GM70 SET

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simon
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#301 Re: GM70 SET

Post by simon »

Coax via Manchester lol!

I've only used the 3116 as subwoofer amps but they work really well as they seem pretty neutral. And they're cheap as chips, well nearly. I'm not sure there's really a good reason not to use them permanently, other than I just like valves. £25 and 25W versus a lot more and a lot more. Mad really.

(But I just have this hankering to run copper GM70s to the 208s and graphite GM70s to the 285GMFs. But that would be about a kW for relatively low output power.)
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IslandPink
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#302 Re: GM70 SET

Post by IslandPink »

Looks good. Do you intend to have the filter slope in the coupling to the output valves ? - ie. after the CF's ?
Ps. what's a WCF ?
I still don't know where the crossover needs to go with the FE208Ez, but you have more flexibility with the bi-amping - and , as we've discussed, you can add a tweeter to the FE208Ez with some sort of passives.
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#303 Re: GM70 SET

Post by simon »

Yes I figured on the input of the output amps, or the output of the drivers. Not sure if there are any pros and cons? The smaller the current the cheaper the chokes?

White cathode follower - what Broskie uses for the output of the Aikido.

I currently use the T90As via a filter from the 208s - is this what you mean, or something else?
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Nick
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#304 Re: GM70 SET

Post by Nick »

There is more to a White Catkode Follower than the output of an Aikido. All the Aikido does in inject power supply noise into the load to provide cancellation. The WCF is closer to a OTL push pull output stage in that some of the signal is injected into the lower valve.

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#305 Re: GM70 SET

Post by IslandPink »

I don't remember the White cathode follower having that good a reputation, sonically. But I couldn't tell you where I read that !
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#306 Re: GM70 SET

Post by simon »

There may be better options than a WCF, and I'm open to suggestions, just that I've built a few and it's a fairly safe bet.

Never tried it in a power amp though TBF, though Dave Dove's 300B amp (which I never heard but was well regarded) had a James' designed pentode Aikido.
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#307 Re: GM70 SET

Post by IslandPink »

More thoughts ...
It seems the WCF is aimed at getting a very low output impedance. I don't think you need that here, so it's more complicated than it needs to be.
As Nick says, as long as the coax isn't too long you might be fine with the pentode's drive if ( like typical with 12GN7 ) it's running 20ma or more ( is it ? ).
The main requirement I see for the buffering is that you need it for at least one channel, if you're going to do the filter slopes in the final coupling. This is what I did with the 2-channel horn amp - I ran the driver ( 4P1L ) direct though the HP filter ( just a smaller coupling cap ) into one 6B4G, then the lower channel had a CF or Aikido buffer into the LP filter ( R, C to ground ) and to the other 6B4G.
If you are just using 1st-order slopes than you can do the filtering without chokes.
I would suggest you could add slope to the bass driver LP by putting a choke in series at the driver to pull it down steeper above 1kHz.

Now, the only reason you could argue the upper channel also needs a CF is that it's easy to put a voltage divider in front of the CF to do some attenuation on the FE208Ez channel - which is the one that needs it. However, I think you can probably do this with a voltage divider before the coupling cap in the HP channel , just need to think about the values so as not to load the driver down too much , but also avoid limiting current to the GM70 grid to avoid slewing.
Is the 12GN7 going to be in pentode or triode mode ?

I will find a link to my 2-channel horn amp for ref.
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Nick
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#308 Re: GM70 SET

Post by Nick »

Maybe I should rephrase what I said. The output of an aikido is not a white cathode follower.
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#309 Re: GM70 SET

Post by simon »

Hmmm, I've obviously misremembered - my tired brain associates the Aikido output with WCF, but you're right. Thanks for persisting!

But yes, perhaps I'm being overly cautious - a metre's worth of low capacity coax might well not need any buffer.

But. The 12GN7 is triode wired (around 20mA IIRC) with a gain of c39, which is enough as I've less than 500V across the GM70. At the moment I'm using the class D amps for the bass drivers so there's lots of attenuation (via the on board volume pots). But the 285GMF is marginally more efficient that the 208 IIRC so it will depend on the relative output of the two amps - it might be the bass amp that needs attenuation I guess.

So with a low pass filter I'll probably need a buffer for the bass amp, and might need one for the main amp unless I need to attenuate and then I'll also probably need one. So perhaps I just wang a couple of Aikido outputs in anyway?
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#310 Re: GM70 SET

Post by IslandPink »

I don't believe the 285GMF is overall more efficient than the FE208Ez, but it probably is in the midrange - but the 208 is upper-90's or over 100dB once you get over 1kHz. I know James claimed around 97dB/w with the AER's.
Course GM70 has more gain than a 6CB5a.
I guess it's belt & braces to allow a buffer for attenuation on the upper channel.

For info -
http://www.audio-talk.co.uk/phpBB3/view ... p&start=30
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#311 Re: GM70 SET

Post by simon »

Thanks Mark, I always like to see a diagram, just the way my brain works. Over 5 years ago - where does the time go?

I have been tempted to try the 4P1L instead of 12GN7 but I think the gain is only around 15? And then I would need a pre stage for sure.
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#312 Re: GM70 SET

Post by IslandPink »

I think at the time I used Spice to calculate mu = 22 without a cathode by-pass cap and mu = 32 with a by-pass cap. Plus you need an (Andrew) filament supply - but it is one of the greatest ever valves.
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#313 Re: GM70 SET

Post by simon »

Oh dear. 32 (as a pentode?) might just be enough gain, and I have 4 of Andrew's boards for such an eventuality. This is not simplifying things lol!
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#314 Re: GM70 SET

Post by IslandPink »

That's as a pentode, mind. As triode it starts as 10x on paper , proba less than that.
But in pentode mode ( it was with ~7.5k anode load ) I had a lovely result with GK-71 , so it should be happy with a GM70 in A1.
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