We are delighted to announce ...

What people are working on at the moment
RhythMick
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#1 We are delighted to announce ...

Post by RhythMick »

... the arrival of a new build in the early hours of Saturday morning.

Weighing in at some arm stretching weight, this is essentially a SE version of my all-DHT, interstage coupled PP amp with the exception that it's 26-71A-300B instead of 26-71A-45.

It's a rough build for now, the PSU especially. Tribute OPT and interstage tx with Lundahl input tx converting from balanced inputs. Each valve circuit PSU is galvanically isolated with separate mains tx, rectifier and LCLC. All chokes are Lundahl, all caps are Mundorf Tubecaps (no electrolytics). Err actually the rough PSU has electrolytics in the first cap positions.

There's a hum which looks like a mix of 100 and 200Hz. Audible and needs chasing down but doesn't detract from the sound of the music, which to be honest is way better than I expected at this stage. I'd almost be happy to bring this to the next Owston!

Name not yet decided...
IMG_20190915_101857_compress13.jpg
steve s
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#2 Re: We are delighted to announce ...

Post by steve s »

Look forward to hearing this one Mick
I'm planning a similar build but with the da100/ 4300a as output.
Just a lack of time at the mo.
The tube manual is quite like a telephone book. The number of it perfect. It is useful to make it possible to speak with a girl. But we can't see her beautiful face from the telephone number
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izzy wizzy
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#3 Re: We are delighted to announce ...

Post by izzy wizzy »

Congratulations. Is this for you or someone else? Be interested to hear what you think in comparison to your PP amp.

Cheers,
Stephen
RhythMick
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#4 Re: We are delighted to announce ...

Post by RhythMick »

Thanks both. Definitely for me. It started out as something to play while I make the improvements to my PP amp. I had the good quality components left over from early SE builds so while the build is rough the components and layout were as good as I could make them.

Compared to my PP amp as it stands today it's easy better. Much much more soulful, but also faster, more attack and snap, more "in the room".

However it's not quite up to Geoffs PP. Damn close and much better than I expected. The temp PSU is the same one I ran Geoffs on while I built his proper PSU and I know there are huge improvements to come from replacing that.

So it's a keeper. Not sure where but...
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IslandPink
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#5 Re: We are delighted to announce ...

Post by IslandPink »

Shoddy !
Nice valve boxes too - I guess they were just hanging around and you had nothing else to prop underneath ? :)
I bought some 71a's years ago to drive 300B but ended up using 45's.
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RhythMick
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#6 Re: We are delighted to announce ...

Post by RhythMick »

45 driving 300B sounds interesting
RhythMick
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#7 Re: We are delighted to announce ...

Post by RhythMick »

It's been a bit of a roller coaster with this SE amp over the past couple of weeks. I initially had a weekend of glorious listening with friends (ignoring the underlying hum & buzz during the quiet passages), but on the Sunday evening the noises became pronounced and we reluctantly had to switch off and take it back into the lab (spare bedroom grabbed after son 1 went to Uni). It's been played again since, with the hum/buzz staying fairly low for a few hours then rising.

Last weekend I made a determined effort to track it down. With the 26 & 71A valves (and their rectifiers) out and the 45 grids grounded, the noise was still present. Good start. Can only be on the anode or the cathode. I used my Picoscope to show the filament voltage from Andrew's DC boards and voila - nasty sawtooth ripple measuring around 180mV.

A lengthy period of investigation followed with boards being numbered for identification, repeat experiments, screen shots from the scope and occasional mails to Andrew (I tried to keep them down, honest Andrew). At one stage we thought the pre-reg wasn't working, but eventually I cottoned on that the results were inconsistent with the same boards and that the ripple was always worse with 2 boards driven from the 2 x 7v toroidal.

I was using the same make of toroidal in the lab which was in the amp. I wonder. Replaced the toroidal with a different make and hey presto, noise gone.

Amp rebuilt and taken downstairs. Turned on. Something wrong. I don't get it, the valves and rectifiers are lit ? What's up ? Oh wow, it's on and SILENT!

Better, it stayed silent through 4 hours of glorious listening last night. Heaven.

Thanks Andrew for your encouragement and support.
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Ali Tait
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#8 Re: We are delighted to announce ...

Post by Ali Tait »

Good result. What were the toroids, so we can avoid them?
RhythMick
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#9 Re: We are delighted to announce ...

Post by RhythMick »

Ali Tait wrote: Sat Sep 28, 2019 9:08 am Good result. What were the toroids, so we can avoid them?
https://uk.rs-online.com/web/p/toroidal ... s/1243873/

They are spec 1.78A per secondary. The 300B draw 1.2A so should be fine.

It's possible that the fault is mine of course. I built the amp upside down and realised after the first weekend that the DC boards were upside down, with heat rising up to the transistors and through the board to the toroidal. I was concerned the fault may have been temperature damage and it's possible that's true, especially as it came on after a few hours. However the toroidal I tested on upstairs was a different one of the same model.

I plan to repeat the measurements with the 2 boards upstairs comparing the blue toroidal with alternatives.
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IslandPink
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#10 Re: We are delighted to announce ...

Post by IslandPink »

They don't look like enough VA, once you've bumped up the voltage with a 4-diode bridge.
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izzy wizzy
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#11 Re: We are delighted to announce ...

Post by izzy wizzy »

I find this helpful
https://www.hammfg.com/electronics/tran ... /rectifier

Your 1.2A DC equates to about 2A AC when bridge rectified into a cap.

Cheers,
Stephen
RhythMick
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#12 Re: We are delighted to announce ...

Post by RhythMick »

izzy wizzy wrote: Sat Sep 28, 2019 3:16 pm I find this helpful
https://www.hammfg.com/electronics/tran ... /rectifier

Your 1.2A DC equates to about 2A AC when bridge rectified into a cap.

Cheers,
Stephen
Ok thanks both. I was calculating required VA based on transformer voltage x valve current. I'll read that page properly later but it looks like for a 1.2A valve the transformer current needs to be at least 1.2/0.62 = 2A
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IslandPink
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#13 Re: We are delighted to announce ...

Post by IslandPink »

Yes, consider that after the rectifiers & first cap, the voltage is probably over 10V, and you lose half of this through the two stages of regulation - so half of your VA goes up in heat on the boards.
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RhythMick
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#14 Re: We are delighted to announce ...

Post by RhythMick »

In terms of expected power lost through heat on the boards, I was working on (7 x 1.414 - 5) x 1.2 which is 5.9W.

I get that the RMS voltage of 7v is 9.9v which (less drop over the Schottky diodes) goes into the cap filter. I guess what I wasn't taking into account was the peak current draw to refill those caps, which results in a higher VA needed. So instead of multiplying by the average draw of 1.2 it needs to be the peak current draw, 1.6 x average.

So in short, double the VA to be safe.

Lesson learned, thanks.
RhythMick
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#15 Re: We are delighted to announce ...

Post by RhythMick »

It's it even worse than that, or am I over thinking this now?

Peak current flow will be when the secondary is at peak voltage? If that's true then peak VA for a 7v secondary with an average current draw of 1.2A will be...

7V x 1.414 x 1.2A x 1.6 = 19VA

Is that right?
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