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COMPLETED! :D modifying laney combo to rack preamp

Started by GuitarLord66, September 27, 2011, 07:34:28 AM

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I had a voltmeter connected when I touched the terminals and the caps dropped to 0 volts so I'm pretty sure I drained them, it was the worst feeling ever, I just stood there shaking and couldn't move for a few minutes. It was the scariest moment of my life!

I should be getting started on the amp project early next week so I will make sure I keep you guys posted about how it goes :)


Sorry it took me so long... I cut the pcb then I couldn't find any wire, but I did about 2 hours ago so I went straight ahead and rewired the small section :) Here's a picture:

I plugged it in for about 10 minutes to test it, sounds the same, at least I think from what I remember haha

Now after I unplugged it, I've never done this before though so it's hard to say but the 2 large black resistors were really hot, and I know it looks brown around their connections but they have always been that way... Is it normal for them to get hot? or should I replace the wires that go to them with a larger gauge wire?

Next job to do is to buy all new pots and jacks and rewire them all up... any tips of removing pots? especially the ones with a dpdt switch... by the way i'm replacing the push-push pots with a standard pot and a small dpdt toggle switch :)

Thought you guys should get a shot from the bottom as well :)


Looks good so far.  Glad to hear it appears to be working.  I'd say the wires are large enough given their short length.  As for the resistors, I'm not totally sure.  How hot is really hot?
Life is what you make it.
Still rockin' the Dean Markley K-20X


Thanks :)

I played the amp for another hour last night, and after I turned it off and unplugged it I tried touching the resistors and they were so hot that if I didn't take my finger off they would have burnt my finger... They haven't exploded though so that's a good sign... and they always had brown burnt looking spots at their solder connections so maybe they have always got that hot?

J M Fahey

Although I don't like that very much, it's "normal" . Oh well.
*If* you want to rack mount only the preamp, you may dispense nwith tham by building a small PSU that supplies +/-15V at reasonable current, without needing to drop them from +/-40 or whatever they use as "big" power rails.


So it's normal for the resistors to get really hot then?
I'm gonna rack mount the whole thing and "slave" the preamp section, it's easier for me that way... and I will still have the speaker output, just in case I do want it again for some reason...

so what's the easiest way to remove pots and push pull pots from a circuit? just slowly unsolder them? and try not to force it and break it?


oh and because the resistors get hot, should I put a small fan in the back of the rack mount enclosure I make? to help cool it down? or is that going overboard?


Quoteoh and because the resistors get hot, should I put a small fan in the back of the rack mount enclosure I make? to help cool it down? or is that going overboard?

I'd rather choose some of the other options that make more sense...

- Substitute the resistors with ones that have higher power rating, or alternatively use a bank of parallel resistor. Anyway, the main point is increasing power handling or decreasing dissipated power per each resistor, which means less heat.

- Redesign the regulator circuit to use integrated regulators instead. Not only is it better option than the discrete shunt zener diode regulator with voltage dropping / current limiting resistor scheme, it also allows more convenient heatsinking of the parts that heat up.

- Do what Fahey suggests and change the power supply to a lower voltage one. You don't need the high power reserve (and high voltage brought down) if you don't need the power amp. Since you say you'll keep the "speaker out", I assume you will actually keep the power amp though, in which case this option is not valid. I hope you have considered the issue of providing sufficient heatsinking of the power amp section in this conversion... otherwise it will say "poof" quite fast.

Quoteso what's the easiest way to remove pots and push pull pots from a circuit? just slowly unsolder them? and try not to force it and break it?
You most certainly do not want to slooooowwlyyyy unsolder them. That means heating up the board drastically, which results to damage.

You want to:
- If possible, clip off the potentiometers and jacks before you start unsoldering their leads. Effectively, you want to minimise the heat-sinking thermal mass as much as possible. If you can, leave only stubs of the leads on the board. Sometimes the body of the component simply just obstructs snipping out the part and you're left with no other option but to deal with considerable thermal mass.
- So, get a big soldering iron that can heat up things fast...
- Then grab that manual vacuum solder sucker and some solder wick and suck off the solder one pin at the time as soon as it melts.


So if your suggesting I change the resistors, I'm just wondering why they get so hot? I never changed them, they are the ones that came with the amp? I just thought maybe because the wire I used was quite thin that could be why they got hot?

