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Messages - phatt

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Well you don't have to turn all the diodes around, just turn the two-pin connector around.

Ha Ha very good. :P
Thanks Enzo trust you to see the simple way to do it. :tu:

BTW I've just been studying the Schematic for this Amp and it's dawned on me that the output of power chip (pin1) connects to both +&- terminals of the speaker.  xP
Maybe I've missed something but might be a mistake.
Also regards to the amp distorting even at low volume;
I just simulated the preamp and a 100mV signal at input does not distort until you turn up past halfway.
But I notice the FB resistor around power chip is 130k. :loco
surely that is rather high and I'm guessing would distort even at low gain. (And that was the main reason for *Lex's* initial post)
The schematic is not very clear and I can't read the part number on the power chip.
Or maybe I'm just not tecky enough to know what I'm doin,. :lmao:

Oh I forgot the obvious page,
Here is a good explanation of the TS9 circuit by R G Keen of Geofex;
It explains how BB diodes work in the fb loop of an opamp which is the type your circuit uses. :tu:

I've come to the conclusion that I'd rather work with a single CLEAN channel amplifier and just use pedals for all the other FX.
My main working amp is not even a guitar amp,, it's an old Keyboard amp with treble bass and volume, but does have a real spring reverb. All the fancy tricks are on my pedal board and forgive me for blowing my horn here but half of those pedals are my own designs or modified brand name units. Took many years to work out what I wanted,, an sure many of those designs ended up as land fill but in the end I got exactly what I wanted.

WHY pedals?
Cause I'm yet to meet a player who uses multi channel amps and totally happy with all channels. :lmao:
Unless you are rich and can afford to have someone custom build exactly what you want then it's likely most of the off the shelf shop rigs will leave you less than impressed. :(

I spent years trying to get an ALL in ONE amp to work for me but in the end I resorted to pedals. I don't like pedals but for me I'm playing many different styles of music and the pedals cover most of what I need.
Sadly pedals are an evil necessity for me,, it's a love hate thing. :-X

Reason 2/
 It's a heck of a lot easier to swap out pedals than it is to have to rebuild amplifiers all the time until you find the sound you want.
My rant over for now,, hope it helps

Yes much better :tu:

There are 2 basic ways to do this Distortion trick.
1/ across the signal path and 2/ in the feedback loop.
Although your setup is in the Fb loop this page might help you understand that it's not just about what fancy pants diodes are used.

This explains the finer details of how diodes work across the signal, often driven by an opamp stage.

In your case you might do well to back off the gain of the input stage as that will slow down how early the amp goes into distortion.
That may well be easier to do that muck around with a whole lot of diodes.

Yes,, what Joe said.  You wire them back to back in that loop.
lay them side by side and they should look like this;
  + -
<     >
  - +
Just be aware that there is no magic diode involved here it's the combination of distortion (Dirt) or whatever diodes you use. The secret to great OD/Distortion/Fuzz is in the tone shaping of the circuits NOT so much the diode type.

So Leds might work for one circuit but sound crap in another amp circuit.
There are just so many combinations of how to do these tricks and then add to that; Each player might want something quite different.
Hence you see Utube clips where someone raves about how good the latest Dirt pedal sounds but you get it home and it sounds nothing like what you heard.
Because your guitar-Amp- speaker- style of music you play- talent level-blah blah.

As mentioned before, a lot of small bedroom amp designs are off the shelf circuits and they don't tend to put much effort into tone shaping to suit the speakers used.

I'm not trying to put you off ,, by all means experiment as that will slowly teach you how to interpret sound/tone.
I've been experimenting for years and it takes a while to find what works and what is a waste of time.

Amplifier Discussion / Re: marshall G30R CD clean channel volume problem
« on: February 16, 2020, 01:42:52 AM »
Oh yeah,, just re reading to check what I may have missed.
You mentioned the input and switch sockets had some issues.
up shot is the control voltage that runs IC2&3 may not be working correctly which would likely cause the issue you have.
I'd be checking that before replacing chips. Control is around TR1.
hope it helps,

Amplifier Discussion / Re: marshall G30R CD clean channel volume problem
« on: February 15, 2020, 11:55:26 PM »
Sorry but it's the wrong forum for anger management issues.

We try to help sometimes we get it wrong,, but we don't have the amp in front of us. xP

If you are well versed in Valve circuits then you would know how important ground return paths are.
Yes every Common point may well be at Ground/Zero **BUT it's the path each section takes back to Main Common point that can make or break a design.**

Tiss why I suggest you take a wire from Volume Common back to to central PSU (where the 2 main filter caps join) and see if it resolves the issue. As you have a new chip coming I hope it works out for you if not then I'd go hunting Ground Paths.

I've worked on enough gear to know that many problems are ground related.
Just as wire layout is important in Valve gear it's no different with SS PCB layout. Unlike lead layout in p-p valve circuits which can be easy rewired,, PCB is Fixed once printed and resolving ground return path issues becomes a lot harder.

I fixed a mates Pro junior Valve amp,, The most sensitive audio trace ran right along side the screen supply trace. :duh
Hence the damn thing hummed like crazy. :trouble
I has to cut a few tracks and hand wire to shut the thing up.
So that was a Fender design layout stuff up.
Without a deep understanding of ground plane layout even the big name gear is often full of suspect design issues.
I wish you well and hope you resolve the issue.

Amplifier Discussion / Re: Marshall Lead 12 testing and repair questions.
« on: February 15, 2020, 08:34:21 PM »
This is a long read for the novice but it may help a lot of folks to understand the basics of *Fault finding* in Power Amp Circuits.
Understand this most basic principal, I quote Rod Elliott from his page:

"If the output voltage is not close to zero, all other voltages are likely to be wrong!
If the output voltage is close to zero, then the amp should be working, but only if it has power."

