Question about bypass caps (in first ever AX84 P1eX build)

[simonallaway]

Good afternoon all,

My wish came true this weekend... i.e. I finished wiring up my P1-eX and it didn't work first time. 

So I am debugging it. I have triple checked the power supply (I started up with a current limiter in series, and have measured voltages at all pins and they're all inside 15% of schematic). I had initially used the wrong wires on the 270EX, but it's good now (2 fuses later).

The amp is silent with preamp and volume down, when I turn them up I get two different types of hum on each (could this be the difference in 120 hz and 60 hz?) and intermittent "putt" noises (very short "p" sounds about 2 seconds apart. If I turn both volumes way up, I get squeal.

My actual question is this: Should the cathode bypass caps be electrolytic? The schematic implies yes, the layout is ambiguous. I bought non electrolytic caps, so I wondering if this could be screwing up the bias for the 12AX7s.

Link to schematic and layout: http://ax84.com/p1x/AX84_P1x_080623.pdf

Thanks,

Simon

[phsyconoodler]

Non-electrolytics are just fine.
  The only thing that can make the amp not work is what you have done.Assuming all the components are ok,you have miswired something.
Your work is nice and neat so it should not be hard to find the issue.
  Triple check your input and output jack wiring.A grounded input will not work.
Check the cathode voltages of the preamp tubes.
  I have seen so many first time builds with mis-wired input jacks or even grounded out shielded wires to grids.

[tubesornothing]

Bypass caps can be either electrolytic or non - wont make a difference.  

As recommended in the p1 build guide - did you trace the schematic with yellow hilighter, and double check all parts, connections, check all grounds?

Squeal could be parasitic oscillation (usually lead dress), or the OT NFB wire is backwards.

To determine the type of hum - a Bb on the guitar on the low 6th string is 120Hz.  120Hz hum is a filter issue, 60Hz is a heater issue (usually a bad ground).


The put-put might be "motor boating".  The B+ filtering is not good enough.  Check all the grounds and connections around your filtering.

[tubesornothing]

There are two tube sockets that have no wires attached - what are they for?

[simonallaway]

  Triple check your input and output jack wiring.A grounded input will not work.
Check the cathode voltages of the preamp tubes.
  I have seen so many first time builds with mis-wired input jacks or even grounded out shielded wires to grids.

That makes sense. I will check (and probably re-wire) the inputs and outputs, and be more careful.

Thanks,

Simon

[simonallaway]

did you trace the schematic with yellow hilighter, and double check all parts, connections, check all grounds?

I did, but I'll check everything again. I used a green hilighter...does that affect tone? 

Squeal could be parasitic oscillation (usually lead dress), or the OT NFB wire is backwards.
To determine the type of hum - a Bb on the guitar on the low 6th string is 120Hz.  120Hz hum is a filter issue, 60Hz is a heater issue (usually a bad ground).
The put-put might be "motor boating".  The B+ filtering is not good enough.  Check all the grounds and connections around your filtering.

I'll check all that too. Perhaps poking around with a chopstick and then re-flowing if I find a dry joint.

Thanks for the hints!

[simonallaway]

There are two tube sockets that have no wires attached - what are they for?

For future modifications. I might different output tube types to listen for any differences. And maybe even reverb or tremolo. 

[bigdaddy]

It looks like you are using a poor grounding scheme. Break the ground from the preamp and power section and tie the preamp ground to the chassis at a different point. the power section and AC ground wire should be tied to one leg of the transformer. Tie the preamp ground to somewhere closer to you preamp, make sure the bolt is tight.

The preamp ground should start with the filter cap and everything should go to there in a star ground or as close as possible as per Merlin's advice.

[simonallaway]

It looks like you are using a poor grounding scheme. Break the ground from the preamp and power section and tie the preamp ground to the chassis at a different point. the power section and AC ground wire should be tied to one leg of the transformer. Tie the preamp ground to somewhere closer to you preamp, make sure the bolt is tight.

The preamp ground should start with the filter cap and everything should go to there in a star ground or as close as possible as per Merlin's advice.

Thanks for the advice on grounding...I'll study the Merlin documents. I just went with the layout from the P1eX drawings, but perhaps I did it wrong.

