Line in to 200S power amp

[Cartoonist1]

Hi,

I have a 200S clone, which is my primary bass amp.  I also want to run my synthesiser through it, actually 4 synths through a 4 channel mixer - giving out a line signal.

I want to try connecting straight into the power amp, to bypass the controls and any unnecessary gain.  So I would appreciate advice:

I'd like to use the bottom input jack to go straight to the power amp. So, would it work if I I disconnect it from the resistor input to the 12AX7 and then put a new line directly into the phase splitter?  And would this be pin 2 of the 6AN8?

Thanks
C

[Isaac]

Interesting question. I'll give you my short answer: I don't know.

More in depth, yes, pin 2 is the input to the inverter. I would think you'd want an isolation resistor between your proposed power amp in and its connection to the tube. I don't know whether or not the 68K resistor on the input jack would be right or not, but it might be fine. I don't know if the voltage would be right or not. The schematic I'm looking at doesn't show a DC blocking cap in line with pin 2. Probably a mistake in the schematic, but, if not, that could be a problem.

What do you mean by "bottom input jack"? My 200S has two input jacks, side by side, labeled 1 and 2.

All in all, it ought to be worth a try. If you know what you're doing, you shouldn't harm the amp or yourself. Of course, if you don't know what you're doing, you could kill yourself, so there's that.

[Cartoonist1]

Thanks Isaac

I used a smaller chassis for this build and placed input 1 above input 2.

No there isn't a cap, and there doesn't appear to be any in the photos of the original 200S internals I have seen online.  I'll look into this and the voltage / resistor values.

[Isaac]

Taking a closer look at the schematic, I see that the DC voltage on pin 2 is 75V, and pin 3 is 90V, so the bias is probably fine. Next question is, is it okay to put 75V on your mixer's output? You might need a blocking cap for your line in.

[Cartoonist1]

Thanks Isaac

I'll check this out. 

[_peter]

Hi,

of course, going to pin 2 of the 6AN8 (i.e. the grid of the triode section) is NOT the way to go.

For one thing, your mixer's output doesn't deliver enough voltage to drive the power amp. The triode phase inverter only has a gain of ~1. So you need another gain stage.

But above all, you will mess with the phase inverter's bias point when connecting something to its direct coupled grid - and possibly even destroy your mixer due to the high DC voltage.

Just go from the input jack to the grid of the 6AN8's pentode section, which is pin 8, if I'm correct. Shielded cable and a grid stopper wouldn't do no harm.

Cheers,
Peter