Rest assured you did the right thing by starting out with a smaller light bulb than necessary for fully operating power. If you had (or have) any fuse-popping tubes or shorted caps, etc., the small bulb will save burnt parts, fuses, and other overload mayhem. Once you had troubleshot the issue was not in the preamp section (removing power tubes caused power supply voltages to approach full value), the small bulb had done all it could.
I usually don't step up that high in bulb size. To be prudent, I would have gone up to the 125 -150 watt range. That way, the power section would try to idle near full voltage like the preamp section did with the smaller bulb. If something decided to run away when it got warm, the mid-sized bulb would allow some time to cut power before magic smoke appeared. Not so much with the 250 watt light.
I usually don't step up that high in bulb size. To be prudent, I would have gone up to the 125 -150 watt range. That way, the power section would try to idle near full voltage like the preamp section did with the smaller bulb. If something decided to run away when it got warm, the mid-sized bulb would allow some time to cut power before magic smoke appeared. Not so much with the 250 watt light.
Remember, the bulb is not for illumination, but instead for its resistance. SO something off your radar like a heat lamp is still a valid limiter bulb. heat lamps used to be relatively common in bathroom light fixtures. Regular bulbs are going away, but special use ones are staying for a while. I think I have a couple coated bulbs made for producing black light (ultraviolet). Very inefficient at it, but they didn't care.
Powered up and ran 1 kHz sine wave test tone into high gain input. All controls were responsive and a majority of the scratchyness was gone.
Looks like the amp is working well! I'll test it further with a guitar to make sure no other issues surface. Guess there probably wasn't anything wrong with it after all
I'll leave the new tubes in just in case.
One odd thing I noticed: preamp tubes 1 and 4 (counting from end of chassis) do not light up as far as I can tell. Is this expected?
Looks like the amp is working well! I'll test it further with a guitar to make sure no other issues surface. Guess there probably wasn't anything wrong with it after all
I'll leave the new tubes in just in case.One odd thing I noticed: preamp tubes 1 and 4 (counting from end of chassis) do not light up as far as I can tell. Is this expected?
They do light up or the amp would be silent. remember the heaters are all wired in series, so if one were not working, they ALL would not be working. Turn out the room lights and look in there again. They are lit, you just can't see it... I bet my lunch money.
What matters about the heaters is the part inside the tube. The part that sticks out is what we can see, and that doesn't matter so much.
What matters about the heaters is the part inside the tube. The part that sticks out is what we can see, and that doesn't matter so much.
Got it, that makes sense. I'll perform the lights out test.
This also may be a good time to practice amp maintenance on my newly acquired scope. I found this video very helpful.
How do y'all normally hook up the scope to the output? I'm a little tentative about hooking the probe (HP 10435A) across the output jack, as the clips will be very close together and at risk of shorting with a small slip.
This also may be a good time to practice amp maintenance on my newly acquired scope. I found this video very helpful.
How do y'all normally hook up the scope to the output? I'm a little tentative about hooking the probe (HP 10435A) across the output jack, as the clips will be very close together and at risk of shorting with a small slip.
Also, my scope can handle a maximum external input of 250V DC or 500V peak-to peak AC at 1 kHz or less. With a 10:1 probe, am I ok to hook the 5150 output to this scope (assuming I'm using a 1 kHz input tone)? 120W output into an 8 ohm speaker should be ~31VAC RMS (~44 VAC peak), so I should be fine, right?
No worries. You should be able to look at every signal inside the amp with your scope. The tube output signals on the primary side of the output xfmr may get into the hundreds of AC volts range, so you want to look at them on the 10X setting. Further upstream in the preamp, you may want to switch to 1X to get a good look at the small signals.
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Sorry, I meant using a scope in parallel with a speaker vs. a scope in parallel with a dummy load. I like the simplicity of the dummy load resistor, but I worry about being able to get my scope leads attached. Then again, I don't really want to hear a 1kHz sine wave at ~100 dB, either...
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So I've finally had a chance to try the 5150 out a bit. Both channels make noise and all pots are responsive. The lead channel sounds amazing, but the rhythm channel is much quieter than the lead channel for the same pre and post gain settings. Is this expected? To be fair, it still gets much louder than necessary for my purposes, but I wonder if there is still a lingering issue.
