JMP 2203 new tube inquiry... which brand of EL34, or stick with the 6550?

jhayat

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Hi all

Just picked up a '81 JMP 2203. Amp is stock, in great shape, and sounds just like it should. It came with Svetlana 6550s

I noticed a rather loud humming noise when the amp was taken out of standby, after the seller had told me the amp was really quiet. I reseated all 7 tubes, but the hum remained. I then swapped the tubes for the EL34s in my other JMP (which also sounds just like it should), and the hum is now gone, and the head is dead quiet. I don't doubt the seller's "quiet" claim; something probably happened during shipping.

Thing is, with the EL34s, the amp no longer sounds all that great. I am not surprised that there is a difference, but that much has me a bit concerned. No, I didn't bias just for testing purposes, but still... And it's not just the level/freq. response, the gain has gotten a bit icky as well.

So, the first Q that I have is whether or not this is normal. I'll post an audio ex. if need be.

Secondly, if I go for new tubes, should I stick with the 6550s, or go EL34? And which brand should I be looking at, regardless of price?

Do I even dare consider something like this?:

https://www.ebay.com/itm/4-NOS-tube...349464?hash=item5944340618:g:OiwAAOSwwJ1eMZsV

Thanks in advance.
 

ampmadscientist

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Hi all

Just picked up a '81 JMP 2203. Amp is stock, in great shape, and sounds just like it should. It came with Svetlana 6550s

I noticed a rather loud humming noise when the amp was taken out of standby, after the seller had told me the amp was really quiet. I reseated all 7 tubes, but the hum remained. I then swapped the tubes for the EL34s in my other JMP (which also sounds just like it should), and the hum is now gone, and the head is dead quiet. I don't doubt the seller's "quiet" claim; something probably happened during shipping.

Thing is, with the EL34s, the amp no longer sounds all that great. I am not surprised that there is a difference, but that much has me a bit concerned. No, I didn't bias just for testing purposes, but still... And it's not just the level/freq. response, the gain has gotten a bit icky as well.

So, the first Q that I have is whether or not this is normal. I'll post an audio ex. if need be.

Secondly, if I go for new tubes, should I stick with the 6550s, or go EL34? And which brand should I be looking at, regardless of price?

Do I even dare consider something like this?:

https://www.ebay.com/itm/4-NOS-tube...349464?hash=item5944340618:g:OiwAAOSwwJ1eMZsV

Thanks in advance.

The hum you heard may have been a shipping damaged 6550 power tube.
You can't put EL34s into a 6550 amp without changing resistors and adjusting the bias. The amp is not set up for EL34.

You need to take the amp to a technician and have the amp changed to EL34, OR you need a new set of 6550s probably.
One or the other is your choice.

Where are you located maybe we can help you find a technician?
 

paul-e-mann

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Hi all

Just picked up a '81 JMP 2203. Amp is stock, in great shape, and sounds just like it should. It came with Svetlana 6550s

I noticed a rather loud humming noise when the amp was taken out of standby, after the seller had told me the amp was really quiet. I reseated all 7 tubes, but the hum remained. I then swapped the tubes for the EL34s in my other JMP (which also sounds just like it should), and the hum is now gone, and the head is dead quiet. I don't doubt the seller's "quiet" claim; something probably happened during shipping.

Thing is, with the EL34s, the amp no longer sounds all that great. I am not surprised that there is a difference, but that much has me a bit concerned. No, I didn't bias just for testing purposes, but still... And it's not just the level/freq. response, the gain has gotten a bit icky as well.

So, the first Q that I have is whether or not this is normal. I'll post an audio ex. if need be.

Secondly, if I go for new tubes, should I stick with the 6550s, or go EL34? And which brand should I be looking at, regardless of price?

Do I even dare consider something like this?:

https://www.ebay.com/itm/4-NOS-tube...349464?hash=item5944340618:g:OiwAAOSwwJ1eMZsV

Thanks in advance.
Your other amp has EL34, I'd stick with the 6550's for variety sake. So swapping out the power tubes got rid of the hum? That was an easy fix! :yesway:
 

MickeyJ

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you cant run el34 too cold and judge them on that ( if you were running them hot, you most cetrainly could)
str-447 el34 sound killer in any marshall, I love em'.
 

jhayat

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Thanks all!

The hum you heard may have been a shipping damaged 6550 power tube.

I was thinking the same.

You can't put EL34s into a 6550 amp without changing resistors and adjusting the bias

Bias, I am aware of. Resistors? Which ones?

So swapping out the power tubes got rid of the hum? That was an easy fix!

Yup!

you cant run el34 too cold and judge them on that

Gotcha. I wasn't trying to judge the tubes; simply trying to find out if the issue was with the 6550s

So, maybe I will leave the head with 6550s - I certainly have no reason to go EL34, if they aren't going to be drastically better (hey - what's "better" anyway?)

Referencing my earlier Q.... If I decide to get new tubes (I am thinking I may leave these for now), do I go this route

https://www.thetubestore.com/jj-6550

or do I go nuts, with something like this

https://www.ebay.com/itm/Quad-GEKT88-6550-OOO-getter-Near-NOS-NIB/163695398545?hash=item261d018e91
 

ampmadscientist

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Thanks all!



I was thinking the same.



Bias, I am aware of. Resistors? Which ones?



Yup!



Gotcha. I wasn't trying to judge the tubes; simply trying to find out if the issue was with the 6550s

So, maybe I will leave the head with 6550s - I certainly have no reason to go EL34, if they aren't going to be drastically better (hey - what's "better" anyway?)

