GU-50 PP

[misko1985]

Hi friends,
I was build this amp: www.jogis-roehrenbude.de/Verstaerker/GU50-Grommes/GU50.htm

It is slightly modified this EL34 amp: www.jogis-roehrenbude.de/Leserbriefe/Roessler-Amp/Bauanleitung.htm

My question: Please, what anode current I can use in this GU50 amp ? I use about 460V DC in anode and -70 in bias input ..

PS: I use on tubes AC heater with this reference because with AC heater was hum in sound ...

An externally hosted image should be here but it was not working when we last tested it.

Sorry for my bad english

 

[artosalo]

At idle some 50...60 mA / tube is OK.

 

[Wavebourn]

I use them with 850V B+, 10K P-P output transformers, and 30 mA idle current, for 100W output power from a pair.

 

[misko1985]

Thanks,
Really? :-D
I tested it on 460V and 30mA and after about 15min are GU tuber very hot. Then, it is normally ...
.. I have 6.6kOhm OT...

 

[Wavebourn]

Tubes suppose to be hot. Gu-50 is rated for 40W anode dissipation, but they can sustain shortly more than 100W dissipation. Anodes are made from a nickel alloy, and covered by a stuff that absorbs gases. It is a genuine Telefunken design of military tubes.

 

[Ketje]

Thanks,
Really? :-D
I tested it on 460V and 30mA and after about 15min are GU tuber very hot. Then, it is normally ...
.. I have 6.6kOhm OT...

If you use UL connection as in the link at the start (g2 at anode voltage) then 460V 30mA is with -Vg1=95V !
If it is -60V what you said before, there is something wrong.
460V with -60V gives 150mA, much to high.
Mona

 

[Wavebourn]

I never tried Gu-50 tubes with more than 270V on G2. Their control grids are dense and fragile. I would understand 460V triode connected, but in UL?!

 

[Nikita63]

Hello Misko, i have here exact the same amp (used the schematics an PCB), it works. Question is - which power supply schematics are You using. There are a few, the last (working) one can be found here:

 

[Ketje]

I never tried Gu-50 tubes with more than 270V on G2. Their control grids are dense and fragile. I would understand 460V triode connected, but in UL?!

Without signal triode or UL gives the same current in the tube.
The schematic the TS uses is for the EL34 but they put the GU-50 in.
I don't like the idea, it seems to funtion but noboddy tells for how long
Mona

 

[Wavebourn]

EL34 has rarer and thicker screen grid, 450V on G2 is withing specs for it in pentode mode. 20V saturation at 150 mA. Gu-50 at that point has about 80 mA G2 current. That'd why in my amps I sense G2 current to control optical input attenuator that decreases gain of the amp flashing red LED.

 

[misko1985]

Thanks you all my friends !
Nikita63: I dont buy "VB408" than I use and build external source with 2x 285V DC. (almost identical to yours scheme)

https://photos.app.goo.gl/NwtvQfmiMdCmdvFM8
https://photos.app.goo.gl/VpMRBNbSZDbYhcUM8

Look, this is voltages which I was today have on test points:
https://photos.app.goo.gl/VpuHp8RTf5o3pSnB9

At these voltages values are anode currents 32-33 mA ... (33mV on catode 1 Ohm resistors) Is someting wrong on that mr. "Ketje" ?

 

[misko1985]

Thanks you all my friends !
Nikita63: I dont buy "VB408" than I use and build external source with 2x 285V DC. (almost identical to yours scheme)

https://photos.app.goo.gl/NwtvQfmiMdCmdvFM8
https://photos.app.goo.gl/VpMRBNbSZDbYhcUM8

Look, this is voltages which I was today have on test points:
https://photos.app.goo.gl/VpuHp8RTf5o3pSnB9

At these voltages values are anode currents 32-33 mA ... (33mV on catode 1 Ohm resistors) Is someting wrong on that mr. "Ketje" ?

EDIT: I see, you told about 460V on G2 but I meant anode voltage. On G2 I have only 271V ...

 

[misko1985]

Sorry, I meant Anode voltage is 460V, not on G2... Voltage on G2 is only about 271V ...

 

[Ketje]

That's a different story, no UL ! I supose you have about -45V on g1 now and a much safer situation for the tubes.
Mona

 

[artosalo]

At these voltages values are anode currents 32-33 mA

That anode current is quite low. Optimum linearity with 6k6 output transformer and 460 V supply voltage is achieved with some 50...60 mA.

 

[misko1985]

Thank Artosalo,
I check this. In my country is almost winter, that will be better radiator.

 

[zintolo]

I use them with 850V B+, 10K P-P output transformers, and 30 mA idle current, for 100W output power from a pair.

I never tried Gu-50 tubes with more than 270V on G2.

I don't use GU-50 without +12V on the 3'Rd grid.

Positive G3 allows higher G2 voltage, for the same G2 current, hence higher anode current swing and lower saturation voltage. I used that in my amps with GU-50, 813 and GK-71 output tubes.

Dear Wavebourn,

we are actually try to see if it's possible to design a big brother of the Baby Huey, and on of the hardest tubes to implement is the GU50, and I think your intervention could bring a very fresh breath to the thread:
Baby Huey enters into puberty: 6550 KT88 possibly GU50?

Thank you in advance.

Regards
Roberto

 

[zdenoeddie]

I want to built the same amplifier as misko1985 but I have OPT Raa 3k with CF. Ub will be around 450V I suppose, what is the right current for gu-50 in this case? Maybe I try also triode connection. Thanks.

 

[artosalo]

That is quite low load impedance. Optimum plate current is that produce lowest THD.

 

[zdenoeddie]

This OPT is formerly for KT88/6550 which is quite near to gu50. I am not sure with Ik only.