Is JCM900 using the same OT as the Plexi?

[soundguruman]

What did you do on the 900? I was planning to gut it and make it a Plexi with a master volume. This is the most unreliable amp I've seen. Blew a plate resistor because it was a 1/4W. All the pots scratches. Now blew the fuse. My Plexi blew the rectifier twice in the 3 years I owned it. Marshall uses 1N4007 and put the cap behind the standby switch. When you switch the standby on, the big surge of current through the small diode proved to be too much. That's the reason old fenders put the reservoir cap before the standby to avoid the surge.

Honestly, I'll give Soundguruman's suggestion a quick try, if it doesn't work out, I'll gut it. It's a lousy amp. I am not going to sell it, I like the strong chassis and the cabinet. Not to mention the two transformer.

Yes, I think the biggest problem with the modern amps are the effects loop. You have to add two extra stages of circuits and that kill the tone. I don't know who in the right mind dream up with this and force other maker to put the effects loop.

"Blew a plate resistor"
Usually that's cause you have a bad preamp tube.

"All the pots scratches."
Just clean the controls. You do that on ANY amp.

"My Plexi blew the rectifier twice in the 3 years I owned it."
That's cause your filter caps are too old...or your speaker impedance is wrong, bad output tube, etc...

You keep blaming problems, related to maintenance or mis- use, on the amplifier.
It's not the amplifier. It's your misunderstanding of how to maintain it, or use it properly.

I see this quite often:
The amp is used as a "universal scapegoat" to blame operator error on the amplifier.

1. I plug a noisy effects pedal into the amp, and the amp buzzes. -blames the amp.
2. Single coil pickup on guitar buzzes. -blames the amp.
3. Wrong speaker impedance, blows fuses. - blames the amp.
4. On and on and on....it's always the amp's fault...

The thing about this is....these type of people WON'T listen to what the tech is telling them.
They just keep on repeating the same errors, and REFUSE to learn.

AND they just blame the amp, for all the mistakes they are making in using it.

[Alan0354]

"Blew a plate resistor"
Usually that's cause you have a bad preamp tube.

"All the pots scratches."
Just clean the controls. You do that on ANY amp.

"My Plexi blew the rectifier twice in the 3 years I owned it."
That's cause your filter caps are too old...or your speaker impedance is wrong, bad output tube, etc...

You keep blaming problems, related to maintenance or mis- use, on the amplifier.
It's not the amplifier. It's your misunderstanding of how to maintain it, or use it properly.

I see this quite often:
The amp is used as a "universal scapegoat" to blame operator error on the amplifier.

1. I plug a noisy effects pedal into the amp, and the amp buzzes. -blames the amp.
2. Single coil pickup on guitar buzzes. -blames the amp.
3. Wrong speaker impedance, blows fuses. - blames the amp.
4. On and on and on....it's always the amp's fault...

The thing about this is....these type of people WON'T listen to what the tech is telling them.
They just keep on repeating the same errors, and REFUSE to learn.

AND they just blame the amp, for all the mistakes they are making in using it.

How do you blow the resistor due to an old tube? 1/4W resistor is not meant to take over 200V across. The body is too short. That has nothing to do with power dissipation. You don't use 1/4W for high voltage, that is BAD practice. I worked with HV for years, I know. The key to resistor using in high voltage is the length of the body. If you can find a long and thin 1/4W, it would be ok.

I bought my Marshall Plexi 100W brand new at the time. It blew the first set of rectifier after 1 1/2 year. I took it to a tech at the time. Then it blew the second time after about another 1 1/2 year before I sold it.

If you don't want to blow rectifier and don't want to change the circuit. At least use a bigger rectifier like 1N5408 that is 3 times the capability than the tiny 1N4001.

Don't blame on me not knowing amps!!!!

[soundguruman]

How do you blow the resistor due to an old tube? 1/4W resistor is not meant to take over 200V across. The body is too short. That has nothing to do with power dissipation. You don't use 1/4W for high voltage, that is BAD practice. I worked with HV for years, I know. The key to resistor using in high voltage is the length of the body. If you can find a long and thin 1/4W, it would be ok.

