Basic Construction and Grounding Questions

[dtbradio]

Hi All!

Happy Thanksgiving to all who celebrate that day tomorrow!

I'm in the process of building my first large tube project, a 3-tube amp built from junk-box tubes and parts just to make it interesting. I have a Marshall-style pre-amp section (minus tone stack for now, and with a few cap values different from Marshall because I don't have those exact values), followed by a single-ended 6V6 power stage. The power stage works great, plenty of volume into my home-grown 2x12 16-ohm stack using a pair of old Leslie 8-ohm series-wired speakers in an old Leslie cabinet. The problem I'm having is high-audio-frequency oscillation at high pre-amp gain levels which sharply drops over-all volume and exhibits a severely-compressed sounding tone. This issue goes away if I back off the gain. Master volume level changes don't seem to make any difference. My main question is this: should I be chassis-grounding everything, or isolate all circuit grounds from chassis except at one spot? My input jack is not isolated, but I have isolated the speaker output jack. I have noticed that if I keep the input cable short, the problem isn't quite as bad. I've also noticed that touching the amp chassis or metal on my guitars will alter the background noise of the system, IE hiss and buzz. My research has shown opinions in both directions on the grounding subject. I do know that my grid stoppers need to be relocated directly to the grid pins which I haven't had a chance to do yet. I'm also thinking of increasing the pre-amp stoppers to about 82K to roll off as much signal above 10KHz  as possible. I don't have the amp here at the time of posting, and I can't recall if I used a grid stopper on the 6V6. Thanks in advance for any insights!

[Strelok]

Hello DTBRadio!

How was thanksgiving?

As far as I understand it is best to create spots where grounding comes together, and then ground one of those to the chassis.
KoC calls this "galactic grounds" because it looks like a set of stars and then one grounding point.

But from what I read you already know that?

The problem with multiple groundings spaces is that ground loops can occur.

So this means that you should insulate the input jack.
Is that easy to do for you? If you have a metal jack for input? Then you must drill a bigger hole and use fibre washers, and maybe you don't have those.
Maybe you could use a plastic jack?


So, you would have one grounding point for the signal. That is signal ground.
Then there should be another grounding point for the AC-receptacle, the ground wire for the power transformer (if it has one) should be connected there as well.
That is chassis ground.

I cannot advise you about the hissing problem.
I had that with one preamp I built bit later the problem was obvious: I was using a cathode follower, but that was a few tubes away.
That distance was way too long.


Strelok