Hi, everyone. I’m looking for some help debugging this amp and you guys were so helpful last time, I thought I'd bug you again. I’m using this project to learn more about tube amps. FWIW, I’m not a total newbie; I promise I can safely take readings and have a basic understanding. I just really want to learn everything about these things.
When we first got this amp, it had a hum, extremely low volume, bad decay and a nasty modulated distortion. The tremolo was also non-functioning. After changing to a three-prong grounded plug, removing the death cap and replacing all of the electrolytic capacitors, as well as reflowing a few old solder joints that looked iffy, the hum was gone and the tremolo started working again. Yay! I also replaced a couple of the old resistors that were failing/failed and, because I didn’t have a 100k on hand for one of those, I changed the input resistors to 68k so I could use one of the 100k's there that tested good, figuring it was more important to have a working 100k in the circuit than at the input jack.
At any rate, the volume is still low. At 3, the strings are louder than the amp and 10 is maybe half what this amp should put out. Also, the higher notes cut out at first, then come in and lower notes start strong, but then decay. The distortion starts between 5 and 6 and hangs throughout all the way to 10. (Now, I just turned the amp on to get those exact numbers and the blasted tremolo isn't working again. Darn it.)
So, I've taken voltage readings off all the tube pins (installed), in the hope that one (or more!) of you all will be able to help me figure this out.
I have a couple of questions right off the bat, too. First, should the entire 6v on the filaments be on one side like that? Second, I wasn't completely sure whether I should be testing for VAC or VDC on some of these and in figuring it out, I realized that I seem to have DC voltage on some of the grids, which I’m pretty sure is a bad thing, since my understanding is that the grid should be the signal coming in from the plate of the previous stage, where a capacitor should block DC. Am I understanding/seeing that right? I’m thinking that might be causing the modulated distortion, maybe?
Thanks so much for any help you can give me. I’m really excited to be learning all about tube amps! Also, please also let me know if you’d like me to take any other readings. I did take readings at all the schematic test points I could figure out, but I didn’t think to write them all down at that point and since I’m learning to “see” the layout from the schematic (because correct wiring diagrams for these are so hard to find anyway). What I can tell you is that I remember seeming to have too high of voltages at the test points along the power tubes and too low along the preamps, but the test point right after the 6X4 was almost dead on. If anyone knows a good resource for learning this, I’d love to go reading up on that. (btw, I’m too old for video learning lol)
Thanks again!
Gina
Tube complement: 2x 6EU7, 2x 6AQ5, 1x 6X4
V1: 6EU7
Pin #1 Htr 0vac
Pin #2 Htr 6.9vac
Pin #3 Not Used X
Pin #4 Cathode 2 .559vdc
Pin #5 Grid .092vac (0vdc)
Pin #6 Plate 2 46.1vdc
Pin #7 Plate 1 45.5vdc
Pin #8 Grid 1 0.0
Pin #9 Cathode 1 0.0
V2: 6EU7
Pin #1 Htr .01v
Pin #2 Htr 6.9vdc
Pin #3 X 99.1vdc **Note, Tube datasheet says not used, but this one is
Pin #4 Cathode 2 16.91vdc
Pin #5 Grid 2 .092 AC (10.57vdc)
Pin #6 Plate 2 62.7vdc
Pin #7 Plate 1 53.5vdc
Pin #8 Grid 1 0.097vac (0vdc)
Pin #9 Cathode 1 0.0
V3: 6AQ5
Pin #1 Grid 1 .085vac (.041vdc)
Pin #2 Cathode 5.92vdc
Pin #3 Heater .004vac
Pin #4 Heater 6.9vac
Pin #5 Plate 301.8vdc (1.9vac)
Pin #6 Screen Grid 99.2vdc
Pin #7 Grid 1 X
V4: 6AQ5
Pin #1 Grid 1 .004vac (.044vdc)
Pin #2 Cathode 5.93vdc
Pin #3 Heater 6.9vac
Pin #4 Heater .10vac
Pin #5 Plate 301vdc (1.9vac)
Pin #6 Screen Grid 99.3vdc
Pin #7 Grid 1 X
V5: 6X4
Pin #1 Plate 252.4vac
Pin #2 Not Used X
Pin #3 Heater 6.94vac
Pin #4 Heater 0.17vac
Pin #5 Not Used X
Pin #6 Plate 252.3vdc
Pin #7 Cathode 304vdc
When we first got this amp, it had a hum, extremely low volume, bad decay and a nasty modulated distortion. The tremolo was also non-functioning. After changing to a three-prong grounded plug, removing the death cap and replacing all of the electrolytic capacitors, as well as reflowing a few old solder joints that looked iffy, the hum was gone and the tremolo started working again. Yay! I also replaced a couple of the old resistors that were failing/failed and, because I didn’t have a 100k on hand for one of those, I changed the input resistors to 68k so I could use one of the 100k's there that tested good, figuring it was more important to have a working 100k in the circuit than at the input jack.
