Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on September 26, 2018, 05:09:51 pm
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Edit: Changed the post since no replies yet and I found one of the answers I was looking for.
Now for another:
I am building a Hoffman AB763. It's my first time using one of his boards. Doing point to point before this I've alwyas grounded the power supply stuff, filter caps HV center tap etc. on one of the transformer lugs.
In his info pages, Hoffman advises the same, with all other grounds going to a bus soldered to the furthest input jack ground and to the back of the pots. I plan to do this too. However I have a question about the last three filter caps. I plan to put them above the board, below the pots, and run the grounds to the preamp bus, as he suggests on the filter cap solutions page.
What would be the practical difference between grounding the last three filter caps this way as opposed to running them to the transformer lug with the first two filter caps? I assume it won't be a problem, but just thought I'd ask.
Thanks!
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A quieter amp.
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Read this, it may help you understand a little more on grounding.
Doug's grounding scheme works fine, Valve Wizard's is a little different but same principles.
http://www.valvewizard.co.uk/Grounding.html
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A quieter amp.
Figured as much. Thanks Sluckey. And thanks for the link Willabe.
As a practical matter has anyone here built this circuit with the last two or three filter caps tied to the ground buss? I'm assuming it's generally quiet and works well as Willabe wrote, or else Doug wouldn't have given it as an option and used it in a lot of builds.
But if I'm much better off going with the filter caps all on the transformer lug, let me know! Very tight under there and I'd have to get creative to find a place to put them. But I'd do it if it would make an appreciable difference. Thanks,
Ted
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As a practical matter has anyone here built this circuit with the last two or three filter caps tied to the ground buss? I'm assuming it's generally quiet and works well as Willabe wrote, or else Doug wouldn't have given it as an option and used it in a lot of builds.
Yes, MANY here and other places have.
Sluckey already told you it's quieter when you separate the power tube grounds from the PI and preamp tube grounds.
But if I'm much better off going with the filter caps all on the transformer lug, let me know! Very tight under there and I'd have to get creative to find a place to put them. But I'd do it if it would make an appreciable difference.
Again, Sluckey already told you it wont, it will be noisier.
Many times guys have built an amp and posted asking how to get rid of the buzz/hum.
After being asked some questions on how they grounded their amp, it would be suggested to them to separate the power tube grounds from the PI and preamp tube grounds.
After they did that, the amp was quieter.
If you read the link I posted many of your grounding questions will be answered. And it explains why. So you will understand what's happening.
A good # of guys here could write it out for you but why, when it's already been written out already?
Here's another link on grounding from a different author;
http://www.aikenamps.com/index.php/grounding (http://www.aikenamps.com/index.php/grounding)
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The first filter cap *must* connect to the Rectifier/PT pulsating-DC source, as short as possible. That may be the PT CT (or FWB - pin) right to the cap, or both on one bolt; whatever, you need this heavy buzz-current contained.
Arguably the preamp filter caps can go "at" the preamp stages. However in tube circuits a foot of wire in an already-filtered filter cap return is pretty negligible. I think Hoffman sometimes put caps AT early stages (easy with modern compact caps); Fender ran everything to a doghouse, no big difference.