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09-19-2018, 02:06 PM
Hi Guys
When we have a tube amp for repair or we have a new build and want to test it, the first thing we should do is REMOVE THE TUBES.
Of course, we never power anything newly built or of unknown status without connecting it through our Power Limiting Safety Socket (PLSS) - this is the mandatory project in TOT (Tonnes of Tone). This test fixture will save you countless $$ on fuses and will keep things from going up in smoke.
At first we limit power as much as possible since there are no tube loads, just caps charging. The surge will cause the lamp to brighten then dim if everything is okay.
We use our DC voltmeter to check voltages within the circuit, to check their polarity and relative sizes. If this has a power section with fixed-bias, check that the bias voltage is present right on the tube socket pins. Sweep the bias pots to assure their function and leave them set to the maximum negative voltage (bias pot '0' end; zero, cold; low bias; CCW)
If the lamp brightens and stays bright, there is a problem in the circuit.
If the voltages seem okay, you can add preamp tubes. This will require changing to a higher wattage lamp in the PLSS. Powering up should be the same as before but the bulb might glow a little bit. You can measure voltages around the tubes and even send a signal through and scope it up to the empty power tube sockets. Check the bias voltage again with the reduced power limiting. Its proportion to the screen or plate voltages should look more typical now.
Add the power tubes and reduce the limiting further using a larger lamp or parallel lamps.
TEST WITHOUT TUBES any time you modify the bias circuitry AND for any new build AND when you add Power Scaling.
When we have a tube amp for repair or we have a new build and want to test it, the first thing we should do is REMOVE THE TUBES.
Of course, we never power anything newly built or of unknown status without connecting it through our Power Limiting Safety Socket (PLSS) - this is the mandatory project in TOT (Tonnes of Tone). This test fixture will save you countless $$ on fuses and will keep things from going up in smoke.
At first we limit power as much as possible since there are no tube loads, just caps charging. The surge will cause the lamp to brighten then dim if everything is okay.
We use our DC voltmeter to check voltages within the circuit, to check their polarity and relative sizes. If this has a power section with fixed-bias, check that the bias voltage is present right on the tube socket pins. Sweep the bias pots to assure their function and leave them set to the maximum negative voltage (bias pot '0' end; zero, cold; low bias; CCW)
If the lamp brightens and stays bright, there is a problem in the circuit.
If the voltages seem okay, you can add preamp tubes. This will require changing to a higher wattage lamp in the PLSS. Powering up should be the same as before but the bulb might glow a little bit. You can measure voltages around the tubes and even send a signal through and scope it up to the empty power tube sockets. Check the bias voltage again with the reduced power limiting. Its proportion to the screen or plate voltages should look more typical now.
Add the power tubes and reduce the limiting further using a larger lamp or parallel lamps.
TEST WITHOUT TUBES any time you modify the bias circuitry AND for any new build AND when you add Power Scaling.
