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quikie22 25th October 2010 12:27 PM

GU-50 in Single-Ended Triode Configuration - What bias points?
Hi all audio fanatic friends,

This may be a silly question, so please forgive me for asking as I am still a NEWBIE to tube amps

I have constructed an RH84 amp with the 12AT7 and 6P14P (Russian EL84 equivalent) in triode mode and find the sound really nice.

As my next project, I want to make a similar circuit (ie Single-ended pentode in Triode mode) but using GU-50 tubes as the output tubes for more power.

Has anyone made a similar set-up with GU-50 in triode mode? Assuming I want to use a B+ of around 700-800 Volts, what would be a suitable bias current be? Any suggestions for cathode resistor values and suitable OPT primary impedances to use? Any role for LED biasing as in the Red Light District (RLD)? I will most likely use the James 6123HS OPT as this is an affordable and acceptable quality OPT in my region.

Thanks in advance.


quikie22 25th October 2010 01:22 PM

Come on guys, I know you have some thoughts. Please help....

azazello 25th October 2010 03:03 PM

I made few years ago SE GU50 with +B=300v and fixed bias was around -40 v,
Ia=95 ma.
Cathode to ground, second and third grid to plate.
Maybe bias in Your ampl. will be 70-80-90 v....
I used 5 kohm Hammond 125 FSE.

ErikdeBest 25th October 2010 03:44 PM

Well, from what you describe you have got no clue about the original purposes of the RH84 circuit: you should, in fact, run the output tube in pentode mode as to obtain more power! Anyway, a GU50 in triode may sound nice as well, and there are even curves for this tube wired in triode mode: you can start from there!
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Wavebourn 25th October 2010 04:28 PM

When I clicked on this topic in order to help you I discovered that ErikdeBest already answered your question. Curves on that page are real, and you can choose your options from them.
What I can add, it is better to use cathode bias since the amp is SE. You will need more than 100V peak to peak from your driver, so a single 12AT7 stage will be too weak to amplify and deliver such a drive.

quikie22 25th October 2010 11:38 PM

Thank you for the replies.

@erik: Indeed I have no clue about the RH84 design. :-( I am totally a newbie. I just followed the schematic and soldered the thing together. I am not an electrical engineer at all. Maybe you could explain? I got lost in all the jargon about the RH84.

@wavebourn: I am thinking of cathode bias for the GU-50. But I am not sure how much cathode bias ie cathode resistor value it should be if I applied a higher B+ of around 700V.
As for the driver tube, what would be a suitable driver tube? Can I use the 6P14P as the driver tube to achieve the voltage swing?

Sorry for so many questions.


Wavebourn 26th October 2010 12:20 AM

You can't apply 700V in triode connection. Please look at that graphs on top of the page Erik provided, if have some questions first of all read some good book about vacuum tubes and how to draw load lines, and what they mean. There are plenty of such books.
Graphs already suggest several versions of regimes: B+, load resistance, idle current, first grid bias. It is very easy to calculate cathode bias resistor's value: just divide bias voltage by idle current. But before that you must understand tubes and load lines, so looking at that graphs you can understand tube's character, how to ask it to provide what you want.

And of course, you must remember: such voltages and power capabilities can kill!

quikie22 26th October 2010 07:40 AM

Hi Wavebourn,

You are one of my most respected forumers :-) The reason I am trying the GU-50 amp in preference to the other more "common" tubes is because of reading about all your experience with it. It seems like a "super tube", very well made and works extremely well.

As for reading about tubes and loadlines, I have tried to read them but end up very confused. I am not trained in engineering except for basic electronics (and that is solid state electronics) while in school. I will give it another try again.

The reason I am thinking of using B+ of 750V is because my recycled power transformers (2 pieces, one for each channel)) provides AC 750V at 60mA (no centre tap). After rectification and losses through rectifier and choke will probably end up around 750V DC too. What would your suggestion be? Get new transformers? Use in pentode mode? Regulate the voltage to something more manageable? I am well aware of the deadly high voltages.

Ian444 26th October 2010 10:50 AM

Not many people use GU-50 which is a great thing as it keeps the price low on ebay. Maybe due to the high voltage needed to make them sing, and high power output, and the sockets aren't cheap either.

The schematics I have found for GU50 single-ended have a 250V regulator on the screen, so they are running pentode.

Try valvewizard for theory that does not turn your brain inside out. Its very practical-orientated.
Do a search on "valvewizard"

Edit: Its easy to build a design that is known to work, but when you want to do your own design you need to know the basics behind it, which isn't that difficult to learn but it requires effort.

quikie22 26th October 2010 02:51 PM

SO you did not have a cathode resistor? How did you "fix" the bias?


Originally Posted by azazello (
I made few years ago SE GU50 with +B=300v and fixed bias was around -40 v,
Ia=95 ma.
Cathode to ground, second and third grid to plate.
Maybe bias in Your ampl. will be 70-80-90 v....
I used 5 kohm Hammond 125 FSE.

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