GK-71 - economical build.

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andrew Ivimey
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#421

Post by andrew Ivimey »

I called in at the home of the old gentleman of Ashen yesterday, ostensibly to compare my now aging GM70 and Philip's new enormous Gk71 ...

Pretty much the same amp except:

1. Phil's is well built - even his nuts & bolts are stainless.
2. Phil's is fully mono-blocked - mine shares a (new) PSU
3. I don't know what Phil's HT was but mine was 666 volts. The numeric symmetry appeals... Instead of damper diodes I am using 10 amp, 1000 volt diodes into CLC. Phil is using damper diodes into choke loading PSU.
4. Phil's 12GN7 is cascode CS loaded - mine is the 'trad' choke load.
5. Phil's 12GN7 is DC heated - mine is raw AC.
6. I'm using 2 computer laptop switched mode power supplies to drive the GM70 heaters - Phil is using... er, can't remember but it is pretty sophisticated and snazzy.( Its T'other Andrew's heater supply of course - very impressive!!! ) We have both used, in the dim and distant past, massive toroids, bridge rect and big caps for heaters.
7. I'm using 7K output trafos and Phil is using 5Ks.
8. Phil's cathode resistor is 820R and mine is 560R
9. I have no idea what caps Phil has here there and everywhere and I've forgotten what I am using except that the coupling cap on mine is rated at 1200 volts - which is sensible.

Phil's amp sounds big, lovely image, and 'warm'. Mine is big, lovely image and crisper, sharper. Using a 465volt power supply on my amp adds a littel 'warmth' (back at home this morning) The sonic difference here is not huge.

Both can play too loud for comfort with no apparent audible distortion in Phil's living room. I would guess some would prefer my amp and some would prefer Phil's. This has been my main listening amp since before the last Owster I attended which is now at least 18 months ago.

Twill do for me ( not bad at all) and now I'm off to get on with several other projects gathering dust in the wings.

(three or so years ago I did an SE version with one channel using GK71 and one channel a GM70. It seemed at the time, if I remember, that a trioded GK71 was not as loud as a GM70 but otherewise sounded the same - but this might not be true ;-)
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#422

Post by simon »

Very interesting as Mike might say. Assuming (and it's a big assumption) that the output valves sound the same what might you put the difference down to?

The obvious candidate would be the driver loading but my very limited experience suggests that they would be the other way round and the CCS might sound crisper and sharper than the choke.

Perhaps it's the filament supply, or maybe the SS v HS diodes?

Or maybe it's just the valves?
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pre65
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#423

Post by pre65 »

Thing is, how can one tell why there is a different slant to those two mighty fine amps ?

Hopefully at the next Owston we can ask the assembled mass of experts. :wink:

If Simon and Andrew can attend there will be 3 to analyse. :lol: :lol:

Or perhaps 4 if Mark J gets busy. :)

Or even 5 if Colin brings his monoblocks.

A GM70 / GK-71 shoot out.

Question for our Thomas - have you got a start date for your GM70 amp .
:?:

PS Anyone else thinking of GM70 or GK-71 ?
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
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shane
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#424

Post by shane »

Thinking, but not likely to be doing in the near future.
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Nick
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#425

Post by Nick »

I would guess this will have an effect:

7. I'm using 7K output trafos and Phil is using 5Ks.
8. Phil's cathode resistor is 820R and mine is 560R
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#426

Post by pre65 »

Thanks Nick. Perhaps I should have tried mine on the 4 ohm tap = 10K.

Here is what Andrews amp looks like. The mains transformer in the power supply is a really meaty Sowter.

The power supply is on a breadboard, literally a breadboard.

Image
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#427

Post by IslandPink »

I recognise that transformer ...... an old friend ....
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andrew Ivimey
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#428

Post by andrew Ivimey »

Oooooo you are coy. Let's have a butchers at your massives. ..
Philosophers have only interpreted the world - the point, however, is to change it. No it isn't ... maybe we should leave it alone for a while.
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#429

Post by simon »

Nick wrote:I would guess this will have an effect:

7. I'm using 7K output trafos and Phil is using 5Ks.
8. Phil's cathode resistor is 820R and mine is 560R
Yes, probably. But interestingly right back on post 1 of this thread James was proposing 5k OPTs. I can't remember now as it's a long time since I did the numbers but my recollection is that the a trioded GK-71 aint the same as a GM70 and the two different operating points give broadly similar performance, or as similar as two different valves can be if you know what I mean.

Anyway, I think it's the stainless steel fixings that make the difference :-).
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andrew Ivimey
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#430

Post by andrew Ivimey »

Nah I don't nowotchumeen, but that's not the point. Stainless steel though, phroarrrrrrrr eh. Phil's uber design deserves an outing, as does you'res our Simon.
Philosophers have only interpreted the world - the point, however, is to change it. No it isn't ... maybe we should leave it alone for a while.
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#431

Post by pre65 »

I think Jamed D originally specified the GK-71 cathode resistor at 900R.

When I did my calculations 820R was "about right". :)
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#432

Post by IslandPink »

andrew Ivimey wrote:Oooooo you are coy. Let's have a butchers at your massives. ..
I have noideawatchaonabout Andrew. There ain't no Santa Claus on the evening stage . I think Mr. Newton has had enough ?
GK-71 @ 5k sounds a good value. 7K for GM70 - also good - given the difference in anode resistance ( 1.2k, 1.7k ? ) . Higher is always OK and reduces 2nd harm and some of the others too , but less power.
Would like to build ( eventually ) or help someone build ( sooner ) a shunt-reg T-Rex-style supply for a GK-71 or GM70 at about 700V . Would be the icing on the cake for these sort of projects . Probably less outlay and real-estate than these monoblocked projects but better .... :oops: oops!
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#433

Post by simon »

IslandPink wrote:Would like to build ( eventually ) or help someone build ( sooner ) a shunt-reg T-Rex-style supply for a GK-71 or GM70 at about 700V .
Now that would be worth hearing.

And I'm using 5k OPTs with GM70 cos that's all I have. I'd love to try something higher one day but it's all brass. I can't hear any difference between 4R and 8R. Must be my cloth ears.
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pre65
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#434

Post by pre65 »

Trust YOUR ears. :)
The only thing necessary for the triumph of evil is for good men to do nothing.

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pre65
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#435

Post by pre65 »

IslandPink wrote: Would like to build ( eventually ) or help someone build ( sooner ) a shunt-reg T-Rex-style supply for a GK-71 or GM70 at about 700V . Would be the icing on the cake for these sort of projects . Probably less outlay and real-estate than these monoblocked projects but better .... :oops: oops!
It would be good to see some details on that Mark.
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
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