11-23-2022, 11:32 PM
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!
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!