Oh No! Another Marshall Amp Thread!

shaunf · 2012-03-13T06:43:47+00:00

A bit of background first. My Marshall JMP 50 watter was working well and sounding great, except for one problem. The turret board had become conductive. So far the only sympton of this is that the pots on the amp sound scratchy as their mechanical movement is being amplified through the speakers, and secondly that the pot movement on my guitars is also being amplified through the speakers, and any switch movements result in a popping noise coming through the amp. Besides that, it hardly hums and is generally very quiet. Over the last couple of months I've been trying to decide what to do about this, and about 2 weeks ago I decided to jump in with both feet. Unfortunately for those of you who accidentally clicked this link, you will have to suffer through watching me, Electronica Ignoramus, attempting something which for me, I believe is fairly ambitious. Firstly, Im going to replace the turret board, and secondly my OCD is going to try to assist me in neatening up all the wiring, which was irritating me. Oh, I'm also going to, as far as possible, do all the work myself, and only involve a tech if I get stuck or have a problem. Here are some Before pics:

shaunf

Thanks for the comments guys. The amp should have been up and running by now, but as Murphy would have it, for some reason I forgot to order some 150k resistors, which happened to be the last component I needed to put on the board. Grrr! >☹

shaunf

And it LIVES!!!!!!!!!!!!!!!

Voltages all seem to be within what is usually deemed acceptable. Biased it, and played a bit at very low volume. Amp is quiet even with the liberal use of carbon comps in many places. Will give it a good crank tomorrow after work and once I'm totally happy, I'm going to connect up the Lar/Mar PPIMV that I've already made up.

makepeace

ActionArnie

Good effort! Looked like a lot of work went into that!

shaunf

I decided to connect up the PPIMV this afternoon:

Rebiased the amp and then cranked the volume to 10 with the PPIMV set at around 3. Sounded good, but there was a bit of fuzziness at really low settings on the PPIMV. I remember reading that this is often caused by a "bright cap" normally fitted to to the Vol 1 pot. The amp never had this cap installed when I got it, but I decided to try a 100pf one when I rebuilt it. Removed this cap, and fired it up again, and the tone is pure 70's rock bliss.

For the record, it's not a pure 1986 Bass circuit. Fiddling with the amp in it's previous form I tried fitting a 0.68uf capacitor on the V2 cathode like the Lead circuit has and I really liked the way it lifted the sound, so I carried that mod through to the new board too.

So, here she is completed now:

There's one last thing that I need to sort out, and that is that the amp is biased a little on the cool side at present. Having calculated what my bias setting based on my B+ voltage of 468V, I should be aiming for a MV reading of around 37-38MV across the 1 ohm bias resistors I installed. Problem is, with my bias pot fully maxed, I can only get them both to 30-31MV. I read somewhere that the bias range can be changed by altering the value of the resistor (currently 47K) running diagonally in between the 2 bias caps, but I'm not sure whether the value of this resistor should increase or decrease to get me in the range I want. Can any of you knowledgable gents confirm firstly whether this is in fact correct and that I'm talking about the correct resistor, and secondly in which direction the value of this resistor should go?

MikeM

Decreasing the value should get you where you want to be (From the description you gave).

Also, I think you mean mA ?

Really great work man!!

shaunf

MikeM wrote Decreasing the value should get you where you want to be (From the description you gave).

Also, I think you mean mA ?

Really great work man!!

Thanks Mike, I'll give a lower value one a try and see what happens.

No I did mean mV. Everything I've read about biasing using a 1ohm resistor connected to pin 1 and 8 on the power tube sockets said to measure the mV across this resistor and then to use the I=VR formula to calculate the mA required. Is that correct?

MikeM

I suppose it's much of a muchness in this situation. 1 ohm resistor means they will be the same, just have never really seen bias measured in mV.

mA = 1ohm x mV so mA = mV in this case. Just the standard when working on these things is mA (Perhaps not on the Marshall side of things?)

I've never actually spent much time investigating Marshalls, besides their adapted tonestack. Interesting to note that the B+ is that high.

shaunf

MikeM wrote I suppose it's much of a muchness in this situation. 1 ohm resistor means they will be the same, just have never really seen bias measured in mV.

mA = 1ohm x mV so mA = mV in this case. Just the standard when working on these things is mA (Perhaps not on the Marshall side of things?)

I've never actually spent much time investigating Marshalls, besides their adapted tonestack. Interesting to note that the B+ is that high.

Ja you're 100% right. Still measured in mA. I don't know why the mA is not just measured directly, instead of measuring mV and converting. Well, one thing I can say is that my multimeter measures small voltages a lot more accurately than it measures small currents, so maybe that has something to do with it.

shaunf

Mike: thanks for the help re the resistor. I dropped the 47K to a 33K and now 37mA is attainable with the pot around halfway set, which is perfect.

Will have to wait until tomorrow to hear what it sounds like.

