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

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

    Hello again friends. Let me start saying I love this forum, it is really fun to do all this stuff, and I really appreciate all the help you guys give.

    I got this marshall amp really cheap. The wooden cabinet is junk, it is infested with moth larvae. It also came with the main fuse and the fuse for power tubes 2 and 3 both blown. Gain and volume pots for channel B are broken.

    So first step, I replaced the fuses and used my current limiter to power on, and determinated that power tube 3 was bad. I don`t currently have 5881 tubes, so I used the 6L6GC from my old twin silver face, these are the original tubes from the 70's. Powered the amp again, and it worked, no blown fuse, and the bulb in the current limiter stayed off. Then I noticed that channel B was at 10 volume all the time, no matter if you turned the knob, so I replaced those bad pots too.

    Now the "stranger things":
    - the amp has a protection in the power amp section, when there is a bad power tube, the amp works half power, and a red led next to the blown fuse lights up. This was the case of my amp, it was working only with tubes 1-4 (before I put the 6l6GC) and the clean channel indeed was at half power, but when switched to the gain channel, it sounded really really loud, like it was at full power. I know the volume pot for the CH-B was bad, but even if I turned the clean to 10, it didn't get to the level of the CH-B.

    - When replaced the power tubes, fuses, and gain/volume pots for the CH-B, the amp worked nicely. But the clean channel stayed with the same output level, like it was working with only 2 Power tubes, and also the CH-B leveled with the clean channel. So now both channels sound with the same level of output, but definatelly the CH-B was louder before changing anything.

    - Changed the Volume pot for the clean channel just in case, with the same result.

    I have checked the voltages really quick, and all seems normal.
    I comapred the output level with the Twin Reverb 2012 you helped me with in the other thread, and the twin sounds much louder that the marshall. Before changing anything, CH-B had same level output than the twin, really really loud, and clean channel like half power.

    - Checked again the fuses and they are good. Also the Fail leds are off, so I think there ir no power tube failure.

    Why would CH-B sound louder with the bad components?
    Why the channels leveled each other but with half power even with the 4 power tubes working?

    what voltages do I need to post?

    Thank you.

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    Senior Member vintagekiki's Avatar
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    Check control voltage at pin1 IC1, IC4, IC6 (M2501) when changing channels.
    The voltage to change the channel is obtained from TR1 point C.
    If no change voltage at pin1 IC1, IC4, IC6 (M2501) when changing channels, check diodes D1, D2, D5 (1N914)

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    https://drtube.com/schematics/marshall/cd0189-iss7-0349.pdf

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    Supporting Member The Dude's Avatar
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    If the volume pot for channel B was open, you had no attenuation at all.

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    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Senior Member Pedro Vecino's Avatar
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    Difference in volume between both channels is normal. Clean channel with a gain adjustment of 5 or 6 (to keep it clean since it has a circuit that generates overdrive above those settings) requires a high volume adjustment to develop power.
    Lead channel with a gain adjustment between 8 and 10 (the most common) requires a much smaller volume adjustment.

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    Don't forget the joker g1's Avatar
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    Are you just going by ear, or measuring power output?
    I agree with Pedro, the clean channel is going to sound quieter. It is possible that it's still not putting out full power as the difference between 50 and 100W is not huge. You would not notice a problem as much when using the dirt channel.
    Did you get good voltage at the screens (pin4) of all the power tubes?

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    Senior Member vintagekiki's Avatar
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    Measuring by ear gives a subjective sense of power, true power is obtained by measuring power output.
    Remove all power tubes (EL34) or remove fuse FS1, FS2 (T500mA) to switch off power tubes (EL34).
    Connect AC Voltmeter to JS4 (Direct out)
    Gain and volume pots for both channels set to max, and bring a signal (10-15 mV/ 1kHz) from the tone generator to JS1 (Input).
    With panel switch or foot switch change channels and compare whether and how much the voltage changes at the power amp input (C7/ 22n).

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    Hello, I was out of town because of my work. This happens quite often. Sometimes I'm out for 2 o 3 weeks.
    Now i'm home, and want to start with this amp again.

