Williamson KT77 Question

[sorenj07]

Is this a reasonable set of voltages for the front end of my KT77 amp? Isn't the first 6SN7 a bit low? Could I switch to two RC's in series for the first two B+ inputs instead of a forked approach?

[erichayes]

Hi Soren,

No. Yes. Yes. I'll give you a call tomorrow evening, your time.

[sorenj07]

ok, sounds good! thanks..

[EWBrown]

The "forked" approach offers better DC power isolation between the VA and PI sections of the first tube

Even though the DC voltages are the same, the current consumptions of the two sections are different, hence the two differing resistance values.

A quick application of Ohm's law indicates about 4.5 mA for the VA and 7 mA for the PI stage.

WIth the resistors in series (let's say 22K and 11K), the VA and PI stages would receive different B+ voltages, and the VA's 47K plate resistor would have ot be decreased, lowering that stage's gain (not a good thing).

Since the plate of the VA is direct-coupled to the grid of the PI, these voltages have to held very closely, in order not to create a serious imbalance inthe PI.

HTH

/ed B in NH

[sorenj07]

All right. My one move with those later B+ stages is to use a 3K 5W resistor after the small choke to drop the voltage down to 350V or so, which is apparently a better zone for it to be.