6B4G - The Resurrection Shuffle

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steve s
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#361 Re: 6B4G - The Resurrection Shuffle

Post by steve s »

Steve I found those 3.5 k sowter transformers last week.. I thought I had them ...just could not remember where they was.
Plenty of px4 in different types..?
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#362 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

I really do need to do PX4s :D
It would be have to be a new amp entirely. Same input and driver stage, no need to change a winning team, but I'm not about to dismantle the pink SET. :)
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Paul Barker
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#363 Re: 6B4G - The Resurrection Shuffle

Post by Paul Barker »

Just make a socket adapteR to English 4 pin and feed it a seperate heater supply and cathode resistor/bypass cap and ground.
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#364 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

Could do that. :)
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#365 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

Did a bit of belt and braces with the rectifier hybrid bridge, by doing the guitar amp tech trick of fitting backup diodes to the anodes of the valve rectifier. Apparently, it stops any power supply or power transformer wrecking nonsense, if the valve rectifier shorts or flashes over, and I have become pretty paranoid about such things lately.
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Cressy Snr
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#366 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

The startup sequence of this amp has been bothering me, ever since I finished the thing a few weeks ago. It had started with rectifier flashovers and blowing fuses. After having "fixed" this by replacing what I thought had been faulty old rectifiers, I was left with a mysterious click from somewhere when the volts came up to speed.

Some time later, it came to my attention that at the same time as the click occured, a small spark arced across the plate to cathode space, in the DHT rectifier. Protecting the valve rectifier with backup diodes appeared to cure the spark when the click happened, but the click itself stubbornly remained.

Then before I went to London this weekend, I stuck my ear to the choke then threw the on switch. The click was clearly coming from what has eventually turned out to be a dodgy choke.

All I can think of, is that the first instance of rectifier flashover, when the choke was in the positive rail, punched a micro hole or holes in the choke wire insulation, causing it to spark internally as it charged up, rendering it a short or break waiting to happen.

I replaced the choke with my other identical spec Danbury one and the click is no more.
Last edited by Cressy Snr on Mon Jan 08, 2018 7:28 pm, edited 2 times in total.
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IslandPink
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#367 Re: 6B4G - The Resurrection Shuffle

Post by IslandPink »

That was a close one ! Well spotted.
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Nick
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#368 Re: 6B4G - The Resurrection Shuffle

Post by Nick »

Remember that cap input supplies put a huge stress on the choke. The choke should be spec'd for cap input use.
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#369 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

It's a choke input supply Nick, albeit with the choke in the negative side of the rectifier bridge.

Chasing the rectifier end of the click and spark issue was a bit of a red herring in the end, but looking on the bright side, the valve rectifier now exists in a mollycoddled little bubble. It's certainly now the most pampered rectifier valve I've ever fitted :)
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Nick
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#370 Re: 6B4G - The Resurrection Shuffle

Post by Nick »

That was brain fade, it should have said:

Remember that choke input supplies put a huge stress on the choke. The choke should be spec'd for choke input use.
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#371 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

Nick wrote: Mon Jan 08, 2018 9:01 pm That was brain fade, it should have said:

Remember that choke input supplies put a huge stress on the choke. The choke should be spec'd for choke input use.
It's a pretty big choke, specced at 10H @ 400mA draw. Whether that's good enough, with a Class A SET drawing approx 170mA, I have insufficient knowledge to be able to answer. Because of this inability I have to resort to over-engineering and hope. Only thing I can say is, it doesn't buzz and it doesn't get hot. Whether that means it is specced right, I can't say.
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IslandPink
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#372 Re: 6B4G - The Resurrection Shuffle

Post by IslandPink »

I'm wondering if it's the voltage (insulating) rating that has to be overspecced for choke-input, since there's no smoothing after the rectifiers ? Haven't had chance to check this on PSUD though.
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Nick
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#373 Re: 6B4G - The Resurrection Shuffle

Post by Nick »

IslandPink wrote: Mon Jan 08, 2018 10:35 pm I'm wondering if it's the voltage (insulating) rating that has to be overspecced for choke-input, since there's no smoothing after the rectifiers ? Haven't had chance to check this on PSUD though.
Its core size and current rating. Remember that the cap after the choke looks like a short circuit at AC. Normally the choke will be wound on a bobbin and that will provide more than enough insulation from the earthed core. MJ has some equations for sizing chokes for choke input.
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#374 Re: 6B4G - The Resurrection Shuffle

Post by simon »

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.
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#375 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

Nick wrote: Mon Jan 08, 2018 10:57 pm Its core size and current rating. Remember that the cap after the choke looks like a short circuit at AC. Normally the choke will be wound on a bobbin and that will provide more than enough insulation from the earthed core. MJ has some equations for sizing chokes for choke input.
Thanks Nick,
Had a look at my copy of MJ and using the equations for my worst case scenario of a 450V supply drawing 200mA idle current, gives a peak current value of 238mA, so my 10H, 400mA choke is adequate for the job it's being asked to do.
Thanks for pointing me towards finding the right formula to use.
Last edited by Cressy Snr on Mon Jan 08, 2018 11:50 pm, edited 1 time in total.
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