[Radioroslyn]

ADMIN NOTE: I split this thread from another thread since this is on a different subject. - RR

Hey Ron,
Just for grins you might want to try a 6SB7 in place of the 6A7. You will have to make up an adapter but it has twice the conversion gain of the 6A7. The grid configuration is a bit different than the 6A7 but can be wired up to work. I think I'm going to give it a try in the 37-620. Should really give the SW bands a boost.
Terry

[Ron Ramirez]

Terry

How would one wire up a 6SB7 to replace a 6A7? I've looked at the grid configurations of the 6SA7 and 6SB7 before, and I can't figure out how to make either work in place of a 6A7 or 6A8.

With the 6A7 and 6A8, it is obvious which pins to use for the oscillator "plate" and oscillator grid. With the 6SA7 and 6SB7, it is not so obvious.

Detailed directions would be very much appreciated. I'm sure others could benefit from this knowledge also.

[Radioroslyn]

Well I think it works out like this.
6A8 gm550............... 6SB7 gm 950!!!!
P1 Shield................. P1 Shield.... Good!
P2 Fi................. P2 Fil .... Good
P3 Plate............... P3 Plate..... "
P4 G3/5............... P4 G2/4..... Leave P4 unconnected at 6A8
P5 G1................ P5 G1....... Good
P6 G2............... P6 Cathode..... P6 of the 6A8 goes to P4 of the 6SB7
P7 Fil............... P7 Fil..... Good
P8 Cathode............... P8 G3..... P8 of the 6a8 goes to P6 6SB7 P8 of the 6SB7 is connect to the grid cap of the 6A8
Think of it this way. G1 is the control grid for the LO
Grid 2 acts like the plate for the LO
Grid 4 on the 6A8 and grid3 on the 6SB7 is the input grid
The 6A8 grid 5 acts like a screen grid. That's why G2 and G4 are hooked together on the 6BS7 both need to have HV on them.
The 6SB7 grid 2(osc plt) and grid 4 Mixer screen grid.
Don't know what grid 3 does in the 6A8.
Hope this make sense to you.
Terry

[Radioroslyn]

Sorry it's a bit hard to read. When I posted it it remove all the empty spaces so the columns are no longer.
Terry
I think I fixed it.
ps For those who don't know the 6SB7 is a high gain mixer tube designed for usage in the early fm receivers and it has the same pin arrangement has a 6SA7. There isn't a 12SB7 it was only made in a 6.3v version. For 12.6 /150ma heaters there is a 12BA7 it's the same tube in a 9pin package.Great for usage in ac/dc sw sets. There is a 6BA7.

[Ron Ramirez]

I see one problem. It is necessary to have G3/5 connected on the 6A7 since these are the "screen grid" of the tube and I doubt it would function correctly with no screen voltage?

Ditto G2/4 on the 6SB7Y.

Since G2 is tied to G4 on the 6SB7Y, I don't see how you could wire this tube to function as a 6A7 or 6A8.

I can think of one single-ended tube that has its grids arranged like a 6A7 or 6A8...the 7J7. And it actually has a separate triode section for the oscillator, unlike the 6A7/6A8 where the five grids perform both mixer and oscillator functions.

[Ron Ramirez]

If you didn't mind using loctals, I see the 7S7 has a higher transconductance (525 umhos) than the 7J7 (290 umhos) and about the same as the 6A7 (520 umhos). Not as high as a 6SB7Y, though.

The 7J7 and the octal 6J8G were used in many 1940 Philco models.

[Radioroslyn]

When the 6A7is returned to the socket it does screen voltage. The 6SB7 receives it's screen voltage from the LO plate supply. It will have HV on it it's just getting it from source. The 6A8 needs two connections to the HV (G2 and G3/5) The 6SB7 only needs one on G2 and G4 which are internally connect.
Terry