2550x Trouble shooting

[SILVERGUN]

The path of the clean signal through that area is just as important as the 47K resistor being there, and just as important as the diodes being OK

Use this diagram to double check that your paths match the paths from the layout....take a close look at the area circled in pink and see how the switching works.....if your path doesn't match this path 100%, then there is a problem

[Jaymz77]

Yes thats zero volts to ground

[Jaymz77]

Ill go over that tonight.
Im leaning towards something failing tho...

[SILVERGUN]

Yes thats zero volts to ground

If there is no voltage (0.0vdc) on that pin, then the triode is not working and there is your signal loss

The path there is simple....
Pin 8 should go to a 470 ohm resistor to ground....if that is happening, you'll see some Vdc on that pin and the triode will work as intended...
If there is any reason that there is not a good solid path to ground through that resistor, that would explain your problem.

[Jaymz77]

Hmm ok.
I am at work today (public holiday yesterday) so I will have to check tonight.
If everything is wired up correctly, can you think of any reason why that would be? Or just a bad tube?

[Jaymz77]

Just reading your post again... Could it be caused by that 470ohm resistor failing if the wrong is correct?

[SILVERGUN]

Could it be caused by that 470ohm resistor failing if the wiring is correct?

Yes, if the resistor is open (or opening after heating up) then there is no path to ground for the cathode.....in a new amp build, it would be more likely that there is a problem with wiring or solder connections...
Check wiring, solder joints, etc....

With no power, you should read 470ohms from pin 8 to ground....

[Jaymz77]

Ok I'll check all that out tonight.
Thanks.

[SILVERGUN]

Thinking more about this Jaymz,,,,
If you had no path to ground for that cathode, the signal would just die there, and you wouldn't get any sound.....I have a feeling you're gonna see approx. 0.7 Vdc on Pin 8.....unless there is some intermittent issue

Looking closer at that stage you've got a 220K plate resistor, with a 470ohm cathode resistor, and you could be pushing that stage through saturation, with too hot of a signal hitting the grid....(that's what it sounds like)
And that would make more sense, based on your results with the ME-50 in the loop.....it was dropping the signal enough to not push that stage as hard as when it wasn't in the loop...

If all goes well with your tests, I would raise the value of your cathode resistor (on that stage) up to 1K and see if that makes a difference in the fizz

It looks to me like they designed that stage to clip as a result of saturation,,,but if the input signal is too big, it could be pushing straight through to cutoff

There are guys here who can get more in depth about this.....consider me troubleshooting 101,,, and read this (to get really confused):
http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

This might help:
http://vimeo.com/33244955

[Jaymz77]

Thanks for that.
I have no idea which is the more likely situation, and I have next to no idea what each parts job in the circuit is.
But based on SOUND alone, that seems to describe what I'm hearing.
The fact that I can now control the buzz with the gain pot kind of makes me wonder if the amp Sounds as it was intended, just the overdrive is not to my ears. Having said that, I find it hard to believe that a real jubilee would buzz like this.

Cheers

[SILVERGUN]

I have no idea which is the more likely situation, and I have next to no idea what each parts job in the circuit is.
But based on SOUND alone, that seems to describe what I'm hearing.
The fact that I can now control the buzz with the gain pot kind of makes me wonder if the amp Sounds as it was intended, just the overdrive is not to my ears. Having said that, I find it hard to believe that a real jubilee would buzz like this.

I am starting to think that I am definitely onto something with your specific issue....If you find that changing the 470ohm resistor to a 1K helps, then you can try a couple of different values there to get it where YOU want it......higher values should get you further away from saturation clipping, but also further away from the intended design..
...we can also consider dropping a little signal by lowering the value of the 1M grid leak resistor that feeds that stage, or installing a pot in it's place to make it adjustable (like an effects return pot)...(the 1M that's shown on the effects return jack in the layout)

From what I've read,,,Frusciante uses his Jubilee for 1 specific sound, so maybe he just doesn't push the gain controls as far as you are  ....and maybe there is a good sound to be had there...
And I remember reading where guys would remove the diode clipping part of the circuit because it sounded too much like artificial distortion.......add that to the saturation clipping stage and maybe you have a buzz machine?

WE can alter the circuit to fit your tastes, but for now we should stick to getting your issues figured out, and make sure you have a healthy amp to start with.

[Jaymz77]

Given my new found controllability since installing the missing resistor I would say you are right re frusciante. He uses his jubilee for crunch only, the major is for cleans. I have a couple of pics of his amp settings and if I match those setting the amp sounds damn like his UNTIL it starts fizzing out. With that in mind I'm leaning towards a bad component.

Like Timbo, I have a switch to bypass the rhythm clipping giving me clean clean, clipped clean and lead. I actually like the diode clipping unlike a lot of people. All of this has been with the rhythm clipping off, so it should not be so buzzy.

I might look at tweaking to taste at some stage but for now I just want to have the amp working  as it should without buzzing.

I will try replacing the resistors with same values first to see if that fixes the buzzing. The. I might change some values.

[Jaymz77]

I just re checked the voltage on p8 V2.
It started at 0.17 volts, i then watched it slowly rise to 0.19 volts before i stopped measuring.
Is this right?

[Jaymz77]

I just left the amp on for about 10 mins and came back.
Voltage on p8 V2 has levelled off at 0.380V.

[SILVERGUN]

Voltage on p8 V2 has levelled off at 0.380V.

That is a low, but OK reading... it doesn't bias the triode ideally....I'm assuming that they designed this stage to clip this way,,,and it could be some of what you're hearing..........this is where a scope comes in handy,,,you see the wave going in and then see the difference of the wave coming out.

