Wow, old school! Humm...If I remember High gain input will distort a wave form. This means the amp increases the wave voltage to pass the maximum available voltage. But since it can't go any higher in voltage, the wave stops at the max (what ever the supply high voltage is rated at) then flattens out at this peak level until the wave drops back down. Your signal distorts any time the feed goes over the limit. Sometimes this is done on purpose as in Guitar amps where there is a separate dial that controls this 'effect'. I'm guessing you are using a regular amp with very simple controls like volume, tone (Bass, Trebble). Some older dedicated amps do not have a volume input but may have a 'Gain' knob which just about serves the same purpose as the volume control. This gain knob is usually set at a level BEFORE distortion can set in from your signal source. Hopefully once properly set you won't overdrive the amp and get unwanted distortion. The problem rises when there are multiple sources of signal say from a Radio Tuner, Turntable (ceramic or Magnetic), and the newer CD, DVD, MP3 players. You must also note that many of the devices after Y2K (wow 18 years ago!) don't have a 'Line' or low level output. Most equip only have earphone outputs which will be too high for use on amplifier. High level output are used to drive head phones or ear buds. You must use anttenuator device to reduce the power for low level use. Donno, if those are still around as Radio Shack stores have closed down.
* Okay assuming you know all this and your amp WAS working normally at one time, AND this is an old 1950-1970's tube amp. Then you probably have a shorted (burnt?) gain or volume knob. Volume Resistor (Pot) cleaner sprayed into component may help. But usually components this old will need replacement. Dirty pots will cause cracks, pops, and scratchy sounds from speaker. This is the most common cause for unstable volume (gain) in amps.
* Another problem may be a shorted or partially bad signal capacitor which is just off the volume/gain pot or directly at the input of the amp section. Capacitors may go bad if a voltage spike (surge) hits it. These are low voltage devices so it does not take much to spike them. Electrolitic caps will sometimes self heal but fail again as input voltages spike near it's limit. You will need an audio electronic tech to go through and check all the components and replace as needed. Yes this is costly as there are not many 'old school' people around. But with some of these antique prices for tube amplifiers (especially bass guitar amps) you have to decide if it's worth it. There are new tube amps out there for audio pureists, so you may want to check that option.
* Okay, worst for last. Tube going bad (very common). Good luck on finding a replacement. I've seen them available on eBay and Amazon but don't know the quality. Very old TV/Radio shops in an old town may have them and worth a check if they are still open. You may also find them at an old music store.
* There are other possible problems (Power supply, Transformer, Bad contacts on tube socket, Bad solder joints, etc.) But really require a tech to trouble shoot and locating replacement parts to initiate repairs.
* since the problem shows up after the amp is heated up and hot. Check the vents for dust and vac out cob webs and debris. Placing a small (small!) fan to blow on the back may help, but over heating usually causes the power supply to heat up as well and may cause arcs in the high voltage section. High heat also breaks down old style capacitors.
* There is a reason why most people switched to transistors and now power ICs. Tubes have a warmer tone that few people can detect as most of us (Rockin '60-70's) have blown out our hearing long ago.
Aloha, ukeboy57
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