My guess is a dirty pot, but I've cleaned them thoroughly and don't hear any scratchiness through their range of motion.
Does this symptom ring any bells with anyone?
My guess is a dirty pot, but I've cleaned them thoroughly and don't hear any scratchiness through their range of motion.
Does this symptom ring any bells with anyone?
The numbers on the controls are only there for your convenience, they do not correspond to anything electronic. You turn them up or down to the loudness and tone you like, then note the numbers. Now next time you see the amp, you can get the same tone by setting the same numbers up. But the channels are different, so don;t expect the controls to mirror one another.
The things that cause diminished sound levels would generally affect both channels. Like a bad Q7.
The lead channel is louder than the rhythm, sounds normal to me. A dirty pot would be scratchy or intermittent, it would not just overall reduce the level.
Look at the schematic. After the first stage, the signal splits into two paths, ultra and "clean". We should stick to "rhythm" because no channel on this amp makes clean. AMp says rhythm, schematic says clean. Anyway, the ultra path has a voltage divider R29 over R41/VR2. This is basically a 2/1 voltage divider, so the signal is cut in half there. On the clean branch, er... rhythm, the divider is R19 plus R21 over VR1, approximately a 5/2 divider, so the signal is reduced further, down to maybe 40% of the first stage. If K4 is energized, then R43 replaces R19, and we rise to a 4/1 divider, meaning the signal is reduced to 1/4 of the first stage.
That is not the whole story, but right there, the ultra has half the first stage while the rhythm has one-quarter the first stage, in terms of signal level. You can go through the rest of the circuit and calculate what channel changes do to signal levels.
The things that cause diminished sound levels would generally affect both channels. Like a bad Q7.
The lead channel is louder than the rhythm, sounds normal to me. A dirty pot would be scratchy or intermittent, it would not just overall reduce the level.
Look at the schematic. After the first stage, the signal splits into two paths, ultra and "clean". We should stick to "rhythm" because no channel on this amp makes clean. AMp says rhythm, schematic says clean. Anyway, the ultra path has a voltage divider R29 over R41/VR2. This is basically a 2/1 voltage divider, so the signal is cut in half there. On the clean branch, er... rhythm, the divider is R19 plus R21 over VR1, approximately a 5/2 divider, so the signal is reduced further, down to maybe 40% of the first stage. If K4 is energized, then R43 replaces R19, and we rise to a 4/1 divider, meaning the signal is reduced to 1/4 of the first stage.
That is not the whole story, but right there, the ultra has half the first stage while the rhythm has one-quarter the first stage, in terms of signal level. You can go through the rest of the circuit and calculate what channel changes do to signal levels.
Whelp, looks like the 5150 is not in full working condition yet. The volume level periodically drops/crackles when playing.
Things I've tried:
Right now my guess is a cold solder joint somewhere in the power tube board. If you look at the second and third pictures in post #1, you see some points with no solder at all. I assume the solder was removed during the same job that added the new grid stopper resistors.
What should my next steps be? Reflow all solder joints on the power tube board? I ordered a new set of preamp tubes just in case, but the guitar-into-effects-return test should have eliminated the preamp tubes as the only cause, correct?
Things I've tried:
- Changing channels: This behavior is apparent on both channels.
- Changing volume levels. I initially thought the signal was dropping because I was playing at low volume, but the symptom occurs with all pre/post gain knobs above 3 on each channel.
- Changed guitar cables, speaker cables, and guitars: the symptom is still present.
- Don't have another cab to try it with, but I swapped heads and didn't hear any fluctuation in volume.
- Ran guitar cable into effects return jack: issue is still present.
Right now my guess is a cold solder joint somewhere in the power tube board. If you look at the second and third pictures in post #1, you see some points with no solder at all. I assume the solder was removed during the same job that added the new grid stopper resistors.
What should my next steps be? Reflow all solder joints on the power tube board? I ordered a new set of preamp tubes just in case, but the guitar-into-effects-return test should have eliminated the preamp tubes as the only cause, correct?
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