GM70 Justification

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IslandPink
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#1 GM70 Justification

Post by IslandPink »

Oooh Errr. :shock: I'm coming over all Thoriated

Since hearing Andrew I's GM70 operating at 300B-like operating point & dissipation, I have some feelings stirring in my loins .
It always seemed that to buy a pair of GM70's would require a serious committment in a 700-800V supply and big output transformers ; or else it would be just an indulgence where I would just get them out of the wardrobe from time to time to have a look at - yum :)
Now I can see some real justifications :
(i) they might sound really fine at that sort of volts and current, better than the 300B's
(ii) I could use the recently-built shunt supply, which could be tweaked-up to nearer 500V quite easily
(iii) It would be only right and proper to have an 'Owston amp' for special occasions, handling less sensitive speakers than the horn set-up
(iii) The KR300B's might go 'ping' at any time, then where would I be ? - They would cost at least £250 to replace , if available :roll:

First step is to buy some , then spend time looking at them ...
Then ask Bud what a pair of level 1 output transformers would cost for say 10W and 100mA-ish max .

Of course the filament supply power is slightly off-putting , but has been done, obviously .
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#2

Post by JamesD »

You could save a bit of money and buy GK-71s and run then trioded :)

But agree that Andrew's amp really raised some questions about spending on output valves....
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#3

Post by Nick »

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#4

Post by andrew Ivimey »

I bought a pair of those coppers 'for best'. Buy now while stocks last!

I started with GM70s with Hammond 125G then they went to MK1 Ongaku (211s) while the 'tractor' (first eggborough do) lay fallow. (Mk3 Ongaku has audionote transformers so no more experimenting there.

I used rectified and smoothed 160va toroids 20-0-20 first and they got very hot. The Maplin's psus were and inspiration. I haven't looked backwards.

GK71s are even cheaper than GM70s (£11 each!) I've tried them and ... they sound the same!

Once the heaters are sorted GM70s are easy and forgiving to work with.

Talking with Mark I remembered I have some 32bs, which were produced before the splendid and worthwhile 300BLxs. I must get round to using them. GM70 is too beguiling. Do the coppers sound better? They are beautiful all lit up and they sound different. But running graphites at 1300volts makes them sound different. Its all there to be (re)discovered! The Boatanchors forum has quite a bit to say about GM70s.
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#5

Post by IslandPink »

Yes, it's an interesting area of endeavour , and thanks for the info ( I saw that very EBay item earlier, Nick ! ) .
I'm thinking for best performance at lower B+ , maybe the extra for the coppers is worthwhile ? They would not break the bank in my (lucky) world, and it seems rude to pass them up while they are relatively common on the auction sites.

I am of course harbouring visions of a 4P1L/GM70 amp that could be awfully fine with the right parts . The latest listening with Andrew's DHT supply confirms that somewhere there is solution to the filament supply on the driver that will deliver the goods without compromise . Looks like I can get away with a pentode driver for A1 use with a grid-leak of 100k or 75k for instance .

What were you using to drive , Andrew ? - I think you said 12GN7, so was it triode or pentode ?
Are the filament supplies something like lighting transformers ?
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#6 Re: GM70 Justification

Post by simon »

Agree with all of this. A very compelling argument indeed. I suspect though that a new pair of KRs would be more than £250

http://www.kraudioproducts.com/Kr/Produ ... roductID=4

GM70 driven by 4P1L with a 4P1L pre sounds like it has to be built...
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#7

Post by andrew Ivimey »

Choke loaded Triode 12GN7 yes ... and I as I was making the tea last night I was thinking about how versatile 4P1Ls would be!
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#8

Post by ed »

I thought the sound beguiling...and it's resurrected my inner turmoil with the BLS transformer saga, vis their third attempt to wind me some transformers for my SV572. I am now spurred to buy 2 of these after the conversation with Andrew.

http://www.ebay.co.uk/itm/6V-5A-5000mA- ... 1c2497f69e

I've spent an awful lot of time and considerable money trying to get Rod Coleman's supply boards to play nice at 4 amps, so far unsuccessfully, lets see what £30 of smps will sound like. £30 versus all those heat sinks and high current caps and large frame transformers..mmmmmm. Then, thanks to Andrew, I must try gm70s as the ps I've got is choke input 500v. It's supposed to be good for 200ma but past experience with the BLS transformers might test that spec, we'll see.
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#9

Post by andrew Ivimey »

As a rule of thumb Ed, if the valve says it wants 3 amps its best to at least double the potential current the PSU can give out. For GM70 I am using 18 volts at 8 amps and they stay coolish. These PSUs are externally switchable up to 22 volts where the max current they will deliver is 6 amps. They get very hot like this.
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#10

Post by pre65 »

I agree with Andrew. My 833a filament SM PSU is rated at 30A although I only needed 10A.

It gets mildly warm. :)
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#11

Post by slowmotion »

I like the idea of using big thoriated valves for low power.
It doesn't really make any sense, so it has to be good! 8)

:)
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#12

Post by ed »

andrew Ivimey wrote:As a rule of thumb Ed, if the valve says it wants 3 amps its best to at least double the potential current the PSU can give out. For GM70 I am using 18 volts at 8 amps and they stay coolish. These PSUs are externally switchable up to 22 volts where the max current they will deliver is 6 amps. They get very hot like this.
Am I right in thinking that you're saying 6v 5a power supply is not going to satisfy 6v 4a heaters?

The last BLS Tx were supposed to be 2 x 9v at 12amps...I measured the draw on them when running and the heaters were drawing 8amps into Rod's boards......the Tx were untouchable after 30 mins....I gave up at this point. I thought it was Rod's boards that were wasting the extra current....

I think I'll leave the amp in the corner of the store room...it can't cost me anymore money there....
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#13

Post by pre65 »

For my 811a filaments I use the big Tent-lab modules, left over from my 845 experiments.

I feed each of them with a 100va 12v transformer and resistor them down a tad.

Transformers are these

http://www.ebay.co.uk/itm/LIGHTING-TRAN ... 35af869a2c
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#14

Post by andrew Ivimey »

Hello Ed, I am at work so not allowed to post ....

I am saying that for whatever reason the stated ratings of switched mode psus just isn't enough. Like Philip with 833 I too have 12volt at 30amp switched modes. He has added super quiet fans. I haven't and nor have I added smoothing and you can tell!

With my GM70s the switched mode psus are rated at 20 volts at six amps and they get hot. At 18 volts they get warm. 4amp jobbies got extremely hot before I gave them back to my colleagues so they could continue with their work.

I'd over rate switched mode psus as much as your wallet and what's on offer can afford. I have bought several pairs of differen voltages and current switched mode computer looking psus from Hongkers and they all are at the very least twice as much amperage as what I had/have in mind.
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#15

Post by IslandPink »

slowmotion wrote:I like the idea of using big thoriated valves for low power.
It doesn't really make any sense, so it has to be good! 8)

:)
Exactly, Jan -
It was a great pleasure to see many things that did not make sense, at the weekend !
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