GU-50 pp guitar amp

[Mark57]

Hello everyone, this is my first post, and I am hoping to get some assistance from the very knowledgeable members out there.
I am planning to build a gu-50 pp guitar amp from scratch using 500v. I have not purchased any iron yet, but I have the gu-50 tubes and sockets

Looking (for the first time) at load lines it appears that a 6.6 to 8 k output transformer will work out well at 8 ohms secondary.

At 8 k
1/4 r = 2000 class a 250ma
1/2 r = 4000 class b 125ma

At 6.6 k
1/4 r = 1650 class a 303ma
1/2 r = 3300 class b 152ma

Bias -35 to -40 v

Am I on the right track?

Also, I am planning cathode bias. Any thoughts or recommendations?

Thanks in advance.
Mark

 

[danFrank]

I don't think that this tube can withstand 500 volts on G2. 250 volts is better suited for G2. Use a 360 vct transformer and wire it up like Marshall does on their old SuperLead amplifiers. Use the transformer's CT for the G2 supply.

 

[Kay Pirinha]

The TELEFUNKEN EL 152/FL 152 datasheet (http://www.mif.pg.gda.pl/homepages/frank/sheets/118/e/EL152.pdf, p.9) which also is applicable to your GU-50's, should set you onto the right pathway (Vg2 = 250-300V, Rpp = 5 kOhms). But observe: Due to it's 1930s' origin, the GU-50 is a rather low gm tube, compared to e.g. an EL 34. Hence it requires about 1.5 to 2 times the grid signal level compared to this more modern power tube (see datasheet: somewhere between 20 and 33 Veff per tube).



I'd not recommend cathode bias for a PP GU-50 geetaar amp, unless you're satisfied by a resticted output power of about 35 W per pair. Tube wear will also increase.

Best regards!

 

[Mark57]

Thank you danFrank and Kay,
I have re done the load line for 450 v and a 6600 ohm opt. I have also started working on a power supply design with a ct split to provide 225 v at g2 as well as preamp tubes. Edcor has a 340v pt with 12.6 v heater supplies and lots of current.
I was planning a phase inverter using a 12au7. Do you think this will be enough to drive the gu50 tubes? Also, I am ok with 35 watts of output. I have a cabinet with a very efficient 102 dB 12 inch speaker. It is very loud with 18 Watts.

 

[Kay Pirinha]

I'd expect the 12AU7/ECC82 to provide the required drive level without any problems. Due to it's low µ characteristics, you most probably need more gain either prior or after this PI, though. IMHO the most appropriate modern miniature tube to drive GU-50's off a LTP PI is the 12AT7/ECC81 which offers both enough gain and output.


Best regards!

 

[Mark57]

Hi Kay,

Thank you for your reply. The 12at7 seems to be a much better choice for the LTP PI.

For the preamp I plan to install 3 9 pin tube sockets although I only have plans to use 2 right now in a high gain configuration. Any thoughts on this?

Thank you,
Best regards,
Mark