6B4G - The Resurrection Shuffle

What people are working on at the moment
simon
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#406 Re: 6B4G - The Resurrection Shuffle

Post by simon »

jack wrote: We've discussed this at length.
Yes I know. I have one of the boards Nick had fabbed. I was more interested in the 3kV rating really, and lack of resistor.
Cressy Snr
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#407 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

simon wrote: Mon Jan 22, 2018 11:35 pm Curious. And a bit frustrating.
Yes, sometimes you just get something that just refuses to work right.
I've some to the conclusion that the basic cause of the rectifier valve eating behaviour was the current draw. I had two problems.
The first was the amount of capacitance and once that was solved, the other problem came up, which was caused by the fact that it is three stages, the first one being a pair of med mu 6AU6s, drawing a total of 18mA, then a pair of small 7-pin power valves, drawing a total of 50mA, followed by the output stages, drawing 140mA in total, then a set of bleeder resistors drawing another 40mA through the supply.

Grand total 248mA - right on the limit of a GZ34, above the limit for a 5U4G and again right on the limit for a 5U4GB.
I refused to risk my Mullard GZ34 or 37, or my MOV U52, and the modern production ones I used, are now all fooked!

With the benefit of hindsight, it ought to have been monoblocks, then there would have been less problems, but a stereo chassis was all I had and funds were seriously lacking, so the amp is a parts bin bitza.
But, the SS recification in the current stereo situation is pretty bomb-proof, so no real damage has been done, apart from a bit of dented pride.

The sound remains top notch and the amp will run "normal" 2A3s on its 375V HT tap, so comparisons with the 2A3-40 can be done on a level playing field, without fear of over-dissipating the conventional ones.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
simon
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#408 Re: 6B4G - The Resurrection Shuffle

Post by simon »

I'm watching with interest Steve, hope you've solved the problem. I've just soldered up a SS bridge for the 45 amp for the kitchen so I'm going to a whirl. I've still put a damper diode in the ground rail. Just can't help it. Maybe once I've got it up and running I might short it out.
Cressy Snr
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#409 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

simon wrote: Tue Jan 23, 2018 9:07 pm I'm watching with interest Steve, hope you've solved the problem. I've just soldered up a SS bridge for the 45 amp for the kitchen so I'm going to a whirl. I've still put a damper diode in the ground rail. Just can't help it. Maybe once I've got it up and running I might short it out.
The 45 amp should be fine with a SS bridge. The filaments on these types of valves come up to temp within a couple of seconds. Way back when they only had DH valves, the DH rectifiers were only slightly slower than a modern SS one.

I reckon, the problem has been eliminated. A bit of sloppiness on my part was the cause really, but no real harm done.
Over the rest of his week, we'll see if anything else rears its head. I'm pretty confident nothing will.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Cressy Snr
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#410 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

simon wrote: Tue Jan 23, 2018 9:07 pm I'm watching with interest Steve, hope you've solved the problem.
Well I think we have cracked it. No bother at all now.
Tube rectification would, I think, have worked perfectly well with a monoblock, but a stereo setup seems to have been a bit too much for it.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
simon
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#411 Re: 6B4G - The Resurrection Shuffle

Post by simon »

That's great Steve :-)
simon
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#412 Re: 6B4G - The Resurrection Shuffle

Post by simon »

simon wrote: Mon Jan 22, 2018 11:39 pm Hmm. I read somewhere t'other day that SS bridges should have the rectifiers snubbed and a 0.01uF 3kV rated cap across the secondary (at this sort of HT) to deal with switching spikes. Didn't get any further researching this but maybe someone can comment?
PSUD really is useful! I had a little play and I think I understand now what was meant - it wasn't the secondary of the trafo but the other side of the bridge.

The first piccy shows a simple cap filter - the voltage rating of the rectifiers is fine
1 - C filter.png

The second only changes the C filter for a LC filter and the voltage rating of the rectifiers is momentarily exceeded
2 - LC filter.png

The third has a .1uF cap before the LC filter and it sorts the over-voltage out
3 - cLC filter.png

Finally, an RC filter (rather than LC) works fine in this case
(Hmmmm, for some reason it won't attach)
simon
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#413 Re: 6B4G - The Resurrection Shuffle

Post by simon »

This is the piccy of the RC filter
4 - RC filter.png
Cressy Snr
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#414 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

Yes, I've got a .22uF spike snubber before the choke. No nasties. This spikiness was one of the problems that destroyed three of my valve rectifiers.

The only thing thing now bothers me about the amp is that blasted hum from the AC filaments. The pink SET is currently out of use until I get DC on them. Being used to the deathly silence of my NVA A20, the heater hum, though low in level, is annoying, as is the mechanical hum from the choke and power transformer.

The chassis, being thin sheet steel, is too resonant. I might try black acrylic sheeting for the top plate. I used acrylic on an earlier PX25 (Chinese) amplifier and it worked a treat; nary a peep out of the bodywork. That's the way to go I think.

I'll have it ready for Owston. The design is far too good not to give it the chance to give of its best.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
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IslandPink
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#415 Re: 6B4G - The Resurrection Shuffle

Post by IslandPink »

Bit late to comment but interesting to hear you explain the rectifier failure due to basic overload - I was a bit mystified about this before.
Glad I went for 2x 6DL3's in the hybrid rectifier for my T-Rex power supply - each of those babies can handle 400ma.
"Once you find out ... the Circumstances ; then you can go out"
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