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Transformer Tap

Setting the transformer tap

The Elecraft documentation includes a table showing three transformer tap options based on your mains voltage. 

115 V Nominal


230 V Nominal

95V to 105V


190V to 210V

106V to 115V


211V to 230V

116V to 125V


231V to 250V

With my mains nearly constant at 248 V my KPA500 should be using the yellow tap....and that's how I left it when I built the kit.  Everything worked fine. 

An Elecraft tech note came out later that indicated the transformer output should be a minimum of 60 V for optimum linearity.  Monitoring my transformer output voltage using the HV menu option, I saw it was dipping to around 57 V. 

To get the transformer output voltage above 60 V when key down, I changed to the red tap.  I was hesitant to do this since the upper end of the 'red' mains voltage range was 18 V lower than what I had so I was fearing an over-voltage that hopefully wouldn't do any damage.  Everything tested ok, the KPA500 didn't complain of an over-volt condition and as desired, the HV was between 66 and 70 V.  Mission accomplished.

What's that buzzing sound on transmit?

An unexpected side-effect of tapping for lower than the mains voltage is a pronounced chassis buzz on transmit.  The buzz (unrelated to mains voltage and tap) came up on the Elecraft reflector and Eric or Wayne indicated a toroidal transformer can't buzz.  I wasn't convinced so I embarked on a quest to mechanically isolate the toroidal transformer from the chassis.  Why?  The KPA500 is already really quiet given the PIN diode TX/RX switching and this, the loudest noise the box emits, seemed unnecessary.

While the chassis has no lateral room around the toroidal transformer, it has a maybe 0.75 inch of vertical headroom.  I added a closed-cell foam cushion underneath the toroid and another cushion under the retaining disk then tightened the toroid retaining nut just enough to secure the toroid but no so much as to crush the closed-cell foam.  The result was a degree of float for the toroid.  I buttoned up the chassis up and re-tested....but the buzz on transmit was still there. 

Wondering if it was related to the tap being out of range for the mains, I reverted to the original yellow tap...buttoned it up and re-tested.  The chassis produced no noticeable buzz.  An iPhone-based SPL meter showed a 12 dB(A) reduction in key-down noise when measured 8 inches from the front of the chassis.  Unfortunately, the HV is right on the threshold for optimum linearity - 59 to 60 V. 

Until I set up a test for linearity to determine the effect or running close to the lower HV threshold, I'll keep the mains-appropriate, no-noise tap.


It's interesting to note that the amplifier output into a dummy load on either tap was the same.  I was expecting the 20% higher HV to result in some additional output for the same input.  So, it's not like the choice of tap makes a difference in the per-band drive levels you've already set (and the K3 remembers).