The LL1620 CFB is a version of the LL1620 where one primary winding on each coil has been split to support Push-Pull Cathode Feedback applications.
The LL1620, LL1623, LL1627 and LL9202 are output transformers for tube amplifiers. All transformers are based on the same core size, winding structure and secondaries, but differ in number of turns (and thus impedance level) of primaries. The transformers are highly sectioned with harmonically sized sections, which results in a minimum leakage inductance. This, combined with a low capacitance coil winding technique results in a wide frequency range.
The transformers have a special audio C-core of our own production, which is gapped for desired DC current. The transformers are of open frame type suitable for mounting inside an amplifier housing.
DC air gap
PP (Push-Pull) version of output transformer have a 25 µm core air gap to allow for a small DC unbalance of output circuit. SE (Single Ended) versions air gap have a bigger size (125 – 250 µm) – the purpose of it, is to accept the DC current of the output tube without saturating the core, leaving enough headroom for the sound signal. As a result of the airgap, the primary inductance is lower for SE output transformers compared to PP. In addition, the inductance tends to vary with DC current. For high quality C-cores with carefully ground surfaces, the variation is within +7% of rated value.
Standard or Amorphous core?
Some of Lundahl tube output transformers are now available with amorphous core. Listening tests, in particular for the PP versions where the core airgap is not as dominating as in SE applications, have reported a more transparent, wider bandwidth character than with silicon iron counterparts.
For connection alternatives and general application information, please refer to data sheets for regular (silicon-iron) output transformers.
The obvious measurable difference between silicon-iron cores and amorphous cores is that the saturation flux for the amorphous core is approximately 33% less than for the silicon-iron counterpart. This is caused partly by a lower saturating flux level, partly by a smaller fill-factor due to the thickness of the amorphous sheets.
As a result, power bandwidth is reduced by about 50%. This means that if the max output power for a standard LL1620/40mA is 25W at 30 Hz, corresponding max. power for LL1620AM/40mA is 13W. This is probably not a problem in most Push-Pull applications, but should possibly be considered in Single End amplifiers.
Most Lundahl transformers are made on request with winding time about 3-5 weeks, so please consider this in your building plans.