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** |
Key
* 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