The cathode resistor size is chosen to bias the tube...the voltage drop that occurs as a result of that resistor puts the cathode more positive than the grid, which is how the tube needs to operate....the larger the cathode resistor, the more positive the cathode will be in relation to the grid, and this will get you away from a bias point that causes clipping........it puts the triode closer to center biasing, and "leaves more room" for a "hotter" input signal.

Did you try replacing the 470ohm cathode resistor with a 1K?......if you really want to hear a change go to a 2.2K, and see how that sounds, and YOU can decide if you like it better that way, with less clipping.

Also,,,did you verify your paths coming off of the clean/rhythm switch?

[Jaymz77]

I had a quick look in the limited time I had and it looks ok.
Didn't have time to solde anything. Had my guitar lesson.
Tonight I'll have more time

[Jaymz77]

So...
given that idiot me had forgotten a resistor, I'm just wondering if that could have put extra strain on any other components causing them to fry/ partly fry?
Or are the 2 resistors mentioned below the only targets?
We have the 470 before p8 V2, the 1m coming off the effects return jack...
Anything else a possibility?

[SILVERGUN]

given that idiot me had forgotten a resistor, I'm just wondering if that could have put extra strain on any other components causing them to fry/ partly fry?

Probably not,,,but I'm most concerned about where you put the 47K when you put it in....go back to reply #100 and confirm that your path looks exactly like that........did you ever replace the diodes?

We have the 470 before p8 V2, the 1m coming off the effects return jack.

The resistors themselves are probably fine....check your wiring and solder joints...the 470 is just low for that spot, and it might work better for you to put a slightly higher value (reduce some buzz)...
As far as the 1M, I suggested replacing it with a pot so you could dial in the signal level hitting the grid of that stage....you might find that slightly reduced signal wont clip as hard......to try it without making it permanent, you could put a 1M across the existing 1M and that will drop the value to 500K  (clip it in or just solder it across the 1M) and see how that sounds.

Anything else a possibility?

Yes, the other 70 components that we haven't ruled out yet 

I would try a new tube in the V2 location (if you had one) ....just because that tube is biased "on the edge" I would want to see if a different tube handled it better....

[Jaymz77]

Yeah that's how it looks. It was the 47k you have not erased that I forgot.
I'll try to get a spare tube today. Or could I swap the tubes around to see if that's the issue?
I'll play around with the resistors tonight.
1M down to 500k
470k up to ??? What do you suggest here?

[SILVERGUN]

Yeah that's how it looks. It was the 47k you have not erased that I forgot.
I'll try to get a spare tube today. Or could I swap the tubes around to see if that's the issue?
I'll play around with the resistors tonight.
1M down to 500k
470k up to ??? What do you suggest here?

Yes, you can try swapping the tubes around and see if that changes anything...
-1M down to 500K is good
-And 470 ohm up to 2.2K and you should definitely hear a difference in the clipping of that stage.......you can always lower it back down and try 1K by putting a 1.5K across that new 2.2K

[Jaymz77]

Oh, and yes I changed the diodes. Changed other diodes too for peace of mind.
Sweet I'll let you know how I go.
Thanks again

[Jaymz77]

Ok here's what I've got...

I swapped V2 and V3... noticeable difference, not MASSIVE but noticeable. Less fizz/ fuzz/ buzz (whatever you want to call it), probably at a level that i would think was normal if i had bought the amp. Voltages much the same though.
I don't have a 500k resistor so i put another 1M in parallel to the existing one just to see what would happen. I think it made a slight difference but so small i can't really be sure if it actually did or not.
I don't have a 2.2K resistor currently either so i tried a 1.5K in place of the 470ohm one... and THERE IT IS! The 1.5K made a huge difference, it seems to have just pulled the gain level back a touch so it no longer sounds fuzzy, just gives that ever so slight break up. Voltage on pin 8 V2 is now 1.275 volts.
With the 470ohm in place (and the missing resistor) i can clean it up by winding the gain back a bit (this affects the tone though) or by using a bit of compression, however that makes the last little bit of the gain dial useless to me. Now, with the gain cranked its seems spot on. Introducing compression now though maybe cleans it up a touch too much (ill have to play around with it to get it right). All that said, i prefer how it is now compared to that fuzzy crappy sound.

Im very happy that the source of the buzz seems to have been located (knowing me i have now spoken too soon) and i thank everyone who has donated their time and knowledge to helping me sort this out. The truth of the matter is that i have learned more from the trouble shooting process than the build itself.

I guess it all boils down to taste now doesn't it? Decisions decisions...
The purist in me would prefer to leave the amp as it was originally designed, but i honestly can't see how that buzzy shithouse sound is useable, I've never liked that fuzz type of OD. I tend to range from that gainey Frusciante pushed clean, to ACDC type crunch, to all out high gain 80's metal type stuff... In other words all MARSHALL, and that fuzz just does not say Marshall to me, it says Hiwatt. Funnily enough i scored a Hiwatt custom 20 at a steal just before i started the build. I had never had anything to do with Hiwatt previously and i only bought it because it was hardwired and cheap. I ended up getting rid of it (for a profit) because i hated the FUZZY OVERDRIVE, and shortly afterwards decided to build my own. I wouldn't say my amp sounded exactly like a Hiwatt but it was heading into that territory.

So again, i hope this victory email is not premature, and again, i would like to thank everyone who has assisted me. MUCH APPRECIATED. Lets hope this is case closed.

[TIMBO]

 

[SILVERGUN]

The truth of the matter is that i have learned more from the trouble shooting process than the build itself.

And that's how it goes Jaymz.....that's the most important part of this.
Since you built that amp,,,it's only right that you fix it.

Congratulations on finding and fixing the problem.....we have all learned this way, by trial and error,,, and we will all continue to learn.
 
Cheers!!!  mate