Cheers,
Shaun

makepeace

Looks good!

shaunf wrote I don't know why the mA is not just measured directly, instead of measuring mV and converting. Well, one thing I can say is that my multimeter measures small voltages a lot more accurately than it measures small currents, so maybe that has something to do with it.

That, and you have to put your ammeter 'in' the circuit. With voltage, you're measuring the potential difference across a part of the circuit. To measure current directly you have to physically break the circuit and put your ammeter in between the two ends (all your ammeter is doing in this case is exactly what you're doing, converting a voltage across a resistance to a current).

shaunf

Ah yes, thanks makepeace. Now I remember that from school. Now you guys know that I wasn't kidding when I said I was an electronics noob ! ?

shaunf

I thought it was time for an update. I've been playing the amp in its current spec for over a month now and although I've been enjoying it and enjoying the sound, I'd have described it as "nice" rather than "excellent" or "outstanding". I've just had this nagging feeling in the back of my mind that there was something missing.

At the start of the long weekend, I decided it was time to break out the soldering iron and experiment a bit. Having spent some time doing bit of research over the last couple of weeks, and in particular reading up on the Negative Feedback (NFB) part of the circuit, I decided that before I changed any components, I'd try repositioning the NFB wire, since it's simply a case of moving a single wire.

When I first got the amp, the NFB wire was routed to the output jack, making the amount of negative feedback into the circuit variable, depending on the impendance of the cab being used. The NFB wire can also be positioned on either the 4 ohm, 8 ohm or 16 ohm taps, thereby making the amount of negative feedback to the circuit constant. When I rebuilt the amp, I simply connected the NFB wire back to the output jack as it always was.

On Saturday, I decided to try moving the NFB wire to the 8 ohm tap, when then in my case, given I run the amp into a 16ohm cab, would halve the amount of NFB sent back into the circuit. Powered the amp up, plugged in and OH MY GOODNESS, all of a sudden the amp is alive! Wow! I cannot believe the different that such a simple change can make. I can't really describe the change in words, except to say that it is simply a huge amount more lively, the bottom ends has tighened up especially in overdrive, where before the bass was a bit on the flubby side. Also, before I hated the sound of the channels being jumpered as it was muddy, now all of a sudden it sounds absolutely amazing.

Any of you out there with one of these type of amps, I can highly recommend you take a look inside your amp, and if the NFB wire (normally purple) goes to the speaker jack or to the 16ohm tap, try connecting it to the 8 ohm tap instead and see if you prefer the sound. I certainly do. The only sad part is that I am now completely blown away with the sound of it, which basically puts a halt to my tinkering with it!

Norman86

well thats good enough reason to keep it as is now?!

makepeace

Norman86 wrote well thats good enough reason to keep it as is now?!

There never is.. ?

TomCat

makepeace wrote
Norman86 wrote well thats good enough reason to keep it as is now?!
There never is.. ?

I've totally rewired and modded my TC-8W a couple of times and I still want to fiddle..... ? ?

nick

shaunf wrote Any of you out there with one of these type of amps, I can highly recommend you take a look inside your amp, and if the NFB wire (normally purple) goes to the speaker jack or to the 16ohm tap, try connecting it to the 8 ohm tap instead and see if you prefer the sound. I certainly do. The only sad part is that I am now completely blown away with the sound of it, which basically puts a halt to my tinkering with it!

hrm, thanks will try this ?

nick

nick wrote
shaunf wrote Any of you out there with one of these type of amps, I can highly recommend you take a look inside your amp, and if the NFB wire (normally purple) goes to the speaker jack or to the 16ohm tap, try connecting it to the 8 ohm tap instead and see if you prefer the sound. I certainly do. The only sad part is that I am now completely blown away with the sound of it, which basically puts a halt to my tinkering with it!
hrm, thanks will try this ?

In this gut shot I can see the purple wire but not sure which tap its going too.

shaunf

nick wrote
nick wrote
shaunf wrote Any of you out there with one of these type of amps, I can highly recommend you take a look inside your amp, and if the NFB wire (normally purple) goes to the speaker jack or to the 16ohm tap, try connecting it to the 8 ohm tap instead and see if you prefer the sound. I certainly do. The only sad part is that I am now completely blown away with the sound of it, which basically puts a halt to my tinkering with it!
hrm, thanks will try this ?
In this gut shot I can see the purple wire but not sure which tap its going too.

Do you have one of those selector pins with the little window in it that shows the selected impedance when you plug it in? If so, it's easy to identify. One pin goes to the centre tap and one pin goes to the selected impedance. In my amp, the 8 ohm tap is the lug at the "top" of the amp when inverted as you have it in your pic.

*Edit* : Looking at your pic again, I'm pretty sure yours is connected to the 16 ohm tap. If you change it, please let me know what you think.

G-Man

Amazing work, I'm late to the party but wow that is great. Have you done anything else to this amp since then or is it still meeting your current needs?

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