2A3 "Pink SET" Amp Rebuild.

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Nick
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#46 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Nick »

Using twin busbars, one running along each bank makes it virtually impossible to ground without introducing noise all over the place.
Both bus bars will have to meet somewhere, that somewhere is your 0v reference, pick the last two caps in the power supply, join their -ve together and call that the 0v point. Take every other power supply cap -ve to that point apart from the first cap in each half of the power supply, take their -ve back to the -ve onto the bridge rectifier. Then take a single connection from the each first cap -ve (directly on the cap, (don't be tempted to use somewhere on the way to the bridge) to the 0v point.

At least thats what I would do.
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#47 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Cressy Snr »

Nick wrote: Sun Feb 11, 2018 7:19 pm
Using twin busbars, one running along each bank makes it virtually impossible to ground without introducing noise all over the place.
Both bus bars will have to meet somewhere, that somewhere is your 0v reference, pick the last two caps in the power supply, join their -ve together and call that the 0v point. Take every other power supply cap -ve to that point apart from the first cap in each half of the power supply, take their -ve back to the -ve onto the bridge rectifier. Then take a single connection from the each first cap -ve (directly on the cap, (don't be tempted to use somewhere on the way to the bridge) to the 0v point.

At least thats what I would do.
Thanks Nick. I'll try that. :)
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#48 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Cressy Snr »

Nick wrote: Sun Feb 11, 2018 7:19 pm
Both bus bars will have to meet somewhere, that somewhere is your 0v reference, pick the last two caps in the power supply, join their -ve together and call that the 0v point. Take every other power supply cap -ve to that point apart from the first cap in each half of the power supply, take their -ve back to the -ve onto the bridge rectifier. Then take a single connection from the each first cap -ve (directly on the cap, (don't be tempted to use somewhere on the way to the bridge) to the 0v point.

At least thats what I would do
Is this what you mean?

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Nick
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#49 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Nick »

Yes, that way the large current spikes charging the first cap can return to the bridge without passing through the common ground point.
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#50 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Cressy Snr »

Nick wrote: Sun Feb 11, 2018 9:06 pm Yes, that way the large current spikes charging the first cap can return to the bridge without passing through the common ground point.
Cheers Nick I'd never have thought of doing it, but now it's been explained.... :)
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#51 Re: 2A3 "Pink SET" Amp Rebuild.

Post by steve s »

the sound of the pink SET is now thin, hyper detailed and and shouty; rather than the lush, real and musical sound it had before I fucked it up.
I don't think you have messed it up
It will need a good few hours to run in
Hyper detailed means there's less cancellation- thats a power supply improvement. . you may not like it...
But stick with it
I think low impedance is important to the bass, that where I have gone off track with my px25 amp, but the musical separation ie the detail needs to be understood as to what you have actually achieved and in my view it not a negative
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#52 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Cressy Snr »

We'll see how it goes Steve :)
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#53 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Cressy Snr »

I've reworked the power supply completely today, restored the AC to the 2A3 filaments, as I couldn't find a suitable transformer for the module AC supplies. The one I had been using got too hot, too quick and I don't do hot.

I have done the grounding scheme as Nick suggested, moved the choke closer to the mains transformer, and added a black perspex trim down the centre of the chassis to hide all the carnage underneath.
The choke is mounted onto the black perspex plate with through holes to the metal chassis beneath, and the filament transformers inside have been mounted on paxolin, in order to try to reduce flux leakage into the steel chassis.

Just one final question for the panel, before I test out the amplifier, if I may:

Where is the best place to ground the 6.3V heater centre-tap?
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#54 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Nick »

Where is the best place to ground the 6.3V heater centre-tap?
What's it heating? What voltage above 0v are the cathodes its heating?

I would normally run the heater 10 or 15v above or below the cathode, or failing that, just connect a 100R + 100R (or some such value) pair of resistors to the star earth point.
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#55 Re: 2A3 "Pink SET" Amp Rebuild.

Post by IslandPink »

Driver valves I would say - 6AQ5 or 6V6 are mentioned.
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#56 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Cressy Snr »

Nick wrote: Mon Feb 12, 2018 5:58 pm
Where is the best place to ground the 6.3V heater centre-tap?
What's it heating? What voltage above 0v are the cathodes its heating?

I would normally run the heater 10 or 15v above or below the cathode, or failing that, just connect a 100R + 100R (or some such value) pair of resistors to the star earth point.
The heater winding is heating the input and driver stages. The 6V6 is running about 12V above the cathode, the 6Au6s are at 2.8V above cathode. and the winding is 3.15-0-3.15V
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#57 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Nick »

I would just connect the centre tap to the star earth then and see how that goes.
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#58 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Cressy Snr »

Fnished the rebuild and it sounds great. Lovely harmonic details.
Hum is there from the AC filaments, but it is no longer audible from the listening position. It can only be heard with an ear to the speaker, so the earthing scheme works perfectly.
Just shows how I really didn't properly understand grounding. Like Merlin Blencowe says in his grounding paper, you might get away with incorrect grounding, but it'll have you eventually and when it does, you won't know how to fix it.

Thanks Nick you're a gent :)

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Here is is playing. I've also changed the feet to a set of tapered 50s style jobbies I made up from a spare bit of mahogany I had lying around. It is running JJ 6V6S drivers at the moment as these are cheap and disposable. I have a pair of Brimars courtesy of Mr I, but I won't use these until the thing has been running a few days.

The amp is running stock 2A3 operating points.
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#59 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Ray P »

Good news Steve, I'm happy for you.

Ray
Sorry, I couldn't resist!
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#60 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Cressy Snr »

Ray P wrote: Mon Feb 12, 2018 8:35 pm Good news Steve, I'm happy for you.

Ray
Cheers Ray,
I really did think I'd fooked it right up. It is going to be left well alone now.

Any more experiments will be strictly done via breadboarding. It's taken 12 years for the penny to drop but, lesson learned as they say.

Onwards!
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