GM70 SET

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Paul Barker
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#256

Post by Paul Barker »

High internal impedance for audio output. Also likely to need positive grid drive.
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Steptoe
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#257 gm 70 set

Post by Steptoe »

Dear Steve, with regard to cv1094, I haven't actually counted them but estimate between 50 and 100. I'm in Australia at the moment but will be back in uk in May and would be pleased to contact you then As I said I did get them to work but they were very quet, probably because I had them single ended and have never been brave enough to wind them up more than 300volts. Unlike the majority of contributors on this forum I usually manage to give myself about one HT shock per month, the last being yesterday so I'm sure you will understand my reluctance to play with high voltages.

I find 300 volts not too bad, as I always wear rubber boots and keep one hand in my pocket but suspect that anything more wouldn't be much fun.
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#258

Post by steve s »

Paul Barker wrote:High internal impedance for audio output. Also likely to need positive grid drive.
I had them as a driver paul..
45 gain is spot on..
Last edited by steve s on Wed Mar 19, 2014 4:14 pm, edited 1 time in total.
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#259 Re: gm 70 set

Post by steve s »

Steptoe wrote:Dear Steve, with regard to cv1094, I haven't actually counted them but estimate between 50 and 100. I'm in Australia at the moment but will be back in uk in May and would be pleased to contact you then As I said I did get them to work but they were very quet, probably because I had them single ended and have never been brave enough to wind them up more than 300volts. Unlike the majority of contributors on this forum I usually manage to give myself about one HT shock per month, the last being yesterday so I'm sure you will understand my reluctance to play with high voltages.

I find 300 volts not too bad, as I always wear rubber boots and keep one hand in my pocket but suspect that anything more wouldn't be much fun.
Yours sincerely Steptoe.
Thats fine, i would guess when they do turn up ther will be a crate full
i'll look forwards to catching up with you in may

300 volts is very sensible..
Thanks..
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#260

Post by Paul Barker »

Your problem using it as a driver is that it is probably a grid positive valve.

Look at this taken from my post on the M7. To read the characteristic curves I have join you have to realise all those grid lines are positive. nearest the cut off at the bottom is +3 and the others are increasingly positive.

It would serve you no useful purpose.

"Lifted from my post on the test results of the M7 transmitting valve: Image

Grid Current chart.

Image

Along the full linear length of the +3v curve the impedance is 30 kohm mu is about 50."
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#261

Post by steve s »

Thats a fair point paul..
And that not really what i imagined

I may try and get the valve i have up to you check out..
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#262

Post by Paul Barker »

Unfortunately I have blown channel A again. Channel B is OK but that is the screen channel, so I can't use the screen as a positive grid voltage supply.

What I could do though is test it at 0v and negative volts, from that we could tell enough to know if it is as above.
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#263 gm70 set

Post by Steptoe »

Dear Pre65 and Paul, thank you for your input re cv 1094. Mine are not exactly the same as the picture as they don't go thinner at the top but they are on the same base. Having seen the graphs I'm amazed that they worked at all the way I had them wired up. Just goes to show how forgiving valves can be.
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#264

Post by Paul Barker »

Steptoe those aren't the graphs for your valves, they are graphs for a valve with much the same purpose and much the same characteristics. I was only wanting to demonstrate that high mu transmitting output valves are not usually fit for purpose as a voltage amplifier in a audio circuit. They can be used as output valves within limits as long as you can provide undistorted current drive to their grid. That is something which is done, but not usually found in the circuits typically found in diy audio schematics.

there is no problem using such a valve but you first need to know how the design has to differ from a design to say choose the 2a3 as the output valve. So you have to toss out everything you have done so far and begin again with an entirely different objective.

then once you have learned everything which is necessary to now design for an output valve which requires power drive, you might aswell take stock and consider is this the most suitable output valve which is dealt with in this way? Your thoughts may then land on the 801a or the GM70 et al.

Or put it another way, start with those better known about valves where you will find a load of schematics which drive these valves properly for better results than the compromised method of bolting a non power drive schematic intended for a 300b or 2a3 on the front to just make use of the A1 region of the characteristic curves.

I am not saying your valves may not be great, but they definitely won't be if they aren't treated appropriately to what they are. Treat them as what they aren't will derive very low power and very poor frequency response to the point of not even worth the bother. I think you found yourself with insufficient power.

