Output transformers are constructed in different configurations. Outputs, designed for Single Ended (SE) amps, contain air-gapped cores. The output cores of Push Pull (PP) amps are gapless. Most currently produced feature Ultralinear (UL) screen taps. UL taps function - much like a local feedback mechanism, reducing distortion and increasing the linearity of pentodes or tetrodes. UL mode is employed in both SE and PP amps.
In UL mode, a special tapping from the output transformer (typically 43% of turns from its center tap) energizes the screens. UL mode reduces the harshness, the "stridency" of the sound, infamously associated with pentode/tetrode mode. In pentode/tetrode mode, the screens are fed a voltage directly from a power supply, lacking any possibility of correction by local feedback.
For an excellent, exhaustive and technical explanation of UL mode, refer to Dennis Grimwood's chapter.
Picture the impedance ratio of an output transformer as a car's gearbox. An output transformer with a 5k input impedance and a 4ohm tap (5000:4 ohms) yields a 1250:1 ratio. So, 8ohm speakers (placed on the 4ohm tap) will appear as a load of (1250 * 8) = 10kohms. If using this transformer with 8ohm speakers on its 8ohm tap, the load is only 5K. A 10k load places much less stress on the output tubes and transformer - than a 5k does.
- Turns ratio
- primary impedance divided by secondary impedance
- Given: The turns ratio of a 5000:8 transformer is: 625:1.
- Problem: By connecting a 4ohm speaker to the 8ohm tap of this 5k output, what will the impedance be?
- Solution: 625 * 4 = 2.5k (The load "seen" by the tubes is only 2.5k plate-to-plate.)
Re-locating speakers from the 8ohm to the 4ohm tap is an expedient trick for constructing an amp with tubes - which "prefer" a high(er) primary impedance. For instance, 807 tubes in class AB1 sound best (less distortion) - when the primary impedance is 10k. It's un-likely that outputs with a 10k p-to-p primary and an 8ohm secondary can be easily sourced - unless via a custom order. Attaching 8ohm speakers to the 4ohm tap of a 5k output is a cheap(er) solution in accomodating this classy-sounding tube.
8ohm speakers operate at a nominal 8ohm impedance. If hooked-up to the 8ohm speaker outlet, they can (and do) dip to 6ohms and below at certain frequencies, straining the tubes and output transformers.
When connecting modern speakers to an old (or a new) tube amp, avoiding the 8ohm speaker outlet - unless old-fashioned speakers which are known to rate above 12ohms - is advised. Following this reasoning, moving the Negative Feedback Loop (NFB) to the 4ohm tap (from the 8 or the 16ohm) - when building or refurbishing an amp is also advised. (Adjustments in resistance and capacitance values for altering the NFB loop are necessary.) Formulas - for implementing these magicks are unpacked - directly below.
The formula for moving the NFB loop from the 16ohm tap to the 8ohm tap, given that there is only a resistor in the loop, is the original resistance value (Ro) divided by the square root of 2.
- Rn = Ro / √2
- the new resistor value equals the orginal divided by 1.414
- Given: An old amp with the NFB coming off of the 16ohm tap and its resistor is 47K.
- Problem: What's the correct value for 8ohms?
- Solution: 47K / 1.414 = 33K
However, if there is a cap in parallel with the resistor, the cap's value must be altered as well by multiplying the orignal cap's value (Co) by 1.414.
- Cn = Co * √2
- the new cap value equals the old times 1.414
- Given: The original value of the cap is 470pF.
- Problem: What's the correct value for 4ohms?
- Solution: 470pF * 1.414 = 680pF = Cn
The formula for moving the NFB loop from the 16ohm tap to the 4ohm, given that there is only a resistor in the loop, is the original value (Ro) divided by 2. Similarly as above - if there is a cap in parallel with the resistor, multiply the cap's original value (Co) by 2.