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Old 25th September 2008, 08:05 PM   #11
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Hey Bas! is that you in the avatar?
Hey Gianluca! It is indeed!

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OH! I must build again something with GM70s ... I loved that tube.
Well you can forget about me selling you those copper plate GM70'S back to you.
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Old 25th September 2008, 09:36 PM   #12
Gluca is offline Gluca  Italy
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Quote:
Originally posted by Bas Horneman

Hey Gianluca! It is indeed!


You cheated! Bring the old avatar back!

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Well you can forget about me selling you those copper plate GM70'S back to you.
HEY! I sent those copper GMs only because you were supposed to use them in a very short while ... stop piling my tubes in a closet!



Teflon socket: I made my own homebrew teflon (well ... not exactly teflon, you can't use teflon as it is but you should be using teflon+other material).

http://www.audiofaidate.it/forum/top...21&whichpage=1

At the end of the game they would have costed something like 40-50 euro for the 304TL (or GM70 or 845 that use big sockets) and 30 euro for the classical UX4s.

Ciao
Gianluca
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Old 26th September 2008, 05:22 AM   #13
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stop piling my tubes in a closet!
LOL!

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Teflon socket: I made my own homebrew teflon
Wow..excellent work.
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Old 26th September 2008, 07:57 AM   #14
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> Teflon flows long term and IMHO is probably not suitable for applications like the GM-70 where the heat is relatively extreme at the socket

I suspected that; better stick with ceramics . .

Cheers
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Old 30th September 2008, 02:59 PM   #15
sycorax is offline sycorax  Netherlands
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I'am using Yamamoto teflon sockets (www.jacmusic.com) in my GM70 PP amp.
Until now no problems at all. You can readjust the contacts in this socket if there is a need for it.
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Old 30th September 2008, 03:41 PM   #16
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Teflon should be good for temperatures up to 260 degrees Celsius, above that it will release toxic gasses. I remember member 316a offering teflon septar sockets and warning they could not be used with 6C33C, as those tubes reach 300 degrees...what is the temperature at the GM70's base? Should be quite high...

Sycorax, can you share your design using the GM70? I am curious! Maybe post it in the 'photo album' thread. I had a look at your website, but could not find the amp.

Erik
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Old 30th September 2008, 04:09 PM   #17
kevinkr is offline kevinkr  United States
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Originally posted by sycorax
I'am using Yamamoto teflon sockets (www.jacmusic.com) in my GM70 PP amp.
Until now no problems at all. You can readjust the contacts in this socket if there is a need for it.
I refrained from mentioning the brand that I was complaining about, but since you have outed them... I don't think it is a big problem if the amplifier in question was one built by you, and used by you, unfortunately I stuck them in a series of rather expensive amplifiers I sold to others, and now 8 years later I am getting reports that the sockets are intermittent - in fact this is the only problem reported with this design so far. The problems are only in the amplifier chassis and that is where I used the teflon sockets. It only takes one insertion of a tube with slightly wider pins to create an ongoing problem.

This could be a really big problem if the contact that goes intermittent or open happens to be the control grid on one of the output tubes...

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Old 2nd October 2008, 02:09 PM   #18
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Sycorax, can you share your design using the GM70? I am curious! Maybe post it in the 'photo album' thread. I had a look at your website, but could not find the amp.
Hello Erik, I have added a small photo to my website. The AMP is not ready yet. The audio is ok ears are happy and audioanalyser is happy. But there is a lot to do. The tubes are running on 800 volts 100mA so some kind of a failsave circuit is a must.

And yes 60 watts of filament power and 80 watts of anode dissipation gives a lot of heat. In the past I used some ceramic sockets from the USSR that i bought on e-bay. I was not happy with them. The Yamamoto's are doing a better job for me. But Kevin is right I'am the builder, owner and I'am doing the maintenance myself. Thats very different from making an Amp for someone else.
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Old 4th October 2008, 05:46 PM   #19
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Hi Sicorax

Thanks for the information: a PP with GM70 is still far from my capacities and necessity, but I am thinking about it anyway. What I am doing now is trying my best at accomplishing a very nice sounding little PP amp in the 5 to 10W range...when that is done and there is some money on the bankaccount I plan to replace the output transformer with an interstage, and drive a pair of GM70's.

And now for another question which can save me a lot of trouble down the road: are you using AC for the heaters or DC? AC can be made to cancel in PP configuration, but I suspect that with 20V AC that will get really hard...

Erik
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Old 5th October 2008, 03:17 PM   #20
kevinkr is offline kevinkr  United States
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Hi Sicorax

<snip>

And now for another question which can save me a lot of trouble down the road: are you using AC for the heaters or DC? AC can be made to cancel in PP configuration, but I suspect that with 20V AC that will get really hard...

Erik
Hi Erik,
It's pretty problematic getting good canceling even with the 5V filaments in 300B. If you are lucky enough to get a pair that cancel well when new they inevitably drift enough that they no longer cancel, and having to regularly adjust a hum balance pot is a nuisance.

Interestingly enough the JJ 300B was consistently good enough for this to work with almost no adjustments, Valve Arts, and Svetlana tubes weren't..

The solution since many of these amplifiers went to customers with limited technical ability was to convert them all to dc heating as they came in for service for other reasons. In the end I ended up recalling the rest - an expensive proposition, but fortunately not too many went far afield with AC filaments.

I strongly recommend dc heating and I IMO you would be well served with a CCS per tube based on the LT1084.. Each transformer should have something like a 24V 5A - 6A rating for this application. This should also help to maximize filament life as there will be no cold inrush filament current. Use plenty of heat sink though as the regulator may be dissipating up to 20W or so..
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