At any rate, the volume is still low. At 3, the strings are louder than the amp and 10 is maybe half what this amp should put out. Also, the higher notes cut out at first, then come in and lower notes start strong, but then decay. The distortion starts between 5 and 6 and hangs throughout all the way to 10. (Now, I just turned the amp on to get those exact numbers and the blasted tremolo isn't working again. Darn it.)
So, I've taken voltage readings off all the tube pins (installed), in the hope that one (or more!) of you all will be able to help me figure this out.
I have a couple of questions right off the bat, too. First, should the entire 6v on the filaments be on one side like that? Second, I wasn't completely sure whether I should be testing for VAC or VDC on some of these and in figuring it out, I realized that I seem to have DC voltage on some of the grids, which I’m pretty sure is a bad thing, since my understanding is that the grid should be the signal coming in from the plate of the previous stage, where a capacitor should block DC. Am I understanding/seeing that right? I’m thinking that might be causing the modulated distortion, maybe?
Thanks so much for any help you can give me. I’m really excited to be learning all about tube amps! Also, please also let me know if you’d like me to take any other readings. I did take readings at all the schematic test points I could figure out, but I didn’t think to write them all down at that point and since I’m learning to “see” the layout from the schematic (because correct wiring diagrams for these are so hard to find anyway). What I can tell you is that I remember seeming to have too high of voltages at the test points along the power tubes and too low along the preamps, but the test point right after the 6X4 was almost dead on. If anyone knows a good resource for learning this, I’d love to go reading up on that. (btw, I’m too old for video learning lol)
Thanks again!
Gina
Tube complement: 2x 6EU7, 2x 6AQ5, 1x 6X4
V1: 6EU7
Pin #1 Htr 0vac
Pin #2 Htr 6.9vac
Pin #3 Not Used X
Pin #4 Cathode 2 .559vdc
Pin #5 Grid .092vac (0vdc)
Pin #6 Plate 2 46.1vdc
Pin #7 Plate 1 45.5vdc
Pin #8 Grid 1 0.0
Pin #9 Cathode 1 0.0
V2: 6EU7
Pin #1 Htr .01v
Pin #2 Htr 6.9vdc
Pin #3 X 99.1vdc **Note, Tube datasheet says not used, but this one is
Pin #4 Cathode 2 16.91vdc
Pin #5 Grid 2 .092 AC (10.57vdc)
Pin #6 Plate 2 62.7vdc
Pin #7 Plate 1 53.5vdc
Pin #8 Grid 1 0.097vac (0vdc)
Pin #9 Cathode 1 0.0
V3: 6AQ5
Pin #1 Grid 1 .085vac (.041vdc)
Pin #2 Cathode 5.92vdc
Pin #3 Heater .004vac
Pin #4 Heater 6.9vac
Pin #5 Plate 301.8vdc (1.9vac)
Pin #6 Screen Grid 99.2vdc
Pin #7 Grid 1 X
V4: 6AQ5
Pin #1 Grid 1 .004vac (.044vdc)
Pin #2 Cathode 5.93vdc
Pin #3 Heater 6.9vac
Pin #4 Heater .10vac
Pin #5 Plate 301vdc (1.9vac)
Pin #6 Screen Grid 99.3vdc
Pin #7 Grid 1 X
V5: 6X4
Pin #1 Plate 252.4vac
Pin #2 Not Used X
Pin #3 Heater 6.94vac
Pin #4 Heater 0.17vac
Pin #5 Not Used X
Pin #6 Plate 252.3vdc
Pin #7 Cathode 304vdc
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