I don't particularly want to redesign any parts of the amp, I wanted to keep the power amp intact also... It's pretty much just a rehousing project for me :)

The power amp has, (which I think are SCR's) on a heat sink that I will be moving the new enclosure.

Ok I should be able to snip the pots off no trouble, there seems to be enough room I think, just the push pull pots will be a little more difficult...
I will look into getting a solder sucker when I buy the new pots, switches and jacks :)


I been thinking about it and how much is involved in making the pre amp dedicated and just getting rid of the power amp section?
If there's 6 wires between the pre amp and power amp section what are they most likely to be? I'm assuming 2 are ground? and then power? and output?
Would I have to make an output circuit as well? or would 2 of the wires be able to be soldered directly to sleeve and tip for output? or would the 2 wires that seem to be ground also be the sleeve of the output? I'm pretty confused on how that would work and what would be involved in salvaging just the pre amp section? can anyway help clear that up for me? :)


QuoteI'm just wondering why they get so hot?
They decrease the voltage from whatever it is to zener voltage of zeners, likely there's about a 15 volt drop across each. ...and in addition they have to supply all the current that the preamp draws. voltage x current = power. power = heat

QuoteIf there's 6 wires between the pre amp and power amp section what are they most likely to be? I'm assuming 2 are ground? and then power? and output?

Don't assume; find out. Where do the wires go in the concerned circuit sections?

- One connects to power supply ground
- One connects a potentiometer lead to ground
- Two connect from their individual voltage dropping resistors to their individual zener diodes and filtering caps. So that's your positive and negative power supply rails for the preamp.
- One wire connects to a potentiometer wired as rheostat, one end grounded. I believe it's the "presence" control, which, according to other pro linebacker series schematics connects to power amp's feedback loop.
- the final one I can't deduce from the photos but since it links the power amp and preamp boards together its likely preamp output signal / power amp input signal.

All those six pins are also used by the molex, which means you have the molex connector feeding:
- positive high voltage
- negative high voltage
(both which are then dropped down to maybe +-15V by the regulator circuitry - resistor + shunt zener + filter caps - on the preamp board you have cut in pieces)
- signal ground
- power ground
- input / output signal
- connection to presence pot

There's your answer.

QuoteI been thinking about it and how much is involved in making the pre amp dedicated and just getting rid of the power amp section?
Above lies your answer. Not much.


Oh whoa thank you very much!

EDIT 4... I'm only doing this so I don't double post haha!

I went ahead and soldered two wires to the last 2 wires that I was unsure about and soldered a jack to them and tried it. It worked! I'm not 100% sure whether it sounds the same as the "send" function on the back of the amp though. Would there be a difference?

So all I need to get is the 15v+ and 15v- power supply from jaycar, and the toroid that goes along with it. 3 jacks, one for input, output and controller. and I'm trying to decide whether or not to replace all the pots... I think I need to replace all the push/pull ones because I want to add dpdt toggle switches to turn the boosts on, not pulling the pot... the volume pots are the ones that are playing up, but maybe it's best to replace the lot? opinions? Thanks for all the help!

Power Circuit - http://www.jaycar.com.au/productView.asp?ID=KC5418&keywords=power+supply&form=KEYWORD
Toroid - http://www.jaycar.com.au/productView.asp?ID=MT2086&keywords=MT2086&form=KEYWORD

Ignore whats below! haha it's just my thoughts and questions that I seemed to work out :)

EDIT 3 before you read the full post I might as well put my latest thoughts at the top haha :)

I'm almost 100% sure that the power supply I posted at the bottom of this post will work :) The only question I really have is whether or not the last 2 wires are the output signal from the pre amp or does it need a buffer circuit or something like that before output? I'm considering just soldering 2 wires to the connections, soldering them to a jack and plugging it straight into my peaveys power amp. I don't want to blow anything up but I think it's the only way I'll find out for sure whether that's what those 2 wires do :P

I'll use a multi meter sometime later tonight and find out what the voltages are along the wires, that should help me work out what wire is what and what voltage the input power is :)

With the molex connection the two center wires are joined in the circuitry as grounding, and the wires I ran from them went to ground on the top and bottom of the circuit, the closest wire was to the presence ground and that's why I linked it there. Would that be grounding or negative voltage?

