Harlon -- If you use the design without the buffer stage or the sensitivity reduction network, and install the unit in the console with that configuration, the biggest difference you would notice is that the volume control would tend to react slightly quicker. Theoretically, the console tuner/preamp provides up to 1.0 vac of audio signal, since that is what it took to drive the original stock design of the amplifier to full power. With the modified design as described above, the amplifier only requires .60 vac to develop full power, so you can see that the volume control would tend to react quicker due to the increased sensitivity. Frankly however, it should be fine, as the increased sensitivity won't be that readily apparent unless you're looking for it. I would only install the sensitivity reduction network if you're a stickler for the details of the console's operation.
On the other hand, if you include the buffer stage, it will ensure the very best performance possible, regardless of how the tuner/preamp circuits are designed. The buffer stage allows the amplifier to interact perfectly with any source impedance driving the amplifier -- even if it is a volume control -- which it likely is, almost surely being driven by the output of the volume control on the tuner preamp chassis. If you take this approach, connect the .01 uF input cap directly to the input jack, as well as the 470K resistor shown connected to ground as well. The series 470K resistor between the input jack and .01 uF cap would not be included. In this configuration, the amplifier will have somewhat more gain than the original amplifier did, but again, it just won't be so different as to be of concern.
I hope this helps!
Dave
Harlon -- If you take this approach, connect the .01 uF input cap directly to the input jack, as well as the 470K resistor shown connected to ground as well. The series 470K resistor between the input jack and .01 uF cap would not be included. In this configuration, the amplifier will have somewhat more gain than the original amplifier did, but again, it just won't be so different as to be of concern.
Dave
3. The schematic you link to is the correct modified version when maintaining the original OPTs. This version was never tested/developed with the buffer circuit added to check for proper HF transient response.
Thank-you Morris. To show the volume control properly, the .01 uF input cap would go to the wiper of the volume control, the low side (min volume) terminal of the control would be grounded, and the high side (max volume) terminal would go to the center pin of the input jack. The two 470K resistors would go away.
If you look at the input circuit as originally drawn, that is the way the two 470K resistors are connected -- with their combined total representing (basically) a 1M pot placed across the input jack, with the .01 uF connected to their junction, which would represent an equivalent level control with its wiper set to mid position if it were a linear control. With the addition of the buffer stage, now any type of attenuation scheme (fixed, variable, stepped, etc.) can be used at its input, and at any setting, without any degradation of audio quality at mid level settings.
Dave
The strongest tube is pulling a of current, and is developing a lot of grid voltage at idle as well.
That sounds like either a leaky coupling cap or a bad tube to me. If you shuffle the tubes around, does it do the same thing in another spot or do all tubes in that location do tthe same thing? If it stays with the socket, the cap is probably leaking or the grid resistor is open. If it goes with the tube, I'd make the tube go. They should not develop any significant grid voltage.
Ah, that's right. I wonder how much the 5U4 drops. Maybe I'll toss a couple diodes in and see just how much the voltage rises as a test. Thanks.The rectifier tube is also being changed to one with higher output voltage. 5AR4 from the 5U4G.