Referencing my earlier Q.... If I decide to get new tubes (I am thinking I may leave these for now), do I go this route

https://www.thetubestore.com/jj-6550

or do I go nuts, with something like this

https://www.ebay.com/itm/Quad-GEKT88-6550-OOO-getter-Near-NOS-NIB/163695398545?hash=item261d018e91

Where are you located?
Maybe we can help you find a tech to set the board up for EL34s.
 

matttornado

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i’d keep the 6550s. probably sounds awesome.
 

matttornado

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I may be wrong, but i don’t think changing to el34s will have too much of a difference in sound.

nothing drastic anyway I wouldn’t think.
 

jhayat

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Where are you located?
Maybe we can help you find a tech to set the board up for EL34s.

Oh yeah - I missed that from earlier.

I live in CT, and have a good Marshall cert. tech who has worked on my gear before.

I will prolly keep it as 6550. I am still curious, tho, as to the resistor comment. Again, bias I am aware of, but I was not aware that you need to swap resistors for a tube change. Do you?
 

ampmadscientist

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Oh yeah - I missed that from earlier.

I live in CT, and have a good Marshall cert. tech who has worked on my gear before.

I will prolly keep it as 6550. I am still curious, tho, as to the resistor comment. Again, bias I am aware of, but I was not aware that you need to swap resistors for a tube change. Do you?
Bias split resistors, bias voltage resistor, bias scale resistor (bias power supply), NFB resistor possibly.
Bias voltage will be approx -42 VDC for EL34, and -52 VDC for 6550. Marshall2203 6550.png

Bias split
2X 150K for 6550
2X 220K for EL34
Bias voltage
27K EL34
15K 6550
Bias Scale
56K 6550
47K EL34
Screen Grid
1K 5W = 6550 or EL34
470 ohm = 6L6, 5881 (if it's been changed check this)
 
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DonP

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Was the hum from the 6550's redplating from them being loose in the sockets?

When you stick in EL34's and don't do the changes, they would be biased very cold and would not sound good compared to a properly biased EL34 Marshall.
 

The Grim Peeper

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I prefer 6550s. They have a deeper growl Every Marshall I’ve owned with 6550s sounded better than my Marshall’s with EL34s. So glad I never did a conversion. 6550s = Bigger balls on the amp. Even though there is only like 2 companies that still produce them :(
 

MickeyJ

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i have a half dozen of these 6550's NOS GE, but i haven't tried them in any of my Marshalls because from memory, they have a low screen voltage, did Marshall lower screen voltage when running these tubes, isn't the screen voltage rated at a mere 300v on the datasheets? And if Marshall did do that, wouldn't they sound cleanish? ( upping screen voltage to match plates gives you a lot of dirt)
 

jhayat

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i have a half dozen of these 6550's NOS GE, but i haven't tried them in any of my Marshalls because from memory, they have a low screen voltage, did Marshall lower screen voltage when running these tubes, isn't the screen voltage rated at a mere 300v on the datasheets? And if Marshall did do that, wouldn't they sound cleanish? ( upping screen voltage to match plates gives you a lot of dirt)

That's interesting. So, tubes with a higher screen voltage gives more dirt?

How do you tell what the rating is by looking at the data sheet?
 

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MickeyJ

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this was taken from another datasheet.

Maximum ratings Operating characteristics
Frequency f 30 MHz
Anode voltage (dc) UA 13 kV
Screen grid voltage (dc) UG2 2200 V
Control grid voltage (dc) UG1 – 1000 V
Cathode current (dc) IK 50 A
Peak cathode current IK M 380 A
Anode dissipation PA 200 kW
Screen grid dissipation PG2 6,6 kW
Control grid dissipation PG1 1,4 kW

you can see that the screen voltage rating on those jj tubes is 400v ( ug2)


yes, that's why tubes that are running lets say 300 on the screens and 600 on the plates sound clean, very clean ( 807 tubes are run like that for example, you'd run kt88 the same in a home stereo or bass amp)

Bring that screen voltage up and things get nice and Pamela Anderson/dirty.
 

Im247frogs

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this was taken from another datasheet.

Maximum ratings Operating characteristics
Frequency f 30 MHz
Anode voltage (dc) UA 13 kV
Screen grid voltage (dc) UG2 2200 V
Control grid voltage (dc) UG1 – 1000 V
Cathode current (dc) IK 50 A
Peak cathode current IK M 380 A
Anode dissipation PA 200 kW
Screen grid dissipation PG2 6,6 kW
Control grid dissipation PG1 1,4 kW

you can see that the screen voltage rating on those jj tubes is 400v ( ug2)


yes, that's why tubes that are running lets say 300 on the screens and 600 on the plates sound clean, very clean ( 807 tubes are run like that for example, you'd run kt88 the same in a home stereo or bass amp)

Bring that screen voltage up and things get nice and Pamela Anderson/dirty.
Palm of My Handerson my rock climbing buddies always called 'er.
 

MickeyJ

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hep b or whatever shes got, old age etc, wouldn't stop me , I'd hump that in a heartbeat
 

jhayat

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Thanks.

So, from the data sheet you posted, it looks like those tubes's screen voltage is 220v? Is that right?

And when you say that the screen voltage rating on those jj tubes is 400v ( ug2), you are looking at the limiting values? B/c there's another ug2 under 'characteristis'
 

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