So positive and negative high voltage, I didn't know about those, I always thought a circuit was positive (voltage) and negative (ground) learn something new everyday :) Also would the signal ground be the same as the power ground?

The guitar inputs are on the front of the amp, with the knobs, so by input/output, are you talking about preamp output/poweramp input? as the same wire? (which makes sense)

So if I wanted to bypass the power amp, I'd need to build a small power circuit? Could I just use say for example a 15volt dv wallwart?

So no more circuitry is involved in outputting the pre amp signal? I could just connect them straight to a jack, and that could go out to my effects (tc g major, dbx equalizer) and then to the power amp of the peavey?

Again thank you so much, you have been very helpful! thank you for putting in the time to help me with this :)


I went ahead and used my multimeter to check out the voltages... Before the large resistors it's -45.8volts and +45.6volts, and after them its -15.55volts and +15.36 volts. The other 2 wires I couldn't work out and I didn't want to try combinations of touching them with the multimeter in case I shorted something out :s So I now understand what you mean with positive and negative power rails, so how would I go about making a circuit that would output 15 volts positive and negative? and those other 2 wires, are they the output? like could they be soldered directly to tip and sleeve of a jack? or is there more involved? thankyou :)

EDIT 2!!

I was looking for a power supply on the jaycar website (australian electronics store) here's what I found

"Headphone Amplifier Power Supply Kit

To ensure the best possible performance to the Headphone Amplifier Kit, this will provide regulated
+/- 15V and +5 outputs.
Toroidal transformer required use MT2086
The kit comes with all specified board components and quality fibreglass tinned PCB.
PC board size: 56.5 x 80mm"

And it says it requires this:

15V-0-15V 20VA Toroidal Transformer - Low Profile

The power supply kit costs $19.95
The toroid costs $23.95

That's $43.90 for the kit and toroid... a bit more then I want to pay for just the power supply...
Because add in the new pots, new jacks, knobs, etc, this will be a well over $100 project... and well I don't have that much money to use :(


This project is far from dead :P sorry I haven't been keeping you guys updated I've only just had the money in the past week or two to get started on it again :) Everything is pretty much done circuit wise, I just have to cut the rack enclosure down to a smaller size and then fit everything in, and finish making the new front panal design in photoshop to stick on the front :) Here's the latest pictures of the amp, sorry their bad quality, I had to take them with my phone.

Here's what's left of the original amp, it's been cut down a fair bit, a few of the pots were replaced and the two gain pots that were also "push pull" pots were replaced with standard pots and a dpdt toggle switch. A few pots were also removed, such as the reverb and presence, which I didn't want or need. All the jacks were taken off, the input, output and control jacks will all be located at the back of the unit.

In this picture you can see the power supply I built and the new torroid transformer, it will be laid out something similar to this eventually

Another angle

This picture is the rack enclosure put together, the bottom of the ruler is where the rack enclosure will be cut down to

All up everything has cost almost $200 so hopefully it all works and is worth it in the end...
I will keep you guys up to date with how it goes, and I will post pictures of it when it's finished :)


Wow, quite the project.  Did you make your own PCB for the power supply or purchase one?
Life is what you make it.
Still rockin' the Dean Markley K-20X


I cut the case down smaller, to a similar size of my tc electronics g major's rack enclosure, because the original case was too long to fit in my rack case, it stuck out the back :s so today I will hopefully mount the torroid and the power supply, get all the wires cut to a appropriate length and joined and then remove everything, spray the case with a black to clean up the spots where it was cut and then stick my new front panel on with all the labels and stuff for the pots :)

I bought a power supply kit :) So I just had to follow the instructions and solder all the components to it :)


All was looking well, I got it all fixed in the case and wired up and when I tested it, the first channel works perfect, but when i changed to the second channel, (which is clean on this amp) doesn't work, if I strum my guitar really hard, it kinda comes through but VERY distorted and only a few crackles really come out of the amp... So I'm going to have to work out whats causing that, once I sort that out and finish it I will post pictures of the final product :)

I spent all night working on the amp and I couldn't work out whats wrong with it... It's only the clean channel that has the problem... the dirty channel works fine...