Cheap Marshall JCM900 4102 "AS IS" - blow fuses, bad power tube and pots - low output

[J M Fahey]

or an MP3 tone to drive amp, and a scope to check clipping and waveforms.

At least a PC Scope and a homemade attenuator.

These 3 offers to solve the measuring problem with free software and $2 total cost in parts were ignored, so ...... what else left to do?

Because in my country

Which is?

with the 250mA fuse the amp sounded really really loud as I was expecting. If I put back the 500mA fuse Fast blow, or higher value, the amp sounds quieter, or less output.

Which is impossible, proof that testing levels are inconsistent.
You need a tone generator for that.

With the twin volume at 5 it hurts because it is so loud. With the marhshall I can go to 10.

Sadly this means little to nothing.

[vintagekiki]

No scope? No problem. Use your amp as a signal tracer.

http://musicfromouterspace.com/analogsynth_new/HOT_TIPS/ampsignaltracer.html

[vintagekiki]

Sadly this means little to nothing.
[ATTACH=CONFIG]54931[/ATTACH]

Exactly
When comparing guitars, amplifiers or speakers, only one thing in the test chain is interchangeable, two must be constant.
Example: two heads are compared with the same guitar and same speaker.

[spunko]

Hello, I'm sorry I was a little busy.

When the mains are disconnected from the wall and the FS1 / FS2 / T500mA fuses removed, when measured by an ohmmeter how much is value resistance between the cathode power tubes (pin8) and the ground (except value R8/R9/100k)

No resistance between pin8 and chasis or other ground points.

How you tested the volume of the amplifier?
Tested the volume the amplifier with a guitar is irrelevant, only any signal source that has a constant output amplitude.

I took out both fuses, pots at max as suggested, fed a 1 Khz / 11.5 mV signal into the amp, AC voltmeter to JS4 and got 5.39 Vac for the clean channel, and 5.2 Vac for the drive. How do I read the voltage at C7?
What it is interesting, is that the amp still had output through the speakers with both fuses out. Is this normal?

I used a tone generator app for iPad, set it for 1Khz and tested with my multimiter to get between 10mV to 15mV adjusting the output volumen of my ipad. (I got 11.5mV between tip and sleeve)

With no fuses the amp indeed has output, really low.

I tested again today, same setup with the tone generator app, this time I got 10.2mV / 1Khz and as you already said, no change in output level when using between different fuses. I think last time I got confused by the distance between me and the cab. If I bend down a little or if I move to the left or right the sound changes.

These 3 offers to solve the measuring problem with free software and $2 total cost in parts were ignored, so ...... what else left to do?

Which is?

I'm sorry, I travel a lot inside my country, Ecuador. I work with hydraulic machinery so I'm always inside the fields, no internet =( sometimes no time between home, kids, work.
Here we lack of electronic supplies, no tubes, tube sockets, Slow blow fuses, no good capacitors, only unbranded chinese stuff because there is no market here. I have to buy via eBay or amazon and get the stuff sent overseas.

I did a little search and got sites where you have to buy both hardware and software to get the scope. I am planning to buy a used scope from the USA, although I have to think about how to bring it from there, it might be heavy with high shipping cost.

Is there any good scope software someone would recommend? It would measure 100 watts without blowing my laptop?
Do I need aditional hardware for a PC scope?
How many watts has to be the load resistor?

[J M Fahey]

Is there any good scope software someone would recommend?

Yes, I´ll use one as a practical example and suggest 2 more which are practically the same,all mimicking a real scope.

There are others chock full of features, but we want to basically see waveforms and not much else so simpler is better-

It would measure 100 watts without blowing my laptop?

Yes because I suggest an attenuator from any voltage to 100/200 mV RMS which is safe and perfect for Laptops´ 3.5" Line input and is diode protected.

Attenuator is nothing more than a few cheap resistors, a couple 1N4002 protective diodes, maybe a rotary switch if you build the fancy version , a couple crocodile clips (ground and input) and a 3.5mm plug.

Do I need aditional hardware for a PC scope?

The simple attenuator I suggest above.

How many watts has to be the load resistor?

