Page 54 - Triwatt Custom Lead Amp Builder's Guide

Severely unmatched output tubes in a push pull amplifier

Push pull amplifiers get by with less power supply filtering because they're supposed to cancel this
ripple in the output transformer. The cancellation can be upset by output tubes that use different
amounts of bias current, allowing the hum to be heard.

Faulty power supply filter caps

There are a limited number of ways for the power supply filter capacitors to be bad. All of the tests
on power filter capacitors must be considered hazardous since they may store lethal amounts of
voltage and charge even with the amplifier unplugged.

Any time you suspect power filter capacitors, before testing be sure to discharge the caps. If you
don’t have bleeder resistors do the following: With the amplifier unplugged and the chassis open,
connect one end of a clip lead to the metallic chassis. Clip the other end of the lead to a 200K 2W
or larger resistor. Holding the resistor with an insulating piece of material, touch the free end of
the resistor to each section of the power filter capacitors for at least 30 seconds. This will safely
discharge the filter capacitors.

Then:

Visually inspect the capacitor(s) for any signs of bulging, leaking, dents and other mechanical
damage. If you have any of these, replace the capacitor. Also note the condition of any series
dropping resistors connected to the capacitors to see if they have been damaged by heat. Replace
them if they have.
Use an ohmmeter to measure the resistance from the (+) terminal of each capacitor to the (-). This
should be over 15K ohms (Ω), preferably much over that. If you get less than that on any
capacitor, unsolder that capacitor and re-measure just the capacitor. Less than 15KΩ indicates a
dead or dying capacitor; replace it. If the resistance is now much higher with the cap unsoldered,
there is a low resistance load pulling current, not a faulty capacitor. Always check all of the power
filter capacitors while you're in there. If one is bad, consider replacing them all.
If there is no obvious mechanical problem and the resistance seems high enough, temporarily
solder a new, known good capacitor of at least as high a capacitance and voltage across the
suspected capacitor or section, then plug in and try the amplifier again. If this fixes the problem,
turn the amplifier off, unplug it, drain the filter capacitors again, and replace at least the bad
section if not all of the filter capacitors.

If you are replacing a multi-section can capacitor, get a replacement can with multiple sections
matching the original before you remove the original capacitor. Once you get it, make yourself a
note of the symbol on each terminal of the old capacitor, such as square □=1uF/500V, triangle
∆=20uF 500V, etc. and then clip the old terminal with the symbol off the old can. Remove the old
can, mount the new one, and use the symbol chart and lugs still on the leads to make sure you
connect the right sections up in the new capacitor.

TrinityAmps TRIWATT Builders Guide Ver. 2.31.docx
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