6B4G - The Resurrection Shuffle

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Cressy Snr
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#376 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

simon wrote: Mon Jan 08, 2018 11:21 pm It's problems like this that make me reluctant to move from an indirectly heated rectifier - the slow start hides a multitude of my sins. Even speccing a bleed resistor to draw enough current to ensure the PS always stays in choke input, and making sure the choke has enough capacity for the DC and AC components, I still can't bring myself to try SS rectifiers (which some say are superior). Just one of my hang ups I guess.
I think my particular problem was a flashover in a pesky new production, GZ34 indirectly heated rectifier.
That firework display buggered up the choke, as being directly in the line of fire, it took the full blast of the flashover. One of the two SS diodes in the hybrid bridge was also destroyed.

What I'm saying is that it wasn't the type of rectifier that was to blame, it was the fact that the valve was faulty in the first place.
More subtly, the aftermath of the flashover also resulted in a sparky choke developing, which in turn, sparked up an old Bentley 5U4G, provoking that into another firework display, destroying it and half of the hybrid bridge again.

Upping the ante with a tough Russian harp filament 5U4G allowed the amp to start up, but if I hadn't discovered the sparky choke, then it was only going to be a matter of time before the startup failed again, and blew up something else, so as Mark said earlier, it was fortunate I spotted the secondary culprit and dealt with it.

So I wouldn't worry about playing around with DH rectifiers when you have DHTs as output valves. They are no worse than an IDH rectifier, as the DHT output valves will heat up and start drawing pretty sharpish. IME there is very little HT overshoot because of the similarity in the rectifier filament type and its heat up characteristics, with those of the DHT.
They have about the same voltage drop as a GZ37.
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simon
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#377 Re: 6B4G - The Resurrection Shuffle

Post by simon »

Sorry Steve, I wasn't trying to say your problem was something different, just chuntering about my prejudices. One day I intend to try SiC rectifiers with a damper in the ground rail to secondary CT. One day.
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#378 Re: 6B4G - The Resurrection Shuffle

Post by IslandPink »

Thanks for the clarifications, Nick.
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Cressy Snr
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#379 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

simon wrote: Tue Jan 09, 2018 12:07 am Sorry Steve, I wasn't trying to say your problem was something different, just chuntering about my prejudices. One day I intend to try SiC rectifiers with a damper in the ground rail to secondary CT. One day.
I wasn't having a go Simon; just trying to clarify what the issue with my amp was. I think the moral is, that if you get a tube rectifier flashover, don't assume that it is merely a matter of replacing the rectifier and getting on with it. There could now be problems elsewhere in the circuit.

I've taken steps with the backup diode mod to the hybrid bridge, that will protect the rest of the amp, if the valve rectifier gives trouble. With the backup diodes in place, a rectifier short, will only cause a rise in HT equal to the voltage drop of the formerly good rectifier, if anything were to go wrong.

I got these mods from "The Immortal Amp Mods" published on Premierguitar.com by RG Keen, Chief Engineer of Visual Sound.

https://www.premierguitar.com/articles/ ... fier-mod-1
The one they publish on there is for centre tapped HT secondaries.

There are other articles pointed to further down the page, that may or may not be worth thinking about, but I haven't investigated those yet.
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#380 Re: 6B4G - The Resurrection Shuffle

Post by simon »

Interesting series of articles. I really should use more fuses.
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#381 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

OK, so hopefully we have the final version of the PSU, complete with the hybrid bridge backup diode modification:
Image
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#382 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

Another useful tweak to the power supply:
Image
20uF added as the first cap after the choke.
The LC filter formed by the L and the 20uF motor run cap, is common to both channels, then the 33R resistor forms an RC filter
with the two 200uF caps feeding the L and R output valves, decoupling the output stage from the stuff behind it.
The 4K7 resistors feeding the 6AU6s have been paralleled with another 4K7, so that a bit more of the HT gets fed to the input stage.
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#383 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

After much exploring in PSUD and a lot of work modifying both the capacitance and the earthing arrangements we have a better power supply.
Image

