Need help with my new amp

[Simpleton]

Hi guys!

I've been fiddling with the circuit, but the sound i get is very faint, 1/2W or less, i guess.
The speakers i'm using have a sensitivity of 93dB, and my KT88 i built after Mikaels schematic rocks the place down, even at low volume.
EF86 anode resistor is 68K, Vdrop 75V->2mA, cathode is 1000uF/3,9K; 6B4G has a cathode resistor of 680 Ohm, which gives me -35V and 50mA (had 750 Ohm, -35V and 46mA) and a 100 Ohm hum pot, around 5,7V filament, loaded, doesn't glow much, at least not as much as a 5U4G ;B+=250V.

What can it be? I suspect it's the EF86, but i'm not sure

[anatech]

Hi Simpleton,
The pentode is wired up as a triode for one. Connect as a pentode and retry.

-Chris

[alexistheo]

what about bypassing the cathode resistor of the 6B4G? wouldn't that help Po in a SET amp?

[Simpleton]

Grid resistor of 6B4 was wrong value. They specify in the datasheet max of 50K, and i was using 150K.
WOW what a difference!
It's a superb sounding amp, the sonics can be diferenciated from Mikael's KT88 for better!

I'm happy!

[billr]

Hi.

if you consider the rough formula for power output of a SE amp is

(Vmax-Vmin)(Imax-Imin)/8

plugging in some numbers, you have lost 35 volts to the bias, plus you have lost say 10v over the transformer, this gives you a Va of something in the order of 200v to play with. If you look at the gif on the link shown below, you will see tha the cut off is going to to be about 50~60 volts or so, so in reality you max swing is going to be about 150v.

http://www.triodeel.com/2a3_p3.gif

If you think that you current swing is going to be about 60ma, lets be generous, then the power out put is approx: 150 *.06/8 = 1.125, pretty low.

Also, if i read the curves right, you need about 50v pkpk swing at the grid of the 6b4g to do the necessary, I think that I am right in saying that the 6b4g is the same as the 2a3.

I haven't had time, but what is the pentode putting out?

FWIW, I would suggest that you lift the VA for the output to about 450V or so, that will give you something to play with, with a fair wind, you can get approx 3.5W out in SE mode.

Hope that this is of some help to you.

kind regards

bill

[kevinkr]

I think you will find further improvements possible if you bypass the 680 ohm cathode resistor of the 6B4G, unbypassed as it is it is both raising the RP of the output tube to the vicinity of maybe 2.5K which is a serious mismatch for your output transformer, and degenerating the gain as well. (Probably close to unity.) I recommend the best quality cap you can fit of at least 100uF/100V.

I believe that as designed you can achieve about 2Wrms output. Boost the supply voltages to 300V and you may achieve up to 3.5W which is typical for cathode biased 2A3 at this supply voltage.

Kevin