Shieled wires - where should I use them?

[Victor Denance]

Hi!

My homemade Super Reverb is pretty quiet, but I'm always looking for ways to improve it.
Would shieled wires *somewhere* decrease the (slight) hiss?

I don't wanna use the damn thing everywhere, it's such a pity to work with...
So far I replaced the wire at the output of the tone stack (from volume pot wiper to next gain stage) whith a shielded one and noticed no improvement.
Maybe I could use it elsewhere with more effectiveness?

Thanks!

Victor

[txstrat]

Some use shielded wires from the input to the first stage. That's where the circuit can pick up the most noise.

[guitician]

Most wires inside a chassis dont need shielding unless it's for protection from coupling to other parts of the circuit to reduce the chance of oscillation. Hiss is from resistors or tubes. Low noise tubes/resistors may help.

[tubeswell]

What txstrat said. I always shield the wires from the input(s) to the grid(s) of V1 and then from the 1st vol pot to the grid of the next stage as well. remember to ground the cable shield at one end only (otherwise you'll make unnecessary ground loops).

For hiss reduction you can install metal-film plate resistors. However if you are a carbon comp fan, I'd at least keep the plate resistor in the stage before the PI as a CC resistor (as I note that this is where R.G. advocates getting the most benefit out of a CC resistor - and having tried it both ways in several amps for myself, I tend to agree). The other thing about hiss is that if you have a high gain 12AX7 stage with (say) a 220k plate resistor, it will naturally hiss more than a 12AX7 stage with a 100k plate resistor anyway. Higher gain = more hiss (all other things being equal).

[Enzo]

Shielded wire is more to protect you from picking up hum rather than hiss.

[Robert M. Martinelli]

If "hiss" is your main concern, then, as already said, you' ll have to look at resistors and tubes, because shielded cable is a countermeasure to reduce hum, not "hiss".

Metal film resistors are quieter, but, if we want to further reduce noise, it must also be remembered that most of the noise generated by resistors is thermal noise, the equation for this noise being En=Sqrroot(4KTR) where K is the Boltzmann's constant, T is the resistor's temperature in degrees K, R is the resistor's value and En is ( quite obviously ) the Equivalent noise.

We cannot change the Boltzmann constant ( at least in this part of the universe, don't know about how things work in other galaxies ), and we have to stick to the resistor's design values, so, in order to lower noise, all we can do is act on the resistors' temperature by lowering it. To achieve this we can simply use resistors with a power rating higher than needed, ( e.g. 1W resistors in place of half watters ) so the resistors' operating temperature will be lower. This is especially helpful with high value resistors, which obviously tend to be noisier.
( this trick is rather commonly used in high end hi-fi amps ).

So, summing up, to reduce "hiss" you' ll have to use low noise/special quality tubes, and metal film resistors of a higher power rating.

Hope this helps

best regards

Bob

[Victor Denance]

Thanks so much for your answers.

I had a pretty noticeable hum with the Super Reverb andI couldn't understand why I had that hum since I took special care in the lead dress and grounds positions.
I realised the top of the inside of the cab was not covered with conductive tape. Taped it with aluminium and it's totally hum free now. I'm glad it's fixed! Shielded wires couldn't do anything better since it's totally hum free, even with all controls turned all the way up.

As for hiss, there's not much of it actually. I used 2W metal film resistors throughout, except for the plates where I sticked with 1/2W Carbon Comp. As RG Keen says, I do find there's a slight difference.

Thanks a million again, your opinions are a pleasure to read.

[Diablo]

I realised the top of the inside of the cab was not covered with conductive tape. Taped it with aluminium and it's totally hum free now. I'm glad it's fixed!

Can you explain how that works, maybe a picture too?

[zhyla]

I think what he is saying is his chassis wasn't completely enclosed so it didn't make a good Faraday cage. My bassman hums like a mother with the lid off.

[Victor Denance]

I think what he is saying is his chassis wasn't completely enclosed so it didn't make a good Faraday cage. My bassman hums like a mother with the lid off.

That's precisely what I was meaning Zhyla, thanks for explaining.