Which transformers are recommended for push-pull EL34 operation?

When selecting an output transformer for EL34 push-pull operation at BTB Elektronik, the first question is the matching, meaning the Raa. According to the Telefunken data sheet (see image on the right), the desired anode voltage provides good orientation. With the usual 350 volts to 500 volts, you will usually end up between 3 kΩ and 5 kΩ. This is also not overly critical, as most loudspeakers are not impedance-linearized. It seems sensible to choose around 3 kΩ – 4 kΩ at lower anode voltage and rather 4 kΩ – 5 kΩ at higher voltages from 450 V – 500 V.

The second question is whether you can live with only one secondary impedance without rewiring, but in return make use of the better audio characteristics of the transformers. Further questions concern the budget, the size, and whether you want to use ultralinear and/or cathode feedback (CFB).

Here is a list of suitable transformers:

Lundahl LL1623 PP Raa = 3.3 kΩ → very wide-bandwidth, very low-distortion, sounds excellent, one load impedance each

Lundahl LL1679 PP Raa = 4.5 kΩ → very wide-bandwidth, very low-distortion, sounds excellent, one load impedance each

Lundahl LL3723 PP Raa = 4.3 kΩ → the finest among high-end push-pull EL34 output transformers. This type has UL taps at 33%, goes up to 160 Wout (@30 Hz) and covers 5 Hz – 60 kHz (−1 dB)

Hammond 1650NA Raa = 4.3 kΩ – good for DIY tube amplifiers with 4 Ω, 8 Ω and 16 Ω connections plus UL up to 60 Wout

Hammond 1750N Raa = 3.2 kΩ – the ideal transformer for guitar amps based on the Marshall JCM800 / 2204 specification

Hammond 125E Raa = 3 kΩ – 6 kΩ – the small entry-level transformer for experiments with lower power up to 15 W