Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: PG000 on September 13, 2016, 10:15:42 am
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I am building a #36/#39 amp and in all schematics I have, the shield of the input cable is connected to the plate. I read in the internet and found out that this is called hotshield and prevents microphony. I am now wondering what cable I should use and if it really helps preventing microphony ?
PG000
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Just use any shielded cable. I'm skeptical about it being worthwhile. A small cap between plate and grid would do the same and might not be as dangerous.
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I would personally NEVER do a hotshield. I see no reason for it and I don't want the ground of a shielded cable with high voltage on it. Not a reasonable risk for me personally when I have had success in making quiet amps in other ways.
Try something. Take a voltmeter and check continuity between your guitar strings and the ground on a guitar cable (without having the guitar cable plugged into an amp but plugged into the guitar).
:think1: :sad2:
With respect, Tubenit
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+1, don't do it
Franco
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If Hotshield is so dangerous what schould I do then to prevent mycrophonics ?
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If Hotshield is so dangerous
first thing, make sure you understand why it's NOT recommended.
my last 3 builds have been silent, to the point the only way you know they're on is the power light says so. Use the search feature here to find threads on shielding, wire dress, placement of tubes, trannies. *best practices* will get you quiet, patience, attention to details, and understanding of how *noise* gets into a build WILL give you good results.
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If Hotshield is so dangerous what schould I do then to prevent mycrophonics
First of all you may have zero problems with microphonics with your amp.
Use a well know proven layout and do a good job with lead dress in soldering.
Use good quality parts. The only significant microphonics problems I've had have all been with a bad tube and a different tube fixed that.
You can use a silicon ring under the tube socket (like is sometimes done on EF86 tubes). You can use a silicon ring around the glass tube itself which helps minimize vibrations and microphonics.
You can build a head (which I presume you're doing) and not a combo to help isolate mechanical causes.
You can use a "snubbing cap" or do what Sluckey said with a small cap like 120-390p range from plate to grid, or grid to cathode, or across the plate resistor.
I would plan on success rather then worry about microphonics. Build it first and then see if there is any issue.
And I would NOT use the hot shielding. With respect, Tubenit
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... I am now wondering ... if it really helps preventing microphony ? ...
It shouldn't do anything for microphonics, which is a problem with an unwanted output happening with vibration of the tube. If you have microphonics, you should attack it at the source with either using a tube which is less susceptible, or finding a way to prevent/damp the vibrations.
The first place I saw it written about was a book by Gerald Weber, and he offered it as a last-ditch to fix oscillations (usually due to wiring and layout). And it's really a bad idea there, too.
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Doing the hot shield correctly does not send voltage out to the guitar cable. We're talking about a shielded cable between the tube grid and the input jack. At the tube socket the shield would be connected to the plate but the shield would not be connected to anything at the input jack. It would need to be insulated with heat shrink or tape.
But the very idea of putting B+ voltage on the shield of a shield cable in an audio amp seems stupid to me. As a technician I expect the shield of a cable to be at ground (zero volt) potential. And I would hate to get shocked by a cable shield.
I have seen some special radar transmitter circuits where a shield had very high voltage on it. But there were plenty of danger signs to let the techs know about it.
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Try something. Take a voltmeter and check continuity between your guitar strings and the ground on a guitar cable (without having the guitar cable plugged into an amp but plugged into the guitar).
Doing the hot shield correctly does not send voltage out to the guitar cable. We're talking about a shielded cable between the tube grid and the input jack. At the tube socket the shield would be connected to the plate but the shield would not be connected to anything at the input jack. It would need to be insulated with heat shrink or tape.
I should have been clearer in my comment above. My point was that the guitar cable is just a short or a mistake away from touching high voltage. I just don't like that as an idea. I agree that IF the hotshield is done correctly that it doesn't send voltage. Given some of the incorrect stupid mistakes I've made building amps, it just seems like an unnecessary and foolish risk.
with respect, Tubenit
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Thanks for the advice not to the hotshield. I would have been to dangerous. Instead of doing hotshield I swaped the tubes around and added some ground resistors. And now the mycrophonics makes no Problem. The amp doesn´t make that squeal noise anymore.
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Most of the time someone is saying they are having trouble with Microphonic Tubes, that is not the issue. It becomes very clear when you actually have a tube go microphonic and it leaves no doubt.
Of course we all want to avoid this from happening, but what I have found helping others. What they call microphonic tubes are most times oscillation issues. This is where nice lead dress and snubbers help.
Carefully construct your amp and troubleshoot if you have problems. It is very difficult to troubleshoot an amp that is not built.