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Old 11th February 2005, 01:38 AM   #21
rdf is offline rdf  Canada
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Quote:
Originally posted by Stabist
I just have to find the reason for this - when somewhere near my room some stuff goes "on" (I suspect the refrigerator)- it can "pop" in speakers??
Hi Stabist. I rectified this problem in my amp with input grid stoppers. It also cleaned up the top end and reduced hiss, which in fact turned out to be RF.
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Old 11th February 2005, 01:40 AM   #22
Stabist is offline Stabist  Slovenia
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Grid stopper? Any picture or link to it - just to visualise what you mean! (I think I know but I'm not sure)
Thanks!
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Old 11th February 2005, 02:09 AM   #23
SY is offline SY  United States
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A grid stopper is a resistor placed in series with the grid, with its lead to the grid as short as possible. It's tough to do in a MC amp without compromising noise.

Are you using wirewound plate resistors?
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And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
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Old 11th February 2005, 02:11 AM   #24
SY is offline SY  United States
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One shielding hint- you can use black anodized perfed aluminum to build a wall between the pre circuitry and the power supply, or even cage it on four sides.
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Old 11th February 2005, 02:35 AM   #25
Stabist is offline Stabist  Slovenia
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Wirewound resistors are only 2 on the PSU +6,3V output ... All others are noninductive ... But I allready plan to change also those two ones ...

Yes - there will be another plate between PSU and circuit - black anodized Al - it allready should be there - if I didn't forget to take it with me when I went to anodizing company ...
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Old 11th February 2005, 03:11 AM   #26
rdf is offline rdf  Canada
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Quote:
Originally posted by SY
A... tough to do in a MC amp without compromising noise...
Ooops, good point. FWIW, the amp I mentioned has 100mv sensitivity for all 3 watts full power output and the grid stopper is a Mills.
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Old 11th February 2005, 07:14 AM   #27
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Quote:
Originally posted by rdf
.... the grid stopper is a Mills.
Hmm. Try replacing this with a non-wirewound type. Even non-inductively wound wirewound types exhibit more inductance than a carbon composition or film type. My choice would be tantalum film.

The inductance of the grid-stopper and the effective input capacitance of the valve form a resonant circuit.

Brian.
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Old 11th February 2005, 04:11 PM   #28
rdf is offline rdf  Canada
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Quote:
Originally posted by Brian Clark
The inductance of the grid-stopper and the effective input capacitance of the valve form a resonant circuit.

Brian.
It has the potential for resonant peak but there's also a lot of resistance in series in this case. I'm using 5.6k in front of an EF86 in pentode mode, gain of ~50 so for a simulation I estimated 200pf Miller capacitance. The body of the Mills is 1/2 long, the inductance of 1" of wire 10 nH, so for the Mills an estimate of 20x was used, or 100 nH.

The result in LTSpice is shown below. It looks clean past 1 GHz. This is a rough estimate and neglects large inductive influences like input coupling caps, meter long interconnects, internal wiring, etc.. The Miller capicitance is also modelled as a perfect cap, i.e. self-inductance below 1 nH. If that's changed to 10 nH the response shows a strong null (no ESR in the model) just above 100 MHz and a clean rise towards 1 GHz, but by that point the model is so far out of it's assumptions it can no longer be trusted.

I think small value grid stoppers would be more vulnerable to this effect.

Edit: Forgot, there's 10 megohm in parallel with the Miller cap, it's a grid leak bias amp.
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Old 11th February 2005, 05:11 PM   #29
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Hi,

Quote:
I'm using 5.6k in front of an EF86 in pentode mode, gain of ~50 so for a simulation I estimated 200pf Miller capacitance.
200pF for a EF86 in penthode mode?
That's seems awfully steep.
None of the interelectrode capacitances rise above 0.5pF for this tube.

Either way, the sim shows the wide bandwidth obtainable form a classic small signal penthode such as the popular EF86.

The graph would be a totally different story if the tube is subbed for a 12AX7A for example.

Also, as far as the pic of the MC preamp allows, it looks as if the shield of the tubes are just clamping the glass.
They don't seem to be grounded hence won't be very effective at shielding.
Should you really need the shielding to bring down humlevels then keep in mind that it adds a little stray capacitance to the circuit which may be responsible for a little roll-off.
Not that it should matter all that much, a MC pre doesn't need to be very wide bandwidth, most of it gets filtered out by the RIAA correction behind it anyhow.

Cheers,
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Old 11th February 2005, 05:26 PM   #30
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Did I displace a decimal? I though I read somewhere the value was 4 pf but that's what you get cranking this stuff while trying to be on time for work. To clarify the plot, it represents voltage to the input of the grid.
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