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JTM45 reissue 6L6s to EL34


 
7/18/1998 7:16 AM
Jupa
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JTM45 reissue 6L6s to EL34
Hi,  
 
Friend of mine bought a JTM45 reissue with "burned" power transformer, due to straight replacement of 6L6s to EL34, or so was said.  
 
Im wondering how it toook out the PT, because there is also a 500mA fuse, which should have burnt first?  
 
Well, this friend of mine wants to have it modified correctly for using EL34s. I believe the right way to do this is to replace 470 ohm screen grid resistors with 1k and readjust the bias for EL34s. Pins 1 & 8 are already connected in the socket.  
 
I read couple of articles sent here earlier this year were it mentioned that GZ34 rectifier cant necessary handle the increased power of EL34s. How is the case with this one?  
 
If there is anyone out there who has actually done this mod, Id like to know how and what was the result?  
 
BTW, does anyone know what was the configuration of the original JTM45?  
 
Thanks,  
 
Jukka
 
7/18/1998 9:29 AM
Paul Markwalter
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6L6s draw .9 amps fil. current  
EL-34s draw 1.5  
The xfrmr might not have been up to the increase in current draw.  
The new prod. Russian & Chinese 5AR4s may not hold up but a Mullard will but I don't think that is the prob. in this case.  
The .5 amp fuse is there to protect the trannys if the power tube plates go short.  
The original had a Mullard GZ-34 and KT-66 or EL-37 power tubes, I think. Good luck.
 
7/20/1998 5:38 AM
Jupa
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Hi,  
 
Thanks for your reply, I have to check those out.  
 
I foung something on that history of the amp from the Unofficial Marshall page http://www.olywa.net/blame/br01340.htm  
 
It says the following:  
 
"Marshall's first amp designs were based on the Fender Bassman tube amp chassis. These models produced 45 watts and were designated as the JTM 45  
series. The first amp was designed and constructed for Marshall by Ken Bran in late 1962.  
 
Did you know that the original design used a pair of 5881 output tubes? Along with three ECC83 (12AX7) preamp tubes, this model produced such nice distortion that guitarists such as Eric Clapton  
adopted the Marshall as their amplifier of choice. The JTM 45-powered 2x12 combo "Bluesbreaker" model amplifier is still considered a classic to this day."  
 
It also refers that KT66s were used later on with 100W models.  
 
So it seems that this reissue (JTM45 MK II) configuration eguals to the original, which is kind of surprising as I though that Marshall has always been a EL34 amp (and 6550 in U.S.). But I guess Im not the only one with that misbelief.  
 
Jupa
 
7/20/1998 7:02 AM
Dave H
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In a Marshall interview in TGM Marshall said that the original JTM45 was a knock off of the 5F6A Bassman and it had 5881s which were later changed to KT66s. The power of the original was 28W. JTM45 was the model # not the power output.  
 
Dave
 
7/20/1998 12:06 PM
Richie
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If you look further you'll find out they used many different output tubes through the years for the same amps, I guess availibility of the tubes was the change, here are a few 5881s,EL34s, 6550s,kt88,kt66, etc.[Richie] different types for english or american models.
 
7/18/1998 9:26 PM
Richie
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You say this is a reissue, does it have a GZ34 rectifier or is it solid state rectifier? Yes you are correct you should change the 470 ohm to a 5 watt 1K [and rebias] also be sure to check the fuse rating,I have had alot of younger guys bring in their amps and said it blew a fuse,so they went to a gas station and bought a car fuse 25 amp and put it in, then they said it worked for a few minutes and then started smoking real bad and had a funny smell [transformer meltdown] so see if your fuse is whats supposed to be. and be sure its the power transformer, not the output transformer.You shouldn't have a problem running the EL34s unless they made this reissue transformer just for the 6L6 current. you could use a fiffty watt marshall transformer if you have to replace this one anyway,and it will run the EL34s with no problem. [Richie]
 
7/19/1998 6:55 AM
Jupa
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Hi,  
 
Yes it is a reissue and it has a GZ34 rectifier. At the moment it came to me it had a proper 500mA fuse in it, but who knows what where in when it blew up :)  
 
Too bad if the transformer is not rated for EL34s, because Ive already orderer one, but I have to try to get the specs somewhere.  
 
Jupa
 
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