I now have the tools to do power supplies properly; silly that it has taken me 12 years to cotton on.
That's the disadvantage of being a poet in an engineering environment; you don't work like anyone else, you don't know the meaning of the word logic, you struggle to even remember the rules, let alone how to apply them, you bumble about like an airhead and take a hundred times longer to get anywhere. But I suppose, in a perverse kind of way, it's entertaining :roll:

I'm harmless though :)
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#384 Re: 6B4G - The Resurrection Shuffle

Post by simon »

Sounds like a lot of us Steve :-D
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izzy wizzy
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#385 Re: 6B4G - The Resurrection Shuffle

Post by izzy wizzy »

Hi Steve,

I have to ask you a question about the power supply. That 22R 5W jobbie feeds both channels in parallel so offers no isolation/decoupling of channels; that's not a value judgement. Some people like to combine these things right up to even sharing the cathode resistor. I'm a fan of mono (not dual mono) power supplies. Anyhoo ...

What would happen if you fed each channel seperately via say a 47R resistor?

cheers,

Stephen
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#386 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

izzy wizzy wrote: Wed Jan 17, 2018 1:34 pm Hi Steve,

I have to ask you a question about the power supply. That 22R 5W jobbie feeds both channels in parallel so offers no isolation/decoupling of channels; that's not a value judgement. Some people like to combine these things right up to even sharing the cathode resistor. I'm a fan of mono (not dual mono) power supplies. Anyhoo ...

What would happen if you fed each channel seperately via say a 47R resistor?

cheers,

Stephen
Hi Stephen,
I wanted to feed each channel separately after the cap, but didn't have anything suitable that I could use. The lowest I had as a pair was a couple of 50W metal clad, 150 ohms, last used in the long defunct 13E1 amp.
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#387 Re: 6B4G - The Resurrection Shuffle

Post by simon »

I've had a quick look through my stash whilst cooking tea Steve (shhhh don't tell the missus) and I have a couple of 47R 3W coffins, and some 22R 3W coffins and vitreous enamels. You're welcome to try them if you want.

I'm buried again but I should be able to post them to you tomorrow, or I'll leave them outside the front door for you if you want to pick them up. Bit of a trek for a couple of resistors like.
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#388 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

Don't worry about it Simon, I'm going to Parkgate retail world with the missus, tomorrow and will get a couple of resistors from Maplins.
Thanks for the offer though and thanks for looking. :)
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#389 Re: 6B4G - The Resurrection Shuffle

Post by simon »

No worries. Enjoy the shopping!
Cressy Snr
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#390 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

izzy wizzy wrote: Wed Jan 17, 2018 1:34 pm Hi Steve,

I have to ask you a question about the power supply. That 22R 5W jobbie feeds both channels in parallel so offers no isolation/decoupling of channels; that's not a value judgement. Some people like to combine these things right up to even sharing the cathode resistor. I'm a fan of mono (not dual mono) power supplies. Anyhoo ...

What would happen if you fed each channel seperately via say a 47R resistor?

cheers,

Stephen
OK, after a trip to Maplins today, I put in the 2 x 47R resistors:
Image

Works nicely and gives a better image and more width to the soundstage, slightly better treble definition. The Pink SET has now gone from great, to fookin' awesome :wink:.
Looking at the number of drawings I've done of this power supply, I'm now on the eleventh attempt to get it right. Must be a record. :D :lol: :lol: ......Crazy man.

Nevertheless I discovered a few comforting words from Herb Reichert, whilst I was researching this whole creation, he said:

If you want an exceptional music reproduction system in your home you must first develop your internal reference for natural sound. Then you must outline your aesthetic and make a series of design decisions that reflect that aesthetic. But remember, you won't be happy if you acquire your aesthetic from reviews and audio pundits. You must discover your own. Trial and error is tedious and it takes a long time to become a wizard, but I am sure you will look good in the wizard's cap.

That's my excuse, and I'm sticking to it, if anyone asks why I'm p*ssing about yet again. :mrgreen:
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