12DW7 in V1 position

[RyanThomas13]

I want to put a 12DW7 in the V1 position where I can choose between the clean 12AU7 side (unbypassed) or the bypassed higher gain 12AX7 side. Would this be a good way to do it?

Switching 12DW7.jpg

[Stevem]

In terms of popping and switching noise from powering up each side you would be better off by adding another plate load resistor and another coupling cap and then switching at the caps.

A 7247 is nice to play around with also if you can get your hands on one!

[RyanThomas13]

So more like this?

Switching 12DW7.jpg

[mhuss]

Hmm, those symbols look familiar.

That'll work, but you still need to tie the open end of the coupling caps to ground with a large resistor if you want to avoid switching pops.

[pompeiisneaks]

Or you could do DPDT and have the other non active one on a resistor to ground as stated. Another interesting idea may be to have a pot there instead to 'blend' the two, so you can try 'all one, all the other or something in between'

~Phil

[Phil_S]

Am I missing something here? All this looks too complicated to me. Is it OK to use one resistor for each input grid and use the switch to connect the input jack to one or the other? BTW, in that case, it strikes me that each resistor should be 33K, but I don't think that's critical and the input resistor might be as low as 10K. You might even "tune" that input resistor to work better with the AU vs. AX side. (Yes, in this config, you need separate plate resistors and coupling caps.)

[sluckey]

I've used this option in several dual channel amps...

[joeboo88]

I've used Steve's switching method and it works great.

[RyanThomas13]

Thank you everyone, these are some really good options. I'm excited to try them out and report back.

Yes Mark I stole them from "Minimalist" schematic I believe

[RyanThomas13]

I went back to sluckey's on/on/on switch schematic and I just want to make sure I understand the important details.

Quick note: I'll be using a Mullard Blackburn era 7247 (I've been looking for a reason to use this tube for years!) I couldn't find a datasheet for this specific tube but I did find one for a Sylvania 12DW7 where pins 6,7,8 are referred to as "section 1" with gain factor 100 meaning ax7 side. I'm going to assume ( ) that the Mullard of similar era is pinned out the same, unless someone has better intel.

If wired like schematic then:

Left Position = 12au7 input engaged (unbypassed), 12ax7 input sent to ground
Right Position = 12ax7 input engaged (bypassed), 12au7 input sent to ground

Center = This is where I had to think for a minute. At first I looked and thought that it turned V1 into cascading gain stages, but this is incorrect. Its actually closer to a paralleled input triode, but one where both sides are biased differently, and that uses different tubes. That would be pretty abnormal under most circumstances correct? Yet this could yield interesting results?! I wonder which will have more gain, the ax7 side or the combined side?

[sluckey]

Hey, it's your idea. Wire it up and tell us what you think. If it's a good idea, someone will quickly steal it.

EDIT... Oh yeah the way I keep my mind right about the 12DW7 is this... The low number pins (1,2,3) has the low gain and the high number pins (6,7,8) has the high gain.

[RyanThomas13]

Ok will do! Thanks again for the help.