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Newbie tube amp question(s)

#43
Rockbobmel said:
I've heard some chatter about not using the standby at all. I guess the idea is that everything has to ramp up anyways. Doesn't sound right to me. Does anyone know what that's all about?
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Ultimately the answer is complicated and varies from one amp to another. An amp is made of multiple circuits that may ramp up at different speeds. In some amps the tubes heat up enough to start conducting before the bias supply is fully charged. So turning the amp on in operate mode may result in red plating until the bias supply charges up enough to control tube current.

Also, the bias supply in some amps puts out extra voltage in standby. So when you switch the amp to operate, the tubes are biased off and then slowly ramp up. If you turn the amp on in operate, the tubes conduct as soon as the heaters warm up. When the tubes start conducting they experience a pretty significant change in temperature, so starting the amp in operate causes the thermal change to happen more suddenly. If you turn on the amp in standby the tubes have a change to warm up a bit, and then if the amp has a soft start feature, it make the change even more gradual. So under this conditions, using the standby as intended decreases tube wear.

I believe the Reeves 225 runs the tubes pretty hard. I suggest using the standby as intended. My 400 has eaten several tubes, but it came loaded with EHX KT88, which have been problematic in my amps.

I have one amp that I turn off without engaging the standby switch. It's my Mesa D180. If I put the amp in standby with the Graphic EQ in the circuit, it produces a very loud clack. This is totally normal for the amp per Mesa's legacy amp tech. I swapped in a new standby switch and the capacitor that bypasses the switch and it made no change to the clack.

Here's is my best guess of what is going on. The Graphic is fed from the negative supply. The Standby circuit interrupts the HT supply after the first filter section, but does not turn off the negative supply to the Graphic. The wires for the negative supply and high voltage supply are bundled together and run down the center of the amp. I believe when the amp is put in standby the choke puts out a big pulse that it induced over into the negative supply wire feeding the Graphic EQ. This causes the graphic to send a transient to the output section, so you hear a loud clack.

If I bypass the Graphic, I can put the amp in standby and it only produces a very mild click. Turning the amp off with the standby switch still in the operate position allows the amp to turn off without making any noise. I chose this option because it avoids wear and tear on the Graphic bypass switch, which is a huge hassle to replace. So I turn the power off and wait a few seconds. Then I set the standby switch to the standby position so it's ready for the next turn-on cycle.

I have found images of D180 that have the negative supply wire feeding the Graphic relocated to run along the front panel. I may eventually try this, but it requires some pretty significant surgery to get the job done. It will involve a lot of work and I could potentially damage the amp, soo I don't plan to do it, unless the amp develops some sort of problem that requires me to tear it down.

I think the only possible consequence in this amp is the power switch may experience a bit more wear and tear. It's a hassle to replace as well, but I bought a spare when I purchased the standby switch...or I could just recycle the good standby switch I pulled out of the amp.
 
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#44
What does your amp's manual say?

According to the Ampeg manual, I've been doing it right, or at least not wrong.

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