    Quote Originally Posted by vintagekiki View Post
    Measuring by ear gives a subjective sense of power, true power is obtained by measuring power output.
    Remove all power tubes (EL34) or remove fuse FS1, FS2 (T500mA) to switch off power tubes (EL34).
    Connect AC Voltmeter to JS4 (Direct out)
    Gain and volume pots for both channels set to max, and bring a signal (10-15 mV/ 1kHz) from the tone generator to JS1 (Input).
    With panel switch or foot switch change channels and compare whether and how much the voltage changes at the power amp input (C7/ 22n).
    This amp uses 5881 tubes. I'm using 6L6 for the moment.
    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?

    Is it necessary to check voltages at the ICs? I'm afraid I can short one
    Quote Originally Posted by vintagekiki View Post
    Check control voltage at pin1 IC1, IC4, IC6 (M2501) when changing channels.
    The voltage to change the channel is obtained from TR1 point C.
    If no change voltage at pin1 IC1, IC4, IC6 (M2501) when changing channels, check diodes D1, D2, D5 (1N914)
    The amp has no problems switching channels, they sound even. What surprised me was that CH-B sounded really really loud with the broken pot, and when I replaced it, the CH-B leveled in output with the clean channel. CH-B was louder with the broken pot. Maybe that is why it sounded so loud.
    Quote Originally Posted by The Dude View Post
    If the volume pot for channel B was open, you had no attenuation at all.
    Here are some voltages:

    All Power tubes
    PIN3: 3.5 Vac / 490 Vdc Is it normal that Pin3 read boths AC and DC?
    PIN4: Standby OFF -90 Vdc / Standby ON -53.18 Vdc Is it normal for the voltage to drop?
    PIN2: 490 Vdc
    PIN1: 3.94 Vac
    PIN7: 3.5 Vac

    The Fender 65 TRR (85 watts) sounds much lourder with the same 2x12 cab than this JCM900 (100 watts). The marshall sounds loud, but is not melting my ears
    I plugged my guitar and tested it with all four power tubes, then just 1-4, and then just 2-3, and I think it didn't make much of a difference. Four tubes definately sound louder than two tubes, but not that much in this amp. And the Twin reverb wins

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    Old Timer J M Fahey's Avatar
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    Melting ears is a very poor power indicator, you´ll have to build a load resistor same impedance as speakers, get a generator or an MP3 tone to drive amp, and a scope to check clipping and waveforms.

    At least a PC Scope and a homemade attenuator.

    Otherwise you are playing Piñata with blindfolded eyes.


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    Juan Manuel Fahey

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    Senior Member vintagekiki's Avatar
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    Originally Posted by spunko
    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?
    JS4 is connected to C7 (22n) via R16 (10k). The voltages you get for clean and drive channel are OK

    Originally Posted by spunko
    What it is interesting, is that the amp still had output through the speakers with both fuses out. Is this normal?
    You should not have output through the speakers if you took out fuses (FS1 and FS2)
    If FS1 and FS2 (in cathode the power tubes) are faulty, then you will not have output through the speakers and the LED1 and LED2 lights.

    Originally Posted by spunko
    Is it necessary to check voltages at the ICs?
    Since channel switching works, it is not necessary to check voltages at the ICs.

    Originally Posted by spunko
    What surprised me was that CH-B sounded really really loud with the broken pot, and when I replaced it, the CH-B leveled in output with the clean channel. CH-B was louder with the broken pot. Maybe that is why it sounded so loud.
    When at volume pots was defective (open), pin which goes to ground is broken, the amlifier sounds very loud, because there is no signal attenuation.
    After replaced the defective pots, the CH-A and CH-B sound should by sound fairly balanced (up to 10%)

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    Unfortunately I don't have an oscilloscope. I know "by ear" you can't measure power, but definitely the Twin sounds louder than the marshall.

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

    Quote Originally Posted by vintagekiki View Post
    You should not have output through the speakers if you took out fuses (FS1 and FS2)
    If FS1 and FS2 (in cathode the power tubes) are faulty, then you will not have output through the speakers and the LED1 and LED2 lights.
    So, it definitely has a problem because the amp gives output even with the fuses out. The both leds light up, but still drives the speakers.

