Hum is the most common amp problem and usually results from AC line noise leaking into the filament wiring or input stages. The primary cause is a bad ground connection. The resistance between any grounded component and the chassis needs be less than 1 ohm (and is typically 0.5 ohm or less). To understand the proper connections for all grounded components, please play close attention to the amp layout.
There are three kinds of grounding:
1. AC grounding (grounding of the AC power cable)
2. AC chassis ground at the power transformer.
This is the main ground point for the chassis and represents the attachment point for the yellow-green wires from the power plug and the power transformer.
3. Internal signal grounding.
4. Wiring layout
Squealing (resonance of the amp) usually occurs when there is coupling between the input and output stages, positive feedback causes the amp to become an oscillator. No proper wire routing often causes a wire with strong signal interfere an adjacent wire because of the great deal of gain of amp.
If squealing occurs, first you can turn round the volume and tone controls to see if it affects the oscillation. This will estimate if the coupling is occurring before or after a given control.
Then squealing is easily corrected by moving the physical location of the wires around. Using a wooden stick (old chop stick works great) move the wires(the wires between the soldered board, the tube sockets, and the controls) around.
Turn the amp on as if you are going to play (The amp must be connected to a cabinet), turn the controls fully up, and with your wooden probe, move the wires in the amp around. Find the locations for these wires that are the quietest (for hum and for potential feedback). You can use a dab of silicone sealant to hold the wires in place if you like.
1. Move output wires away from input wires.
2. Shorten excessively long wires.
3.Use shielded wire on the input jack and other important connections (The shielded wires are indicated on the layout figures for your amp).
5. Bias of power tube amplifier
If you want to bias power tubes be aware that there are lethal voltages inside the chassis. If you do not feel absolutely confident that you understand how to perform this procedure and keep yourself safe, please consult a professional.
5.1 The meaning of tube bias
Bias voltage is the voltage between a tube's cathode and control grid.
Bias current is the current flowing from the cathode to the plate with no audio signal applied to the control grid (idle current).
The current through an amplifier tube is controlled by the bias. Current can be stopped totally (cutoff), or it can be at a maximum (saturation), depending on the bias setting and the signal.
5.2 Why bias adjustments are necessary
A) Changing the bias can change the tone of amp.
B) Different kinds of output tubes need different bias levels.
You don't always need to bias tube amplifiers after a tube change.
5.3 Meaning of 'hot' and 'cold' bias
A)'Hot' bias means that the amp has "more" idle current than the optimum. The result is 'fatter' and warmer tone and shorter valve life.
B) "Clod' bias means that amp has "less" than the optimum idle current. The result is a 'thinner' tone and longer tube life.
5.4 The biasing procedure
A) Set the tube's idle, or no-signal current flow at a point to keep from overheating the tube's plate.
B) Set the idle current.
C) Measure the voltage across the cathode
resistor (R24) (refer to the layout and schematic diagrams for your amp to determine the location of R24). The bias adjustment is accomplished with a variable resistor. The location is indicated on the layout.
For 6V6 tubes, the voltage across the cathode resistor should be between 20 mV and 30 mV. We recommend 30 mV.
For 6L6/EL34/KT77/KT88 tubes, the voltage across the cathode resistor should be between 20 mV and 35 mV. We recommend 35 mV.
6. Some DO NOTS
(A) Never, Never, Never run the amp without a speaker plugged in. This can cause major damage.
(B) Do not flip the power switch off and then back on rapidly. This can cause damage to the power supply.
(C) Never replace a blown fuse with one that has a higher amperage rating.
(D) Never ignore signs of overly high heat. A wisp of smoke or a burning smell is NOT normal.
(E) Never ignore a red tube glow other than the small orange ends of the filaments. A red glow over a large part of the internal plates of the output tubes means they are about to melt. If you notice this, shut the amp down immediately and get a technician to help you find out what is wrong.
7. Some DOs
Add another speaker into the "external speaker"jack; a mismatched speaker load won't damage the amp, while an open circuit (disconnected speakers) may do so.
Overdrive the stuffing out of it. Tubes are very forgiving of massive overdrives, unlike solid-state circuits. As long as the tubes don’t overheat or stay overdriven for long periods, it is not fatal.