5881 to EL34 Conversion in the JTM-45

daveghr · Post by **daveghr** » Fri Feb 24, 2006 9:15 pm

Is there a mod you can do to put EL34s in the JTM-45? Is it as simple as just changing the tubes or do you need to mod the board?

Flames1950 · Post by **Flames1950** » Fri Feb 24, 2006 11:11 pm

You'll need to make sure you have 1K 5watt screen resistors mounted on the power tube sockets in place of the 470ohm 5watt resistors common to 6L6/KT66/5881 tubes.
Then it's just a matter of setting the bias range right -- which may involve changing one resistor on the board......

daveghr · Post by **daveghr** » Sat Feb 25, 2006 8:41 am

any ideas on which one and to what rating?

Billy Batz · Post by **Billy Batz** » Sat Feb 25, 2006 9:51 am

You wont know that until you try and bias them. Most likely you wont have to.

daveghr · Post by **daveghr** » Sat Feb 25, 2006 10:53 am

is the EL34 pin configuration the same as the 5881s?

Billy Batz · Post by **Billy Batz** » Sat Feb 25, 2006 10:55 am

Almost. Not enough taht youd need to change anything. You need a 1K screen grid resistor jumping pins 4 and 6 on each socket if it doesnt have that value already.

daveghr · Post by **daveghr** » Sat Feb 25, 2006 12:09 pm

so i can use the sockets that came with the kit? and instead of using the default 470 ohm resistor in the kit just substitute it with a 1k 5 watt one?

Flames1950 · Post by **Flames1950** » Sat Feb 25, 2006 1:29 pm

Yep.......

daveghr · Post by **daveghr** » Sat Feb 25, 2006 1:34 pm

thanks so much... it'll be a sweet power section

jbzoso2002 · Post by **jbzoso2002** » Sun Feb 26, 2006 1:49 pm

another thing to think about the standard heyborer output trans
has an 8k primary impeadence. el34's like 3.4k. the el34's will
work and wont damage anything but wont sound as good as they
can. if you have a radio spares replica opt it has a 6.6k primary
and sounds good with either tube. i put el34's in my jtm45 clone
and i didnot like the tone. kt66's sounded way better in that amp

jbzoso2002 · Post by **jbzoso2002** » Sun Feb 26, 2006 1:54 pm

dont forget what flames said about the 1k 5watters
if you dont change them and an el34 blowes it will
take your output transformer with it. besides after you
change these resistors you can use el34.s 6l6's kt66"s
5881's with the 1k 5w

jbzoso2002 · Post by **jbzoso2002** » Sun Feb 26, 2006 2:05 pm

dont forget what flames said about the 1k 5watters
if you dont change them and an el34 blowes it will
take your output transformer with it. besides after you
change these resistors you can use el34.s 6l6's kt66"s
5881's with the 1k 5w

daveghr · Post by **daveghr** » Sun Feb 26, 2006 5:34 pm

i was gonna get the mercury magnetics upgrade for the kit. does that have a lower primary impedance? what does the tone with the el34s sound like in comparison with the kt66s?

jbzoso2002 · Post by **jbzoso2002** » Tue Feb 28, 2006 8:25 am

read the output tube description at the top of the tube forum
very good desciption of the tube's sound