Jcm800 4010 Assistance...

[Mchome25]

I have a question that I could use some help on... I recently purchased a used jcm800 4010 (S prefix serial number so 1984 I believe) and when I got it it had EL34s in it. With the jcm800 4010 most came with 6550s with only certain few being made to take EL34s stock ftom the factory. I know that they can be converted to accept EL34s but I also know that there are unknowledgable folks out there who might just think they can swap the two.

Is there a way to know what tubes my particular amp is made to run...either EL34 or 6550? How can this be determined if it was made from the factory to run EL34s, modified to run EL34s, or should I be running 5881s and someone just stuck in the wrong tubes? Can I tell by owning her up and looking at the eletronics inside (if so could you tell me what to look for)? There are no stickers indicating which tubes should be used.

Many thanks in advance from this new menber!

 

[blues]

4010 most came with 5881s

Actually 6550 not 5881. Marshall used 6550 or El34 in that amp

4010 is a good amp. Enjoy it.

 

[Musicmaniac]

Not sure if this will help https://drtube.com/library/schematics/69-marshall-schemas#JCM800 I have one and love it!

 

[Jethro Rocker]

A picture might help. Is it American market or Canadian or European? The Canadian ones of that vintage were EL34 equppied and have a metal power toggle on the front. Some UK and Euro ones would be EL34 as well I believe.

 

[Mchome25]

I can provide pics this evening but it's not a Canadian amp and I don't believe it to be European. From what I've read certain US market amps also were factory set for EL34s.

From what I've been researching I'm going slide out the chassis and see what resistor is across the top of the power tube socker...if its a 1K then it's set up for EL34s and if its a 470 ohm it's set up for 6550... does that sound like a plan that should work? Or have I got things incorrect??

 

[blues]

set up for 6550/5881

6550 is a completley different tube that a 5881. 5881 was not a factory option.

 

[Mchome25]

Thanks blues...corrected.

 

[Mitchell Pearrow]

Thanks blues...corrected.

To the best forum of Marshall amps, I have no idea on how to answer your questions concerning your amp, but there are more knowledgeable members here that can help you! Cheers Mitch

 

[Jethro Rocker]

I can provide pics this evening but it's not a Canadian amp and I don't believe it to be European. From what I've read certain US market amps also were factory set for EL34s.

From what I've been researching I'm going slide out the chassis and see what resistor is across the top of the power tube socker...if its a 1K then it's set up for EL34s and if its a 470 ohm it's set up for 6550... does that sound like a plan that should work? Or have I got things incorrect??

If that's what the research tells you. I'm not that smart...

 

[ampmadscientist]

I have a question that I could use some help on... I recently purchased a used jcm800 4010 (S prefix serial number so 1984 I believe) and when I got it it had EL34s in it. With the jcm800 4010 most came with 6550s with only certain few being made to take EL34s stock ftom the factory. I know that they can be converted to accept EL34s but I also know that there are unknowledgable folks out there who might just think they can swap the two.

Is there a way to know what tubes my particular amp is made to run...either EL34 or 6550? How can this be determined if it was made from the factory to run EL34s, modified to run EL34s, or should I be running 5881s and someone just stuck in the wrong tubes? Can I tell by owning her up and looking at the eletronics inside (if so could you tell me what to look for)? There are no stickers indicating which tubes should be used.

Many thanks in advance from this new menber!

You can tell which tubes it is set for. 6550 will be biased with about -54 volts DC. If it is set for EL34, the bias will be about -42 volts DC.
5881 will be biased also somewhere in the fifty volt range.
To check the bias voltage:
Take out the power tubes, turn the power on, set standby to "warm-up," do not turn standby to operate position.
Set meter to DC volts, black probe connect to metal chassis. Red probe use to test pin 5 of the output tube sockets.

Now measure the voltage on pin 5 as above.
This will tell you what tubes it's set up for...(or not set up for).
It is possible somebody set up the amp for 5881.

Clear pictures of the whole insides, will also help since the stock tubes will be set up with the stock resistors.

 

[ampmadscientist]

I have a question that I could use some help on... I recently purchased a used jcm800 4010 (S prefix serial number so 1984 I believe) and when I got it it had EL34s in it. With the jcm800 4010 most came with 6550s with only certain few being made to take EL34s stock ftom the factory. I know that they can be converted to accept EL34s but I also know that there are unknowledgable folks out there who might just think they can swap the two.

Is there a way to know what tubes my particular amp is made to run...either EL34 or 6550? How can this be determined if it was made from the factory to run EL34s, modified to run EL34s, or should I be running 5881s and someone just stuck in the wrong tubes? Can I tell by owning her up and looking at the eletronics inside (if so could you tell me what to look for)? There are no stickers indicating which tubes should be used.

Many thanks in advance from this new menber!

And welcome to the forum

 

[Mchome25]

Here are some pics of the insides...hope this helps

 

[ampmadscientist]

Here are some pics of the insides...hope this helps

The amp is still set up for 6550.
However it is possible for 5881 to be used because the bias voltage is very similar.

 

[MarshallDog]

If still set up for 6550s pop some in and go for it, they sound great in this amp. I have one that is stock with 6550s and love it!

 

[ampmadscientist]

If still set up for 6550s pop some in and go for it, they sound great in this amp. I have one that is stock with 6550s and love it!

I think so yeah.

 

[Mchome25]

Thanks guys...the fact that the guy before me was using el34s it in... would that have done any damage? It looks like a 1K resistor across the tube socket, which i think would be the correct resistor for the el34...might that have prevented any damage?...or is it already screwed and should I try to get my money back?

 

[mickeydg5]

MD approves 6550 use in model 4010!

 

[MarshallDog]

Thanks guys...the fact that the guy before me was using el34s it in... would that have done any damage? It looks like a 1K resistor across the tube socket, which i think would be the correct resistor for the el34...might that have prevented any damage?...or is it already screwed and should I try to get my money back?

Correct me if Im wrong but if the amp is still stock and someone was running el34s instead of the 6550s it came with I believe it should be fine. The only thing that would have been going on is that the el34s would not have been biased high enough. Maybe AMS can verify this???

 

[Jethro Rocker]

Correct me if Im wrong but if the amp is still stock and someone was running el34s instead of the 6550s it came with I believe it should be fine. The only thing that would have been going on is that the el34s would not have been biased high enough. Maybe AMS can verify this???

Good point! If it were doing any damage you'd think it would have happened sometime between 1984and now. If it's working now....

 

[Mchome25]

I spoke to my amp tech today and he basically said the same thing as marshalldog... shouldn't be any damage by having the el34 in a 6550 setup in the jcm800 4010 - the issue might be getting a good bias set without any additional mods.

I'm going to have him check everything out to include the bias next week... thanks for everyone's thoughts!