[Mark Korda]

I am using a Dyna-35 for my amp,and I love it.The Dyna SCA-35 has 2 hum controls while mine has none.I know they use 2 different tubes,the 35 uses the 7247 vs. the SCA-35 uses the 7199 which I read a long time ago was looked unfavorably by Frank VanAlstine and was getting unavailable at the time.Were the hum controls used for adjusting bias by ear or something,I have no clue?Anyone out there with a clue?Thanks...Mark Korda

[rlee8394]

Mark,

The hum controls were for adjusting the balance between the output power tubes.

Ron

[Mark Korda]

Hi Ron,thanks for the answer,but when you balance the power between the 2 output tubes,what does that accually mean?Is it because the tubes may not be matched?,the bias setting?,you see Ron I don't get how my ST-35 does that without hum controls.Thanks for replying...Mark.

[rlee8394]

The bias voltage will be the same for both tubes. If both tubes were perfectly matched, then they would draw the same current. Looking at the hum controls, the pots are connected across the filaments with the center tap connected to the cathode connections. By varying the cathode point across the filament AC voltage, it's possible to minimize the hum due to slightly mismatched tubes.

[avahifi]

In the SCA-50 the hum control simply injects power supply hum into the output, out of phase with the output hum, a kind of "two hums are better than one" scheme.

The circuit conditions that cause the hum are still there, just masked.

Not a good engineering solution.

Regards,

Frank Van Alstine

[Mark Korda]

Dear Ron and Frank,great answers,thanks so much....sincerely...Mark K.