100W for a 100W amplifier.
Or a smaller one IF you drop it in a water bucket.

I´ll also show one made out of cheap electric heater resistors, as used in:



I showed the whole idea in The Gear Page but sadly I´m blocked there, can´t even *look* inside, not because of myself but my whole ISP ´provider IP range is.

Please search there: scope died, anybody uses software type? and you´ll find it.

Post shows the simplest 2 resistor 2 diode attenuator as example, it can be expanded as much as needed.

[vintagekiki]

No resistance between pin8 and chasis or other ground points.

Correctly

With no fuses the amp indeed has output, really low.

I tested again today, same setup with the tone generator app, this time I got 10.2mV / 1Khz and as you already said, no change in output level when using between different fuses. I think last time I got confused by the distance between me and the cab. If I bend down a little or if I move to the left or right the sound changes.

With no fuses in output tubes, the amp should have no output
To be sure, test an amp without output tubes, it's the same as when amp no fuses.

The ear is a very unreliable "instrument" it has a directed characteristic.
To be sure what you are measuring, connect an AC multimiter 20- 50V ~ in parallel to the speaker.
By measuring the AC voltage on speaker, you can get enough information about the power output.


http://www.sengpielaudio.com/calculator-ohm.htm

Is there any good scope software someone would recommend? It would measure 100 watts without blowing my laptop?
Do I need aditional hardware for a PC scope?
How many watts has to be the load resistor?

Scope software
https://www.google.co.uk/search?hl=en&as_q=oscilloscope+software+pc-laptop (People also ask)
https://www.instructables.com/id/Use-Your-Laptop-as-Oscilloscope/
https://windowsreport.com/oscilloscope-software-pc-laptop/
http://homediyelectronics.com/projects/howtomakeafreesoundcardpcoscilloscope/
Each Scope software application describes aditional hardware for a PC scope.

For fault finding and repair of amplifiers it is quite sufficient decent software that emulates osciloscope on PC, multimeter and resistor or combination of multiple resistors whose total resistance is 8 ohm/100W

[vintagekiki]

Please search there: scope died, anybody uses software type? and you´ll find it.

J M really wonderful, all in one place.

https://www.thegearpage.net/board/index.php?threads/scope-died-anyone-use-the-pc-based-ones.1620645/

https://www.thegearpage.net/board/index.php?threads/scope-died-anyone-use-the-pc-based-ones.1620645/#post-20628962

https://www.thegearpage.net/board/index.php?threads/scope-died-anyone-use-the-pc-based-ones.1620645/#post-20633904

https://www.thegearpage.net/board/index.php?threads/scope-died-anyone-use-the-pc-based-ones.1620645/#post-20636188

etc

[g1]

With no fuses in output tubes, the amp should have no output
To be sure, test an amp without output tubes, it's the same as when amp no fuses.

I think with this particular amp, even with output fuses removed, there is still the 100K and LED path from cathode to ground. So maybe a very weak sound with fuses removed is possible?

[vintagekiki]

I think with this particular amp, even with output fuses removed, there is still the 100K and LED path from cathode to ground. So maybe a very weak sound with fuses removed is possible?

With Ua=490Vdc and R8=100k, if there was a any current, it would be of the order Ik=4.9mA, which at R8=100k on the cathode of the output tube would generate a voltage of 490Vdc. With this Uk each tube is non-conductive. Personally, I think with this "bias" output tubes anything could be heard from the speakers.
In any case, it's worth a try. New knowledge should never be rejected.

[Helmholtz]

With Ua=490Vdc and R8=100k, if there was a any current, it would be of the order Ik=4.9mA

How this? You are ignoring the very high series resistance of the tubes.

which at R8=100k on the cathode of the output tube would generate a voltage of 490Vdc.

True, this shows that there can't be 490V at the cathode (across the 100k).

I would expect a cathode voltage in the cut-off region, maybe around 50V (cathode voltage adds to the fixed bias voltage for effective bias). Easy to measure.

[spunko]

Thank you very much for all the answers.
I'm going to check all the links, and try the attenuator and the scope software this weekend, I hope.

