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I have an Ampeg reissue from the early 2000's, has 2 6L6 and three preamp tubes, 60 watts, master volume......When I first turn on the amp it has a normal volume, the gain and volume controls work normally...but after a few minutes, there is a sudden reduction in volume and the gain control seems to no longer effect the volume or the amount of gain, so I have to turn the master volume up to reach the volume level that the amp had prior to the volume reduction. It seems like after the amp heats up something goes haywire, reducing volume....one of the preamp tubes? Also, it seems that just before the volume reduction there is a kind of click sound, like a switch kicked in..This volume issue repeats whenever the amp cools down and it's turned on again...Any help appreciated.
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Last edited by ctone; 02-23-2023, 06:11 PM. -
Do you mean two 6L6 ?
Like schematic attached?Originally posted by Enzo
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Is it doing the same as if you were engaging the clean setting with footswitch? Are you using a footswitch? If not, have you cleaned the footswitch jack?Originally posted by Enzo
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It has two 6l6's, and it's like the schematic. Not using a footswitch, don't seem to have one. I just tested the issue again. It's a popping sound and then the volume cuts out, and it seems it's the gain channel that's effected, not entirely, as there is still some distortion from the gain, but no real volume change as the gain knob is turned up. Before the popping sound, the gain control added lots of volume as it's turned up, now I can turn it all the way up and it just adds a little more distortion and almost no additional volume. Will try cleaning the footswitch jack. I did jump the effects loop, but that didn't do anything. I had had the effects loop jacks replaced a while back. Could the pre-amp tube that controls the gain channel have issues as it heats up? Not sure which pre-amp tube that would be.Originally posted by g1 View PostComment
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If not using footswitch, try the channel switch. Does clean channel work normally?
What is the difference between clean channel, and when fault occurs with drive channel?Originally posted by Enzo
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It doesn't seem to affect the clean channel, didn't notice a difference in volume. It's the drive channel, specifically the drive control knob that appears to lose most of it's power. I cleaned the footswitch input jack, and at first it seemed to fix the issue, but after about ten minutes, instead of just a few, the popping sound happened and the drive channel lost volume. The master volume doesn't seem to be affected either, just the drive control. On this amp there is a volume control for the clean channel (doesn't seem affected), a master volume control (not affected), and a drive control - is affected.Originally posted by g1 View PostComment
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There is a diode, D11, across the relay coils. Measure the DC voltage across D11 (one meter probe to each end) with amp switched to 'dirt' channel. Does voltage stay the same when working right as when fault occurs, or does voltage change?Originally posted by Enzo
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This is probably beyond my ability.....I think i'm going to have to take it into someone to repair. What would that measurement at D11 show? Is that testing the channel switch? Is there something I can relay to a tech that may help him? Thanks.Originally posted by g1 View PostComment
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It's just verifying some of the voltages listed on the schematic for the switching circuit. I thought that would be easier to describe if you do not read schematics.
The tech can verify if the K1 relay that switches the gain pot in and out is getting it's appropriate control voltage. It may be, and the relay is defective. Or it may be getting an improper command from a defective relay control circuit.
One other thing you could try is tapping on the K1 relay when the fault occurs and see if the proper volume comes back.Originally posted by Enzo
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I found the K! and K2 and D11 parts you pointed out on the amp. Will try tapping the K2 relay.....Sorry for the stupid question...but to test the D11 diode, I turn the amp on, put the channel on clean, test, then switch the amp to gain, test....compare readings......Originally posted by g1 View PostComment
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I'm more concerned with the amp in gain channel. Does the D11 voltage in gain mode change from when working correct (cold) to when the fault occurs?Originally posted by ctone View Post
Originally posted by Enzo
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Do I test set to AC or DC? I hit the K2 and it didn't affect the volume......Originally posted by g1 View PostComment
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K1, not K2 is the one you want to tap on. And measurement is DC volts.Originally posted by Enzo
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I measured D11. When first turned on and on the gain channel, there is no voltage reading, when switched to the clean channel, it reads 28....I used two different meters to confirm. After leaving the amp on for a half hour the readings are the same. Oddly, the gain channel operated normally this time, even after a half hour (the volume didn't lesson over time)....was measuring DC.Originally posted by g1 View PostComment
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It's possible that there is a bad solder connection in the area you are probing. You may have temporarily restored the bad connection.Originally posted by Enzo
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