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Does using shielded grid wires really help much?

William S.

New Member
Messages
4
I have noticed that some people use shielded wires on all the grids of the preamp tubes. Does this really make much of a difference? Should the PI tube also have shielded wires on the grids? I am building a BF Super Reverb head and would like to keep it as quiet as possible.

Thanks,
William
 

OlAndrew

Member
Messages
2,347
it can make considerable difference. Also, some recommend attaching the shield to the plate, rather than ground. Makes a very very low value cap out of the wire, helps cut RF interference and prevent parasitic oscillations. I've always just used shield to ground, myself.
 

GearHeadFred

Member
Messages
1,651
I found it quieted my BF Super build when I used shielded wire from the input jacks to the first grids. Only ground 1 end of this -- not both -- or you can create a ground loop. I experimented with this and found that grounding the jack side was quieter.
 

Tonic Amps

Member
Messages
480
BF Supers do not require shielded cable for the preamp tubes unless you have noise issues. It's quite critical on high gain amps to keep the signal leads isolated from the rest of the amp, so you'll often see shielded cable in these types of amps. Some builders use it as a matter of eliminating issues before you have them as well. If you keep the signal leads as short as possible and route them as you see in photos of old BF Supers, you should be fine. Straying too far from a known good layout and you are asking for trouble.
 

jellewelagen

Member
Messages
235
I agree with TONIC. The Blackface layout is pretty smart about this. Just look at the two grid wires running to the first preamptube. The signals on these wires are opposite in phase yet they are running parallel a good stretch. What do you think is going on here? That is NFB into the grid wire of the first gain stage.

Yes that will sound different than a high capacitance shielded wire like the silverface amps have.

Take a good look at pics of Blackface leaddress. Note how wires run, and experiment.

Have fun!

Jelle
 

mooreamps

Senior Member
Messages
375
I have noticed that some people use shielded wires on all the grids of the preamp tubes. Does this really make much of a difference? Should the PI tube also have shielded wires on the grids? I am building a BF Super Reverb head and would like to keep it as quiet as possible.

Thanks,
William
Does it make a difference? Absolutely, because the input impedance to most gain stages are very high, and thus sensitive to noise ; including the PI if you are using the standard value of 1 meg on the grid resistors. This sounds like a great opportunity for you do do a real good first class job when using all shielded cable for the signal path.




-g
 

VacuumVoodoo

Member
Messages
1,547
it can make considerable difference. Also, some recommend attaching the shield to the plate, rather than ground. Makes a very very low value cap out of the wire, helps cut RF interference and prevent parasitic oscillations. I've always just used shield to ground, myself.
In this case cable capacitance is in parallel with the grid-plate capacitance and is effectively multiplied by the Miller effect. 10pF cable capacitance thus becomes a virtual 100-300pF and DOES affect treble response.

One should also have in mind that there will be 100 to 250V DC between shield and signal wire and many shielded cables designed for audio signal routing are not designed to withstand this. With time and elevated temperature the isolation between signal wire and shield may deteriorate with nasty crackling noises as results, and in the worst case a tube failure.
PTFE isolated shielded wire is the minimum required if you want to put the shield at high DC potential.

If you really need to tame RF/high treble with Miller capacitance use 5pF to 25pF ceramic or mica capacitor mounted directly on the tube socket and ground the cable shield in the traditional way. A compulsive tweaker may want to use a variable trimmer capacitor here and tune the high end response to his hearts delight.
 

hasserl

Member
Messages
4,711
Good advice as usual Alex. Besides I hate the idea of applying HV to a shield that someone else down the road may reasonably be expecting to be at ground potential. Seems like a possible accident waiting to happen. May just be me, but I don't like it.
 

OlAndrew

Member
Messages
2,347
Wasn't recommending the idea, and don't do it myself, just ground on end of the shield. It's mentioned on some of the books, so I thought I'd include it.
 

mooreamps

Senior Member
Messages
375
In this case cable capacitance is in parallel with the grid-plate capacitance and is effectively multiplied by the Miller effect. 10pF cable capacitance thus becomes a virtual 100-300pF and DOES affect treble response.
I'm not trying to imply anything here, but I'm just not seeing this one. The signal cable, lets say an a couple inches in length from the input jack to the card, less than one puff if that, and in parallel with the tube. I just don't see how that plays into the miller effect of the tube.



