Colin's gm70 Boat Anchor monoblocs

What people are working on at the moment
Darren
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#16

Post by Darren »

Paul Barker wrote:What are you saying about my building Darren? I'm offended :!:
Not at all Paul.....Open plan is all the rage these days I believe... :D
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andrew Ivimey
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#17

Post by andrew Ivimey »

You know it takes two of these very smart amps to Tango!

Oh yes, Colin has a pair for stereo and that reminds me ... just how big are so many of Nick's realisations.

Still, if we wanted small we'd relish BC109s.

I'm in the doghouse tonight; not quite sure what I've done wrong but I will find out (to my cost) eventually.

All things come and go - the point is not to try to hang on!

mumble mumble mumble and my new virgin.net telly thing doesn't work properly.
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Greg
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#18

Post by Greg »

Hi Colin,

Lovely professional build. Well done. I'm looking forward to the final pics when the whole chassis is stuffed and I really hope you don't have any issues with hum etc. If you do I hope that'll be easily sorted. Frankly, this design visually deserves to suceed aurally. Your outcome could easily be of interest to many, regardless of panel speakers. Please keep us fully updated as you prgress.

Best wishes,

Greg
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Nick
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#19

Post by Nick »

Andrew: They are the new potting enclosures AE are using, they used to make their own, but decided they were better off using their time to make transformers and buy in the enclosures.

Greg: We will have got something wrong if the hum at the output is anywhere near 1mv, I would hope for less than 0.5mv
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andrew Ivimey
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#20

Post by andrew Ivimey »

Philip @ Bluebell is now getting the Hammond range of transformers potted. They do look neater!
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Toppsy
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#21

Post by Toppsy »

Hi Greg,
Your outcome could easily be of interest to many, regardless of panel speakers.
This was the very reason I and Nick opted for the conventional output trannies so the amp could be used with conventional cone drive speakers as well as panels.


I'll post more pics when the innerds have been installed.

Colin
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#22

Post by simon »

andrew Ivimey wrote:Philip @ Bluebell is now getting the Hammond range of transformers potted. They do look neater!
Is this an extra over Andrew, or something that Hammond are now offering? Any photos?
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andrew Ivimey
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#23

Post by andrew Ivimey »

I'm afraid you will have to speak to Philip. He was wrestling with a new project (and so was I) as we spoke so didn't take in many of the details.

Are you up to anything wicked, Comrade Simonski?
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#24

Post by simon »

Comrade Simonski is dabbling in very uncommunistic practices - we've just put the house up for sale. So all my mdf and clipleads have been packed away for a while.

But the phono stage is slowly being fettled, and I've plans to build the mythical ECL82 spud amp, and a Steve Bench type series reg experimental PSU (as I like to run before I can crawl :D )

How's your devil's work, sorry PP amp, coming along? :wink:
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Dave the bass
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#25

Post by Dave the bass »

simon wrote:...and I've plans to build the mythical ECL82 spud amp,
Simon,

BOBA Mk2 was an ECL82 that NickG drew up for me. I learnedededed about Parafeeding too on that project. Mine was good but not as good as 6EM7 in my experience.

Just IMHO 'n all that loike.

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#26

Post by Rob »

That is one pair of sexy amps you have there Colin :D
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#27

Post by simon »

Thanks for the perspective Dave.

The amp I want to try is James D's circuit - he reckons it's the best sound he's ever achieved with his Quasars. Not sure why he isn't using one now, perhaps the quest to achieve something better? There's not a lot to it in the way of resistors and caps but there are a few quids worth of chokes unfortunately.

Of course, what I didn't say was after the ECL82 spud amp I want to build a 6EM7 spud. :wink:
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andrew Ivimey
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#28

Post by andrew Ivimey »

well a 6EM7 spud I found was not bad but prefer the 6DN7 - why? I'm biased! I've just finished a mk1 version of a push pull 6DN7 amp. I've ordered some booteek coupling caps but ultimately this rather odd design is limited, I suspect by the incredibly tiny Hammond output trafos.

The little amp sounds very good and I appreciate you don't need big outputs for decent pushpull but ...

Anyone got a decent pair of 8k or 10k push pull outputs?
Last edited by andrew Ivimey on Fri Jul 27, 2007 2:28 pm, edited 1 time in total.
Gerry
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#29

Post by Gerry »

Colin/Nick

They look great.
I like that copper top sheet look! Reminds me of some moster Marantz Project T1 with its 300Bs & 845s....just need the little amp meter!

If you don't mind me asking, what are the specs?

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Gerry
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Nick
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#30

Post by Nick »

Ok,

Its a c3g pentode, with a regulated screen supply, driving a 300b. 300b via 1:1 amorphous core interstage to gm70. gm70 running at 980v 120ma. Sounds far far better when using copper gm70's and despite previously thinking you had to run the Cu ones at lower diss they are happy at 120w diss (russion supplier confirmed the specs are the same). Output transformer again amorphous.

All iron by AE (two power TX's two chokes, interstage and output).

300b and c3g are powered by a valve series regulator (6080+ecl82+od2). gm70 supply is CLC with SS rectifiers.

300b and gm70 are heated with tent lab DC supplies. The prototype used a lash up regulated supply for the gm70 but the tent labs supplies are a far better solution, and don't need adjusting when changing valves.

On the bench it seems good for about 36w at 3% distortion, mostly 2nd harmonic, the real one is at a slightly better op point so I expect a little more power, but for Colins speakers its "enough" it will drive the panels to there limits. The interstage with the 300b behind it is able to drive the gm70 grid at lesat 20v into A2, there is scope for a bit more but thats working it very hard and up to 40w or so output.

I have been testing and tweeking the prototype using a pair of NS1000's as my OB's were just too easy to drive for it.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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