Colin's gm70 Boat Anchor monoblocs

What people are working on at the moment
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Toppsy
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#31

Post by Toppsy »

I managed to pick up the chromed copper top plates late Friday after work and have to say I'm a little disappointed. They show up every minor flaw and are a liitle too bright for my liking but it's done now so no going back. Anyone know how to dull back the chrome shine?

So having spent Saturday morning and most the afternoon finshing off the second cabinet and one tag board with the power supply front end the daunting task of takling the innards had come.

As my skill and understanding in this type of high powered and high voltage amp is very limited, I took the cowards way out and have just spent most Saturday night and Sunday over at Nick's place assisting him wiring up the first monoblock. Well I say assisted. I held the end of wires for him to tin and cut wires to length and trimmed the ends. But it was an education watching the master at work.

I'll take some pics tonight but only for a talking point mind you. I'm sure we'll get lots of comments about Rats nests and the like, in true Nick build fashion. But what the heck non of it is seen and the end sound result is all that matters. I did force Nicks arm up his back a little to use some cable ties all in the interests of neatness you understand and also to help me duplicate all the runs on the second amp.

We fired it up late last night and it sounds fine and measures well. A little hard sounding perhaps but Nick put this down to the typical Fostex driver sound if they are overdriven or driven really hard and the newnes of all the components and tubes. He does not expect this when they are played through my ESL's. This amp is really far too powerful for his OB's in fairness.

We have a little 50Hz hum but nothing that cannot be cured. We simply ran out of time last night to sort this.

Unfortunately one of the Tentlabs 300b heater modules died on us so this will hamper the final build and testing the second amp until a replacement is received.

Colin
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Toppsy
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#32 The Rossendale lighthouse!

Post by Toppsy »

Well,
I couldn't wait any longer for the second monoblock to be wired up so I dug out some old 60's mono LPs and hooked the finished amp up to one of my Acorns.

I could not detect the hardness to the sound that was apparent at Nicks through his OB's, so I have put this down to the fostex drivers. The recordings are not the best but the sound was impressive with very clear vocals and instrument seperation. It's odd really listening to a single speaker that is offset to one side of your room. Anyway the amp warmed the room up rather nicely and gave a wonderful orange glow from the copper anode gm70.

Couldn't resist a pic, but unfortunately my digital camera focused on the brightness of the gm70 and makes the whole thing look like a lighthouse beacon. It doen't glow that bright, honestly! Must work out how to set up the thing in fully manual mode.
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Cressy Snr
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#33

Post by Cressy Snr »

Hi Colin
That looks fantastic.

Strange how these new amps of yours and mine are having such a painful birth. What with my amp's impression of a canal barge and your Tent Labs heater supply failure.

Have your results so far been worth it.?

Steve.
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#34

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Hi Steve,

In a word YES. Like you, it's a lot of time and effort we put into these things but I have to say so far I have a big cheesy grin from ear to ear. If memory serves Nick started the development of the design back in December and I've spent around 3-months of spare time sourcing the components and building the chassis etc. To think that the nearest equivalent commercial amp to this is the DeHavland gm70 monoblocks and these retail for around $9K USD!

The nice thing I like about this amp is how quiet it is on switch-on. Not a hint of a TX thump or anything, just the sudden lighting up of the gm70 followed by a slight squeal like sound as the OD3's light up with their wonderful irridesance purple glow to contrast the orange glow the gm70.

We have a slight hum issue to sort (or should I say Nick is to sort) and I have the daunting task of following Nick's internal wiring of the first.

Hope you sort your instability problems quickly. At these you have the guys with the know-how on this forum to help. Good luck.

Colin
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#35

Post by Paul Barker »

I sorted my instability problems by evicting some drug dealer tenants, and leaving my job as a nurse.
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#36

Post by Darren »

And how did plumbing fit into this pattern?
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#37

Post by Toppsy »

As promised guys some pics of the innards.

One down, one to go. Unfortunately this will be delayed waiting the return of the repaired/replacement 300b TentLabs heater supply module.
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#38

Post by Nick »

I have just been running a few numbers from one of the monoblocks.

First I have to admit defeat, I can't get the hum down as far as I hoped, its at 2.5mv at the terminals, hopefully good enough. I could maybe get lower with DC of the c3g, but I don't think its a major issue. I could also get the hum down by altering the gain structure of the amp, if I used a 2:1 interstage, it would be lower, but on the breadboard stage, that didn't sound as good as 1:1 as the driver stage and pentode were havig to work harder for the same output.

Freqency response (-3dB points) at 5w output is 18Hz and 42kHz

THD at 1w output (into 8R) is just under 0.1%
11w is 0.6%
1% is reached at 23w
1.8% at 35w
and 3% at 43w

So I am happy with the power output, the goal was to build a high power SET for Colins stats.

I am very happy with the way the power supply has worked out, the 300b is running at 50ma and 330v, and the gm70 at just the point I was aiming at (higher than the breadboard) B+ is 1100v, so the gm70 is running at 112ma @ 1020v so 115w diss.
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#39

Post by Cressy Snr »

Must be those big power supplies Nick.
The 6AS7 pp, which of course has a PSU way out of proportion to its size has 2.8mV at the speaker terminals.

Your PSUs on the GM70s must be 3 times the size of even this, so I would have thought that at 2.5mV you would be doing remarkably well there.

Steve
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#40

Post by Toppsy »

Well the amps are now finished but for one replacement Tentlabs unit, which should arrive this week. So hopefully if Mr Posty gets his finger out I'll be able to fire them up in Stereo this weekend. To date I've only ran the one fully wired up amp in Mono.

Must get on with those 6m interconnect leads and the two small Oak tables the missus wants me to make to sit each monoblock on!

For those interested Nick has posted some pics of the final amps here:

http://www.lurcher.org/nick/images/gm70/

Report on how they sound to come...........

Colin :lol: :D :D
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Paul Barker
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#41

Post by Paul Barker »

You can definately build my final amp at a fee when I decide on it.

You could sell those for 6 grand or so couldn't you?
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Nick
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#42

Post by Nick »

Well the remaining question is how do they sound, I have only heard them in mono on my OB's, so I hope it at least lives up to the breadboard version.

Time will tell.

I do have to admit to being very chuffed whith them though. As you know Paul, I have been trying to build a GM70 amp for several years now, I must have had at leaet four runs at it :-).
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#43

Post by Dave the bass »

I've said it on WD and I'll say it here, they look bloomin' amazing.

Congratulations you two, brilliant work.

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#44

Post by Cressy Snr »

I'll second that :D

Steve
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al newall
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#45

Post by al newall »

I just wish i had the patience to produce something like that.
My first attempt at building a valve amp resulted in something that looks ok from a distance, but really the "bling" type appearance is only there to detract from the shambles underneath.
I would be far better suited to building stuff on breadboards.
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