1983 jcm800 4210 to el34 question

[lidxv0n]

I am a new first time Marshall owner. I picked up a 4210 from guitar center that had el34s in but am skeptical that the bias resistor has been changed. I bought one of the bias tools from amp head and getting a plate voltage of 470 on both tubes and a reading of 19.2 and 17.8 ma for the two tubes.

I have read in other postings that a resistor needs to be changed to 220k. Can someone tell me what the r number on the PCB this corresponds to? I am new to this and read that its in line with the bias control but I could use a little hand holding as to if I am looking at the correct area. I know about draining that caps before hand. I have been on a crash course of learning over the past couple of weeks and wanting to know how to work on/maintain my new amp.

Thanks for any input you can give to a newbie.

 

[john l]

Hi and welcome. Post pics if your circuit board please.

 

[VanR]

I am a new first time Marshall owner. I picked up a 4210 from guitar center that had el34s in but am skeptical that the bias resistor has been changed. I bought one of the bias tools from amp head and getting a plate voltage of 470 on both tubes and a reading of 19.2 and 17.8 ma for the two tubes.

I have read in other postings that a resistor needs to be changed to 220k. Can someone tell me what the r number on the PCB this corresponds to? I am new to this and read that its in line with the bias control but I could use a little hand holding as to if I am looking at the correct area. I know about draining that caps before hand. I have been on a crash course of learning over the past couple of weeks and wanting to know how to work on/maintain my new amp.

Thanks for any input you can give to a newbie.

Could be, but it could also be the tubes. For instance I received a new pair of JJ EL34's from tubedepot the other day. My old tubes were biased at 30mA at 490 volts. Dropped the new tubes in at the same setting on the bias pot and they draw 11 mA. Had to adjust the bias pot upwards to get them to 30 mA. You could have a cold set of tubes. Did you attempt to set them higher?

 

[Mxzx]

Did you see my post on "Getting '79 JMP 2204 to bias in range"?

Guessing you have the same issue.

 

[lidxv0n]

Hey guys,

I am going to open it up tomorrow while the wife and kids are away. I will take pictures and try adjusting it. They are JJ el34s that are in there so perhaps the cold tube thing is all that it is.

Having read about the needed mod and my luck (or should say lack there of ), I figure that is someone slapped in tubes without changing it over

I will update how things go and post pics.

thank you
David

 

[lidxv0n]

Hey guys,

After maxing out the bias adjustment, the highest I can go is 27/26 ma. I took a photo of the pcb, but seem to be having trouble trying to add the attachment to thread. I've tried the paper clip, and the manage attachments button. Do I need to use another site and link to it?

thanks,
David

 

[john l]

Yeah host it on photobucket or image shack and link it. Seams to be guys comming out of the wood work with bias range issues lately. The common denominator has been the range resistor in line with the bias trimmer most of the time

 

[lidxv0n]

John, et al

Here is the link.

http://i993.photobucket.com/albums/af52/lidxv0n/83_4210_zps78906260.jpg


Please let me know if this doesn't get to you what you need. The original was able to be zoomed in on quite nicely, but not sure what the photobucket upload process does.

thanks,
David

 

[john l]

John, et al

Here is the link.

http://i993.photobucket.com/albums/af52/lidxv0n/83_4210_zps78906260.jpg


Please let me know if this doesn't get to you what you need. The original was able to be zoomed in on quite nicely, but not sure what the photobucket upload process does.

thanks,
David

If you still have it opened up take a pic with your flash on. I'm on a stupid tablet thingy and can't see for worth a damn. It does not look to have been converted for el34s from what I can make out.

 

[lidxv0n]

John,


Thanks for the reply. Believe it or not but the flash was on. It seems like the metal reflected it all back at the camera. If you could sort of explain the general area to me, I could crop, enlarge and add some brightness to the original file to give you a better look.
I really appreciate your help.

thank you,
David

 

[mickeydg5]

I like to use good lighting, no flash and place the camera in macro mode for closeups.
Take a good one at the right side of the board while catching the power tube sockets in the photo.