Quick test: No power tubes = no output // with tubes and no fuses = low output

Should I post voltages without the fuses, but with the power tubes plugged in?

Also, is this ok?? That pin8 gets different voltages because of the fuse in use?

FS1 500mA Slow blow / FS2 500mA Fast blow
Tubes 1 - 4 at pin8: 39.5 mV
Tubes 2 - 3 at pin8: 9.3 mV

Now, if I switch the fuses:
FS1 500mA Fast blow / FS2 500mA Slow blow
Tubes 1 - 4 at pin8: 9.5 mV
Tubes 2 - 3 at pin8: 39.3 mV(

[Helmholtz]

which at R8=100k on the cathode of the output tube would generate a voltage of 490Vdc.

Yes, please do. Will help to find out why you're getting sound without the fuses.

Also, is this ok?? That pin8 gets different voltages because of the fuse in use?

With fuses in, pin 8 voltages should be in the mV range. Absolute numbers are not relevant as they depend on individual fuse and fuse holder contact resistances. So nothing to worry here.

[vintagekiki]

Yes, please do. Will help to find out why you're getting sound without the fuses.

Please if you quote something, then quote it in its entirety, so build your construction on the basis of the entire quote.
Do not draw conclusions from context.

With Ua=490Vdc and R8=100k, if there was a any current, it would be of the order Ik=4.9mA, which at R8=100k on the cathode of the output tube would generate a voltage of 490Vdc. With this Uk each tube is non-conductive. Personally, I think with this "bias" output tubes anything could be heard from the speakers.

if there was does not mean it was
anything could be heard is negation

How this? You are ignoring the very high series resistance of the tubes.

Very high series resistance has a tube only if it is cold respectively if it is not heated.
The current does not flow through the "cold tube"
If the tube is hot, respectively if it is heated, it has real resistance


PS: The translation is machine, so I let some of the terms not be literary.
In any case, from a technical side, all is absolutely understandable

[Helmholtz]

Sorry, most of your reply I don't understand (I am no native speaker either). Especially it is not clear what you mean with "cold tube". In our case the tube is heated and has a cathode resistor of 100k, so a small auto-bias cathode current will flow and cause a correspondent voltage drop across the 100k. The cathode current cannot be as high as 4.9mA as that would mean a shorted tube. I rather expect a cathode current below 1mA.

If the tube is hot, respectively if it is heated, it has real resistance

Yes, that is the case and I expect the tube's resistance to be several 100k. This will constitute a voltage divider together with the cathode resistor. Consequently the voltage at the resistor will be a fraction of Ua=490V.

To be more specific, you wrote: "With Ua=490Vdc and R8=100k, if there was a any current, it would be of the order Ik=4.9mA", which I interprete as there is either zero current or something close to 4.9mA.

I say the cathode current will not be zero and will be considerably lower than 4.9mA.

Let's wait for the voltage measurements at pin 8.

[vintagekiki]

¹⁾ Especially it is not clear what you mean with "cold tube"

²⁾ To be more specific, you wrote: "With Ua=490Vdc and R8=100k, if there was a any current, it would be of the order Ik=4.9mA", which I interprete as there is either zero current or something close to 4.9mA.

¹⁾ Hot tube = Tube is heated, and has some real resistance, the tube conducts the current.
Cold tube = Tube is not heated, and has very high resistance, the tube not conducts the current.

²⁾ Let me explain. If R8=100k were connected to Ua=490Vdc, current through R8=100k will flow 4.9mA.
In other words, if the output tube is 100% open, (bias is 0V) internal resistance is approximately 0 ohms. In this case, current through R8=100k weighs to maximum value of 4.9mA

As the negative bias increases, the internal resistance of the output tube increases and the current through the output tubes decreases. the internal resistance of the output tube and R8 will be the voltage divider, and as the current through the output tube decreases, the voltage on R8 will also decrease

As the negative bias increases, the internal resistance of the output tube increases, and the current through the output tube (Ik) decreases. The internal resistance of the output tube and R8 presents the voltage divider, and as current through the output tube (Ik) decreases, the voltage at R8 will also decrease.