I also noticed I soldered the shield of the input wire to the tip of the input socket *cough*. I will have time this weekend to check and remedy all this stuff. This is what I wanted though right? A broken amp to debug 

[tubesornothing]

I also noticed I soldered the shield of the input wire to the tip of the input socket *cough*. I will have time this weekend to check and remedy all this stuff. This is what I wanted though right? A broken amp to debug 

Nothing like a good learning experience!  I suggest you stick with the P1ex build guide.  It has worked well for a lot of people.  Once you get it working, then mod it if you like.

[simonallaway]

I also noticed I soldered the shield of the input wire to the tip of the input socket *cough*. I will have time this weekend to check and remedy all this stuff.

Well, fixing the input jack wiring gave me a result.

I powered it back on and gently turned up both pre and power amp volumes...and heard...nothing. Before I was hearing screeching and hiss from beyond the grave. So I reach over and flick the strings of the guitar I had plugged in, and CLANNNGGGG! there it was.... glorious music!

I now "get it". I thought I was addicted before, but now I've assembled an amp, it's different. Thanks again for all the help 

I plan to revisit the grounding based on reading some Merlin docs. It's pretty quiet now, even when cranked, but I'll do it anyway just to see what difference it makes.

[tubesornothing]

I plan to revisit the grounding based on reading some Merlin docs. It's pretty quiet now, even when cranked, but I'll do it anyway just to see what difference it makes.

Congratulations.  That's a good way to approach it, one changes at a time between tests.

[labb]

Re: Groundiing of the P1ex...The grounding as shown for that amp is buss grounding with one ground point on a grounding lug right at the input jack.( and a seperate AC input power ground where the AC enters). The board is laid out such that the sections to be grounded are in the correct order..It is fun to mess with the grounding scheme but in the end I think that you will be hard pressed to improve on what you have. Phil from over on the Marshall 18watt board designed that amp. If it is your first, I think you will really enjoy it. So now get ready. Next will probably be the HO, then come the SEl and then comes.............So many amps to build, so little time.

[simonallaway]

I am sure the design is excellent, labb, but you are assuming I wired it correctly 

Also I'm determined to learn more, so tweaking around with it myself and using it as a place to study is always going to be useful. And as tubesornothing said (and all my experience in software), change one thing at a time.

Here's a picture of the P1eX is assembled on its highly sophisticated work-stand:

[labb]

The P1EX grew out of the P1. Take a look at that schematic and compare it to the P1EX and you will see the changes that were made to increase the gain. And Welcome to the club..Good luck.

[tubesornothing]

Simon, do you have the P1EX tweakers guide?  Lots of useful tweaks in there...

[simonallaway]

The P1EX grew out of the P1. Take a look at that schematic and compare it to the P1EX and you will see the changes that were made to increase the gain. And Welcome to the club..Good luck.

That'll be my homework for tonight 

[simonallaway]

Simon, do you have the P1EX tweakers guide?  Lots of useful tweaks in there...

No sir I do not. I can't find it online either, using those terms. Do you have a link?

[tubesornothing]

Oops, turns out I was talking about the p1 HO, not the p1 extreme.  Here is the HO guide anyways...

[simonallaway]

Oops, turns out I was talking about the p1 HO, not the p1 extreme.  Here is the HO guide anyways...

Thanks for that. It looks like I would benefit from studying the evolution from P1 -> P1eX -> HO (if indeed that's how it went). Then I'd be in a better position to attempt any mods.

The amp sounds really good right now. Much better than I imagined it would. I was expecting ...well, I don't know what I was expecting, but this thing is usable even now. For the output stage I used a 6V6, but I'll try it with an EL34M too. I hope to record the before and after for comparison. Gives me an excuse to just play the damned thing 

[labb]

Here is how to get to some of the Mods:

Go over to AX84
open Classic Amp Projects
open AX84 P1-Extreme
go to the bottom of the page and open(The Older Version)
open P1-Extreme
go down to Varation Schematics.....That will give you a few different versions of the amp.

[simonallaway]

...go down to Varation Schematics.....That will give you a few different versions of the amp.

Did you mean this document? http://ax84.com/index.php/oldprojects.html?project_id=p1-extreme

[simonallaway]

I've posted my build experience and pictures here:

http://hotbottles.wordpress.com/2010/05/11/finally-a-functioning-ax84-p1ex-tube-amp/

Thanks to everyone on the forum. I couldn't have done it without you.