Peter Alerich
2004-04-20 01:39:30 UTC
Last week I decided to retube my Marshall JMP-1 preamp and Marshall
EL84 20/20 power amp. The preamp is 8 years old and had the factory
tubes and the power amp is probably about 5-6 years old (I bought it
used) and also had the factory tubes. The power tubes were Ei/Yugo and
all the preamp tubes were branded Marshall so I'm not sure of the
manufacturer, possibly Ei/Yugo as well.
I placed my order with LV and since I had a few days before they would
arrive I decided to pull the two 12AX7 tubes out of the JMP-1 and
temporarily replace them with the almost new JJ/Tesla and SovtekLPS
tubes from the first two positions on my Super Reverb. All I can say
is -man- what a difference. The overall tone didn't change that much
but the quality of the tone got much tighter. Less sizzle with the JJ
tubes and the notes both individually and in chords were much more
defined. Not sure how much of this was because of tube age or just
cheaper factory tubes but the difference was impressive. I never
really thought the tubes in this thing had much influence on the tone
since it has so much solid state stuff in it but now I know
differently. If you own a JMP-1 and have never retubed it I'd
recommend it. Once the Teslas arrived I installed 2 in the JMP-1 and
returned my subs to the SR.
The 20/20 has 3 preamp tubes and a quartet of EL84s. The first
position is a 12AU7 so I picked an NOS Phillips for that slot, all
others were Tesla. Again, I noticed the same effect on the power amp
as the preamp - not a major tone shift but much tighter response. My
rig responds more to my actual pick attack now. Less sloppy than it
was before. Maybe just a tad less bass than before but it's more
focused so the tradeoff is a value.
Marshall provides two test points and two variable pots to set bias -
you simply set each test point/pot for 70mV DC at idle. I'm guessing
this is some sort of sense resistor since the voltage on the VOM will
vary as you play much like the bias does on my Super Reverb. Im not
sure what the actual idle current is, perhaps 35ma per tube (just a
guess) at about 370VDC on the plates. These little jokers get hot -
I've read some reports of them overheating for some folks but if you
put the power amp in a slot in your rack with a blank space above it
you should have no trouble.
In the end, I really like these tubes and I'm highly likely to
purchase them again. I figured I'd like the Tesla preamp tubes from
having used the one in the normal channel on my Super Reverb. It even
makes that Fender bark. One note: if you own an EL84 20/20 it might
not hurt to open it up and check the solder joints on the tube socket
pins. I found several questionable solder joints in mine and retouched
all of them (on the tube sockets) since I was in there anyway.
Peter
email - alerichatbellsouthdotnet
EL84 20/20 power amp. The preamp is 8 years old and had the factory
tubes and the power amp is probably about 5-6 years old (I bought it
used) and also had the factory tubes. The power tubes were Ei/Yugo and
all the preamp tubes were branded Marshall so I'm not sure of the
manufacturer, possibly Ei/Yugo as well.
I placed my order with LV and since I had a few days before they would
arrive I decided to pull the two 12AX7 tubes out of the JMP-1 and
temporarily replace them with the almost new JJ/Tesla and SovtekLPS
tubes from the first two positions on my Super Reverb. All I can say
is -man- what a difference. The overall tone didn't change that much
but the quality of the tone got much tighter. Less sizzle with the JJ
tubes and the notes both individually and in chords were much more
defined. Not sure how much of this was because of tube age or just
cheaper factory tubes but the difference was impressive. I never
really thought the tubes in this thing had much influence on the tone
since it has so much solid state stuff in it but now I know
differently. If you own a JMP-1 and have never retubed it I'd
recommend it. Once the Teslas arrived I installed 2 in the JMP-1 and
returned my subs to the SR.
The 20/20 has 3 preamp tubes and a quartet of EL84s. The first
position is a 12AU7 so I picked an NOS Phillips for that slot, all
others were Tesla. Again, I noticed the same effect on the power amp
as the preamp - not a major tone shift but much tighter response. My
rig responds more to my actual pick attack now. Less sloppy than it
was before. Maybe just a tad less bass than before but it's more
focused so the tradeoff is a value.
Marshall provides two test points and two variable pots to set bias -
you simply set each test point/pot for 70mV DC at idle. I'm guessing
this is some sort of sense resistor since the voltage on the VOM will
vary as you play much like the bias does on my Super Reverb. Im not
sure what the actual idle current is, perhaps 35ma per tube (just a
guess) at about 370VDC on the plates. These little jokers get hot -
I've read some reports of them overheating for some folks but if you
put the power amp in a slot in your rack with a blank space above it
you should have no trouble.
In the end, I really like these tubes and I'm highly likely to
purchase them again. I figured I'd like the Tesla preamp tubes from
having used the one in the normal channel on my Super Reverb. It even
makes that Fender bark. One note: if you own an EL84 20/20 it might
not hurt to open it up and check the solder joints on the tube socket
pins. I found several questionable solder joints in mine and retouched
all of them (on the tube sockets) since I was in there anyway.
Peter
email - alerichatbellsouthdotnet