I bought my Marshall Plexi 100W brand new at the time. It blew the first set of rectifier after 1 1/2 year. I took it to a tech at the time. Then it blew the second time after about another 1 1/2 year before I sold it.

If you don't want to blow rectifier and don't want to change the circuit. At least use a bigger rectifier like 1N5408 that is 3 times the capability than the tiny 1N4001.

Don't blame on me not knowing amps!!!!

SEE what I mean?

LOTS of amps use 1/4 watt plate resistors, including Fender and Marshall. Tens of thousands of them...and they keep working, just fine.
If the tube draws too much current, the resistor BLOWS.
but, you blame that on the amp. You ignore the fact that the tube is probably bad. There is no way you are going to LEARN that. You REFUSE to learn.

The rectifier Blows. It blows for a REASON. the amp was drawing too much current.
But you blame that on the amp too.
YOU are making the amp draw too much current. Wrong speaker impedance, bad speaker cable, bad speaker, etc...etc...maybe you had 1 bad output tube...

It's not the amp. It's YOU.

AND since there is no way you will ever realize that amp abuse = amp problem...

No matter WHAT amp you have, if you abuse it, it will stop working. AND you will blame the amp, for the result of your own non- understanding.
YOU will just keep repeating the same mistakes, and blaming the amp for your own misuse.

There are lots of amp owners just like you. You WILL NOT learn.

The problem is YOU, not the amp.

No matter WHAT amp you have...no matter who repairs it...you will just keep doing the abusive operation....the wrong impedance, the bad cable, the bad speaker, the bad tube...
and blaming the amp for the problems that you refuse to correct.

There is no hope for people like you. Buy an acoustic guitar...and give up.

AND NOW you want to tear the amp apart, and rebuild the entire thing, to compensate for a problem, that YOU are causing.
Truly hopeless.

Then you will rebuild the entire thing, and the same problems will happen again!
Because of: YOU.

[soundguruman]

IN CASE you wanted to know:

Some models of Marshall Plexi were built without SCREEN RESISTORS.

When the design of EL34 was changed, this made the amp draw too much current.

IF you have the plexi without SCREEN RESISTORS,
you have to install the screen resistors, to use the new style EL34!

Otherwise, it will blow the fuse, rectifier, etc...over and over again...

NOW maybe you will LEARN something, but I predict NOT.

[g1]

Of course this is all only because it is a Marshall.
If it was Crate it would be completely different.
sheesh

Edit: Pray tell when the EL34 specs changed.

[soundguruman]

Crates are not designed for continuous operation.
This according to the factory and techs that built them.
"Crate amps are designed for occasional use only."

Which explains why they overheat and self destruct...

CONTINUOUS OPERATION:
Is defined by: an appliance that operates 3 or more hours without being shut OFF to cool DOWN.

Marshall amps ARE designed for continuous operation.
That's the difference. Marshalls are designed to keep working.
Provided they are maintained and operated properly, they usually WILL.

EL34 was improved, the efficiency was increased. This causes more screen current...
Plexis built before this improvement were designed for the old style EL34.

Plexi designed before the improvement in tubes, did not have screen grid resistors.
You can see this on the old plexi schematic. NO screen resistors.

If you install the NEW style tube, In the OLD plexi, without screen grid resistors, the fuse will blow, over and over again.
It will also damage other components, such as the rectifier, transformers, etc...
THIS is because the screen grids are drawing too much current.

If you install the screen grid resistors in the OLD plexi, you can use the NEW EL34 no problem.
It won't blow fuses anymore.

This information was provided by the engineers, at the Marshall Factory in Milton Keynes, UK.
They advised me that the screen grid resistors must be installed in the OLD plexi,
to use the NEW tubes. Otherwise, too much current, blown fuses, etc...

Therefore, if you need more information about it,
I suggest you contact Marshall UK.
They will tell you the same information, that they passed on to me.

As far as "when" the new EL 34 was replacing the OLD style EL34, you should ask them that, too.

However, several old plexis have been updated - using the information from the Marshall engineers...
and they no longer have the over-current problem.

