Preamp stage question...

so this is what i got. doing a homebrew, following a schematic and was wondering if this is a no-no or if i can get away with it. below i drew (rudimentary at least) what is layed out on the left and how i wired it up on the right. i have my homebrew all wired up, got sound through my speaker output (static actually) and i can intermittenly hear my plug going into the input jack though most of the time nothing. i'm in the process of quadruple-checking my wiring and i noticed that i wired my preamp stage cathodes like this.

will this adversely affect my grounds and cause no sound? or is this not the issue and i should move onto the next possible mistake.

thanks for your help as always.

(don't mind the black rectangle, its a cutNpaste void)

http://home.comcast.net/~zjcsiplon/pwpimages/preamp%20question%201.jpg

my .02 cents:

All preamp tubes I have seen in schematics use separate cathode resistors...unless this preamp tube is used for a long tail pair type phase inverter. When the cathodes are tied together, the triodes (assuming 12ax7) will work in parrallel, meaning, when the grid of triode 1 starts conducting, the 2nd triode will be affected likewise by the increase in electons present at the cathodes. Now, if you are not using cathode resistors, then it's all good.

Here, this may help explain more of what is going on. This was taken from the schematic i'm following.

http://home.comcast.net/~zjcsiplon/pwpimages/preamp%20stage%20question1a.jpg

I've never seen anything like that. Even the plate resistor is shared between the triodes. Is that a guitar amp schematic?

It's essentially running the tube as a single triode, right? I've heard mentions of that type of thing. I think I read about it being possible in a Gerald Weber book, but I've never seen it done in a guitar amp. I believe that Weber's assessment was that it was a good high-headroom, low-noise setup, but much less efficient in terms of gain than using the two triodes separately.

I'm not sure what you are trying to do here. Pins 3 & 8 are the cathodes of the two triodes. They can share a cathode resistor (and bypass cap); this is often done, and then you use a resistor value half of what you would use for a single triode (Ohm's law). Looking at your drawing, you have the cathodes jumped, going through two resistors in parallel (half the value, then) and two capacitors in parallel (double the value then). No reason why this won't work, though you are wasting a resistor and capacitor.

Fully parallel valves (shared cathode, grid, and plate) are common enough. Look at a Matchless schematic, the Clubman stage 1, for instance.

The advantage of using a common cathode resistor/capacitor for both triodes is you save a few cents, which is important if you are building 1000 amps. Advantage of keeping them separate is so you can tune each triode the way you want. Running the two triodes in parallel gives you a different effect; slightly more gain (about 30%) and what is often described as a 'fatter' tone.

In my amps, the dual triodes are either completely split (independent cathode resistor/cap, independent load resistor) or I am running them parallel (shared Rk/Ck, shared Ra).

In the second schematic, we see the two triodes are run in parallel with a shared 100K Ra (each tube 'sees' half of that, so gain is lowered); a shared Rk (2k7, again, this will be rather low gain), a shared Ck of .68, and a switch to parallel in other Cks. The paralleled Ck's change the frequency-dependent gain of the stage. This looks like one of Tubenit's drawings for his hot switch. I've done this many times, it works great.

As a matter of fact, my 'standard' first stage now looks a lot like this, and I got the idea from Tubenit, too.

steven

it's actually from a Bogen conversion schematic, two 12ax7's and two 6V6's. I wish i could remember which site i got it from, maybe schematic heaven. Interesting, never really knew what i was hooking up so now you got me curious to what it may sound like once i get it up. If your description Ben is anything close to what it may produce, its exactly what i was looking for. Any ideas on my OP?

it's actually from a Bogen conversion schematic, two 12ax7's and two 6V6's. I wish i could remember which site i got it from, maybe schematic heaven. Interesting, never really knew what i was hooking up so now you got me curious to what it may sound like once i get it up. If your description Ben is anything close to what it may produce, its exactly what i was looking for. Any ideas on my OP?

You got it from the hoffman forum, http://www.el34world.com

Schematics section. Probably the Carolina Blues Special or as you say one of the Bogen conversions. When the switch is in center-off, you will have a clean, low-noise stage. Switching in the (22uF) will increase gain; the 47uF will increase it more.

steven

sjhusting, you must have posted while i was typing. thanks for the input also, all this is getting me curious, i'll recheck my work tonight and see what i get.

There ya go sjhusting and its the Texas Blues Special. oh, i also removed the reverb stage, just to make things simpler for me or at least try too.

The hot switch is a great simple way to add flexibility; you can also parallel in another 2K7 Rk (for a total of ~1K3) which will change things, too. It's so simple and adds so much, it's just a standard part of my builds.

steven