GM70

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Paul Barker
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#16 Re: GM70

Post by Paul Barker »

It’s Ok I’ve trod this path before and have a plan.
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#17 Re: GM70

Post by Paul Barker »

Image

This layout only just works. Hammond 432 x 203 x 76. Filament transformers within the 75mm and laptop 20v supply with resistor relayed out when valve warm which I might do automatically with an ntc sensing bottle heat.

Hammond steel chassis is a budget constraint, £41 each. Horses for courses. All iron I own already.

Big white area reserved for Tribute output; a beast.

No room for a gk71 base and a gm70 base. So I’ll build it as a gm70 amplifier. I must be single minded this time. The 801a amp has far too many unused valve based it looks like a Swiss cheese from above.
Driver on the AE Europe bifilar interstage may be 2a3 or 6v6 triode. Shown is a 45 but AE not ideal then. With 3 transformers I can’t let any stage to go with insufficient inductance. 2a3 or 300b probs.

First stage triode D3a as it’s used in 801a amp 150v 15mA.

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#18 Re: GM70

Post by Paul Barker »

Oh dear, my pair of GM70 valves with silver collet valve bases are in Russia, and tracking has said since 26th “arrived at local distribution centre Russian place name I defo can’t spell !”

With Russia at war I’ll not hold my breath. Sh!t US$160 I shouldn’t have spent in first place.

I have two gk71 bases. Might turn into gk71 thread!

See if those bases arrive. Plenty else to do for now.
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#19 Re: GM70

Post by Paul Barker »

Anyway, staying on the GM70 trail with blinkers on, I know gm70 and I know copper plate is extremely close sounding to 212. So I’m fixing on that. Now I have used 212’s so why not make it with those. Mainly because I want future affordability. But I’m not saying in my impending retirement I won’t produce a marketable Uber expensive 212 amplifier sold with valves. Which circuit may not differ much from this!

which valve that will work optimally with AE Europe bifilar interstage, 3kohm 30mA?

Sovtek 2a3? I promoted these to you 20 years ago, they are monoplate, whereas most Western nos is biplate. The Sovtek mono is today £152 a pair. A truly viable option.

But I have NOS PX4’s. As the sound of the driver is influential, they’re best I have that fits criteria of interstage. But if you want to follow just get Sovtek 2a3.

I see now Brimar are trying to resurrect px4 production in UK. At the moment they have a product but they aren’t making it themselves yet and it has ux4 base. That would be PSVaine then I imagine. £445 a pair
https://brimaruk.com/product/brimar-tp- ... ched-pair/

It’s a big step in the right direction. They hope for production in UK with volunteers. Sounds like a plan. I wish them well.

For the moment I’ll use the best of what I possess the non globe px4’s. But if you are following, I’d suggest Sovtek 2a3 while there are still stocks, but if they’re ever made again expect Chinese rebadged.
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#20 Re: GM70

Post by JamesD »

Hi Paul,
I find the russian 6C4C to be as good as the Sovtek 2A3 and both are happy at the same op. points with 5c4c being much cheaper. As you know from the Sovteks they are not an exact match to the US 2A3s and benefit from running at there own optimised op point. Both have slightly higher anode resistance (1000R versus 800R) so that needs taking into account and both sound better loaded with 5K rather than 3K or 3.5k in my experience.. YMMV - probably will :D
ciao James
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#21 Re: GM70

Post by simon »

FWIW I prefer RCA dual plates to the Sovtek monos, and the JJ 40W jobs to the RCAs, thought they're not really 2A3s.
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#22 Re: GM70

Post by Paul Barker »

Thanks both. I’ve learned a few things from both.

I may not have mentioned yet I’ve decided to go PX4 instead.
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#23 Re: GM70

Post by simon »

You're just showing off :-D
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#24 Re: GM70

Post by Paul Barker »

I guess, or is it sharing knowledge and experience with people growing up into the knowledge? people today may never have hears of a px4.

