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Marshall 5010 Master Lead Combo - Issues

Started by Skydog75, July 05, 2014, 02:52:47 PM

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Skydog75

So I picked one of these up from a local music store after listening to some videos and trying it out in the store.
Here is a schematic for it:    http://www.drtube.com/schematics/marshall/5010.gif

There are two issues that I have with it. First, the preamp control doesn't gradually increase the overdrive. The overdrive suddenly "kicks in" at about 9-10 on the knob. I'd like the overdrive to have a more gradual increase instead of that jump at 9-10.

Second, the overdrive has a decent Marshall-like sound to it, but there is a fizziness that colors the sound over top of it. Also, when a chord is left to ring out until it fades, the fizz will suddenly drop off.

I plugged into both the high and low inputs, but STILL hear the fizz.

I have tried plugging into the headphone jack to eliminate the possibility of the speaker being the culprit and I STILL hear the fizz.

Finally, here's a sound sample of a guy who apparently had the same exact problem:       https://www.youtube.com/watch?v=y1osT39OSjs

The amp seems to have really good potential, so any help would be greatly appreciated!!

J M Fahey

What fizz?
That´s plain old distortion, above a certain voume you have it, below that you don´t.
By the way that´s a killer SS amp, but you must get used to it.

Just one sample of a guy who already learnt to use it well:
http://youtu.be/4RoO_jQEflc

Ok, here´s another:
http://youtu.be/9kY3PNz4JA0

Couldn´t resist:
http://youtu.be/8ucBVsWYvmk

phatt

Hey skydog,
I can't guarantee the outcome but I'd do this to help reduce the fizz,
Make R5 4k7
Add 2n2 cap at node of R5,R6,C6 then connect other end of cap to ground.
Make R7 15k if you do not have enough gain.

As to all the gain at the top of dial problem,
Increase the size of R4 to say 300 Ohm but you may then need to raise the value of R3 to match.
Try one thing at a time,, R4 first ,,you may not need more gain.
Have fun, Phil.

Roly

Quote from: Skydog75the preamp control doesn't gradually increase the overdrive. The overdrive suddenly "kicks in" at about 9-10 on the knob.

First up, try cleaning the pot.
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

Sorry but in this particular case it´s a design problem, only solved by a reverse log pot.
Look at the schematic: on full gain, the feedback resistor to ground is only 100 ohms, but the gain pot is 10 k linear.
It means that on 10 you have max gain, and on 9 you have extra 1K in series, 10X the former value, 10X less gain or -20 dB
So going from 9 to 10 on the pot you vary gain by huge 20 dB .
Talk about nonlinear and hard to control gain !!!!
While a reverse log pot would get to 1K seres value on "5" , effectively spreading that 20dB range along 5 scale points ... very usable.

Skydog75

Quote from: J M Fahey on July 07, 2014, 08:53:54 AM
Sorry but in this particular case it´s a design problem, only solved by a reverse log pot.
Look at the schematic: on full gain, the feedback resistor to ground is only 100 ohms, but the gain pot is 10 k linear.
It means that on 10 you have max gain, and on 9 you have extra 1K in series, 10X the former value, 10X less gain or -20 dB
So going from 9 to 10 on the pot you vary gain by huge 20 dB .
Talk about nonlinear and hard to control gain !!!!
While a reverse log pot would get to 1K seres value on "5" , effectively spreading that 20dB range along 5 scale points ... very usable.

Forgive me, since I am still a complete noob at electronics and reading schematic diagrams, but I thought the gain pot was labelled "VR1" and the value of that pot reads "22k LIN". Am I reading this wrong? Sorry if I am. Would replacing that pot with a 22k LOG (not reverse LOG) be worth trying first? What makes a reverse LOG give the gain a better sweep from 1-10?

Quote from: J M Fahey on July 06, 2014, 12:41:10 AM
What fizz?
That´s plain old distortion, above a certain voume you have it, below that you don´t.
By the way that´s a killer SS amp, but you must get used to it.

Just one sample of a guy who already learnt to use it well:
http://youtu.be/4RoO_jQEflc

Ok, here´s another:
http://youtu.be/9kY3PNz4JA0

Couldn´t resist:
http://youtu.be/8ucBVsWYvmk

Those sound samples are how I want mine to sound  :dbtu: But mine definitely has a sort of crackly/fizzy sound along with the distortion. Are the diodes at D1 and D2 the source of the overdrive, or does it come from the opamps? Could either of those be the problem with the fizz?

BTW, I am still learning, so please forgive me for stupid questions and I thank all of you for your patience.

