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-   -   GU50 SE vs 300B SE tube amp?? (

gaborbela 29th December 2012 02:20 PM

GU50 SE vs 300B SE tube amp??
I turn to the forum ( to some expert guys) about my SE tube project.
I start from the beginning so it would be easy to understand my case.
I wanted to build a GU50 SE triode tube amp, so I collected almost all the necessary parts, let say 90%
Power transformer 325V 2.5A, a pair 3.5K SE 10W 100mA continuous bias USA made transformer, driver and power tubes, sockets, choke, PS capacitors etc
Now I don't know do I complete the amplifier or to use those parts to build a 300B SE amplifier.
Do any one has experience with these two power tube, built or heard their sound?
I could use the same transformers, for the 300B , I have D3a tube to drive the 300B, only has one choke 200mA tested up to 2000V (may be I need two)..
What would be the best think build a 300B and sell the GU50 tubes or finish the GU50 and probably I never get close to the 300B performance??Any advise pro and contra welcome.
Also please keep in mind probably later I'll sell the amp because my wish to build a GM70 SE. I just do not want to start with 1000V now, last few years I built only SS amplifier.
Of course all depend the amplifier performance, if sound great I'll keep her.:)
Also if the amp sound better probably easier to sell in case/.
I do wanted to post pictures from the parts I collected but I have to charge my camera's battery.
So pictures will follow soon.
Thanks for your help
Happy New Year!
Greetings Gabor

Vinylsavor 29th December 2012 02:58 PM


The output tube is only one part of the equation. The overall sound of the amp will be influenced by the circuit, parts quality, driver and PSU. Some of these aspects even more than the output tube. I'd say stick with your plan and build with the tube you had in mind.


andyjevans 29th December 2012 03:11 PM

I don't like building with high voltages, so I see your point. I'd go up to 450v or so. But at that kind of HT you can use various big tubes at "low voltage" operating points. The 813 is one that many like. Looks like you can put up to 100mA through your OPTs so you may get enough power - what speakers are you driving?

gaborbela 29th December 2012 04:40 PM

3 Attachment(s)
Here is the two circuit I think about
First the GU50 what I planed to build but mean while I collected the parts I get several negative or just OK feedback on these tube using as SE...
The GU50 I like with the 6E5P driver tube but one power tube only..
The 300B is from Andrea reference amp all do I have to do several mode because he use higher primary OPT and almost dual mono PSU.
Right now I have only one choke 6H 200mA, he uses two choke. I don't know for start if I can connect both channel to one PSU.. At least to test the circuit.
After I would buy a pair choke because the two must be the same.
I wrote to Andrea to get some info about his amp but unfortunately I didn't got answer yet. Also one capacitor value missing from the circuit..
Please take a look the 300B circuit, can I use it with my 3.5K transformer and for start can I use only one choke in the PSU?? I know these is a great amp, I know if Andrea say is a reference amp that is not bad.
I read good think about the 300B but... that how it was with the GU50 when I started. Now those guys who praised the GU50 tube they gave up and build something else.
To me 6 to 8 Watt it would be plenty to drive my speakers.
Thanks for the help.
Greetings Gabor

gaborbela 29th December 2012 04:54 PM

If you look at the 300B circuit there is two arrow
First I don't know the capacitor value and voltage
second arrow - I'm not sure if I can leave out that part of the PSU and connect both channel for the one 400V.
These would be temporary until I would get some idea about the amp performance after I would buy two 10H choke. Also I plan to use SS bridge rectifier, not tube
Only I question the amp performance because my OP transformer has 3.5K primary. Probably I get higher distortion and higher bias with out mode.
Please let me know what would you do in my case.
About the GU50 tubes if I solder them (specially the socket cost more) I can't sell those..
Thanks one more time
Greetings Gabor

quikie22 29th December 2012 05:03 PM

I have built the GU-50... and I think it is pretty awesome.
You can try running it in triode mode, or switching to pentode mode but use some plate to grid feedback.
Since you already have all the parts you need, just proceed with the GU-50... you may end up liking it. I certainly do like mine.
It won't sound like a 300B, but I could say no 300B could ever sound like it, with the solid bass and bite and yet maintain a sweet midrange especially in pentode mode.
BTW, my driver stage is with 6E5P too but using 3 red LED bias bypassed with Elna Silmics2, and the GU-50 run in pentode mode with screen grid voltage regulated by neon lamp voltage reference at 210V.

enzoastro 29th December 2012 06:09 PM

i'd build the gu50 and when funds allow get some 300b's, the money is in the ps and opt's so swapping sockets and bias components is a pretty economic way to get a new amp. i would not fret too much re your choke, worst case you get a little cross talk and imho thats the least important spec when your listening.
p.s both those circuits could be much improved with 2 resisters and a dn2540 mosfet!

Wavebourn 29th December 2012 09:42 PM

I would recommend you to use quikie22's schematic and practical advises - he made great amp. I helped him a bit along the way.

quikie22 30th December 2012 02:26 AM

1 Attachment(s)
This is the circuit that Wavebourn helped me with.

The plate load resistor for the 6E5P will be upgraded to gyrators once I get the time to do so.

Wavebourn 30th December 2012 02:31 AM

Are you sure 100 KOhm feedback resistor goes directly to the grid, instead of going to the anode of 6E5P tube?

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