Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mike_J on March 02, 2021, 11:49:44 am
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I have a high degree of respect for members of this forum and would consider the opinion concerning Aiken, sorry don't know the first name. Reason is the last phase of my rebuild will deal with proper grounding techniques. He has some comments regarding this matter.
I know Doug has comments concerning this matter and his layouts are based on what he knows has worked in the past. Sound reasoning no doubt but I question everything. I can't help it and nobody should feel upset about it. Always wise to question everything in my opinion. Don't like anyone questioning me any more than any of you like to be questioned. Don't know this Aiken from Adam but he has a nice website for his amplifier business and I have heard people speak of him in the past but don't remember what is was about or whether it was good or bad.
Anyway, I hope you will come along with me on this little journey into amplifier grounding, presuming Aiken knows what he is talking about.
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He says the same things that I say, so I have a very high opinion of him.
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good information presented in an accessible way
when it comes specifically to grounding schemes: the right way is the quiet way, however you might achieve that. there are plenty of amps out there with "wrong" grounding schemes that sound great. my grounding schemes, layout, and lead dress have improved from build-to-build-to-build
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Randall Aiken has been around a long time and knows his stuff inside-out and back-to-front.
Merlin Blencowe (a.k.a. Valve Wizard) also knows his stuff and has an excellent article on grounding http://www.valvewizard.co.uk/Grounding.pdf (http://www.valvewizard.co.uk/Grounding.pdf)
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Randall Aiken has been around a long time and knows his stuff inside-out and back-to-front.
Merlin Blencowe (a.k.a. Valve Wizard) also knows his stuff and has an excellent article on grounding http://www.valvewizard.co.uk/Grounding.pdf (http://www.valvewizard.co.uk/Grounding.pdf)
Thank you for the reference. The only place I can think the Hoffman method is at all questionable is in the first three filter supplies. Most likely a lot about nothing. In the past I have tried to place ecaps with the stages they feed. Not entirely sure it wouldn't be better to tie the first filter stage to the power tube cathodes then on to the PT bolt lug. Read where some think it is important and it wouldn't be hard to do now. More difficult once the board is in and could always change back to the Hoffman method with little effort if the experiment proves unsuccessful. Going to read everything I can find on the subject.
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good information presented in an accessible way
when it comes specifically to grounding schemes: the right way is the quiet way, however you might achieve that. there are plenty of amps out there with "wrong" grounding schemes that sound great. my grounding schemes, layout, and lead dress have improved from build-to-build-to-build
That is the objective. To get better with every build. That is why I am doing this rebuild. Could have changed the ecaps which were starting to create a little hum in the amp and let it go at that. Amp sounded pretty close to the same as my '59 Bassman LTD which is a good amp and of course they should sound about the same with essentially the same component values, cabinet and speakers. But no not me I am on a quest for a growler. Feel a little like Chevy Chase when he was going nuts on his way to see Walley Moose.
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The use of a power transformer mounting fastener (especially a laydown type) to attach the safety ground lug seems a poor choice to me, about the worst possible option.
As it’s entrusting the only safety critical connection in an amp to one of the most mechanically stressed fasteners. And PT fasteners tend to become slack over time, due to such mechanical stress, and from the laminations compressing together.
Regarding Aiken, it’s apparent to me that he’s an extremely competent EE who shares our (near pathological) obsession with valve guitar amps :icon_biggrin:
The ‘tech info’ area of his site is an invaluable, gold standard reference source for this stuff, that’s been online since the late 90s.
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I believe European electrical security standards mandate (second hand information, I admit I have no primary source for this):
- Safety ground must be connected to a dedicated bolt which may not be used for anything else.
- The length of the cables should be such that if the power cord is physically ripped out of the chassis, hot and neutral connections should be ripped out first. Safety ground must be the last connection to go.
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good information presented in an accessible way
when it comes specifically to grounding schemes: the right way is the quiet way, however you might achieve that. there are plenty of amps out there with "wrong" grounding schemes that sound great. my grounding schemes, layout, and lead dress have improved from build-to-build-to-build
I agree!
My understanding of grounding is that even an arbitrary grounding scheme, with potential loops and unknown current paths (through the chassis) can work great, but it may take a bit of luck and experimentation to get that good result.
The grounding schemes suggested by Aiken and Blencowe use a more scientific approach. My experience is that the scientific approach allows you to get a good result every time, without having to relying on luck and without having to adjust and experiment.
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The use of a power transformer mounting fastener (especially a laydown type) to attach the safety ground lug seems a poor choice to me, about the worst possible option.
As it’s entrusting the only safety critical connection in an amp to one of the most mechanically stressed fasteners. And PT fasteners tend to become slack over time, due to such mechanical stress, and from the laminations compressing together.
Regarding Aiken, it’s apparent to me that he’s an extremely competent EE who shares our (near pathological) obsession with valve guitar amps :icon_biggrin:
The ‘tech info’ area of his site is an invaluable, gold standard reference source for this stuff, that’s been online since the late 90s.
Totally agree with pdf64 here. Bonding the Earth conductor to a Transformer mounting bolt is about the worst "shared" connection you could pick. Power transformers physically vibrate at the mains frequency due to an effect called magnetostriction (all magnetic materials physically change shape during the process of magnetization).
This can and does cause the fasteners to back off the threads of the transformer mounting bolts over time.
The solution is simple, and much safer. The earth conductor gets it's own dedicated connection to the chassis.
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I believe European electrical security standards mandate (second hand information, I admit I have no primary source for this):
- Safety ground must be connected to a dedicated bolt which may not be used for anything else.
- The length of the cables should be such that if the power cord is physically ripped out of the chassis, hot and neutral connections should be ripped out first. Safety ground must be the last connection to go.
Sold on the dedicated bolt from many comments concerning the matter. Really appreciate the extra long ground wire comment. I obsessed over my daughter doing the same as she makes this lamp out of pipe. Not practicing what I am preaching.
Thanks
Mike
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Randall Aiken has been around a long time and knows his stuff inside-out and back-to-front.
Merlin Blencowe (a.k.a. Valve Wizard) also knows his stuff and has an excellent article on grounding http://www.valvewizard.co.uk/Grounding.pdf (http://www.valvewizard.co.uk/Grounding.pdf)
Now I am learning just enough to be dangerous. Will follow his suggestions concerning earth ground with the exception of the extra length to the ground wire as mentioned by d95err. That is essential in my opinion.
Anyway, I am reading along and finding out most valve amps consist of a power transformer, rectifier and reservoir capacitor he called a block where physical connections should be kept as short as possible. Well realizing my first ecap is in a dog house which is more like down the block than as close as possible I am already a little concerned. Another thing, it doesn't have a fancy dropping resistor on both the positive side and negative side of the cap. Bet it would be harder to get that V1 and V2 noise he is talking about in Fig 15.6 if I had a reservoir cap with a dropping resistor on the negative side.
Question is do I need one and if so how do I get one?
{edit: munged Quote-- PRR}
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not something i would worry about
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“Now I am learning just enough to be dangerous” …. Perhaps with a question like the one you gave asked it may be worth outlining your own experience.
While it is worth questioning a lot of what you read on the internet, for the obvious reasons that needed not be gone into here, questioning the experience and knowledge of an established professional amp builder is not the most direct path to wisdom.
There are many many more I’ll informed and anonymous, that state they’re opinions as if they were a simple fact.