Oddball Valco Vega amp schematic inside

[shzmm]

Hello everyone,
Thought I'd throw something out there that might be interesting/generate some discussion.
Here's a schematic of a Vega V-70 built by Valco probably sometime in the early 60's. It's somewhat similar to other Valco amps of the period like a Supro 1696tn I think.
I would be interested to see an explanation of the strange (to me) input channels. What is exactly going on with that 6.8M/470K voltage divider leading to the grid on one side and the cathode on the other?
I will post some photos and finish the vibrato side of the schematic in the next days-- if anyone is interested let me know.

By the way, the amp is in really nice condition and is very loud-- I haven't been able to turn it past 3 in my flat!

[teemuk]

Originally Posted by shzmm

I would be interested to see an explanation of the strange (to me) input channels. What is exactly going on with that 6.8M/470K voltage divider leading to the grid on one side and the cathode on the other?

I believe the simple explanation is a mistake made in sketching out the circuit.

Go to schematicheaven.com. Download the Supro 1969TN schematic from the bargain bin section and compare.

[Enzo]

I agree with teemu, the drawing is wrong. But you know what would make it sensible? if they left out the ground connection to pin 3 of the input tube.

Ground pin 3 in that drawing, and though it is oddly laid out, that would turn the 470k into a simple grid return resistor for pin7, and with the grounded cathode of pin 3, that would leave the 6.8 meg resistor for a grid leak bias stage at pin 2.

And that would be similar in approach to that SUpro 1969.

[shzmm]

Ah yes now I see. Thanks for the heads up on the schematic. I had to check continuity on the 6.8M pin3 connection to see if it really went to ground. Sure enough-- I guess it's a grid leak bias set-up? I need to find more about those... While I have someone here, I was wonedering if I could ask a couple more questions. Why the partial bypassing on the plate resistors? Another thing I don't remember seeing before is after the LTPi the different values in coupling caps.
It's an interesting amp but the layout is a terror, everything lying on top of each other and going in every which way.

[teemuk]

As said: Errors in sketching out the circuit.

Again, refer to the Supro 1969TN schematic and see how that phase splitter works: It uses a resistive divider to attenuate the signal that drives tube V5. The attenuated signal is then fed to the driver V2b that inverts the signal, amplifies and drives the output tube V4. Makes sense?

The coupling cap values are different because they couple to different load impedances and the circuit still needs to achieve the same response for both sides of the push-pull output.

But, it's kind of useless to talk too much about your circuit as long as the schematic has several major errors. Just resketch the circuit and it will make much more sense.

Edit:
About the "partial bypassing". It's not.
It may look like insufficient filtering when your mind is adjusted to RC low-pass filters that have the R in kilo-ohm range. In this one the R is 100 kilo-ohms, almost hundred times higher than usually in such circuits. Correspondingly, the C element can have a much lower capacitance. When you throw it into equation

f=1/(2piRC)

you get

f=1/[2*pi*100000*(47*10^-9)]=33.86 Hz.

That is perfectly enough filtering for 100 or 120 Hz ripple frequencies present in the B+.