    What do you thnik about the voltages?

    All Power tubes
    PIN3: 3.5 Vac / 490 Vdc Is it normal that Pin3 read both AC and DC?
    PIN4: Standby OFF -90 Vdc / Standby ON -53.18 Vdc Is it normal for the voltage to drop?
    PIN2: 490 Vdc
    PIN1: 3.94 Vac
    PIN7: 3.5 Vac

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    Senior Member vintagekiki's Avatar
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    Originally Posted by spunko
    So, it definitely has a problem because the amp gives output even with the fuses out. The both leds light up, but still drives the speakers.
    Check that LED1 and LED2 go through the R8 and R9 resistors (100k) to the power tubes cathodes.
    If the leds go directly to the cathodes of the power tubes without R8 and R9, it lights up when the fuses out.
    This is not good because the power tubes protection does not work in this case.
    Check circuit FS1, R8, LED1 and FS2, R9, LED2.

    https://drtube.com/schematics/marshall/4100-60-02-iss14.pdf

    Originally Posted by spunko
    What do you thnik about the voltages?

    All Power tubes
    PIN3: 3.5 Vac / 490 Vdc Is it normal that Pin3 read both AC and DC?
    PIN4: Standby OFF -90 Vdc / Standby ON -53.18 Vdc Is it normal for the voltage to drop?
    PIN2: 490 Vdc
    PIN1: 3.94 Vac
    PIN7: 3.5 Vac
    I'm afraid you're not the best count pins on power tubes.
    When you look at the power tubes from below, the pins are counted from the nose (pin 1) clockwise (from left to right)

    Click image for larger version. 

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    I was trying to read the numbers in the socket, I guess I'm not the best I should have definitely check tube by tube and write down every value.

    I get continuity between FS1 - R8 - LED1 and FS2 - R9 - LED2, both fuses out.

    This are the voltages I get with all fuses IN (voltages in the picture)
    Click image for larger version. 

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    Because in my country you won't find slow blow fuses, I replaced the FS2 with a fast blow fuse, and FS1 has its original slow blow fuse.

    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

    So, I took some fuses to test, all of them fast blow. If the fuse is to high, like 4A, the voltage drop even more (like 5.2 mV), if I swap it with a 250mA the voltage rises to 44.5 mV

    Also, 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.

    I am waiting for the slow blow fuses to come, I got them from eBay, but they will get to my hands like in 35 days.

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    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by spunko View Post
    I get continuity between FS1 - R8 - LED1 and FS2 - R9 - LED2, both fuses out.
    I have a question.
    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)

    Quote Originally Posted by spunko View Post
    So, I took some fuses to test, all of them fast blow. If the fuse is to high, like 4A, the voltage drop even more (like 5.2 mV), if I swap it with a 250mA the voltage rises to 44.5 mV
    Fuses FS1/FS2/T500mA are located between the cathode power tubes (pin8) and ground.
    The different voltage drop on fuses of different values is due to the different resistance of the of the wire contained in the fuse tube.
    It doesn't have to burden you.

    Quote Originally Posted by spunko View Post
    Also, 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.
    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.

    EDIT
    Click image for larger version. 

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    Senior Member vintagekiki's Avatar
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    Online Tone Generator

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    In the absence of a signal source with constant output amplitude, the Online Tone Generator can serve.
    Enter the desired frequency in the Hertz field.
    The tone generator can play four different waveforms: Sine, Square, Sawtooth and Triangle. Click on the buttons to select which waveform you would like to generate. The tone will continue until the stop button is pushed.
    You can make a whole range of frequencies for the test amplifier.

    To be you independent of http it is enough to download the desired signal on HD in duration 1 min and then play it through Winamp.
    To allow the test signal to run indefinitely, enable Toggle Repeat in Winamp.

    Sound from sound card output jack, connect via the shield cable to the amplifier.
    Please always make sure amplifier volume potentiometer are set to a low volume to avoid damage to amp.

    http://onlinetonegenerator.com/
    Online Tone Generator



    EDIT (190825)

    https://www.szynalski.com/tone-generator/
    Online Tone Generator

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    Last edited by vintagekiki; 08-25-2019 at 12:37 PM.

