2550x Fiddling...

[Jaymz77]

Hello all, its been a while since i posted.
I have been enjoying my 2550x for the last few months and damn it sounds good. I have held off posting pics or sound clips until i reach that NEVER TO BE OPENED AGAIN stage. Im ALMOST there.
I had the amp open because i built a new cabinet for it so i decided to play around with the bright cap.
I had previously removed it because i thought the amp was too bright. However i also had teething issues at the time which have long since been resolved.
I thought the amp might sound different with the bright cap installed now that the amp is burned in etc.
Well it does sound better i think. Not ear slitting bright like i remember it, but it has more "life" to it.


Anyway, i went back through my trouble shooting thread to refresh my memory ("2550 trouble shooting").
It was concluded by TIMBO and SILVERGUN that my V2 pin 8 voltage was too low causing saturation. The original schem shows a 470 ohm resistor, i upped it to 1.5k.
Today I swapped the 1.5k v2 pin8 resistor back to the 470ohm one just to see what the effect was and it went back to sounding like shit. So back in went the 1.5k. NEVER TO BE TOUCHED AGAIN.
However, today i was just browsing schems and i noticed that the re-written schem that i downloaded with the layout drawing ACTUALLY SHOWS A 1.5k RESISTOR!!!
I wonder what happened there? Whether its a mistake or if the schem was deliberately modified for some reason.
I then started looking to see if anything else was different between the 2 schems.
The marshall schem shows a 100k resistor connected to v2 pin 6, but the schem my layout was based on shows a 220k resistor, which is what i installed.


What sort of effect is this 220k having over a 100k resistor?


I thought i would try putting a 220k resistor in parallel to the existing one to drop it closer to 100k. I can't say i really noticed a difference, but then again the solder joint was only temporary so might not have been real good.


Im just curious as to why the modifications were made to start with (which i don't expect anyone here to be able to answer) but also what effect changing those values will have.
It seems strange to me that the marshall schem shows a 470ohm off v2 pin8 but when its installed it sounds like shit. No Slash or John Frusciante recording i have ever heard sound muddy and buzzy like that, yet that value is what the schem shows.


I have no idea what difference the 220k/ 100k off v2 pin6 would do so if anyone can enlighten me….


Thanks

[SILVERGUN]

Hey Jaymz, glad to see you back...
My 'fix' for you was basically a Band-Aid that was intended to get you up and running...
Anytime you change the value of those resistors you are changing the bias point of the triode, which affects how clean or clipped that stage tuirns out.

Take a look at this video and see if it answers any of your questions:
http://vimeo.com/33244955

[Jaymz77]

Thanks Silvergun.
Can you see how the original shows a 470 and the other a 1.5k on v2 pin 8? Out of pin 6 v2 the original has a 220k and the other a 100k.
What will dropping to 100k do? I have about 200v on that pin with the 220 in place.
With a second 220 in parallel it seems to (possibly) clean up a smidge and voltage drops to 150-160v.

Cheers

[SILVERGUN]

Can you see how the original shows a 470 and the other a 1.5k on v2 pin 8? Out of pin 6 v2 the original has a 220k and the other a 100k.

No...I'm not following...
They appear to be the same values to me, but one triode is labeled V2A on the one schematic and the other is calling that same triode V2B
They are just labeling the tube differently in the schematic.

What will dropping to 100k do? I have about 200v on that pin with the 220 in place.
With a second 220 in parallel it seems to (possibly) clean up a smidge and voltage drops to 150-160v.

Dropping the plate load resistor will lower the gain of the stage, increase the current drawn by the tube and in turn show the increased voltage drop that you are seeing.

[Jaymz77]

You can't see it?
V2b shows a 470 on pin 8 and a 220k on pin 6 in the marshall drawing.
V2b shows a 1.5k on pin 8 and a 100k on pin 6 the other drawing.

When I put the other 220k in parallel with the existing one on pin 6 the effect was only very slight but it did seem to give it a touch more clarity. Didn't seem to drop the gain too much but I guess it must have to get the extra clarity.

In my trouble shooting thread when I gave the voltages BEFORE we solved my issue I had about 150v on pin 8 v2, now with the problem solved (1.5k in place of the 470 on v2 pin 8) it has about 200. But as I mentioned above it drops to 155 or so with another 220k in parallel.

