Lowering Noise and Microphonics

[Lonely Raven]

skyboltone, do you feel those resistors help keep the noise floor down?

[skyboltone]

skyboltone, do you feel those resistors help keep the noise floor down?

Boy I don't know Lonely. It's such an inherently noisy amp that one would have to try to build an exact duplicate with carbon resistors to be able to tell anything.

I use them because they are forever. They will never drift, and they don't fail.

Dan

[Lonely Raven]

Good reason to use them, huh?

[skyboltone]

OK!!!!!

All of my noise problems are gone. 33K grid resistor on V1A. Shielded cable for V1B grid.

And here's the big surprise. I had taken a voltage divider off the screen supply (270K/18K) to provide a 14v DC reference for the filament balance pot. Well guess what, it injects a very nice 60hz signal directly on the power tube screens. That didn't make 120hz hum, it made just plain irritating and unacceptable noise. Not good. Took that out and grounded the wiper of a 250ohm 2 watt wirewound pot and balanced the filaments. By the way the pot is 3/4 of the way off of center to get to near 0 hum. Anyway, with everything set at 1 o'clock it's as quite as a mouse chewing on a cigarette lighter. (who said that the other day?) At 12 o'clock you could bring it into the studio for a recording session it's that quiet.

And it absolutely roars. Turn up the treble slightly and get some of the most amazing harmonics you've ever heard anywhere. Whooooo Hoooooo!

Dan H

[muchxs]

With everything set at 1 o'clock it's as quite as a mouse chewing on a cigarette lighter. (who said that the other day?)

Totally off topic but here's a pic of the actual mouse chewed Bic. Had that been a desert rat or one of the good doctor's squirrels it would have surely eaten the whole thing!

I suppose that's the ultimate solution to any noisy amp. If you can't quiet it down just smear peanut butter all over it and put it out on the porch, the rodents will take care of it!

[skyboltone]

Yeah, I get them on the boat from time to time depending on where we're moored. These aren't those cute little suckers they make cartoons out of though. These are wharf rats. The only way to go after them is with a Rat Terrier whose been to the Michael Vick school of dog training. Or DeCon and find them when they get good and rotten.


But that's not important right now........The amp works the amp works!

How many times do you run into arcing OT primaries AND one of your fixes (the screen supply hum discombobulator) making the same sound?

But that's not important right now......... Anyway, gotta get it and a speaker into a box, cover same with tolex and get it to Mr FedEx before the Christmas deadline. It's going to a 14 year old whizbang guitarist who better give me at least $100K for it when he gets famous because he's gonna.

But that's not important right now......

Gotta go after my Hybrid A finally.

[dartanion]

My garage mice have taken a liking to tolex! They started chewing on the top edge of my SFVC.

About the need to shield the NFB lead. I think I need to do that as from the pictures I've seen in this thread would never work in my Liverpool. I've rerouted it 4 times and tried a few varieties and gauges of wire. Now I think I'm going to try shielded. Once this is tamed a bit, this bitch is going to scream! I can't turn it up to loud or it oscillates wildly. Take the NFB loop out, no squeal but with MORE NOISE. Sounds stellar at lower volumes though.

[Tubetwang]

I tried cable tv wire yesterday with good results...

Can't solder the ground but...

For mice on dry land we have used a Scottish Terrier...

Life on the boat requires a Cairn Terrier.

Others will prefer Poodles

[Structo]

How about a Schipperke.
Used for ages on canal boats.

[Tubetwang]

Captain love them.

Don't mind the sheddings but...

[Trace]

I just finished my Ceriatone build as well. I noticed how sensitive the v1 is. If you *breath* on it you hear it! LOL

As I’m sure many are already aware the layout is critical with the Express circuit. If the input jack is too close the circuit board or the controls themselves are too close to the circuit board the first stage will be very susceptible to noise, microphonics and high frequency oscillation. One solution is to shield all wires going to grids and this may be all that is needed. Another option would be to add a resistor going into the grid of the second stage. Depending on what your lay out is, etc there are numerous options.

Another problem lately is that tube manufactures are not burning in tubes. As a result trying to put a non-burned-in tube into V1 of a plexi, Express, etc usually results in a microphonic tube. The positive side to this is that burned-in tubes actually do sound better and less prone to going microphonic.

Metal film types are the quietest but I don't think they've been too well received by those that have tried them in a wreck.

We’ve used them in numerous builds and in my humble opinion they sound great. Same tone, less noise and tighter tolerances (based on our experience).

Has anybody seen the need to shield the NFB loop?

I cannot think of a time when shielded wire was necessary for the negative feedback wire, at least not from my experience but mileage varies.


Have a very Merry Christmas!
Trace

[novosibir]

And here's the big surprise. I had taken a voltage divider off the screen supply (270K/18K) to provide a 14v DC reference for the filament balance pot. Well guess what, it injects a very nice 60hz signal directly on the power tube screens. That didn't make 120hz hum, it made just plain irritating and unacceptable noise. Not good.

That must have been a Liverpool with the output's cathodes on about this positive potential! Why didn't you change the voltage divider, to rise up the heater's reference voltage some more to about 35V?

Larry

[novosibir]

Has anybody seen the need to shield the NFB loop?

Shielding a FB wire means, to have created an additional (parallel) presence control - though with a very small presence cap, but always dimed

Never do it, it may lead to RF oscillation in the power stage.

Larry

[skyboltone]

And here's the big surprise. I had taken a voltage divider off the screen supply (270K/18K) to provide a 14v DC reference for the filament balance pot. Well guess what, it injects a very nice 60hz signal directly on the power tube screens. That didn't make 120hz hum, it made just plain irritating and unacceptable noise. Not good.

That must have been a Liverpool with the output's cathodes on about this positive potential! Why didn't you change the voltage divider, to rise up the heater's reference voltage some more to about 35V?

Larry

Good thinking Larry. Ha! Cathodes at about 11 volts. I started with a 270k and a 39K and just felt that 50 volts was too high so I grabbed a 18K. Live and learn.

[mlp-mx6]

Interesting updates from Randall Aiken embedded in this Ampage thread.

http://music-electronics-forum.com/show ... php?t=5213