Installing the screen grid resistors allows the OLD amp to use the NEW style tubes.
and that I have confirmed.
No more problem after the update. They are working just fine after the update.

[The Dude]

Crates are not designed for continuous operation.
This according to the factory and techs that built them.
"Crate amps are designed for occasional use only."

Please post a link or some kind of proof as to where you aquired this "information". I've scoured the web and can only find one jacka$$ who continues to assert this as fact on various forums. It is not mentioned anywhere else on the web that I can find. Also, having been an authorized SLM service center for years, I do not recall EVER having been told this or hearing anything of the sort mentioned.

[soundguruman]

Please post a link or some kind of proof as to where you aquired this "information". I've scoured the web and can only find one jacka$$ who continues to assert this as fact on various forums. It is not mentioned anywhere else on the web that I can find. Also, having been an authorized SLM service center for years, I do not recall EVER having been told this or hearing anything of the sort mentioned.

AS I said BEFORE:
you are welcome to call the Marshall Factory, speak with the engineers, and CONFIRM it YOURSELF.

Just because there is no "link" does not mean that it is NOT TRUE. CHECK FOR YOURSELF.
Prove me a liar, go right ahead.

Believe it or not:
Not every fact in the world is on the internet...although YOU imagine it is.

Yes, there is a world, outside of the internet.
YOU should try it sometime.

Did YOU learn anything new from this?
of course not.

[Alan0354]

SEE what I mean?

LOTS of amps use 1/4 watt plate resistors, including Fender and Marshall. Tens of thousands of them...and they keep working, just fine.
If the tube draws too much current, the resistor BLOWS.
but, you blame that on the amp. You ignore the fact that the tube is probably bad. There is no way you are going to LEARN that. You REFUSE to learn.

The rectifier Blows. It blows for a REASON. the amp was drawing too much current.
But you blame that on the amp too.
YOU are making the amp draw too much current. Wrong speaker impedance, bad speaker cable, bad speaker, etc...etc...maybe you had 1 bad output tube...

It's not the amp. It's YOU.

AND since there is no way you will ever realize that amp abuse = amp problem...

No matter WHAT amp you have, if you abuse it, it will stop working. AND you will blame the amp, for the result of your own non- understanding.
YOU will just keep repeating the same mistakes, and blaming the amp for your own misuse.

There are lots of amp owners just like you. You WILL NOT learn.

The problem is YOU, not the amp.

No matter WHAT amp you have...no matter who repairs it...you will just keep doing the abusive operation....the wrong impedance, the bad cable, the bad speaker, the bad tube...
and blaming the amp for the problems that you refuse to correct.

There is no hope for people like you. Buy an acoustic guitar...and give up.

AND NOW you want to tear the amp apart, and rebuild the entire thing, to compensate for a problem, that YOU are causing.
Truly hopeless.

Then you will rebuild the entire thing, and the same problems will happen again!
Because of: YOU.

You must not be an engineer, I don't think you know the first thing I talked about. I designed circuits for 30 years and I know what is the meaning of designing with MARGIN. Your definition of abusing the amp is........Turn it on!!!!

I don't think you know the spec of junction temperature, that it's not the thermal dissipation, it's the surge that burn the junction while the component is stone cold. The tubes in the amp worked just fine after replacing the rectifier. Use a bigger rectifier with larger surge current spec and you are done. Instead of beefing up the rectifier, you blame the person.

It is offensive that you assume others don't know anything about electronics. You don't design a circuit that anything goes over the limit and the circuit burn. That is BAD DESIGN. Marshall did not short out the OT if people forget to plug in the speaker, the OT becomes an inductor, it fly back and spike the primary over voltage and burn the transformer. Fender short out the OT, at least there will be no arcing. Hopefully people notice that and plug in the speaker.

Do you know why some 1/4w burn and some does not? If it is bulk resistors ( like carbon comp), it is more rugged. Because of the cross sectional area is big, the junction temp rise is low. For the film resistor, the resistor material is only a thin layer on top, cross section area is small. You can pop those resistor by surge and burn it when it is stone cold. I bet you never even work on high voltages. That's the reason we don't use any film resistors, only the bulk resistors.