If there is a worthy goal to make use of a venerable old valve to somehow give animation to an inanimate object you have sufficient reason to try. For me it is a bunch of metal in a vacuum with some special materials technology which was made for a different purpose than the purposes which drive me. Sell them to the transmitting fraternity if there are any old enough for there to remain a boyant market.

On the whole the market for old valves has to be driven by fitness for a modern day purpose. Try and unearth something the music industry cherishes. Or the other market is the restorers of old receivers, but be quick because when the generation interested in that stuff die off (and they are fast doing so) I wouldn't like to second guess what will happen to that market.

Take the old projection equipment or the old film camera equipment. Though it holds a residual value of a fraction of it's value the day before digital video recording and digital camera recording, the market just dropped like a doodle bug instantly the new technology arose. Only those things which are superior for purpose or at the very least can hold their own continue to retain the same value as they did when they were the only way to do the thing.

So you have to look for people who currently want what you have so they can actually use it not just collect it, to drive the prices and create scarcity value when everyone has run out. Take away the need for it, and it's value will die with those who used to cherish it.
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#265

Post by simon »

What a nice surprise! I got to work this morning and a bright yellow box from Ukraine Post was waiting for me. Of course I told everyone it was a warhead as that was less embarrassing than the truth...

Delivered in 6 days including the weekend, not bad at all, better than some stuff I've had from this country.

So, who wants to see what the coppers look like? Okay, it's a poor excuse to show some glowing valves. I mean, everyone knows I'm a sucker for valves that glow :-).

So here's the "before" with two graphite anodes
Image

The right valve changed to a copper
Image

And now both coppers
Image

Okay, I've cheated with the exposure a little, but it's my thread :-D.

And here's a close up of a copper
Image

Lovely.
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Paul Barker
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#266

Post by Paul Barker »

That there is a lovely sight.

How about a review of the sound?
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#267

Post by simon »

They've got less than an hour on them but I'm very pleased to report I can hear the same differences I heard at Steve's two Saturdays ago.

I find the graphites to be proper party animals - the bass is immense: big, bold, powerful, full of drive. And it perhaps overpowers the treble which seems a little unsophisticated. Don't get me wrong, I really like them and was in the process of putting them in a domestically acceptable(ish) box whilst I sorted out some other projects.

Hearing Nick's coppers at Steve's was a bit of an ear-opener. They make the graphites sound a little agricultural. Unsophisticated. Top to bottom, everything is a little better defined. The bass isn't quite as prominent and I think might be a smidge better balanced. But does it grip the music! Rock solid.

The treble is the revelation for me though. I noticed it immediately at Steve's - it just opened up completely. With my relatively limited experience of valves I'd say the wonderful bit of 300Bs is here now, and 2A3s, why not. But somehow, it's better than either of them. These valves are visceral, the music they produce tangible. The combination of the grip, bass extension and wondrous fluid treble is shockingly good. I never thought I'd make an amp as good as this. Certainly not one so simple. I think the best compliment I can pay it is that it's very easy just to let the music play, there's no need to track hop, the next one's perfectly good. I don't know if I'll ever get back to 300B now. (Well, okay, course I will cos this is a hobby, but you know what I mean.)

As you know, I'm not one for writing stuff about how things sound, but these stir my soul. Don't you buggers go and buy them all up now!

So here's a photo of the proposed 4 chassis amp.
Image

Now don't go expecting The Shadow's build quality cos you'll be sorely disappointed, but the plan is an audio chassis (on the left) and three chassis for the power supply (GM70 filaments, GM70 HT and driver LT, and driver HT - bottom to top). It has to be this way as it's the only way I can fit it in to the space available. And it has the benefit that I lift the damned things!

And now I'm going for a lie down. After I've cooked tea.
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#268

Post by Paul Barker »

That is a real interesting review.

I also recall the coppers a lot more refined at the top end than the graphites.
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#269

Post by Greg »

Excellent review Simon, and what you are doing to box it all looks really good. Now, being a happy 300B PP and 2A3 PSE user, am I tempted........hmmmm, maybe :wink:
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#270

Post by pre65 »

Your woodwork looks good so far Simon.

My old GM70 monoblocks were quite heavy, but not too much so. I found a photo of them, see here.

Image

It will be interesting to compare your GM70 with my GK-71, Owston perhaps ?
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