GK-71 - economical build.

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Cressy Snr
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#466 Re: GK-71 - economical build.

Post by Cressy Snr »

Have a look here:
http://www.emissionlabs.com/datasheets/EML274.htm
This is where I got interested in chokes in the ground rail, whilst looking at and drooling over Emission Labs tubes.
Half way down the page is a bit of useful info on power supplies.
"Circuit 4" and the comment at the side about it, was the interesting one. Pursuing this, I ended up on Turner Audio, where there was a bit more detail.
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Paul Barker
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#467 Re: GK-71 - economical build.

Post by Paul Barker »

Nothing new under the sun. I read about it in Terman.

https://db.audioasylum.com/mhtml/m.html ... orum%3DALL

“I have spent some time studying the ways of reducing ripple with minimum filtering. That is by targeting the frequency which for me is 100hz. There are a number of means in early texts that they had to resort to before the advent of cheap electrolytic caps.

The simplest is a balancing resistor which sends the ac ripple to the end of the pi filter out of phase by 180 degrees with the original so cancellling it. This is dead easy to set up. You need a pi filter consisting of CLC, place a small value resistor like 50 ohm on the end of that and a final cap. Right from the start of the pi filter to that final cap bipass the whole lot with a 100k pot. Trim the pot to eliminate the ripple. There is a draw back the 2nd harmonic and beyond are less effectively filtered, 200hz sounds actually worse to me than 100hz and I am not currently using this method. It would work if you built a normal pi filter after it and just treated the raw 100hz ripple with this sytem in front of that. There is also the tank which would trap the ripple frequency, consisting of a choke and ac cap to ground tuned to the ripple frequency. Beyond that is the tapped choke which is similar to the above.

In other words there's a lot more to tube power supplies to explore. In my view it is worth the journey of discovery. I will never forget the experience of tuning my 300b power supply and hearing a holographic impression of Nat King Cole projected in front of one speaker, listening to one channel only. All achieved by power supply tweaking. But it's back breaking work and you tune in the best sound so subtley it's easy to drift and not realise you've lost it. Then once you've got it you find it doesn't fit in your case”
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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pre65
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#468 Re: GK-71 - economical build.

Post by pre65 »

Good as it is, my GK-71 amp has become surplus to requirements. :shock:

I've decided to make the 833a my "statement" amp from now on, so if anyone has GK-71 aspirations perhaps I may be able to help ?

The output transformers must stay (for 833a) but anything else could go. I'll advertise things individually at some time in the future.
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andrew Ivimey
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#469 Re: GK-71 - economical build.

Post by andrew Ivimey »

As far statements go I'd say 833 is pretty unambiguous! I'll have the ccs circuits if you're breaking things up. I'd like to direct compare with the anode chokes I use with Gm70.
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#470 Re: GK-71 - economical build.

Post by pre65 »

As the GK-71 works well with the ESL57 speakers, I've decided not to break them up.

I was interested in the discussions between RythmicMick and Nick re the 22R virtual cathode resistors, but try as I might I just can't get my head around the maths.

This morning I ended up with a bad headache trying to understand how much current (and hence wattage needed) the 22R resistors would pass. My brain really did hurt.

I have 4 X 50W Arcol 22R resistors, and my "gut" feeling is that (on a heatsink) they will be enough.

Am I anyway near right ? :oops: :?
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Nick
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#471 Re: GK-71 - economical build.

Post by Nick »

Whats the heater voltage?
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pre65
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#472 Re: GK-71 - economical build.

Post by pre65 »

Nick wrote: Fri May 25, 2018 12:05 pm Whats the heater voltage?
20v @ 2.7A _ 3.5A
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Nick
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#473 Re: GK-71 - economical build.

Post by Nick »

10v across 22R = 460ma = 4.6W

Not precise, but close enough for what you need.

Assuming you heater supply can handle another half amp.
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pre65
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#474 Re: GK-71 - economical build.

Post by pre65 »

Nick wrote: Fri May 25, 2018 12:37 pm 10v across 22R = 460ma = 4.6W

Not precise, but close enough for what you need.

Assuming you heater supply can handle another half amp.
That's a good point. It's one of Andrews designs, like what Mark uses for his GM70.
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pre65
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#475 Re: GK-71 - economical build.

Post by pre65 »

If Ray (ratbagp) is reading this, I sent you a PM last November and you don't seem to have read it yet.

It's about your GK71 amp.
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#476 Re: GK-71 - economical build.

Post by ratbagp »

PM sent.

Occasionally I look at the site. The 6C33C OTL looks interesting and since I have pretty well everything needed to build it,I might have a go later in the year when it gets a bit cooler.

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#477 Re: GK-71 - economical build.

Post by RhythMick »

pre65 wrote: Fri May 25, 2018 11:14 am As the GK-71 works well with the ESL57 speakers, I've decided not to break them up.

I was interested in the discussions between RythmicMick and Nick re the 22R virtual cathode resistors, but try as I might I just can't get my head around the maths.

This morning I ended up with a bad headache trying to understand how much current (and hence wattage needed) the 22R resistors would pass. My brain really did hurt.

I have 4 X 50W Arcol 22R resistors, and my "gut" feeling is that (on a heatsink) they will be enough.

Am I anyway near right ? :oops: :?
Nick did a great job of putting my head back on straight. I did a spreadsheet which might help - very happy to share if you want it.

I just plugged the numbers in for the GK-71 using Ia = 125mA.

R1 has the heater current and the cathode current going in the same direction. If you go with 22R that will see 517mA and thus dissipate 5.88W. That's a beast. Might be better using larger resistors, eg a pair of 100R, in which case R1 has 163mA and 2.64W.
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#478 Re: GK-71 - economical build.

Post by IslandPink »

Can you remind me - what was the reason for the resistor pair ?
What fil supply are you intending to use ? - because with Andrew's units I can't see the need for a virtual centre-tap.
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#479 Re: GK-71 - economical build.

Post by RhythMick »

IslandPink wrote: Tue Jun 19, 2018 3:28 pm Can you remind me - what was the reason for the resistor pair ?
What fil supply are you intending to use ? - because with Andrew's units I can't see the need for a virtual centre-tap.
Next weekend I hope to complete the build of my amps, which will use Andrew's boards. I will be using Virtual Cathode pairs. I'll let you know how I get on.
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#480 Re: GK-71 - economical build.

Post by Nick »

IslandPink wrote: Tue Jun 19, 2018 3:28 pm Can you remind me - what was the reason for the resistor pair ?
What fil supply are you intending to use ? - because with Andrew's units I can't see the need for a virtual centre-tap.
Its back to my burst of maths on another thread. Other than the removal of any remaining noise, it also ensures that equal signal current passes out of the two ends of the filament (see the maths).
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