1. The GA-9 power amp is close to the BR-9 which uses a 6SN7 as a preamp tube. I would like to add a second stage with the 6SN7. My thought was to use two 220k mixing resistors like I have seen on the Bassman. However, I'm unsure how this would work right into the power tube. My idea is listed on my schematic. Thoughts?
Do you want the added channel to be very clean?
The 6SN7 is the octal precursor to the 12AU7. I'd expect the cascaded 6SN7 to have a similar gain as the 6SJ7, when the volume is near max. A higher-gain dual-triode might give you a wider range of useful volume settings. You could use a 6SL7, which has a gain more akin to a 5751.
2. The GA-9 uses a field coil speaker that I don't have and won't be using. From the schematic, it looks like I need about 275 on the plates of the 6V6's (seems low to me) so I thought I would use a 275-0-275 I have to get about 315. Sound ok? From the schematic, it looks like the speaker field knocks the B+ down from 342 to 275.
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I picked a Gibson GA-9 which has a 6SJ7 preamp and uses two 6v6's in parallel.
Your new amp doesn't have its speaker? Or its power transformer?
If you're minus the field-coil speaker (because the amp has been cannibalized or butchered), you could just use a 1kΩ resistor in place of the field coil. The schematic shows 342v before the field coil, and 275v after; 342v-275v = 67v. That implies 0.067A across the 1kΩ, and it will dissipate 67v*0.067A = ~4.5w. Use a 1kΩ 10w resistor minimum.
3. The GA-9 has a 470 ohm resistor going from pin 5 of one 6V6 to pin 5 of another (signal path). I was going to omit this as I had not seen it on any other parallel schematics. What would this resistor be used for? Should I keep it?
It's a grid-stopper. Presumably, one of the 6V6's is closer to the volume control than the other, and they added the grid stop to the further tube to prevent oscillation. If it were me, I'd probably use 1.5kΩ 1/2w resistors going to both 6V6 grids, and feed both from the common input. I'd probably also add screen resistors (say 470Ω 2w) for each 6V6 screen, to connect to the screen B+ node.
3. Finally, I was thinking about my OT. My understanding is that I need half the "normal" primary impedance for parallel tubes. I have a single ended / heavy duty OT with 8k primary. Can't I just plug a 4ohm speaker in the 8ohm output and the tubes will "see" 4k?
Again, you're confusing me. Is the OT missing from your GA-9, or was it mounted on the (now missing?) field coil speaker?
You didn't ask, but I recommend changing your mix network. You need a 220k-470k resistor to ground per 6V6 (or you can combine two 470k's into one actual 220k resistor to ground). Previously, the 500k volume control performed this function. But you'll be adding a channel, and attempting to isolate them somewhat, so you now need to add that grid reference resistor.
You may consider boosting the existing volume controls to 1MΩ each. The original was 500k, but now you'll be adding that 220k mix network and a 220k-470k resistor to ground. All this is gonna amount to something of a voltage divider and will reduce the effective signal at the 6V6 grids. Using a 1M pot in place of 500k will allow you to recoup a little of that gain (it boosts preamp stage gain because it is a lighter load).
The other reason to use it is that the 500k volume pot needed to be small to be an effective grid reference for the 6V6's if you turned up to max volume. You're now not going to have that situation, so you can use a larger-valued pot.
Topic: Adding an Additional Octal Preamp and Other Questions GA-9 (Read 4076 times)