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Messages - mensur

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Preamps and Effects / Re: LND150 Jose style preamp
« on: July 14, 2019, 10:34:50 AM »
The junction od D23 and R32 i the point where R33 should be attached, not to ground, the purpose of C19 and R33 is to isolate dc from follower(which is about 150-200VDC) from clipping diodes, so they will survive, and create clipping in the signal. Also I would advise to change C19 to about 220-680nF(at aleast 250VDC rating).

Amplifier Discussion / Re: Marshall Code50 speaker
« on: May 19, 2018, 04:03:26 PM »
From Marshall website:
Speaker configuration
Speaker model
Custom (4Ω, 50w)
Unit impedance

...Mensur, I just saw your latest edit to your post (about routing things from the middle leg of the red channel master pot to, presumably, the power amp input?). Can you be more specific about how I would achieve that?
Can you read the schematics?

Thanks for having a look, DrGonz - I don't have the amp in front of me right now but I will look more carefully at those components a little later today. Going from visual memory of yesterday's chassis-opening, I believe those are two block resistors to the right of the clipping diode / pot, and I think the "crack" is just a surface-level scoring of the component that the flash brought out, but I'll look more carefully at it now that I see it.

And sure, I know that just looking at things is rarely likely to produce results but the last few SS amps I've had to fix did in fact have visually obvious issues - axial-lead caps with one end blown clean free of the cap body, power transistors with burn marks and/or holes, etc. Sometimes you get lucky. :)
I think Juan made a great point that looking at the components of the amp is not gonna fix it in most cases.

If I was to question one two things by your pictures alone it would be by the double potted tone potentiometer and those two IC chips(not sure what those are right of the Tone Clipping pot...) I think they are IC's I have an RG80 combo but it does not have that component there so I have no idea what it is used for... Anyway that thing looks cracked or something...

Another thing thing is the green 1KV 202M signal cap looks like the leg has torn and might be leaking... But that is probably not the problem but still can be replaced later on. However, I know that those ceramic caps when the legs are torn can cause a huge loss to signal overall. Not always the case but something to chew on just by looking at pics.
Those two *IC* are 220nF tonestack caps, I guess that one is broken so it need to be replaced along the 2.2nF treble cap if it's leg is cut offed.If that's the case, signal only goes throughout mid pot, and energy is extremely reduced.Again from the pics, that's the only problem.Here's one trick, route the signal form master pot.middle leg to the amp input, and let's see is the tonestack the problem.

Several things,shot guts inside amplifier,replace filter caps in power supply(that's where that hum comes) 2x6800uF.
Single coils are not good for this amp, so try dirty ch with humbuckers.
Do these thing and well take it fro there.

Tubes and Hybrids / Re: 18w minimalist
« on: February 08, 2012, 11:18:03 AM »
hi everyone,

i have some questions:
id like to build a 18w minimalist, but with a preamp volume. is the schematic alright or do i have to make some changes(i havent changed any values but i just adden a 1m pot in front of the first tube.

i live in belgium and i have a tranny with 230 on primary, do i have to get another fuse value?


gr guitarfreak
1.You don't need to change anything, it already has preamp volume,500K after first tube.What you did is just another volume pot of your guitar on amp.
2.1A slo-blow fuse will do the job.

Amplifier Discussion / Re: Marshall mg50fx master voulme pot
« on: January 26, 2012, 07:30:17 AM »
I can't tell, is the middle left board digital preamp or analog with smd or both?

Amplifier Discussion / Re: Marshall mg50fx master voulme pot
« on: January 24, 2012, 11:49:40 AM »
Hello I have a friends amp here. His master volume potentiometer is shot. Some dude sprayed something in there to clean it and no it just spins freely.

I can solder, and I know what I'm doing but the thing is I can't find a replacement pot! I believe it's a 11mm B10K vertical pot, but I can't find that value anywhere. All 9 pots are labeled B10K.

Any help?? Who to call??

Can you take hi res pics of the amp inside, and upolad it here?

Preamps and Effects / Re: jfet emulator, buzzing - help me
« on: September 20, 2011, 04:29:56 PM »
Maybe you don't find this of any help ... now .... , but I remember my first amps were buzzy, oscillated, got all kinds of radios, worked as buried mine or gold detectors, you name it.
I used solid copper busbars, shielded everything, star grounding,  set all wiring parallel or at perfect 90º , Hiwatt style, all the tricks in the book, and still had problems.
Plus hiss.
Now I do not even use shielded wire , sometimes barely twist a couple wires ... and my amps do not hum , oscillate or hiss.
Maybe it can be called experience.
Nothing magic to it, just keeping doing it and things get better.
Oh well.
So, go on and step by step things get better  :tu:

You're right, it is called experience :)
Just replace trimpots with fixed resistor values close the pots.
It's that simple :tu:

Schematics and Layouts / Re: Ultra Gain Tube Preamp
« on: August 07, 2011, 05:59:41 PM »
After treble put 470K in series and 47K volume pot, or 1MEG/100K volume, this will tame off the signal and low the impedance.

Hi Findeton, any update on this? ;)
Heres te simpler version for the SS triode:

Findeton's idea is good although it's a bit complicated , so quick solution to that problem is to make a buffer which will not change the input signal, and which will isolate the input from the feedback DC voltage, and set the right amount of current.

Triode MOS

Triode JFET
Triode MOS.jpg is high voltage counterpart of real triode(with amplifcations factor of 100-130 times), Triode JFET.jpg is low voltage counterpart of a triode(with amplifcations factor of 22 times) for pedals, and low voltage preamps.

Amplifier Discussion / 55W Fet Poweramp
« on: June 18, 2011, 09:53:18 AM »
Hi guys, I'm asking the advice for the  power amp that I designed.
It's a almost all FET discrete amp(except the bipolar current source and mirror, which only improves the linearity of the amp, and doesn't affect the sound).
I'm going to use the kind of Marshall MF350 negative feedback which is also used in MG series.

I know how to design voltage feedback, but I'm not familiar with current feedback.Can someone explain me how this feedback works, so that I can design PCB and share whole project with you guys.


Preamps and Effects / Re: Randall RG80/RG100 Preamp DIY
« on: May 21, 2011, 02:44:08 PM »
What charge pump was used to get 24v from 9v?

Amplifier Discussion / Re: Zoom 15watt Modeling Amp BUZZES
« on: May 04, 2011, 06:01:22 PM »
It's blown amp, you have to replace the whole power section.That buzz you hear is DC voltage, and it can fry your speakers.

You say "whole power section", but that should just be a single chip since most cheap amps use an IC for their power amp section.  Should be a TDA2030, LM1875 or similar.
I know it's a chip amp, but maybe there are faulty caps, resistors, again it's only few components.

Amplifier Discussion / Re: Zoom 15watt Modeling Amp BUZZES
« on: May 04, 2011, 01:14:01 PM »
It's blown amp, you have to replace the whole power section.That buzz you hear is DC voltage, and it can fry your speakers.

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