Recommended Output Tube and Output Transformer Combinations

Transformer Manufacturer Part Number P-P Impedance Ultra-Linear Recommended Connection Tube Type Tube Type
Dynaco (SCA-35, ST-35) Z565 7100 ohms Yes Triode 6CK4** 6BQ5
Dynaco (ST-70) Z326 4300 ohms Yes Triode EL34** 7581
Dynaco (MKII) A430 4300 ohms Yes Triode KT88CZ 6550C
Dynaco (MKII/MKIII) A431 4300 ohms Yes Triode KT88CZ 6550C
Dynaco (ST-70) A470 4300 ohms Yes Triode EL34** 7581
Acrosound TO-300 6600 ohms Yes UL & Cathode Coupling 7581** EL34
Acrosound TO-330 3800 ohms Yes UL & Cathode Coupling KT88CZ 6550C
Harman Kardon Citation II* 3800 ohms Yes UL & Cathode Coupling KT88CZ** KT90**
Harman Kardon Citation II*** 3800 ohms Yes Triode 30Wpc Pushpull SV300B** XXXX
Heathkit 51-36 (W-7A) ~3800 ohms Yes Triode or UL EL34** XXXX
Heathkit 51-29 (SA-2) 8000 ohms Yes Triode or UL 6BQ5** 6CK4**
* There are 3 variants of this transformer, the later types are best. Early transformers had 3800 ohm primaries, late ones 3200 ohms - based on units on hand. This transformer can be used with a quartet of tubes and will produce more than 100W out in this configuration.

** Optimum sounding tube type with this transformer - my opinion...

*** Earliest version was used in this application. Performed excellently in every respect.

The recommended connection in most cases produces the best overall performance with the particular transformer cited. Some transformers are really excellent when used in triode / pseudo triode applications while others perform better in Ultra-linear (partial triode to the British amongst you) or a combination of UL and cathode coupling. Where cathode coupling is recommended the output should be padded down 6dB by resistive dividers made up of resistors in the range of 30 - 40 ohms each to minimize power loss in the cathode resistors. Omitting the resistors is not recommended except on the Acro T0-300.
I have used these transformers in a variety of different amplifier projects with good to excellent results. Many of the transformers employed came from units that were cosmetically and/or electrically challenged... I don't recommend destroying pristine units to get these items...

©1998, 2001 By Kevin Kennedy