5
phrarod is offline phrarod  United States
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The plate is loaded by the output transformer so I can only work with bias resistor/cap. Are you saying the OT may be what is needing to be helped in loading the tube? IE possibly this OT isn't loading the tube properly on its own? These are a pair of Audio Note double C-core I bought and there's no specs on them but I figured since its audio not they should have been built for something like a 5687 whereas I'm using a 7062 tube
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Old 22nd August 2009, 09:07 PM   #6
disco is offline disco  Netherlands
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Ah, it's not about a power amp. Is this what you want to achieve?
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Old 22nd August 2009, 09:23 PM   #7
rdf is offline rdf  Canada
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Quote:
Originally Posted by disco View Post
Ah, it's not about a power amp. Is this what you want to achieve?
If that is the circuit the low frequency response will vary by pot value and rotation.
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Old 22nd August 2009, 09:38 PM   #8
phrarod is offline phrarod  United States
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Wow! I can't draw schematic without it taking me forever. That's he circuit. The pots are 50K
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Old 22nd August 2009, 10:05 PM   #9
disco is offline disco  Netherlands
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Alright then. Some questions:

- what's the output impedance of the source
- what's the impedance of the circuit you are driving
- what's the turns ratio of Tr1 (apply 10.00Vac on the primary and measure the voltage on the secundary)
- at which voltage is the 7062 katode
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Old 22nd August 2009, 10:30 PM   #10
phrarod is offline phrarod  United States
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Source - measuring output jacks I get 1M
linestage - 50K
I'll have to measure T1 its in circuit at the moment
cathode is 193VDC running ~10ma 220R
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