Tubes

# Output types

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.

##### Transformer and speaker matching

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.

##### NFB calculations

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.