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    Don't forget the joker g1's Avatar
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    Any progress on this spunko?
    Especially this question:
    Quote Originally Posted by vintagekiki View Post
    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)

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    Old Timer J M Fahey's Avatar
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    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.

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    Last edited by J M Fahey; 08-28-2019 at 10:26 PM.
    Juan Manuel Fahey

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    Senior Member vintagekiki's Avatar
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    No scope? No problem. Use your amp as a signal tracer.

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

    Click image for larger version. 

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    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Sadly this means little to nothing.
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    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.

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  19. #19
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    Hello, I'm sorry I was a little busy.

    Quote Originally Posted by vintagekiki View Post
    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.

    Quote Originally Posted by vintagekiki View Post
    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.
    Quote Originally Posted by spunko View Post
    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.

    Quote Originally Posted by J M Fahey View Post
    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?
    Quote Originally Posted by J M Fahey View Post
    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?

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    Old Timer J M Fahey's Avatar
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    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.

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    Juan Manuel Fahey

  21. #21
    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by spunko View Post
    No resistance between pin8 and chasis or other ground points.
    Correctly

    Quote Originally Posted by spunko View Post
    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.

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

    Quote Originally Posted by spunko View Post
    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

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  23. #23
    Don't forget the joker g1's Avatar
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    Quote Originally Posted by vintagekiki View Post
    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?

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    Quote Originally Posted by g1 View Post
    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.

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    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.

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    Last edited by Helmholtz; 08-31-2019 at 02:13 PM.
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    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?
    Quote Originally Posted by spunko View Post
    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(

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    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.

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    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by Helmholtz View Post
    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.

    Quote Originally Posted by vintagekiki View Post
    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

    Quote Originally Posted by Helmholtz View Post
    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

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    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.

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    Last edited by Helmholtz; 08-31-2019 at 02:08 PM.
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    Quote Originally Posted by Helmholtz View Post
    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.

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    Old Timer J M Fahey's Avatar
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    Given the different native anguages and the nuances implied in these borderline undocumented bias and setup conditions I suggest you should both reset the counter to 0, not even try to explain earlier misunderstandings and simply start from 0.

    Ignore what was said before, don´t try to explain it, just suggest and refer to new measurements and tests.
    Start with a white sheet of paper.

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    Juan Manuel Fahey

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    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by spunko View Post
    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?
    Each fuse value has some resistance, so measuring the voltage drop on the fuse does not indicate anything.

    Look tube schematics
    http://www.drtube.com/en/library/schematics/69-marshall-schemas

    In 99% amplifier schematics pin 8 (cathode) is connected directly to the chassis, so nothing bad can happen to the amplifier, if in parallel with FS1 / FS2 solder resistor 1 ohm to adjust the bias (until you get the original fuse)

    Measuring the voltage drop on 1 Ohm will give true information about the status of the output tubes.

    Make sure look at the output tubes and the sockets between pin 2 and pin 3 for a lampblack (soot, char)

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    Quote Originally Posted by J M Fahey View Post
    just suggest and refer to new measurements and tests.
    Start with a white sheet of paper.
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    Well done master

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    Senior Member vintagekiki's Avatar
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    Any progress on 4102 spunko?

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    I'm sorry for the late repply.

    This is what I found for the PC scope. I will have to order some parts from ebay.

    Found:
    - 470K
    - 470 Ohm
    - plug for the pc
    - 1N400 x2
    - metal box and a plastic one for the case

    NOT found:
    - 600 volt =/> 0.1uf or 0.22uf in fact only found 400v in every single store.
    - 20 watts or > 4 ohm resistors for the dummy load

    My laptop has only one jack that serves as headphones and mic at the same time. There is no option to change from Mic to Line. It runs Win 10.
    I have an old PC that ran Win Vista, but the HDD died, I will find a cheap replacement, and try with it.

    There are no signs of lampblack or burnt/black spots in any part of the amp.

    Someone will borrow me some new 6L6s to try, because mine are too old.

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