Also, you don't design with component that is not spec for the operating voltage. You try to give some margin!!!! DESIGN WITH MARGIN.

Not to be offensive, you don't want to talk electronic theory with me. You better study up before you talk like this. Spend a few years studying, and work a few years in the general electronic industry, you might learn something. No wonder everyone joke about you. I try to be respectful.............. AND I was even trying to defend you a little last night.

Quite a few people here are VERY KNOWLEDGEABLE. If you just stop talking long enough and listen/read, you might just learn something.

[J M Fahey]

I wonder who that jackass might be

I also found him all over the net, desperately asking for help on simple repairs he could not handle.
So much for the "years of experience" .
Or insulting people who told him things he did not want to hear.
Here's a couple free samples (I have a Ton available if needed):

Free sample #1 .
Soundguruman, who among many other claims sets himself up as a Hartke amp expert, was in 2011/2012 ("yesterday") asking noob questions, that no mildly experienced Tech would make.
>>>>>>>>>>>>>>>>>>>
at BassDogs.com
SOUNDGURUMAN says:
Results 1 to 2 of 2
07-21-2011, 09:40 PM #1
soundguruman
Join Date
Jul 2011

Default 5500 HAS EXPLODING CAPACITORS, (any mod or parts to prevent this?)

Yo dogs,
Is there any Hartke genius(es) that have a mod for the 5500 power amp module? Parts be esplodin and stuff!
This is a HA3500 power amp board V1.2
Does anybody stock these transistors, or do you just use NTE whatever...?
C 3198
C 4370
A 1837
A 1266

FWIW those are easy to find 2SC 3198/4370 or 2SA 1837/1266
Obviously he IGNORES the Japanese convention of usually not writing the 2S prefix on transistor body because it's a given, ANYBODY knows that.

So far I got 11 parts on this board that burned, is this normal?
YOU-MUST-BE-KIDDING
Obviously you have zero experience to ask such a dumb question

But all the outputs are good. Seems like the PNP final driver shorted and took everything previous out.
Is there any replacement P amp modules, or are you on your own out there? Any insight appreciated, thanks.
(from repair shop swine)

Sample #2:

01-24-2012, 07:02 AM #2
soundguruman

I think after looking at this I will answer my own question. I have posted this for future repair techs... Oh the generosity
1. The amp was designed to run on 100 volts AC (Japan), in reality... running it on 120 V is pushing it. It is labeled 115 volt, and with normal house current in USA, this is 120 volt.
This is one of SGM pet theories, that Hartke (now extended to Crate) sold in USA are built for 100VAC power lines.
Probably these dumb Japanese know nothing about USA or only export unsold surplus, built to be used inside Japan.
And what about Koreans, Chinese, Vietnamese?
Who do NOT have 100V power lines
Well, they must all be the same as Japanese .... they all eat rice with chopsticks after all
2. This causes the amp to run hot.
3. The heatsink design is IMHO inadequate, not beefy enough.
4. The fan speed is controlled by a thermistor, and the thermistor is mounted in a spot where it does not detect the full heat of the heat sink. This results in the fan running too slow, and it does not speed up when it needs to.
5. The heat sink is designed so that the first transistor (close to the fan) is being cooled, but all the heat is being moved (to the exhaust vent) to the last transistor, at the end of the heatsink. This makes the end (last) transistor on the heatsink, as hot...
6. There is bias, but it is designed like a Hi Fi amp, not a guitar amp.
There is nothing wrong. I have compared it side by side with a stock amp, and there is no difference, they both get hot, identical design and operation, in both amps.

There are NO replacement amp modules, although it was built as a modular, plug in, field serviceable amplifier.

