Another Ground and Hiss Thread

[fiftynine]

I've tried star grounds (and slight variations of), logical short cuts to the same ground, tinkered about with all sorts and found the classic Fender system to be quiet enough or quieter. Can anyone summarize their findings and opinions on this?


Do I really need a buss wire and to terminate the pre-amp signal ground at the input jacks only or can I use the pot lugs soldered to the back of the pot (and a number of resistors) with the pot shields and effectively the whole front of the chassis as a ground plane along with the input jacks? It simplifies things that way, keeps everything to a minimum and you can get a bad pot off more easily. It's quicker and it looks neater.


Also, what are the main sources of hiss if grounding is good, tubes are good, shielded cable used etc?

[fiftynine]

I forgot to add, when I first power up an AC30 style amp I have here, it hums a bit more than normal then it pops a few times and the extra hum goes away. Any ideas?

[jjasilli]

geofex.com > tube amp de-bug page > hiss

[fiftynine]

Oh yeah.

[HotBluePlates]

... what are the main sources of hiss if grounding is good ...

I would never look at grounding as a way to cure hiss...

Assuming the basics are good (good soldering, good grounds, all parts functional), resistors are your #2 source of hiss. Some types have more hiss than others, larger-values (1MΩ vs 820Ω) have more hiss than lower values, some circuit positions are more sensitive to hissy resistors than others.

Your #1 source of hiss are the tubes, but especially amp circuits with a lot of gain. All tubes have inherent noise, pentode being noisier than triodes. And if you build up a circuit with a lot of gain stages, you have more tubes to contribute to noise. If your lot-of-tubes circuit has high voltage gain, then the noise early in the circuit gets amplified a lot.

People normally quote noisy plate resistors as the cause of noise/hiss, but I also wonder about those 1MΩ grid resistors in early gain stages. And one forum member used a listening amp to find that is was a low-value cathode resistor which was to blame for hiss in his amp. A tube exhibits more gain from cathode-to-plate than it does from grid-to-plate... So resistor noise times big voltage gain equaled big output noise.

[jjasilli]

+1 to HotblueIn the first post 59 talks about hiss; then hum in post2.  Hiss and hum are 2 different things.  They must be addressed separately and differently.  BTW in the USA, hum will always (almost always???) be 60Hz or 120Hz.  60Hz corresponds to mains power and usually arises from bad ground or caps in half-wave rectified AC:   in B+,  or in fixed bias supplies.   Or, in the filament supply.  120Hz usually arises in full wave rectified AC in the B+ supply - again from bad ground or caps.