Dx Blame MKIII-Hx - Builder's thread

[bonfis]

Originally Posted by destroyer X

Several years ago i have used these transistors (2SD669A and 2SB649A).... a very high gain transistor but die like a bird....they die easy..fell down almost dead when sick...poor bird close eyes and fell down..when reach the floor it is already dead..... this transistor die so easy as a bird... i have lost all six 6 pairs i had...this seems, at least to me, they are not that reliable...or strong...or resistant.

regards,

Carlos

Thanks Carlos for sharing you experience with the 649,669. My MK III has somewhat lower rail voltages (55 v) so maybe the bird will be less stressed. I will keep a close eye on her.
Steve

[destroyer X]

Carlos

[evette]

Mr. Carlos sir,, have you ever tried to use 2sc3245 transistors I have and got very good results. Regards Joe

[destroyer X]

For this power amplifier.

The VAS main transistor, in stand by mode, will dissipate 1.8 Watts of power... without signal.

This one is 900 Miliwatts maximum...it will not survive to this amplifier...maybe will be good to the Dx Super A and lower voltage and lower current models.

2SA1285 is the complementary and the capacitance is less than 3 picofarads....good transistor..high speed and high voltage device (120 to 150 volts)

The gain can be too much...can go to 800.... this amplifier was not made to all this gain.... more than 250 may produce troubles.

regards,

Carlos

[evette]

It Works great in the st. Joe

[destroyer X]

Yes...i am feeling curious about that...will search this transistor in Brazilian electronic shops.... we still have this kind of things here.

regards,

Carlos

[evette]

I use 1 of my st. for headphone setup works great. Evette

[destroyer X]

Enjoy your amplifier....Blame ST is great too...but watch Juan post.

regards,

Carlos

[canonnica]

I received my 2SA968B and 2SC2238B for the drivers replacement and some 2SC1819 for VAS position.

The 2SC2238 seems to be genuine Toshibas, the 2SA968B are ISC copies, and the 2SC1819 seems genuine Matsushitas, but they're used.

The joy of grey market components...

I tried them on one of my boards, but one of these parts doesn't fit the bill as now I have some ringing on the positive side of the wave (like Bonfis depicted before increasing base-stoppers from 22 to 56r (I have 100r)...
The dent on the positive side at clipping is gone though...

... to be continued

[canonnica]

For the specific design of this HX amp with miller compensation, is it very important to have the lowest possible Cob for VAS transistor?

Let's say I have the choice between:
A) a transistor with a Cob of 5pF and a fT of 100Mhz
B) a transistor with a Cob of 16pF and a fT of 200Mhz
... and all the other specs being similar, what would be the best choice?

We can read all over this board that the lowest the Cob and the highest the fT, the better the results, but in this particular case, what spec shoud be preferred over the other?

Martin.