And so now we will run it.
<<<<<<<<<<<<<<<<<<<<<<<<<

To this late statement, a Forum Member answered:

07-23-2011, 11:35 AM #2
tank's Avatar
tank
tank is offline Member Big Guppy

I got on the Sampson website and just asked if there was any way they could help me out finding the part. Within the day I had a response back that gave the phone number and name of one of their parts people. I gave the person a call and explained what had happened and he asked for my mailing address and said he was mailing me the part. I ask how much I needed to pay and his response was "NOTHING". Nothing for the part or for postage. Granted, the part probably cost them very little and postage was probably also very little.... But the response time amazed me. I had the part and my amp repaired within 2 days. I'm not one of their sponsored musicians or one of their dealers, just a regular guy using one their products and they treated me great.

FWIW SAMSON/HARTKE offers full power boards for straight replacement for very reasonable prices .... for authorized service shops, of course, not tyros trying to repair what they can't on the kitchen table.

[The Dude]

AS I said BEFORE:
you are welcome to call the Marshall Factory, speak with the engineers, and CONFIRM it YOURSELF.

Why would I call the Marshall factory regarding a Crate issue? Learn how to read!

Just because there is no "link" does not mean that it is NOT TRUE. CHECK FOR YOURSELF.

I asked for "a link or some kind of proof". Choose the source you wish. It doesn't have to be the web.

Prove me a liar, go right ahead.

I just did.

Did YOU learn anything new from this?
of course not.

When seeking knowledge, most do not consult the village idiot.

[g1]

soundbaronmunchausen, the most interesting man in the world, with the world being his own mind.

[minim]

In all seriousness, I never realized this forum had a crazy aunt.

[minim]

Double post

[soundguruman]

You must not be an engineer, I don't think you know the first thing I talked about. I designed circuits for 30 years and I know what is the meaning of designing with MARGIN. Your definition of abusing the amp is........Turn it on!!!!

I don't think you know the spec of junction temperature, that it's not the thermal dissipation, it's the surge that burn the junction while the component is stone cold. The tubes in the amp worked just fine after replacing the rectifier. Use a bigger rectifier with larger surge current spec and you are done. Instead of beefing up the rectifier, you blame the person.

It is offensive that you assume others don't know anything about electronics. You don't design a circuit that anything goes over the limit and the circuit burn. That is BAD DESIGN. Marshall did not short out the OT if people forget to plug in the speaker, the OT becomes an inductor, it fly back and spike the primary over voltage and burn the transformer. Fender short out the OT, at least there will be no arcing. Hopefully people notice that and plug in the speaker.

Do you know why some 1/4w burn and some does not? If it is bulk resistors ( like carbon comp), it is more rugged. Because of the cross sectional area is big, the junction temp rise is low. For the film resistor, the resistor material is only a thin layer on top, cross section area is small. You can pop those resistor by surge and burn it when it is stone cold. I bet you never even work on high voltages. That's the reason we don't use any film resistors, only the bulk resistors.

Also, you don't design with component that is not spec for the operating voltage. You try to give some margin!!!! DESIGN WITH MARGIN.

Not to be offensive, you don't want to talk electronic theory with me. You better study up before you talk like this. Spend a few years studying, and work a few years in the general electronic industry, you might learn something. No wonder everyone joke about you. I try to be respectful.............. AND I was even trying to defend you a little last night.

Quite a few people here are VERY KNOWLEDGEABLE. If you just stop talking long enough and listen/read, you might just learn something.

Yeah, it's really common for people to blame the amp when they are having problems.
But most of the time, they are creating the problem and blaming the amp for it...
That's pretty normal.

And although Fender and Marshall have been using 1/4 watt in tens of thousands of amplifiers...
I will be sure to tell them that their design is wrong. (according to YOU)

I prefer 1 watt plate resistors, but I also know that if a tube blows the resistor is no longer a viable fuse.
So I use 1 watt for lower noise according to my own preference.

They use 1/4 watt because if something goes wrong, they want the 1/4 watt to blow.
Which is understandable, to limit the damage.

But you don't agree with that. So Fender and Marshall must really be idiots, and you are a genius.

Yes I have repaired guitar amps for 34 years, and I better study up on it...
(That's why Fender made me authorized technician "for life." Because I need to study up on it.)

So, you think Marshall is a "BAD DESIGN"
and i think Crate is a bad design...
touch'e.
So, I guess you should play Crate amps instead. You won't have to endure that Marshall "bad design" any longer...