Vibroluxish build

[überfuzz]

I'm planning to build an amp that is going to more or less a straight up copy of the 6G11 vibrolux amplifier. At the moment I'm trying to interpret the schematics. If anyone has the time to look at my values, circuitry etc. For instance, is the cap C17 properly placed?

Cheers!

[Chuck H]

C17 looks ok. Your PI is missing a tail. Your NFB is being applied to the grid coupling node. You should have a resistor between the R11 and R10 junction and the NFB should enter the circuit at the node between the new resistor and R10. The new resistor should be something like 10k to 22k.

[überfuzz]

I'm not sure I follow you... I try to 'decipher' this scheme: http://elektrotanya.com/PREVIEWS/634...chem.pdf_1.png

I think it's very hard to see everything in this picture. Are you sure there's a resistor between R10 and R11?

[g1]

Your PI is the same as the 6G11 with the exception of R10 and R21. R21 can be adjusted for taste, but R10 will affect the bias of the PI tube, so I think you will want to use the 1.5K stock value.

[Jazz P Bass]

That 100K 'feedback resistor' grabbed me as odd.

[überfuzz]

ok then, my next guess is 56k on the feedback resistor. Closer?

Thanks for all input, it really helps a lot.

How does the C17 capacitor affect the tone stack?

[Chuck H]

There's still a resistor missing from the PI. There needs to be a PI "tail" and then the resistor following would be the shunt for the NFB circuit. Simply changing the NFB series resistor to 56k while retaining the circuit as it is drawn won't fix the problem. I shot typical values for the tail resistor. It looks like the 6G11 uses a 6.8k tail and then a 1.5k NFB shunt with a 56k NFB series resistor. The 56k series resistor and the 1.5k shunt form a divider with a specific ratio. If you veer from this without understanding what's happening (or with an incomplete circuit) you're just shooting in the dark. With the 100k resistor you have in the schematic you could simply use a 3.3k shunt resistor for a very similar ratio to the stock circuit. But it doesn't matter a lick if you don't add a tail to your PI.

[g1]

I'm not following you Chuck, his R11 (6K8) is in the same position as the 6800 shown as the tail for the 6G11 (shown in link from post #3). Are you looking at a different 6G11 schematic or am I missing something?
As I said earlier, altering the 1500 (to 4K7) will mess with the bias of the tube, so I think he should stick with the 1500. Otherwise the only difference I can see is the 56K is changed to 100K.

[JoeM]

Was there a different schematic posted originally? I'm seeing the same tail, R11, as g-one.

[Chuck H]

Ignore all my above posting. Somehow I managed to see the circuit posted incorrectly and made a mistake.

As per the NFB ratio... The 100k series resistor is fine. The larger shunt resistor will INCREASE the NFB ratio. Go with a 3.3k instead of the 4.7k on the bottom of the tail. That'll be similar to the original. Or, as suggested, go with the 56k and the 1.5k shunt like the original amp.

[überfuzz]

I did change the resistors to the stock values. The thing is, im having a hard time reading the pixly schematics i found. Your input really helps a lot.

[Jazz P Bass]

Fender Vibrolux 6G11 Schematic

6G11 schematic.

[überfuzz]

6G11 schematic.

That's the one I have. Eh well lets just say I'm having a hard time figuring out what's on it... Your input has helped a lot!

[überfuzz]

At the moment I'm leaning towards ab763ing it a bit. I.e. add a mid control and a merge the bright/normal channels into one channel with a bright switch.

[Audiotexan]

Here's a much cleaner schem for those that might have trouble reading the others floating about.
vibrolux_6g11 (layout, schematic).pdf