Truth is between the two, and I know Simon was just having fun. We all have egos, and were feeding them aswell as sharing for altruistic reasons which then also feed our egos. So were happy for a moment or two, then its same old same old.

The decision is because the driver valve is such an influence on the output tube sound. I was disappointed that the interstage transformer would produce a bass role off in a series of three transformers, so they all have to nail the bass otherwise the problem gets out of hand, as its additive three times. (addendum on something I said earlier about AE Europe they’re limited to 30 mA NO they're clearly happy at 50mA from the upturned picture above where I was marking out the possible monoblock chassis) When I was building the 801a I measured the inductance, though Ive forgotten the number, it didn't excite me for use at 10k, whereas the hammond is ideal. Without wanting to repeat same concept in same post, reason is obvious.

Otherwise I wouldnt use PX4, I would use PX25 and I still have two balloon’s just have no px4 balloons. (globes are the USA term, Jim Fish and other old timers when I started called them ballons, and actually the British valves look more like Hot air balloon’s in shape than globes. Or 10Y, 45, 371a Cunninghams (I just have two “globes” as our cousins say). Im not being pedantic, its nice for learners to understand the subtleties, within the history across the oceans. Its like today knowledge has become homogenised like milk (another thing I dont like). true differences and knowledge are lost and newbies are being treated as one world lemmings. As if all differences are immoral jail-worthy or racially inspired. Political correctness has taken over from truth, and blinds the eyes of the beholder.

So at the primary inductance of the AE, I suggest the px4 is my optimum. Im not saying the 2a3’s Simon suggests arent just as good for the task. I dont know, and am avoiding new purchases or Id order a pair of those 2a3 40 watters. But I know that out of what I have my px4’s sound better than any 2a3 I own.

Newbies take note of the 2a3 options which would be affordable. Such as Russuan versions not fogettimg 6.3v heater versions in octal bases and in four pin bases.

chinese px4 available evidently from Brimar, though not British (is that racist?) and rather expensive, buyer beware, but Im really glad at last someone is trying to redress the attrition of British valves.

Long way of saying I want this GM70 amp to have the best chance! otherwise I may aswell not bother and build another 212 amplifier.
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#25 Re: GM70

Post by Nick »

is that racist?
I think the word you are looking for is xenophobic.

From what I can see the Brimar thing started with lots of optimism but is now built along the same lines as a group of steam railway enthusiasts. Good luck to them. Selling Far East valves for them is the equivalent of the York Railway Museum having "made in china" things in its gift shop.
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#26 Re: GM70

Post by andrew Ivimey »

I have a brimar nos 2a3 in a very battered box. 1950s? Dunno. If I remember it's twin plate like my RCAs.

Now then young Simon ( patronising air... sniggers), am I showing off!?!
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#27 Re: GM70

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Id have to look that word up! But don't care.

Yes they are volunteers more power to their elbow.
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#28 Re: GM70

Post by Paul Barker »

andrew Ivimey wrote: Wed Mar 02, 2022 10:04 am I have a brimar nos 2a3 in a very battered box. 1950s? Dunno. If I remember it's twin plate like my RCAs.

Now then young Simon ( patronising air... sniggers), am I showing off!?!
Probably USA made badged by Brimar so they could supply spares to the theatres using push pull parallel Western Electrick 2a3 theatre amps. I have a pair of those amps myself.Wish the speakers were offered to me. No such luck.
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#29 Re: GM70

Post by andrew Ivimey »

Was it USA then who 'invented' the twin plate (2a3)? I've tried the Soviet 6s4s variant and didnt like them. I use shuagang single plates in my pp amp but the graphite gm70s get much more use. ( coppers are 'for best'!)
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#30 Re: GM70

Post by steve s »

simon wrote: Tue Mar 01, 2022 11:47 pm FWIW I prefer RCA dual plates to the Sovtek monos, and the JJ 40W jobs to the RCAs, thought they're not really 2A3s.
Hope my memory is not playing tricks on me, but we had a 2a3 rolling session at mine with a variety of types a few years ago?
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