Version 3.2
Page: 57
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, 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
10K 1/2W 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 □=1
uF
/450V, triangle ∆=20
uF 450V, 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.
Faulty bias supply in fixed bias amplifiers
A bias supply with excessive ripple injects hum directly into the grids of the output tubes. Check that
the bias supply diode is not shorted or leaky, and then bridge the bias capacitor with another one of
equal value to see if the hum goes away.
Unbalanced or not-ground-referenced filament winding
The filament power must be referenced to the DC in the tubes in some way, otherwise you may get a
lot of hum. The filaments are usually a center tapped 6.3VAC winding, with the CT grounded for the
necessary reference.
Summary of Contents for Trinity Tweed Amp
Page 2: ...Version 3 2 Page 2 ...
Page 16: ......
Page 20: ...Version 3 2 Page 20 Input Jack Theory from 18watt com ...
Page 25: ...Version 3 2 Page 25 ...
Page 49: ...Version 3 2 Page 49 ...
Page 65: ...Version 3 2 Page 65 HEYBOER OT for TWEED 6L6GT CONVERSION ...
Page 66: ...Version 3 2 Page 66 ...
Page 75: ...Version 3 2 Page 75 ...
Page 76: ...Version 3 2 Page 76 ...
Page 77: ...Version 3 2 Page 77 Trinity Amps Schematics and Layouts ...