GK-71 - economical build.

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IslandPink
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#436

Post by IslandPink »

There's a few pages of struggling to design the thing here :
http://www.audio-talk.co.uk/phpBB2/view ... &start=105
....then some hardware here :
http://www.audio-talk.co.uk/phpBB2/view ... c&start=60
.. on the next page actually ..

So basically, Phil, it would be a scaled-up version of that circuit, as in all the voltages would be 1.5x higher . I did think about whether the shunt valve could still be a KT88 and it's probably still OK as they can take 800 on the anode I think ... but maybe there are other options. KT120 would give a bit more current headroom . Going to a transmitter like GK-71 as the shunt will add complexity with the direct-heated filament though, so IDHT or IDHP is still easiest .
You'd probably have to uprate the feedback (cascode) valve to something that's happier with 250 or 300v across each section too - shouldn't be too difficult .
"Once you find out ... the Circumstances ; then you can go out"
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pre65
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#437 Re: GK-71 - economical build.

Post by pre65 »

The GK-71 amps are still alive and well. :)

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#438 Re: GK-71 - economical build.

Post by pre65 »

The grid leak chokes have been delivered. :bounce:

I'm just familiarising myself with the circuit, and I notice the grid stopper (on the 12HG7) is 1K (carbon) whereas some GM70 circuits specify 220R, does that matter in practice ?

After I've woken up properly and broken my fast I'll take the top section off and examine the wiring and choke placement options. :)
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#439 Re: GK-71 - economical build.

Post by pre65 »

The modifications are complete. :)

There was a little confusion about how I had wired up the 12GH7, but I printed out the base pin data and went through it methodically to check and all was OK.

I'm now sitting in my normal listening position (with the laptop) and trying to ascertain any differences.

1) It's "better".

2) The lower notes seem a tad more"forward", not louder, but perhaps better defined and easier to follow without smothering the mids.

3) Sound-stage seems slightly bigger.

4) Seems a smidge more "clarity".

Apart from that it's just "nice".

5) there is no 5. (copyright A Ivimey)

6) I could be imagining it all. :lol:
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andrew Ivimey
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#440 Re: GK-71 - economical build.

Post by andrew Ivimey »

You added the grid choke before the grid stopper and down to earth?
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#441 Re: GK-71 - economical build.

Post by pre65 »

andrew Ivimey wrote:You added the grid choke before the grid stopper and down to earth?
Oh yes. :)
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#442 Re: GK-71 - economical build.

Post by ratbagp »

As a new member, I would like to thank this group for all the information I gleaned from your forum.

Here is a link to my version of an economical GK71 SE amp. You will notice that I used your group's indirect heater ideas as well as Alex Kitic's AC heating. It was a fun project and the amp sounds really good.

http://audioratbag.blogspot.com/2016/01 ... 17k-v.html

ray
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#443 Re: GK-71 - economical build.

Post by Cressy Snr »

An interesting read :)
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#444 Re: GK-71 - economical build.

Post by pre65 »

Yes, interesting. Thanks for posting it.

I've looked at "powerdrive" before, but never got round to trying it. :(
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#445 Re: GK-71 - economical build.

Post by ratbagp »

It is usually interesting reading how others have gone about doing the same thing you are doing but with different methods. I really enjoyed wending my way through this long discussion.

Powerdrive is well worth playing with. I must admit I found it much easier using a pcb rather than the strip boards I have used beforehand.

ray
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#446 Re: GK-71 - economical build.

Post by pre65 »

On the 12GN7 driver valve it has a cascaded CCS on the anode and a grid choke.

Possibly an impossible question to answer, but how much improvement might I expect if I regulated the anode voltage/current ?
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#447 Re: GK-71 - economical build.

Post by IslandPink »

Can you remind us of the current circuit ?
I'm not quite clear what the cascaded CCS is.
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#448 Re: GK-71 - economical build.

Post by Paul Barker »

Oh poop. Catching up here is first I realised my photobucket restricted. Would cost $39 a month to get back what was free.

Sooner or later a competitor will blow photobucket out.

Until then. I have no solution.
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#449 Re: GK-71 - economical build.

Post by jack »

Paul Barker wrote: Sun Aug 20, 2017 12:46 am Oh poop. Catching up here is first I realised my photobucket restricted. Would cost $39 a month to get back what was free.

Sooner or later a competitor will blow photobucket out.

Until then. I have no solution.
It's a major pain in the scrotum. Some forums I use have switched to imgur and there are tools out there for forum administrators to migrate images linked in posts by directly editing the forum's MySQL db, e.g. https://github.com/pastudan/photobucket-imgur

If you need those photos, best bet is to pay a one month fee (can you just pay for one month?) and get the files, mapping them to the shared URLs as you go. Then speak nicely to the forum admin and give them the mapping file as a CSV (oldimageurl,newimageurl) file.

Major, major pain. For everyone.
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Paul Barker
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#450 Re: GK-71 - economical build.

Post by Paul Barker »

I think I can download them to my device. Haven't tried yet. Then forums that host photos can host them.

You tube hasn't blocked this use yet afaik, so could put still pics on you tube and embed that?

Or just host stuff within a Google doc and instead of an embedded image post the link to a Google doc (accessible to anyone who has the link). That's probably the way forward until a competitor to photobucket springs up.
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