GU-72 Snigel ended

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GU-72 Snigel ended

Postby soundbrigade » Tue Jul 19, 2016 8:37 pm

Snigel in Swedish is Snail in English, so it was a deliberate Freudian wrongwriting ... :|

Well, when you want muscles, a PP with KT88 or even 6L6 will fix most problems with low sensivity speakers but if you want to go SE, what to do?
There are tubes like KT120 and 150, you may find the EL156 not to mention 6S33S (or 6C33C as some say). Did I just see a 12E- somting project in here? Yoy could go for 211, VT4C etc .... I won't continue this namesdropping.

Many of us like to play with odd tubes and one of those is the Russian GU72. A typical radio tune with a lot of testosteron to release.

GU72 is a big and heavy tube, it's a indirectly heated tetrode that in audio application likes to be fed some 700V. It is easy to drive and well ... look at some data.. One drawback - anode's on top.

I got two of those tubes from Bulgaria including sockets. Anode connectors are lying around as long copper strips that will be fastened around the top with a wire going down to the circuits. To avoid kids, cats and adults from being electrocuted I will build a cage around the tubes.

I ordered special transformers for the 700V plate voltage the 26V heater and 6.3V for the driver tubes. However I missed a winding for the bias voltage as well as a low voltage ( :| ) tap for the driver tubes.
Ordered a lower voltage transformer and a bias transformer just to find put that I could arrange things differently. Don't like a lot of iron.
Drivers will be 6SN7 or 6E5P or C3g, haven't decided yet.
Chassis are some slightly larger (two chassis, monoblocks) Hammond chassis 3" high. However I will mont tubes, transformers and connectors on a 1/2" thick composite material. Looks like black marble but the material has a hard and reflecting surface but can be tooled with drills, saws and router. Wanted something different for this one.

Finally, if and when I get things workibg, what will I get? Approximately 25W of single end power. Will get the membranes of my un-ortjodox two-way speakers move a little.

More to come.
Magnus

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Re: GU-72 Snigel ended

Postby soundbrigade » Tue Jul 19, 2016 8:38 pm

Magnus

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Re: GU-72 Snigel ended

Postby EWBrown » Tue Jul 19, 2016 8:46 pm

Wow! That's a "beast" of a tube, and reasonably priced, too!

Image
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Re: GU-72 Snigel ended

Postby soundbrigade » Tue Jul 19, 2016 8:59 pm

It's like that Kinder Egg, you get a tube that will attract attention, that is powerful and easy to drive. And still doesn't call for 1500V plate voltage.

So far I have drilled/routed/carved the holes for the tubes and propably use 6SN7 as it is almost a twin 6S2S used in a Russian applikation.

Some schematics here: http://www.hififorum.nu/forum/topic.asp?TOPIC_ID=89644
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Re: GU-72 Snigel ended

Postby Geek » Tue Jul 19, 2016 10:10 pm

Hmmm, I know what I'm using with da CB radio next sunspot cycle (666)

(waits for Ed's two cents (lol) )
-= Gregg =-
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Re: GU-72 Snigel ended

Postby kheper » Fri Nov 11, 2016 10:56 am

soundbrigade wrote:It's like that Kinder Egg, you get a tube that will attract attention, that is powerful and easy to drive. And still doesn't call for 1500V plate voltage.

So far I have drilled/routed/carved the holes for the tubes and propably use 6SN7 as it is almost a twin 6S2S used in a Russian applikation.

Some schematics here: http://www.hififorum.nu/forum/topic.asp?TOPIC_ID=89644


If you haven't finished the amp, a C3m might be a good driver.

http://www.jacmusic.com/techcorner/ARTI ... -info.html

http://www.mif.pg.gda.pl/homepages/fran ... g/GU72.pdf

These GU-72s are 85W tubes. The screens have a max voltage of 700. The heaters run at 22-27V. Since you need 20+ volts on the heaters, a C3m with a 20V heater might be an option. The C3m can operate up to a 78 voltage gain with low distortion.

My calcs for the C3m with a "modest" 47 voltage gain as a pentode volt amp:

Ep = 225
Eg2 = 155
gm = 6500 µmhos
Rip (internal plate resistance) = 250k
Rep (external plate resistor) = 7.5k

Av = 6500 * 250000 * 7500 / 1000000 * (250000 + 7500)

Av = 12187500000000 / 257500000000 = 47


There's a commercial amp, featuring the GU-72. 22W per channel is claimed.

http://vinnytsia.all.biz/en/lamp-gu-72- ... er-g661110
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Re: GU-72 Snigel ended

Postby soundbrigade » Sat Nov 12, 2016 2:05 am

This project hasn't stalled really. It's just awaiting some enthusiasm from me. And I am enthusiastic but hasn't really got the energy to go on.

I am very thankful for the figures you gave me. I still haven't decided what driver tube to use. If I am into some experimenting I'd propably pick the Russian 6E5P.

However I dug up some brass mesh metal and made myself a cage for the tubes. Got to find some good solder to attach some mounting screws to the cages. I cannot come up with a good enough connector to the anodes that stops sneaky fingers feeling a soft touch of 700V, so the mesh will hopefully keep fingers away and allow enough air to flow around the tubes.

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PS. My kbd is getting slobby and floppy, missing letters and adding letterss.
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