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 Post subject: 18watt loud hiss
PostPosted: Thu Nov 17, 2016 2:47 am 
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Location: Adelaide, Australia
Hi all,

I hope this is ok because I have a query about an amp that is not BYOC. I've started using my Ceriatone 18watt TMB EF86 a bit more and I've noticed that it has a pretty loud hiss in the TMB channel. The hiss gets louder as you increase the master volume quite considerably. You don't really notice once you start playing and it has a very nice tone but the hiss is pretty annoying when you stop playing. I've just put new preamp tubes in and use the tube Shields which doesn't seem to make a difference.

I'm also using shielded wire on both the TMB input and the EF86 input and they are properly grounded. I've checked over everything in the layout and it all seems exactly as it should be so I'm a bit stumped. Are they just naturally noisy Amps? It also uses carbon comp resistors for the whole thing so I'm wondering if that is causing issues?

I'd ask on the Ceriatone forum but that thing is dead.

Any ideas? I'll probably get some photos up soon.

Thanks for any help.

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 Post subject: Re: 18watt loud hiss
PostPosted: Thu Nov 17, 2016 8:56 am 
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Just to be sure we're on the same page--the amp's problem is hiss, not hum, correct?

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 Post subject: Re: 18watt loud hiss
PostPosted: Thu Nov 17, 2016 12:38 pm 
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You have a bad cap in your TMB preamp. The fact that the noise goes away when you start playing is indicative of this problem. This is usually caused by the silver mica cap the tone stack or the silver mica high pass cap in parallel with the volume knob, but it could be any of the caps in the TMB preamp.

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 Post subject: Re: 18watt loud hiss
PostPosted: Thu Nov 17, 2016 3:22 pm 
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Location: Adelaide, Australia
Hmmm interesting thanks for that. Yes definitely hiss not hum. I'm not sure whether the hiss actually goes away when I start playing exactly just that the volume overpowers the hiss. So just to be clear the silver micas I've got one on the EF86 volume knob and a large yellow one one the board. Should I start with those? I guess there isn't really a way to tell I'll have to change all the caps one at a time.

It almost sounds like a windy day at the beach or something.

I'm going to redo the shielded wire coming from the inputs and change those resistors to carbon film on the weekend just because looking at that years later I'm not happy with the job I did and I have a feeling it could be contributing. If that doesn't do it I'll start trouble shooting the caps.

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 Post subject: Re: 18watt loud hiss
PostPosted: Sat Nov 26, 2016 7:45 am 
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Location: Adelaide, Australia
Just a little follow up. I've posted this on the 18watt forum and have gotten some useful feedback but still haven't solved the hiss.

I've tried redoing the shielded wiring, rewiring all the wires on the tube side of the board, tried different preamp tubes, replaced most of the carbon comps with carbon film, changing to a better grounding scheme. I've narrowed it down to something with the cathode follower which is V3 on the layout. If I pull that tube the hiss goes away. But ive replaced pretty much everything before that part of the circuit.

I did a bit of a YouTube clip that shows what it is like. Does it help narrow anything down? I have signal caps and New power tubes ordered but I don't think they are the problem. I've tested for DC on the pots too.


https://youtu.be/0EAEvD88QAg

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