One should also have in mind that there will be 100 to 250V DC between shield and signal wire.
I'm not seeing this one either. That would imply there is voltage potential of 100 to 250v present on the input grid, and that just is not the case.






-g
 

Swarty

Member
Messages
1,130
Do shielded leads help noise? Yes.
Do they noticably impact the top end? I'd think most ears would say yes.
I'd recommend using them only after standard wire was used and the noise was an issue, and then as few as possible, hopefully only on the first gain stage.
Good lead dress should really be all that is needed in a Super Reverb. Fender added a sheilded lead to the first gain stage in the early '70s to overcome inconsistent/sloppy lead dress. This along with the top end robbing suppressor caps on the output tubes really muffled the amps (they lost a lot of the openess).

FWIW, the plate connected shield was stock on lots of Marshall 2203/2204s.
 

VacuumVoodoo

Member
Messages
1,547
I'm not trying to imply anything here, but I'm just not seeing this one. The signal cable, lets say an a couple inches in length from the input jack to the card, less than one puff if that, and in parallel with the tube. I just don't see how that plays into the miller effect of the tube.
It doesn't matter where a capacitance between grid and plate is located physically or what form or shape it has. Any external capacitance applied between grid and plate pins is in parallel with the internal grid-plate capacitance. Miller effect applies to any capacitance between grid and plate in a common cathode amplifying stage. In cathode follower or common grid stage Miller effect is negligible. It's all in the old books and many are available for download from the web.

In HF applications we used to solder a piece of sleeved wire to the grid and plate each, then twist them tighter or looser to trim HF response. Ask any old HAMster.

I'm not seeing this one either. That would imply there is voltage potential of 100 to 250v present on the input grid, and that just is not the case.
Signal wire on the grid is at 0V (usually), plate is at whatever DC operating point the stage is designed for. Cable shield connected directly to plate is at the same DC potential as the plate. Therefore there is a potential difference (otherwise known as voltage) between signal wire and shield. It's obvious, isn't it?

The answer OPs original question if shielded wire helps much is both yes and no. It can help a lot or screw thing up, it depends on how and where it is applied. Miller and Maxwell rule unthreatened.
 
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mooreamps

Senior Member
Messages
375
no. any cable capacitance is in parallel to the tube. The miller effect is only those components, either resistive or reactive which is in "series" with control grid.


-g
 

VacuumVoodoo

Member
Messages
1,547
no. any cable capacitance is in parallel to the tube. The miller effect is only those components, either resistive or reactive which is in "series" with control grid.


-g
Gary, for reasons known only to yourself you seem to refuse to accept my explanations. I could be rude and say you don't have the faintest idea what Miller effect is. However, I'd rather blame myself for being unable to convey the idea clearly enough for an esteemed rocket scientist to grasp.

Perhaps you'll find Mr. Millers original analysis acceptable?

It's here:
http://web.mit.edu/klund/www/papers/jmiller.pdf

If that is too hard to swallow there's a simpler explanation courtesy of Randall Aiken:
http://www.aikenamps.com/MillerCapacitance.html

If it is still too chewy here is a very basic one with pretty colorful pictures:
http://www.zen118213.zen.co.uk/RFIC_Theory_Files/Miller_Effect.pdf
 
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mooreamps

Senior Member
Messages
375
yes, all very good articles. But, I hope this is something you would consider spending some time to ponder.
The miller effect is only those components, either resistive or reactive which is in "series" with control grid.


-g
 

VacuumVoodoo

Member
Messages
1,547
yes, all very good articles. But, I hope this is something you would consider spending some time to ponder.
The miller effect is only those components, either resistive or reactive which is in "series" with control grid.


-g
Perhaps you would be so kind as to enlighten us by providing a mathematical analysis of how Miller effect operates on a grid stopper resistor, it being in series with the grid as per your statement. A simple schematic drawing illustrating this will help. I am sure this is childishly simple for you to do.
 

reaiken

Member
Messages
1,880
yes, all very good articles. But, I hope this is something you would consider spending some time to ponder.
The miller effect is only those components, either resistive or reactive which is in "series" with control grid.


-g

:messedup


:munch
 




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