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Post subject: Amp Built - Hiss/Whistle
Posted: Fri Oct 14, 2011 7:34 am
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Ok, the story: I have an old 60s all tube reel to reel and the motor is very tired, so much that I decided to give the unit a second life as a single ended tube amp, 3W. I removed the tape part and everything obsolete for the project. I am using the Phono In to plug my guitar and to compensate for the massive gain I would get from that input I wired a 561pF cap to the ground input jack.
Now, while it sounds killer when I play through it, I get a high pitch whistle or hiss but only when nothing is plugged in the amp, the hiss gets louder with the volume up. It goes from silent at 0 to unbearable at max volume. As soon as I plug a cable to that input it is completely silent and I can play normally with no hiss or hum.
Question for the amp jedi masters is, what causes the hiss?


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Post subject: Re: Amp Built - Hiss/Whistle
Posted: Fri Oct 14, 2011 11:49 am
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Sounds like you have a microphonic tube. You can try this. Unplug the guitar. Turn on amp, low volume. Now GENTLY tap each tube with a wooden chop stick or tongue depresser. If you hear tapping in the speaker, louder with one tube than any other--- you prolly found the culprit.

Hopefully, you have a sub tube that you can tryout in that socket.

JOC--- did you alter the global feedback off the output tranny, when making this SE amp?


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Post subject: Re: Amp Built - Hiss/Whistle
Posted: Fri Oct 14, 2011 12:07 pm
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It started happening after I soldered the cap to the jack. I do have spare tubes so I can rule microphonics out though, good idea. I did not change anything on the tranny. The mods were very simple, the cap on the phono jack, trimming all of the unnecessary electronics ( ie. tape drive), hooking up a speaker and locking the amp in recording mode.


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Post subject: Re: Amp Built - Hiss/Whistle
Posted: Fri Oct 14, 2011 12:18 pm
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You should use shielded wire between you input jack to the capacitor. Ground shield wire at one end only.

Or capacitor short as possible from the input jack and the output of the capacitor a shield wire .

Your capacitor, long wire is like an antenna


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Post subject: Re: Amp Built - Hiss/Whistle
Posted: Fri Oct 14, 2011 12:35 pm
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Yea, that must be it, good call, the ground wire at the amp input jack is wire shielded but I stripped the wire and shield to soldered the cap right between the jack and the ground wire. Now there's some exposed unshielded wire, I can easily shorten the exposed wire as much as possible and wrap the rest with electrical tape. Will try that tonight. :)


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Post subject: Re: Amp Built - Hiss/Whistle
Posted: Fri Oct 14, 2011 1:04 pm
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The input signal path should as short as possible ( if not completely shielded ) and solder with the shortest wire to the tube socket.


All non shielded signal wires on the amp should cross themself in a 90 degre way never parallele.


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Post subject: Re: Amp Built - Hiss/Whistle
Posted: Fri Oct 14, 2011 2:16 pm
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Should the capacitor be right at the jack or at the other end straight before the tube (kinda inaccessible)?


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Post subject: Re: Amp Built - Hiss/Whistle
Posted: Fri Oct 14, 2011 3:20 pm
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Well, there is no longer any exposed bare wire and it still sounds like a siren as the volume goes up, then I plug the guitar cable in and it's silent again. Dont even have to hook guitar up, just the cable does it. I guess it completes some kind of circuit. Weirdest thing. I guess I can live with it as long it's fine when I play. I dont know about long term if something may blow up but will find out eventually. Oh and I changed the tubes as well, no difference. If anyone has any bright idea now is the time.


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Post subject: Re: Amp Built - Hiss/Whistle
Posted: Fri Oct 14, 2011 3:38 pm
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atolleter wrote:
Should the capacitor be right at the jack or at the other end straight before the tube (kinda inaccessible)?



Does't mater if the long wire end is shielded


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Post subject: Re: Amp Built - Hiss/Whistle
Posted: Fri Oct 14, 2011 4:44 pm
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No more exposed wires but the whistle is still going on.


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Post subject: Re: Amp Built - Hiss/Whistle
Posted: Sat Oct 15, 2011 1:30 am
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whistle !!!! hard for me to imagine. a kind of feedback ?

Your phono input with a 561 capacitor to ground ??? I can't understant the use for the hi gain you talking about . That is not you need. Look at any guitar amp schematic and built your phono input like guitar input to the first tube with 68 k ohms and 1 meg.ohms. You must have this impedance.

For me you make some mistake in circuit wiring or wrong value components. You have the schematic of your work ?


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Post subject: Re: Amp Built - Hiss/Whistle
Posted: Sat Oct 15, 2011 1:51 am
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stratele52 wrote:
Your phono input with a 561 capacitor to ground ??? I can't understant the use for the hi gain you talking about . That is not you need. Look at any guitar amp schematic and built your phono input like guitar input to the first tube with 68 k ohms and 1 meg.ohms. You must have this impedance.


+1

You have a severe coupling mismatch between your guitar's pickups and V1's screen. And I believe that cap is acting for all the world like an oscillator whenever the input jack is open.

JMO

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Amp Built - Hiss/Whistle
Posted: Sat Oct 15, 2011 4:46 am
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One other thing you may wanna try is a jack that grounds out when not plugged in. This may help you may help STOP picking up UFO signals and amplifying them. See Hoffman/Switchcraft 12A jack.

http://www.hoffmanamps.com/MyStore/perl ... =343521972


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Post subject: Re: Amp Built - Hiss/Whistle
Posted: Sat Oct 15, 2011 6:21 am
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It's like a very high pitch hiss to goes louder when the volume is increased. Although of course it wasnt my first choice I had to use the phono input because that's the only working input on that reel. The phono input goes to the first tube. You mean I should add resistors to the ground as well?
I do not have a schematic, it's purely experimental with very limited to no knowledge. But hey that's how you learn, also that's how you get electrocuted :)
Weird that when the circuit is closed it works great. The phono input jack seems to be a jack that is grounded out when not plugged in already. There are other jacks not in use on that reel anymore that I may swap.


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Post subject: Re: Amp Built - Hiss/Whistle
Posted: Sat Oct 15, 2011 6:50 am
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Well, problem solved. When you said grounded out jack and I looked closely at mine, found out it was, I bent the ground tab that closes the circuit when not jack is in and that did it. The high pitch came from the circuit being closed on itself I guess. Thanks to all.
Amp sounds killer by the way, so gritty, gets my blues so dirty they're brown.


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