Specifications

4. The 100 mfd/25v cap on the output of the EFB cathode regulator circuit is actually the original

cathode bypass cap used in the Dynaco design. Its presence is not needed to ensure a low imped-

ance virtual ground for the output tubes to operate from, but it surely reinforces it, and aids in

stability of the 337.

5. Preference for the manufacturer of various tubes is a personal choice. However, after rigorous

testing of currently available 6BQ5/EL84 tubes, I strongly suggest the Russian made EL84M,

seen on page-4. These tubes are rugged, economical, readily available (currently), and allow all

power output and performance specifications to be fully met. If you are looking to replace the

output tubes in your SCA-35, these are a very good choice.

6. The 5.0 ohm 1W cathode resistors actually

consist of two parallel connected, hand

picked, 10 ohm ½ W resistors in each channel

to obtain the desired value and rating. In Fig-

ure-8, these can be seen mounted on a four lug

terminal strip on top of the chassis, between

the power transformer and front output trans-

former. The terminal strip mounting and

wiring access to it is provided by the cooling

slots that already exist in the chassis. A three

lug strip is all that’s required, but a four lug

strip is what populated my “box o’ stuff”.

7. The 360K 2W resistor from the B+ actually

consists of two 180K 1W resistors connected in

series to produce the value and rating shown.

You can see one of them in Figure-9 and the

other in Figure-4. I used these simply because

they are components I had on hand. The bottom

lug of the two lug terminal strip on the back

panel is used as the tie point for the two resis-

tors I used (the top lug is a ground point for

components associated with the bias control).

Any combination of values to provide the

desired value will work, but it is important to

maintain the 2W overall rating to minimize any

drifting of value due to component heating. In

my SCA-35, bias drift was minimal (< 1ma)

within just a few minutes of operation.

8. The bias control should be set for an average of 0.27 vdc across the 5.0 ohm cathode resistors

under quiescent conditions. This will produce a current flow of 27 mA through each output tube,

and a plate dissipation of just under 10 watts.

9. Analysis of Measured Performance: Looking at the performance of the original design shows

that the amplifier met published specifications when each channel was measured individually,

Figure-8. Pairs of parallel 10 ohm values form the cathode

resistors. They are mounted in this slot on top of the chassis.

Figure-9. Connections between the EFB circuit and the

existing amplifier are made at “can” capacitor C24. Notice

the symbols which identify the terminals of C24.