A 5U4G will operate in place of a 5Y3G but it draws 1 amp more filament current and is larger. Doing this may cause a power transformer to run hot.
6V6 is a beam pentode, has more gain than a 6K6, straight pentode. The 6K6 has a third grid while 6V6 has two beam forming plates instead of this grid.
Both substitutes would be acceptable if proper tubes weren't available. Only use a 5U4 if the power transformer was heavy duty enough.
Norm
:I beginning wotk on the Crosley 66TA today, I noticed that it came to me with a 6K6GT subbed for the 6V6GT rectifier and a 5U4GB subbed for the 5Y3GT output tube. In looking up these tubes, they appear to have the same pin configurations, but neither sub is a 'recommended' substitute. I used 6V6GT and 5Y3GT and the radio aligned beautifully after recap. Now I'm curious as to why the subs were used and whether the radio is 'better' or 'worse' with the subs. I have not tried them in the set. Thanks for the info...