Skydog75

I didn't see anything in the forum rules saying I can't post info from another forum, so here's a thread from the same guy (I assume from the name) as in the link to the video that I posted above:

http://www.marshallforum.com/marshall-amps/61260-new-marshall-5010-master-lead-combo-thinking-some-mods.html

Apparently, he had the same EXACT issues that I'm having. Maybe I should PM him to see if he made any mods that helped?

Enzo

In general:

don't bother the guy with a PM, just post in their forum.  That way others who might be wondering will benefit as well.

J M Fahey

Sorry, I trusted my memory  xP
Yes, the pot is 22 lin which makes it worse.

As to the ear-percieved linearity the problem is as follows<:
that VR1 gain pot is very clever, it does the work of two pots.  8)

See that the mid leg is grounded, so we effectively have 2 pots (linked) working at the same time
From center leg to right we have a variable 22K resistor, and from center to leg we have another variable 22K resistor.

Full end to end value is, of course, always 22K so on 0 right is 0 ohms, left is 22K (in both cases to ground), on 5 each is 11K to ground and on 10 right is 22K to ground and left is 0.

Although both halves are always linked (you vary one, you also vary the other ... but in opposite ways), each half can be used in a different circuit.

Here the right one forms a voltage divider together with R6, think how signal varies (even down to 0) by varying the value of the pot. works almost like a regular volume pot.

The left half, being itself in series with R4 , varies the feedback (gain) of Ic1a (the left one, incorrectly labelled Ic1b).

Problem is linear pots vary "too fast" from 9 to 10 (2200 ohms compared to 100 ohms).

Reverse log vary much less from 9 to 10.

A regular log will be smooth from 0 to 5, not what you need here.

phatt

Thanks Jaun, for explaining the details. :tu:
I was going to try and have a go at it but I don't know the terminology well enough.  xP

I certainly have experienced how wacky and unstable opamps can get as that value goes down below ~300 Ohms. 100 Ohms is on the bleeding edge of instability IME.

I only suggested changing the value of R4 as sourcing a reverse log pot may not be so easy.
I notice R8 is set to a more sane value of 470R.

I know from testing ideas like this that the freq response goes berserkaz :o once you get down to such low values and maybe partly the source of the fizz noted.
Phil.

acmeinc

Sorry for reopening an old topic, however, this is an old amp :)

I'm thinking about converting the 22k linear Pre-amp pot to a reverse(anti)log pot as discussed above using the methods described here http://www.geofex.com/article_folders/potsecrets/potscret.htm.  I have 2 ideas in mind:

If I'm doing the math right, I'll need a 500k resistor across the first 2 pins of the original 22k pot.  My concern is, is doing this going to give me enough of a curve?  Either a 500k will give a huge curve, or a straight line... the math is escaping me, but I'm leaning towards almost straight.

Another option I was considering, is replacing the 22k pot with a 50k pot and placing a 50k resistor on the first 2 pins.  This should give me a reverse log curve at 25k.

Any feedback on the above is appreciated!

Enzo

Good luck finding a 500k resistor, but 470k is close enough and is a standard value - about as common as it gets.

Do you have such a resistor?  In the time it took to post this, you could have tacked a resistor to the pot and found out if it worked well or not.

J M Fahey

Quote from: acmeinc on March 17, 2016, 03:58:05 PM
I'm thinking about converting the 22k linear Pre-amp pot to a reverse(anti)log pot as discussed above using the methods described here http://www.geofex.com/article_folders/potsecrets/potscret.htm.
Sorry but, short answer: getb the proper reverse log pot.

The cheesy idea of "converting" a pot from Lin to Log or Reverse Log does not work, period.

*Only* place where its problems can be somewhat tolerated (and curve is horrible anyway)  is at the output of an Op Amp or similar low impedance stage (because impedance load and pot value varies 10:1  :o ) but nowhere else, specially not in a tone control or an active gain/attenuation as shown here.

If it were that easy/functional, all amp makers would fake log pots and save a few cents.

I suggest you check Mouser and get the proper Reverse Log pot.

Clyde

#13
Hard to locate anyone selling reverse log pots.  Closest I could find in Mouser's inventory was BI Technologies / TT Electronics 20K and even they are on order & not due in until 4/16.  They list 25K Alpha's but min order is 1000 & non-stock item. 

This discussion got me thinking of trying one on a 5005 Lead 12.

phatt

You would be better off raising the value of R3 and raising the value of R4 up to at least 470R.
I'd go with 1k for stability. Below ~300 Ohms R4 is likely on the verge of instablity.

Trying to get a fancy pot For VR1 will drive you nuts as the curve would need to be ##EXTREME##
I doubt they even make one that would even come close.
Without any resistance.

If you have room make R3 a 50k gain  pot and save yourself all the fuss.  8)

Phil.