component layout

[blues man]

The guitar signal goes through the first stage and flips 180 and then though the next stage and flips 180 which puts it back in phase with the guitar input. so I read in Kendrick's book keep a 3" distance apart if running wires parallel and the wires are in phase. No problem I understand.
In a typical fender design the signal goes through the first stage then flips 180 and then goes through the next stage and flips 180 but its not the same tube so its easier to keep a 3" distance on wire layout.
On a single input amp with 4 stages of gain ( 2 pre-amp tubes ) it becomes harder to maintain a 3" distance because you have 4 stages of gain and the tubes are next to each other.
My real question is what about the cathode cap and resistor in the layout. I see in a typical fender layout they are keeping a 3" distance between the cathode cap and resistor of one stage to another. a fender typically shares a cathode cap & res between 2 stages which seems to help maintain that 3" distance.
So can I put the 2 sets of cathode cap and resistors from stage 1 and 2 closer to each other or do they have to maintain a 3" distance? 
Or should the cathode cap & res of the second stage be 3" apart from 1rst stage which would put it right next to the cathode cap of the third stage.
Does it even matter?
I have built a few amps and they all work well but I didn't really consider maintaining  a 3" distance apart  on the cathode caps in the layout. I have been working on another layout on a high gain amp and I would like to keep parasitic oscillation down to a minimum. I hope this didn't sound to confusing.     

[Paul1453]

3" sounds rather arbitrary to me.   

HBP posted something about distances between signal wires before, I can't recall where.

But there are a number of factors that calculate into reasonable separation distances.

Voltage level is a significant factor.

You could shield wires you thought might be too close.

The board layout of components is something I'm not sure about.

But I think there are many examples of layouts where multiple preamp tubes don't have 3" separation of their components.   

[eleventeen]

This is something I think you can comfortably forget about. A typical Fender cathode sits at under 2 volts, on top of a very small impedance 820/1500 ohm and bypass cap in parallel...while the tube is passing 1 milliamp. This is just miniscule power, tiny. This is not where you are going to be seeing big voltage swings that could broadcast to somewhere else in the amp.


Now it *IS* absolutely possible that if you share a cap between two tube sections whose signals are in phase, yes, that can produce feedback...because it IS feedback. This does not happen in a real Fender but of course can in a homebrew amp, and, it HAS caused problems with amps that forum members have built. It's also possible that if you run a plate circuit (where we would expect to see much larger swings (working under a 100K resistor, 50 times as large as the cathode resistor, right next to somebody else' grid circuit, yes, you could have bleed.


Most builders here do not like to share cathode caps UNLESS it is a direct copy of a no-problem proven design. Just one of those things, a 25/25 uf is such a cheap part, better to avoid the problem altogether by not sharing.

[blues man]

Good answer. I wasn't even thinking about what the voltage is there. I just started another build using everything left over from previous builds and was considering using terminal strips instead of fiber board and was looking at what i would do with the layout. I just built a super reverb without the vibro and used the xtra tube for 2 more stages of gain with vol controls for each stage and made a 5 stage pre-amp. It sounds way better than I thought it would. I'm going to build this one without reverb but with vol controls on drive stage.
Ive never shared cathode cap&res with any of my builds.

[eleventeen]

I did that once, on a SF PR about 30 years ago. Pulled out the trem (which I never use) and repurposed that tube into a fairly ordinary add'l gain stage. I had many years of electronics experience but really knew almost nothing in terms of book knowledge about how to do so in terms of selecting components, it was simply an extra triode stage with a 100K plate resistor and 1.5K cathode R and a blocking cap on the output, shoved into the signal path like a fuzzbox. 500K "volume" control leading into the extra stage. Very vanilla. I had already boosted the amp to 6L6 power with transformers pulled from a Dynaco Mk IV 35-watt hi-fi amp and an Altec 12" speaker that I somehow shoehorned into the cabinet.


Holy crap! The amp became a flaming, on the edge-of-out-of-control loud, loud monster with a VERY aggressive sound and would feed back on a whim. It was actually quite fun for the stuff I was playing back then. It was a very rude little brat!


As for building the extra stages on term strips....well you sort of have to since you are going to depart from the stock circuit which the standard parts board demands. There is no question you're going to have to work for "best practices" when it comes to boosting the gain(s) in order to avoid hum and noise issues. Yes, try to maintain separation between wires, yes, you'll have to pay attention to how you lead the wires for the volume controls to the front panel and what those wires cross....probably have to use shielded cable and you will almost certainly have to whack the gains of the add'l stages you put in. Keep leads short....and try to document what you do so you can backtrack if you work yourself into a corner. It's really all that complicated but it *can* be a challenge to not end up with a hum and noise monster.

[blues man]

when I added the 2 xtra stages to the super reverb I used the drive section from the humble overdraft except I didn't put in the xtra tone stack or the clean master. I did use a 1M res in place of clean master. I also used 2.5mf &2.7res cathode bypass on both sections. It worked really well. I really learned good  info doing this mod. very nice distortion.