[sluckey]

You can't see it?
V2b shows a 470 on pin 8 and a 220k on pin 6 in the marshall drawing.
V2b shows a 1.5k on pin 8 and a 100k on pin 6 the other drawing.

As SG said, the two circuits are identical. Only the names of the tubes are changed.

[SILVERGUN]

Look again...
The different schematics are just using different names for the same stage.

So what is V2B on the Marshall drawing is actually V3A on the other drawing.

There are no constants when drawing schematics, and different people will name the same items differently,,,including using the pin numbers in no consistent order,,,,unfortunately

When I put the other 220k in parallel with the existing one on pin 6 the effect was only very slight but it did seem to give it a touch more clarity. Didn't seem to drop the gain too much but I guess it must have to get the extra clarity.

In my trouble shooting thread when I gave the voltages BEFORE we solved my issue I had about 150v on pin 8 v2, now with the problem solved (1.5k in place of the 470 on v2 pin 8) it has about 200. But as I mentioned above it drops to 155 or so with another 220k in parallel.

Read my previous response again and watch that entire video 10 -12 times until it becomes clear. (I did)
The answer is there.

[Jaymz77]

I think I see what you are saying.
The marshall drawing has v1 a and b, v2 a and b, v3 a
The other drawing has v1a, v2 a and b, v3 a, v4a

The other drawing has not labelled v1b which is why there is a v4a, correct?

I'll watch that video.
The voltages are of no concern? 150 or 200 on v2 pin6. Just alters the gain at that stage?

Cheers

[Jaymz77]

Ok, i watched that video and it makes sense.
However, i still don't know why…
a) the amp sound like shit when built EXACTLY to the drawing. I find it impossible that the buzzy shit sound I'm getting is what people wet their pants over.
b) if i make it sound correct (or as close to as i can so far) by changing resistor values i get double the voltage on v2 pin6 ( i mean, i know why, but why does it takes that to sound right).


I found a pic that shows what each pin voltage should be with a B+ of 460v. My v2 is way off. I have 200v on pin 6
See attached

[SILVERGUN]

I found a pic that shows what each pin voltage should be with a B+ of 460v. My v2 is way off. I have 200v on pin 6

And what voltage on Pin #1 ?

I ask that because of the same scenario we talked about yesterday......your Pin 6 could be their Pin 1 

And we should get a reading from 3 and 8 also...

[SILVERGUN]

However, i still don't know why…
a) the amp sound like shit when built EXACTLY to the drawing. I find it impossible that the buzzy shit sound I'm getting is what people wet their pants over.

A simple wiring error would be hard to spot...
An incorrect resistor value somewhere could wreak havoc...

If you go over everything with the finest tooth comb you can find, and find zero errors,,,and are 100% sure that you have built an exact replica of the original circuit, then the amp will sound how the amp is supposed to sound.

If I did all that and it still sounded bad I would start the process of making it sound better by eliminating the diodes that we were looking at last time....
That's just me.
 

[Jaymz77]

I actually DID replace all the diodes last time.

Looks like I'm going to have to check EVERY component. Argh...
The way it is right now it sounds fantastic. Maybe I just leave it.
Will having 200v on v2 pin 6 cause any dramas when I'm supposed to have 100v?
Would I be right in assuming it will just be cleaner?

[SILVERGUN]

1)Will having 200v on v2 pin 6 cause any dramas when I'm supposed to have 100v?
2)Would I be right in assuming it will just be cleaner?

1) no
2) yes

I actually DID replace all the diodes last time.

I was suggesting getting rid of them all together or at least bypassing them temporarily to see how it sounds...
You would have to be able to pull that off.

If it sounds great, I wouldn't change a thing....play it mate

[Jaymz77]

Alright. Cool
For now I won't touch it but I do want to check each component so I understand why. It's just my nature to want to know.
Thanks bro.

[TIMBO]

Hey James, Sorry to hear that your are having problems.

I tweaked the dropping resistor so that the preamp sees similar voltages as the original. I think..........

[Jaymz77]

G'day TIMBO, how are you mate?
Im not really having problems, the thing sounds great, its more that I'm trying to understand WHY it was necessary to move away from the original design slightly to make it sound right.
I just can't for the life of me conceive that that buzzy, crap distortion i was experiencing previously is what people rave about with these amps.
As you may recall, i only had to change one resistor value in the end (pin8 v2 from 470 to 1.5k), but even that shouldn't have been necessary.
I should be able to recreate the original design and get the same sound but thats not whats happened.


