gk71 Matrix amp

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Paul Barker
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#1 gk71 Matrix amp

Post by Paul Barker »

GK71 instead of 813 because I have a new pair but all my 813’s are transmitter batterd used ones. I’ll just have to juxta position the two valves characteristic curves to ensure I’m at a suitable operating point, adjust as required.

Pretty sure I have a pair of Lundahl 1620pp. So no spending on OPT’S.

I was intending to buy input transformers for the input of the ACVII level shifting input/direct driver cirquit. Could use Steve’s Llundal’s for both amps, housed in their own box to plug into either.

So only extra spending for Matrix interstage transformers. But the way Stephie connects them I’m hoping normal SE interstages could sustitute, of which I have two options, so no spending there.
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#2 Re: gk71 Matrix amp

Post by Ant »

Paul, wouldn't those sowter 3575 input transformers in that pre I did for you a couple of years ago work in place of the 1540s?
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Paul Barker
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#3 Re: gk71 Matrix amp

Post by Paul Barker »

Yes you’re right Anthony if I configure them as such unbalanced to unbalanced they’ll do same job. I hadn’t remembered those even while I’m sat listening to them I’m sat studying input transformers for the acf2 from Sowter and I missed these, then I’m wandering about building Matrix Still listening to same preamp. There’s non as blind as them who can’t see!

It’s a very valuable friend who bothers to put himself into youre situation and help you find knowledge you’ve lost through age trials and present circumstances. You’ve helped a lot right there. I have to be honest there’s a lot of cobwebs obscuring clarity of thought about all that I once had in my foremost conscious thinking nut!

So as long as the 1620’s are where I last put them, a place I think I remember.....

So let’s stand back and look at this...... so I have three stages of audio transformers. ✔️

Haven’t studied whether I need to alter operating point for gk71 but for sure I can make a b+ around 800v with headroom choke input and regulated. The various pto’s required for 813 Matrix should all come out at levels of the shunt vr chaina controlling the final shunt regulation valves (one each channel). I.e. I’ll use my own power supply practices (which came from the fact I’ve lived and breathed Stephies pages since last century when I first discovered then that excellent award winning website. It hasn’t changed much but it remains the strongest influence upon me.

So maybe out of 1/ ACFII class A/A2 SE design aimed at 801a and SV 811-10. (same base similar needs) all easily adjustable! 2/ gk71 Matrix.

I have two of the 6mb8’s on order. Not to be confused with 6bm8! Stephie said the 6bm8’s 4 out of 5 were thin at low level and changed over to 6mb8 for 813 Matrix.

The two gk71 and bases I have.

Yes let’s put rest of bits for project 1/ in Jiffy bags for another time and build a flipping Matrix at last.

Side note, you know I was in a small room where Stephie and Broskie were talking Stephie was carrying in a big heavy amp, pretty sure it was probably the 813 Matrix back in 2003. It is such a shame atm I hadn’t known Stephie was the person in a frock with big ear rings and bright makeup was S. Bench. Bu**er! One of life’s great missed opportunities!
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#4 Re: gk71 Matrix amp

Post by Ant »

Well when I looked at the circuit it reminded me of that barbarik amp and it clicked
The 3575s are very versatile

Glad I could help
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#5 Re: gk71 Matrix amp

Post by Ant »

Also, if I remember correctly, I soldered all the TX wiring onto tag strips of some sort fixed to the front panel so you should be able to just take your wiring straight off the tag strip and out of a hole in the back panel rather than having to strip them out
The baseplate unscrews so you can have a look, you'll see what I mean
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Paul Barker
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#6 Re: gk71 Matrix amp

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I darent say publicly I considered just reconfiguring Barbaric as a Matrix, don’t tell you’re Dad.

But no worries, after I looked at the Matrix designs of Stephie it was immediately clear that the 813 design is taylored for me. Though I was drawn to the KT88 as it “sounds better than the 845 amp”. So a KT88 which I’ve never heard sound anything like as good as an 845 amp gets that accolade the old nut is processing that thought, and in the way of dyslexic mind mapping it went like **** off a shovel and landed from KT88 Matrix to 813 Matrix on warp 6!

While I’m typing this Anthony I see you’ve added instructions how to implement changes. Thank you!
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#7 Re: gk71 Matrix amp

Post by Paul Barker »

I’ve just made mention Of this desire to finally build a Matrix at my part of Intact Audio (pb Pb place). Totally neglected it and probably only posted drivel previously so if you promise to follow only what I’ve posted today onwards, why not look over there aswell just in case Stephie <3 joins in.

http://www.intactaudio.com/forum/viewforum.php?f=14
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#8 Re: gk71 Matrix amp

Post by Paul Barker »

I see Nick still having fun

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#9 Re: gk71 Matrix amp

Post by IslandPink »

Paul Barker wrote: Sun Jun 21, 2020 9:48 am
Side note, you know I was in a small room where Stephie and Broskie were talking Stephie was carrying in a big heavy amp, pretty sure it was probably the 813 Matrix back in 2003. It is such a shame atm I hadn’t known Stephie was the person in a frock with big ear rings and bright makeup was S. Bench. Bu**er! One of life’s great missed opportunities!
Interesting .. he was still Steve Bench is 2005, when I saw him at ETF. Perhaps he was dressing differently at times, even in 2003.

Does sound like a very interesting project though. I don't think I ever fully understood how it works.
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Paul Barker
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#10 Re: gk71 Matrix amp

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Don’t tell anyone but I don’t understand it either! It’ll be the first time I’ve built an amp I don’t understand hardly at all. But I’m building it from trust!

I don’t think it’s healthy to talk about Stephies journey we know so little of it. But these changes of mind and body evolve and unwrap over time.

I only mentioned it to set the scene of why it never occurred to me at the time that the man with feminine choice of dress makeup and jewellery was the authority I so much admired!
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#11 Re: gk71 Matrix amp

Post by Nick »

I see Nick still having fun
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Ray P
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#12 Re: gk71 Matrix amp

Post by Ray P »

Prompted by the earlier posts I had a quick look around at some of the various resources on Steve/Stephie Bench's work - something to follow up on when I have a bit more time. One of the things that caught my eye was a simple valve-based line level crossover.
Sorry, I couldn't resist!
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Paul Barker
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#13 Re: gk71 Matrix amp

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Yes Ray that active crossover has long been an aim of many. Never heard from anyone having built it.
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#14 Re: gk71 Matrix amp

Post by Paul Barker »

It just happens I have that experience of the early Roberts radio. Doubt its the same today.
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#15 Re: gk71 Matrix amp

Post by IslandPink »

Oh, looks like I think Roberts radios sound nice, too. Had missed that.
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