Input Options for parallel first stage

[turtle441]

I'm working on a modified Matchless Spitfire/Lightning style build with a parallel first gain stage.  As I was thinking through actually drilling wholes for the physical layout, the question occurred to me, do I really need 2 of the 68k resistors on the V1 socket for this?  Couldn't I just run both input wires into the same resistor before it got to the socket?  (Or, is this somehow tieing that side to the 1M resistor mounted on the input jack, making 2 hi-input jacks?)  The more I stare at this, the more I think I'm overthinking it. 

I don't think any of the Spitfire or Lightning layouts that I've seen only used 1 resistor, but I guess that doesn't mean that it wouldn't work.   

Real reason this has come up is because I'm putting this in an old deluxe reverb chassis and the placement of the 2 resistors is a little awkward (going to either cross a bias wire or a heater wire) because of the orientation of the sockets.  Yes, I know I could just drill 2 new wholes and rotate the socket... but where's the fun in that?

[PRR]

The reason for two jacks and two resistors:

You can mix two guitars with less interaction one to another
You can cut-down a too-hot guitar so it does not overload the first stage.

I believe these are useful functions. If you don't, then wire one jack and one 33K (not 68K; the jack switching defaults to two parallel 68K).

[sluckey]

Here's my Lightning with only one input jack and one grid stopper resistor...

     http://sluckeyamps.com/lightning/matchless_lightning.pdf

But if you have two input jacks you really should use two resistors to provide some isolation between jacks. You never know when someone may want to plug in and play along with you. If you don't see this ever happening then you should really consider using only one jack.

[turtle441]

Awesome.  Thanks for the info.  I think I'll be sticking with the 2 resistors for now. 

[dude]

Here's my Lightning with only one input jack and one grid stopper resistor...

     http://sluckeyamps.com/lightning/matchless_lightning.pdf

But if you have two input jacks you really should use two resistors to provide some isolation between jacks. You never know when someone may want to plug in and play along with you. If you don't see this ever happening then you should really consider using only one jack.

Could you use that M/V with the cut and "fixed bias"?

al

[sluckey]

Yes. I don't care for that MV though.

[dude]

OK, in that schematic, putting a M/V just before the PI (just a 500K single pot) in place of that 200ohm or putting a gang pot after the PI, like you have (I believe it's call Lamars...?). In your opinion, what would the difference be?


al 

[sluckey]

OK, in that schematic, putting a M/V just before the PI (just a 500K single pot) in place of that 200ohm or putting a gang pot after the PI, like you have (I believe it's call Lamars...?). In your opinion, what would the difference be?

I'm not sure what you are talking about? There is no 200Ω just before the PI. And I don't have a dual gang pot after the PI. What I have in that schematic is a single pot after the PI and it's called a "CROSSLINE MASTER VOLUME". And I don't like it. And there are a lot more people that don't like it either.

[dude]

Sorry my mistake, the 220K and .01 cap going to the grid of the PI tube (forgetting the effects loop), a M/V, 500K pot in place of that 220K. I should have explained better. What I was asking is comparing, in your opinion and maybe most others, is a master volume as written above (in place of that 220K) or a master volume after the PI, a gang 1 Meg pot as below. The attached schematic is cathode biased but I assume it can be used for fixed bias too.

[sluckey]

If your goal is simply to turn the volume down then I would use the simple pot before the PI. But if you're all about overdrive like most guitar pickers are, then the simple pot before the PI (let's call it pre PI MV) will allow you to really crank the preamp volume for preamp distortion and drop the signal level feeding into the PI back to a sane level. The dual gang pot placed after the PI (let's call it post PI MV) will allow you to also overdrive the PI stage and then drop the signal back to a sane level before it hits the power tubes. For a really versatile machine, use both MVs. And for the ultimate distortion machine, use both MVs and some kind of speaker attenuator.