Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: cusebassman on March 16, 2019, 01:38:21 pm

Title: Grounding scheme for stereo 5F2A design
Post by: cusebassman on March 16, 2019, 01:38:21 pm
Hey all - I'd like to get the forum's thoughts on how to properly ground this stereo 5F2A design. I built an iteration of this amp in a re-purposed Fender DeVille chassis two years ago that works well, but the grounding scheme is, frankly, all wrong, according to the reading I've done more recently around here. Subsequent attempts to build this amp in a Hammond 17x10x3 chassis have all been fraught with distortion and (I believe) oscillation problems, so I want to get some advice on:
I've attached a DIYLC file and image with no ground bus, in case anyone would be so kind as to take a look. Thanks!
Title: Re: Grounding scheme for stereo 5F2A design
Post by: cusebassman on March 16, 2019, 01:47:27 pm
Attached updated wiring diagram.
Title: Re: Grounding scheme for stereo 5F2A design
Post by: Willabe on March 16, 2019, 04:03:52 pm
Have you read Merlin's web site page on grounding?

http://www.valvewizard.co.uk/Grounding.html (http://www.valvewizard.co.uk/Grounding.html)

There's different ways to ground an amp. But basically what you want to do is get rid of 'random' chassis grounds and use/make a 'wired' ground scheme.

Separate the power amp/power tubes and their filter caps grounds from the preamp tubes and their filter cap grounds. You make ground 'stars' for each B+ filter cap ground. What ever tube is fed from a filter cap you tie those grounds together as a 'star' and then run a wire to the next ground star.

I have 2 chassis ground connections when I build an amp, 1. Circuit ground 2. Safety ground. But other guys use 1 or 2 more chassis grounds and have very quite amps. 

The 1st filter cap should be grounded to the PT's B+ wind CT,  -nothing else-  , then run a wire from there to the power tube ground star.   

If the amp has a -FB loop that goes back to the PI, then the OT speaker jack ground should go to the PI ground star.     
Title: Re: Grounding scheme for stereo 5F2A design
Post by: cusebassman on March 16, 2019, 04:07:48 pm
I haven't seen that article - thanks! I've got some reading to do.
Title: Re: Grounding scheme for stereo 5F2A design
Post by: Willabe on March 16, 2019, 04:08:41 pm
Here's how I did my tweed Princeton.

http://el34world.com/Forum/index.php?action=dlattach;topic=21122.0;attach=61998
 
Title: Re: Grounding scheme for stereo 5F2A design
Post by: Papa Jim on March 16, 2019, 05:37:15 pm
I recently built a deluxe reverb using the scheme Willabe mentioned and it turned out very quiet.
Title: Re: Grounding scheme for stereo 5F2A design
Post by: Papa Jim on March 20, 2019, 07:44:52 am
Quote from: Willabe on March 16, 2019, 04:08:41 pm
Here's how I did my tweed Princeton.

http://el34world.com/Forum/index.php?action=dlattach;topic=21122.0;attach=61998

PM sent Willabe, curious about your NFB pot.
Title: Re: Grounding scheme for stereo 5F2A design
Post by: silverfox on March 21, 2019, 01:50:46 pm
Quote from: Willabe on March 16, 2019, 04:08:41 pm
Here's how I did my tweed Princeton.

http://el34world.com/Forum/index.php?action=dlattach;topic=21122.0;attach=61998 (http://el34world.com/Forum/index.php?action=dlattach;topic=21122.0;attach=61998)
First of all, I previously used the Star grounding scheme so I'm not disputing it. I would like to know however, how the noise problems arise if ultimately all the grounds are connected together through essentially zero Ohms resistance and the chassis is connected to the power grid ground. I suspect in many cases the several different ground points make a poor connection to chassis, in a noisy amp.

For a problem to arise, I have to believe the inductance and capacitance distributed over the chassis allow the small coursing currents to develop noise voltages above the threshold necessary to be amplified by the preamp circuit. So in a poorly grounded chassis scheme, small impedance networks develop within the chassis and various grounding wires raising the noise above the threshold?
Or do the antenna characteristics of the chassis pick up noise and that noise passes through preamp ground points to the circuit?

The ground lift circuit is also a noise reduction technique as well the Humdinger pot.
silverfox.
Title: Re: Grounding scheme for stereo 5F2A design
Post by: Willabe on March 21, 2019, 10:34:18 pm
Quote from: silverfox on March 21, 2019, 01:50:46 pm
First of all, I previously used the Star grounding scheme so I'm not disputing it. I would like to know however, how the noise problems arise if ultimately all the grounds are connected together through essentially zero Ohms resistance and the chassis is connected to the power grid ground. I suspect in many cases the several different ground points make a poor connection to chassis, in a noisy amp.

For a problem to arise, I have to believe the inductance and capacitance distributed over the chassis allow the small coursing currents to develop noise voltages above the threshold necessary to be amplified by the preamp circuit. So in a poorly grounded chassis scheme, small impedance networks develop within the chassis and various grounding wires raising the noise above the threshold?
Or do the antenna characteristics of the chassis pick up noise and that noise passes through preamp ground points to the circuit?

The ground lift circuit is also a noise reduction technique as well the Humdinger pot.
silverfox.

This is from Aiken amps on grounding. He talks about grounding through the chassis problems.

http://www.aikenamps.com/index.php/grounding