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Junm 9th January 2013 03:46 AM

GU-50 PP + 10k:16-8-4 Ohm ,20w OPT..what is the best to do?
4 Attachment(s)
I Bought this 6AQ5-12au7-12ax7 DIY tubeamp from a seller for 85USD, it's still operational and plays music but not to my liking...maybe because of old caps and components needs to be upgraded. It's 10w pp amp with 4 6aq5 as output...I plant to dismantle it and make use of the bulky power trafo,OPT,5U4G rect and the choke...

Now, the plan i have in mind is to maximize OPT power of 20w. the trafo is japanese made intended for 6AQ5,6bq5 and 6L6. impedance is 10k: 16-8-4ohms terminal. The power trafo is from General transformers with 350vac x 2, 5v,6.3v and still unknown secondary. I only have GU-50 power pentodes and i think overkill for the as not to destroy the opt i seek your suggestion of what gu-50 pp schematics is best fo the said opt.

I'm also thinking of GU-50 PP in triode mode.self bias..which will make use of the tube rectifier to lower down the B+ to around 320vdc as seen in the schema.Which is also allowable for triode connection which has 400v max for gu-50 IMHO.
Now, gentlemen i would ask your suggestion.. I'm no tube designer and I'm just an amatuer builder for tuibe amps and i want this amp to stay in my music room so i want the best if possible...TIA

matejsirk 9th January 2013 06:24 AM


this is my PP version with GU50 in triode. For input/splitter I used 6350 (like 12AU7) and for driver 12B4A. I get around 14W.

Junm 9th January 2013 08:07 AM


Originally Posted by matejsirk (

this is my PP version with GU50 in triode. For input/splitter I used 6350 (like 12AU7) and for driver 12B4A. I get around 14W.

wow nice build...Thanks for sharing and probably candidate number one...with 430v supply i might not use the 5U4gb rectifier instead use a solid state rectifier and higher filter caps...I intend to use 6N6P as driver tube and 6n1p as predriver...14w is already okay to me but if it can do 17-20W is much better since my speakers are not that efficient..

Junm 10th January 2013 03:12 AM

hi matej... ive noticed that you use 3.5k:8ohm OPT....mine is 10k:16/8/4 ohms will it fit s the bill? If i use the 16ohm for 8 ohm speaker it will reflect 5K impedance so nearer the specs you have...will there be increase or decrease in power? Thanks

Wavebourn 10th January 2013 06:00 AM

Let's calculate. A first, what voltage swing is needed for 20W on 10K load. Then, check if tubes can provide such swing on given B+ voltage.

20W on 10K means RMS voltage = sqrt (20*10,000) = sqrt(200,000) = 100*sqrt(20)=447V, or 224V per tube.
Peak value is 224* sqrt(2)=316V.
If B+ is 430V that means minimal voltage between anode and cathode is 430-316=114V.

Yes, doable. But it is better to print plate curves and draw load line, instead of such calculations on a napkin, you will see better.

Junm 10th January 2013 08:09 AM

Thanks for the calculations sir anatoly...I will try to, but my knowledge on loadlines of tubes is very limited...i will print the triode and pentode curves and try to see what will come up.

BTW, 400vdc is the maximum triode voltage for gu-50? if it will not doable in triode i will shift it to pentode mode...but as you said it is doable...I hope you will help me design this thing..

Wavebourn 11th January 2013 12:18 AM

Honestly, I never tried to find the maximum. But it is easy, to measure slowly increasing anode voltage and negative control grid voltage measuring anode current and keeping anode dissipation below 40W, and measuring screen grid current. When this current multiplied by voltage approaches max power specified for G2, this is the maximum.

Junm 11th January 2013 03:58 AM

that good suggestion tnx.

Also, how about putting higher value resistor between g2 and plate so as not to exceed the g2 dissipation. this value might be experimental but on approximation it might be around 1k or higher depends on the B+ used. Will this be okay or it will turn out to be pentode mode region?

Junm 11th January 2013 04:04 AM

I'm a little bit confused on plotting loadline for GU-50 triode mode. can somebody help me draw loadline for 10k load and 450v B+..TIA

Junm 16th January 2013 03:41 AM

10K Loadlines
1 Attachment(s)
I've tried plotting the loadlines for GU-50 PP triode for B+=400v and Ia at 60mA. I also do calculations based on Turner Audio computation and assumption and i came up with the attached.
I just wandering why is it that the AB1 power is lower than Class A power...Does it mean that the tube is only a PP class A? or Am I doing the correct calculations and plotting? any comments please?

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