Anyway, like SILVERGUN said, its not going to cause other dramas so if it sounds good just play it. Thats is what i plan to do for now UNTIL i can get to the bottom of it.
Its one of my pet hates when things don't work as they should and i have to get to the bottom of it.

[Jaymz77]

Im going to have to get that program…
Cant read those .sch files

[Jaymz77]

Here's one for the boffins… at least those more knowledgable than me 
I found a thread on Metroamp where a guy was complaining of similar symptoms as me and it was determined he had an early version of the amp.
Steve SDM (who is the guy i got the layout and schem that i used from and who seems to be hard to get in touch with these days) walked this guy through how to convert to the later revision of the preamp.
Anyway i stepped through his instructions and it struck me that i may have build the pre-revision preamp.
He told this guy to change R10 from 100k to 47k, and remove R8 and jumper it. There were a few other things he changed to, one of which i don't yet understand, one doesn't apply to my layout and one I'm not sure what location to apply the change to yet.


What can you guys see this achieving (R8, R10)?

[Jaymz77]

Anyone got any ideas on the R8, R10 thing?

I might post up some more voltages too

[TIMBO]

Hey James, My knowledge on a lot of this stuff is very limited but as I see it by reducing R10 to 47k and removing R8 would allow more signal to pass when the lead switch bypasses the clipping circuit. 

[Jaymz77]

Hmmm... Ok.
That's probably not what I want to do then.

[Jaymz77]

I can actually see that now myself I think TIMBO.

What effect would changing r22 (next to bias pot) from 56k to 68k?

[sluckey]

What effect would changing r22 (next to bias pot) from 56k to 68k?

R22 is a 220K. Did you mean to say R23? If so, increasing R23 will cause the bias voltage to increase, allowing the output tubes to run cooler. The bias pot may still be able to set the bias properly.

[Jaymz77]

WARNING!!!! OLD THREAD REVIVAL!!!!



Hi all. Well, i just can't leave well enough alone! I need to know why my amp sounds this way with the correct values (see previous posts) when i don't believe it should.
Basically, when you crank the clean gain up above 8, the amp starts by sounding very punchy, extremely punchy in fact. Then over the course of the next couple of minutes the sound gradually degrades and gets very buzzy/ fuzzy. It essentially sounds like a stomp box dying as you listen to it. It becomes very grainy and the notes are full of noise and is basically unusable. If you turn the gain back down it will slowly come good again.
I had the opportunity to a/b with a real jube last week and it did not have this issue, so it can't be a trait of the circuit.


There MUST be something causing this. The band aid fix suggested by TIMBO works, perhaps a little too well, the original i tried had a little more hair than mine however mine was a smidge brighter. The brightness is probably due to the age of the components.


If anyone has any ideas on what might be causing this i would greatly appreciate their input.


I should note, it MUST have something to do with the signal level coming into the amp because the problem is worse with humbuckers than singles. It seems to have settled slightly since i last tried the original value back in, but the problem still exists.

[Jaymz77]

No takers?
Heres an update.
I tried a couple of EH12ax7's in v1 and v2 to see if there was a tube issue… If anything it made the issue worse.


The more i think about this the more it seems like there is simply too much input signal.


* With my Strat, it seems to be right on the edge of the problem, with humbuckers it is buzzing and decaying its head off.


* A compressor up front shuts it up, but I don't want to HAVE to use a comp and don't usually use one with HB's anyway.


* I know there should be a volume difference between singles and HB's but it is quite a big difference in this amp. And punchy as hell.


Im still learning about these circuits but would it have something to do with the voltage divider on the input or the grid stopper resistor on V1? Maybe i put too much heat into one of the resistors when building.


PLEASE HELP!!!!

[TIMBO]

Hey James, I still can't give you an answer to what could be the problem. I just chatted to my mate thats got mine and he has just got a firebird with hot pups and compared to his Gibson with p90s there is a lot of extra tweaking needed to get that right amount of hair on the notes with out loosing control.
He also added that now having the amp for awhile the harshness has diminished and playing so much better.

One suggestion you may try is the input resistor at the input jack can play a big part of the amps sound.

Sorry, that's about it.