 

[john l]

Let's make this easy. I think I see a 47k (yellow purple orange) in line with you bias trimmer. Turn down your bias pot all the way and bleed down the amp completely. Get 2 insulated alligator clips and a 47k 1/2 watt resistor and clip one end of each lead on it like this

>------<47k>------<

Take the free ends and clip them onto the 47k on the board making sure there's a good connection and that they aren't making any contact with anything else. Only after double checking your bias pot is all the way off fire up your amp and see if you can get a useable sweep now.

 

[Marshall Art]

Maybe this will help. I adjusted the exposure in PS.

 

[mickeydg5]

Yes it seems people and places like Guitar Center are just sticking EL34 tubes in Marshalls without identifying what should be in there, not realizing it is setup for 6550 tubes.

MarshallArt, it is cool of you to do that. I forget or am too lazy to remember to try that.

Lidvx0n
What gets changed is up to you. The easiest thing to do is piggyback a resistor value of about 47k across the other 47k to get a decent negative voltage bias range, as John_l had mentioned. It looks like it may be marked R41 on that board and kind of inline with RV1.

 

[john l]

Yes much better and it that confirms it. Sucks they did that to you. yeah now you can either convert it the easy way, the official way, or grab some 6550s and see if they're your thing OP.

 

[lidxv0n]

Hey guys,

Thank you for brightening up the photo and taking a look. I've attempted another photo of the area: lidxv0n's Library | Photobucket

As far as my options, the amp was a gift from my wife so it will be a keeper. I would be interested in hearing what is involved for the "official" conversion to Marshall specs for this model running el34s. I understand that some changes might "work", but I would rather do the job right regardless of time or some additional parts.

Thank you,
David

 

[john l]

Hey guys,

Thank you for brightening up the photo and taking a look. I've attempted another photo of the area: lidxv0n's Library | Photobucket

As far as my options, the amp was a gift from my wife so it will be a keeper. I would be interested in hearing what is involved for the "official" conversion to Marshall specs for this model running el34s. I understand that some changes might "work", but I would rather do the job right regardless of time or some additional parts.

Thank you,
David

Hi David the easy way just involves altering the range resistor to get you an acceptable sweep for el34s you would do this like the test I posted above accept you would simply solder the resistor instead of clipping it.

The marshall way involves altering the range resistor, changing the two bias splitter resistors from 150k to 220k and the changing the supply resistor from 120k to 220k.

When your amp is set up for 6550s you can run EL34s safely it's just a matter of getting them to an appropriate bias but you CANNOT run 6550s in an amp set up EL34s if you see what I mean. Some guys say the amp sounds better if you go with the full conversion, I am not one of these people though and would just as soon do the range resistor and call it done unless you know that you'll never want to try out 6550s later then why not go the whole way.


Edit so to answer your original question and using this last pic you posted

The 2 150k (brown green yellow) bias splitters are sitting together to the left of the orange and green wires going to the board. these get changed to 220k (red red yellow)

The supply resistor (same colors) is sitting in between the diodes and directly under the big bias electrolytic on the left of the other one. His gets changed to 220k as well

The bias range resistor is the 47k we spoke of earlier that's directly on top and in line with the bias trimmer.

If you do go the whole way you should replace the 2 big black 10uf/100v caps with new fresh ones an consider changing the giant 10k 2w carbon comp dropping resistors to 2watt metal oxides. Comps aren't the best choice in this part of an amp and they don't look original anyway. I say this only because you'll be under the board anyway by going this route.

 

[lidxv0n]

John,

Thank you!

very much appreciated!

David

 

[john l]

John,

Thank you!

very much appreciated!

David

 

[lidxv0n]

John,

One more quick question. I think I've read elsewhere that the 220k, etc should be 1/2 watt resistors. Is that correct?


Thanks,
David