50w metro output in a Classic 50

[waznew]

hi guys,
i'm toying with the idea of installing one of george's 50w output transformers into a peavey classic 50 (4 x EL84's). i have a friend who really likes the amp...and wants to upgrade the output transformer. would the primary impedance of the metro work ok with a amp with 4 el84's? whats the ideal primary impedance for such a transformer? i was thinking it was about 4k... am i way off?
thanks

[flemingmras]

hi guys,
i'm toying with the idea of installing one of george's 50w output transformers into a peavey classic 50 (4 x EL84's). i have a friend who really likes the amp...and wants to upgrade the output transformer. would the primary impedance of the metro work ok with a amp with 4 el84's? whats the ideal primary impedance for such a transformer? i was thinking it was about 4k... am i way off?
thanks

According to the datasheets, 4-5K is the ideal load for 4 EL84s. 3.4K is typically what a 50 watt EL34 OT is rated at.

With the slightly lower plate load, you may/may not have issues with redplating at full crank when biasing to 70%, so you may have to bias a little colder.

[waznew]

thankyou for the reply..
yeah, i think i might try it out with a used 50w i've got here..better than fryin one of george's.
if i was going to get one wound, what would be the difference between 4 and 5k? which would be more reliable?

[flemingmras]

thankyou for the reply..
yeah, i think i might try it out with a used 50w i've got here..better than fryin one of george's.
if i was going to get one wound, what would be the difference between 4 and 5k? which would be more reliable?

Both.

The impedance that the tubes see isn't a constant value. 4-5K is a nominal figure...but the impedance ratio has to be within a certain "range". 4-5K is the range that it has to be in. On an 8 ohm load, a turns ratio of about 23.72:1 will give you about 4.5K load plate to plate.

Here's a turns ratio chart for different impedance loads to get you a 4.5K plate load -

4 Ohm Load - 33.54:1

8 Ohm Load - 23.72:1

16 Ohm Load - 16.77:1

[waznew]

awesome, thats exactly what i needed to know... might get one wound up... for the meantime i'll put a marshall-ish 50 in there and see what happens.
thankyou flemingmras, great help.