So check Your power supplies FIRST otherwise you can go in circles and quote these voltages in your postings will help speed up the repairs. :tu:
So many folks just start replacing parts and actually complicate the problem even further. :duh :duh :duh :duh

Rod Elliott's page on trouble shooting:

Amplifier Discussion / Re: Marshall Lead 12 testing and repair questions.
« on: February 15, 2020, 03:40:49 AM »
How can I tell, in the circuit, which lines are the emitters of the PTs?

All you want to do is measure the voltage drop across those resistors,, so set your DMM to DCV and measure the voltage *Across* those Resistors. so one probe on one side and the other probe on the other side of those resistors.
They are 3Watt (Big) and most of the others will be 1/4Watt (Small).

Amplifier Discussion / Re: Marshall Lead 12 testing and repair questions.
« on: February 14, 2020, 06:59:49 AM »
Alright, kids.  Replaced the PTs with no change.

I did inject an ipod signal and, with the neg lead of the spkr tied to the chassis, I can red probe pin 1 and 7 and get audio.  At pin 1, the audio is clear.  At pin 7, it is a bit distorted and louder.

My next suspect is TR3.  Can anyone tell me what i should be hearing at the bce of that TR?

Bu the way...yes, with the spkr disconnected, both output TRs remain cool.

This is starting to get a bit confusing xP

Can i suggest you just do some measurements of the power amp and report your findings then folks here might be able to pinpoint the problem.
I may have missed it but what schematic are you working from? Maybe wise to post it so we are all on the same page.

Amplifier Discussion / Re: Marshall Lead 12 testing and repair questions.
« on: February 14, 2020, 06:53:20 AM »
Hmmm... there is no C8.

There is a spot for a C9 but no cap was ever installed there.

R25 and R26 are both directly behind the headphone/line jack but there is no R27.

Maybe we are reading different schematics?
What ever cap connects to the base of TR1 will be the input to the power amp section.

As for the 2 x OR33 that *LoudThud* mentioned they are 3Watt resistors (they will be Larger than the others).
They connect the Emitters of the 2 power transistors to the speaker output.
so maybe a good time to go and measure the voltages as was asked.

Hello Phil,

thank you for the reaction.

Still a few questions;
Just remove CR1 & CR2 for clean.
   Or you could wire up a switch to switch those 2 diodes in or out of circuit for clean or dirt.

  • being a small amp as 'small' aka SS 15w or small because of the speaker?
    I will use a single 12" or (2x) 10" - so to an external cab.


Small Amp ; means small wattage and small speaker = not much clean headroom.
small chip amps distort very quickly once you try to get a large volume due to the simple fact that they run small transformers and low wattage chips.

Regards to how diodes work to create dist just study some of the OD dist pedal circuits.
The Back to back diodes has been around long before distortion pedals were invented.
basically they limit/clamp the signal swing to the voltage breakdown of whatever diode you use.
As Joe has already mentioned you can use Leds if you want as they have a higher breakdown voltage so distort later. the big gotcha with any diode dirt circuit is the dreaded fizz that often comes with the dirt and most of the low wattage bedroom amps don't bother to filter out the fizz and they often sound quite harsh.

Software / Re: Zoom G5 software
« on: February 12, 2020, 07:03:50 AM »
I doubt you will need an old version of software to do what you ask.
Many of those Audio programs will do the same job.

Amplifier Discussion / Re: marshall G30R CD clean channel volume problem
« on: February 12, 2020, 07:00:13 AM »
I would recommend 'grounding' the clean channel wiper.
That should kill the signal.
As yet, I have seen no resistance measurements of the circuit.
I would look at the junction of C49 & C 22 to ground.
What are the readings when the clean volume control is swept from min to max?

Bingo  I Agree   :tu:
What you think might be ground may not actually be at ground.
That is why we use meters. 8|

I'd add to that,, and suggest measure the DC resistance from volume wiper to the track ground on PCB and then also to the main filter caps common/ground point,, and then again to Chassis/Case.
They should ALL be within a bees dick of of Zero Ohms.

THEN and only then will you know it's not the volume control.

Frankly I'd suggest save yourself the pain and $$$,,just leave it and pull the plug half out as that will ground the input and kill all sound. How hard is that?

Amplifier Discussion / Re: Peavey Stereo Chorus NEW noise problem
« on: February 12, 2020, 06:35:02 AM »
Yes the 2 Ch's are interconnected via the switching sockets, as shown on schematic.
Signal at Pre out sockets also crosses over to the other poweramp via internal switching,, look at links around R70 & R80 for clues.
So yes nothing to worry about.
I'm wondering as Enzo already mentioned, a Common supply to both Amps.Hum?
The chances of the identical issue on both power amps is rare but still possible.
Maybe keep monitoring those 42 volt rails while the amp is working to see if one of those rails fluctuates. I'm not the expert but I can only assume that if one side sags too much it could cause the bias to change giving the crossover fizz,, on both circuits.
BTW, in case you are not aware the vertical string of diodes (on the schematic) running down from the Collector of Q119 sets the bias. The dotted line around CR148 normally denotes that it is strapped to the heat sink.
Maybe check that both are intact and in full contact with the heat sink.

Software / Re: Zoom G5 software
« on: February 11, 2020, 09:38:57 AM »
If it has a USB port then it's likely to work with most software and on most of the later win versions. You might need to update the USB interface.
Others here will likely know more.

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