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Graaf GM100

161.142.34.233

Posted on January 9, 2019 at 02:19:37
rkenned02@gmail.com
Audiophile

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Location: PULAU PINANG
Joined: January 9, 2019
My dealer tells me my GRAAF GM100 requires a new Output Device for the right channel. They know the amp quite well but this 'output device' seems to have them stumped. Does anyone know where I can source this part? At the moment two of my output tubes illuminate brightly and burn out - only when a speaker is attached. If I can't get this part does anyone know what it consists of and if it can be rebuilt?

 

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PL504, posted on January 9, 2019 at 10:49:18
Ralph
Manufacturer

Posts: 3862
Location: Minnesota
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EI and JJ both made a version of this tube. I think you can use the 27F12 as a sub.

The EL504 is the same tube but with a 6.3Volt filament.

 

RE: PL504, posted on January 9, 2019 at 15:57:12
rkenned02@gmail.com
Audiophile

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Location: PULAU PINANG
Joined: January 9, 2019
Hi Ralph, Appreciate the help but the tubes are available and OK. The dealer suspects the issue lies in a mystery circuit that is only specified as an 'output device' on the diagrams. Looking at posts on the GM200 I suspect that this circuit may involve a varistor. I am trying to contact Graaf.

 

RE: PL504, posted on January 19, 2019 at 21:42:30
rkenned02@gmail.com
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Location: PULAU PINANG
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Apparently it is a transmission line transformer which shifts the output tubes impedance from 72 ohm to 8 ohms

 

RE: PL504, posted on January 20, 2019 at 09:50:23
cpotl
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Posts: 944
Location: Texas
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"Apparently it is a transmission line transformer which shifts the output tubes impedance from 72 ohm to 8 ohms"

So it is not an OTL, then. It is a bit surprising, in fact, since the Graaf GM20 and the Graaf GM200 are both genuine OTL amplifiers.

 

SEE URL Para 2, posted on January 22, 2019 at 01:37:38
Frihed89
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I'm not sure what to call this? Or where it is in the figure, if it even is?

 

RE: SEE URL Para 2, posted on January 22, 2019 at 09:12:22
cpotl
Audiophile

Posts: 944
Location: Texas
Joined: December 6, 2009
"I'm not sure what to call this"

Well, I think one calls it an output transformer. Namely, an impedance-matching transformer between the output tubes and the loudspeaker. To quote from Graaf's own description of the GM 100:

"The GM 1OO circuit derives from the OTL configuration and uses a new type of interfacing between the output tubes and the loudspeakers. This system adds to the OTL configuration a transmission line transformer which shifts the tubes impedance from 36 / 72 ohm to 4 / 8 ohm."

It has a smaller primary/secondary turns ratio than a conventional output transformer, but the purpose is the same; impedance matching.

 

Thanks nt, posted on January 23, 2019 at 01:23:21
Frihed89
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Location: Copenhagen
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nt

 

It's an Un-OTL, posted on January 28, 2019 at 12:46:54
Lew
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Posts: 10265
Location: Bethesda, Maryland
Joined: December 11, 2000
Which could also be called an OTC.

 

new type of interfacing" sigh... nothing new under the sun"~nT, posted on July 1, 2019 at 00:49:04
Cleantimestream
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~!
The Mind has No Firewall~ U.S. Army War College.

 

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