[Jaymz77]

Hey Timbo, how are you mate?

After some reading I'm wondering if it simply because there is only one input rather than a high and low input. Does your mate say that his/ yours craps out with the clean gain on 10 with humbuckers? I mean it really sounds bad, completely unusable.
It HAS settled down some. It would do it with singles previously where now it seems to be right on the edge with singles, but still useable.
If I turn the gain down to suit hb's I don't get the same warmth as I do with singles with the gain on 10.

I a/b'd mine with a real jube recently and it sounded REALLY close, using my strat. Wish I tried a lp as well...

[SILVERGUN]

Hey Jaymz, sorry to hear you're still not happy with it...

I looked back at the schematic and have a suggestion.
Snip the 47p cap off of V1A from plate to cathode and see what effect that has.
If you damaged it with heat while soldering it, it could've been the problem all along.
I'm not sure if this was covered in a previous post, but it's worth a shot.

After some reading I'm wondering if it simply because there is only one input rather than a high and low input. Does your mate say that his/ yours craps out with the clean gain on 10 with humbuckers? I mean it really sounds bad, completely unusable.
It HAS settled down some. It would do it with singles previously where now it seems to be right on the edge with singles, but still useable.
If I turn the gain down to suit hb's I don't get the same warmth as I do with singles with the gain on 10.

YES,,,some high output humbuckers can put out 3-5 times the signal voltage of a single coil.

Example: (from Dimarzio's site)
Dimarzio Area 67 single = 120mV
Dimarzio Crunch Lab humbucker = 410mV

[TIMBO]

Hey mate, Alls good in Brissy. I think you just said it, the SJ DOES NOT like to be cranked to 10 on that clean channel and if you are using HBs you need to trim the VOL on your guitar. Give that ago.

[Jaymz77]

Hey SG, how are you mate?
I use vintage pickups in all of my guitars except the ESP which has EMG's. My Strat has SD SSL-1's, my LP and SG both have custom shop Pearly Gates in them, a Billy Gibbons set in the LP and an Angus set in the SG. The point being they are not super hot pickups.


It wasn't so much that I wasn't happy, i was very happy actually, especially considering it sounds so close to the original i compared it to. It was just a splinter in my mind knowing that it wasn't quite as per schematic with the changed value of R37 (490-1k). It was really me just being picky.


It seems as though this might be the reality with this amp. Timbo's information seems to support that.


It does sound bloody good, it just looks like its set up so that only low output singles can use all of the available gain. I have to knock it down to about 7-8 with HB's or its crap out city.

[TIMBO]

Hey James, the more I play with these amps the more it becomes clear that there is no amp that they are a one trick pony and are setup that way.
I have several new and old amps and they all sound good/bad with different guitars.
My suggestion would be, build more amps

[Jaymz77]

Heh heh heh, I'm already thinking of building a 2204! Possibly followed by a dumble.

I actually built a volume pedal last week. Just a simple switched 1M audio volume pot. This should allow me to have it set for use with my HB guitars. I put a treble bleed cap in it so it doesn't choke the tone when used. I originally built it to be used like a boost pedal but in reverse for solos. Knowing the amp was on the verge of too much signal even with singles, I planned to use it as a cut pedal for rhythm, then turn it off for solos.
There is a question... Has anyone ever successfully used a boost pedal in front of a jube?

I'll get some pics of the finished jube up tonight if I remember. I've promised before and forgot.

[TIMBO]

Cool

[Jaymz77]

Well, I'm pleased to say that i am 99% sure i have solved my problem.
I spent (another) day going over my drawings with the amp open in front of me and by mid afternoon the answer presented itself.
Long story short, there was a bad earth on the effects return jack. So essentially, when things heated up a bit, the earth failed and then there was no ground reference for v2b grid, causing v2b to go into runaway…. at least thats my version of it.


Like i said tho…99% sure...

[TIMBO]

Hey James, Merry Christmas and happy new year, well so far it sounds like its off to a good start.
Just to prove you got it sorted, I think you need to put it to the test. A SOUND TEST, ya know what I mean. 

[Jaymz77]

Thanks Timbo. Merry Christmas and happy new year to you too mate.

I've done the sound test and under the same conditions the issue hasn't come back yet. Over all the amp became louder slightly and just sound a bit happier.

I think it's right now.

[TIMBO]

GREAT.