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bitsandvolts
PostPosted: Mon May 21, 2007 4:15 pm 
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Hey folks,

First things first...welcome 18w refugees!! :D

Alright, brass tacks!...My sIII, based on one of the first of Stephen's and s2's schematics...has always been a tad 'buzzy'. For the longest time I just dealt with it and figured I'd return to address it at another time. School's over and I've got some time before grad school starts in the fall...so *nows* the time.

First off...has anyone had to deal with the buzzies with their sIII? Did you employ the ruby zener-fix? Did it work?

a week or two ago, when I had some time to putz with this thing, I raised the cathode resistor from 130r to 150r....I got marginal improvement...but I'm still getting the buzzies.

I almost pulled the noval sockets and drilled em open to replace em with octals to convert the amp to a full-blown 6v6 sIII. In fact, I *still* might do it...since I actually like the 6v6 better in a 2X configuration. I find that the el84 in a 2X config doesn't hold up as well as 4Xel84's.

The problem with drilling bigger holes, is that it's alot harder to go back after you drill the holes...lol..

So let's hear it. Anyone have any comments re: this?

Thanks,
Frank
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coco
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PostPosted: Tue May 22, 2007 7:19 am 
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I've not had to resolve this one, but have heard it spoken of. One customer referred to this problem, but in the end, got it [pretty well] resolved by reversing the OT leads and installing Groove Tubes instead of EH. That was after a long period of troubleshooting.

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Emohawk
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PostPosted: Tue May 22, 2007 8:53 am 
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(Kirb's mindless ramblings deleted)


Last edited by Emohawk on Sat Jun 23, 2007 11:19 pm, edited 1 time in total.

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zaphod
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PostPosted: Wed May 23, 2007 4:56 am 
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The big issue I had was a harsh buzz in the top end. I'm not talking about the "fizz" that many high gain amps are notorious for. The was a nasty artifact that was very unpleasant, and it was happening on both channels. I don't know how to describe it actually. It had a kind of "twangy" voice to it.

.... In the end I totally fluked into a solution. Actually, it's more of a work-around ....At that point, I was connected to the output from the PI (basically a ground from one of the 8.2k resistors to the pre-amp ground).

So now you're grounding the grid of one of the power tubes? :shock:
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On the other hand, if you're talking about the fizziness you get when you wind the volume out (especially on the TMB side), that's just what EL84's do when you really hammer them (as this design does). I know a lot of people don't really like that, and I don't know how you could eliminate it without adding a feedback loop to the output stage or something like that.

There are simpler ways. One is to use a huge bypass cap on the EL84s, around 1000uF to 2200uF. Some 18W OTs, such as Trinity's, also seem to help reduce this kind of fizzyness or 18W buzz. In bad cases, you can use Paul Ruby's buzz mod, using a apair od zeners and diodes.

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Emohawk
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PostPosted: Wed May 23, 2007 10:28 am 
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Last edited by Emohawk on Sat Jun 23, 2007 11:21 pm, edited 1 time in total.

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Emohawk
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PostPosted: Wed May 23, 2007 7:21 pm 
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Last edited by Emohawk on Sat Jun 23, 2007 11:21 pm, edited 1 time in total.

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zaphod
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PostPosted: Sun May 27, 2007 11:53 am 
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It appears all it has really done is reduce the input level to the power stage, kind of like increasing a grid stopper or grid leak.

No, it's shut down the grid drive to that power tube, so it's no longer doing anything.
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.... the ground trick works pretty much anywhere on the either of the power tube grid resistors (8.2k) or the top side of the .01uf coupling caps (i.e towards the front of the amp). The effect is exactly the same regardless of where it's connected in this area. ... It makes no difference if connected to the ground side of the 470k resistors, so I guess that ground connection is solid.

So your amp only works right when you kill either of the power tubes by grounding its grid. :shock: So it's not running push-pull anymore. Dude, that amp's got some serious problems.

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Emohawk
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PostPosted: Fri Jun 01, 2007 1:36 pm 
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(last batch of mindless ramblings deleted...)
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