4 x el84 opt question

jbzoso2002 · Post by **jbzoso2002** » Sun Jul 22, 2007 3:23 pm

i have a crate vintage club w/4 el84's and 3 10" 16 ohm
speakers in for repair. it has a blown opt. i dont know
shit about these, does anybody know what the primary
impeadance should be?? i need to know this so i can
select an opt for this. i also read before that old ac30's
used the radio spares opt thats in the original jtm 45's
is this true, and what primary impeadance was that?

thanks in advance

jimmy

novosibir · Post by **novosibir** » Sun Jul 22, 2007 5:07 pm

3 16ohms speakers wired in parallel do have a resulting impedance of 5.3ohms

The AC30 OT has taps for 8 & 16ohms

For the 3 speakers wired in parallel (as in the Fender Vibro King) a OT's impedance of 4ohms is required, which the AC30's OT doesn't have

Larry

LordAli · Post by **LordAli** » Sun Jul 22, 2007 5:11 pm

What's amp power output and EL84's plate voltage?

Datasheets for EL84 says
two tubes in push-pull, class AB and B
Va = 250V, Raa = 8kOhm
Va = 300V, Raa = 8kOhm

Because you got 4xEL84, I guess Raa could be 4kOhm or some close value and amp power output somewhere around 30W.

One question: Didn't you connect speakers to output transformer 100V winding?

jbzoso2002 · Post by **jbzoso2002** » Sun Jul 22, 2007 5:33 pm

well thanks for the replies, i cant connect anything because
the opt is blown/shorted out. what im realy trying to do is
see if a readily available opt like a jtm 45 reissue stock opt
or a radio spares jtm 45 clone opt w a 4 ohm tap will work
w this amp. i emailed merc mag and they suggested an opt
that costs $185.00 and my buddy doesnt have alot of cash
to spend and i may have a stock jtm 45 reissue opt available
for free. so whats the primary impedance? if its 8k for 4
el84's im good to go.

jimmy

LordAli · Post by **LordAli** » Sun Jul 22, 2007 6:31 pm

No it isn't.

Raa = 8kOhms for 2 EL84s
Raa = 4kOhms for 4 EL84s
....

it's same as EL34
Raa = 3.4kOhms for 2 EL34s (50W)
Raa = 1.75kOhms for 4 EL34s (100W)

novosibir · Post by **novosibir** » Sun Jul 22, 2007 8:34 pm

You can use a JTM50 OT, the Drake 784-139 or any clone of it!

This OT has a Raa of 3.4kOhms

4 * EL84 require a Raa of 4.0 kOhms, as LordAli already said

When you now hook the 3 speakers with an impedance of 5.3 ohms to the 4 ohms tap, the to the primary side reflected load will be 4.5 kOhms with this OT, what is close enough and will work properly!

Larry

jbzoso2002 · Post by **jbzoso2002** » Mon Jul 23, 2007 8:37 pm

thanks very much fella's for you answers now i know what
to do with this amp. on the primary side a 50w el34 opt will
work. output side 4 ohms will work with a 5.3 ohm speaker
load. man you guy's great

this is by far the best forum
on the web for amp stuff. thanks again

jimmy