| Electronics > Projects, Designs, and Technical Stuff |
| PLEASE, can you help me with this absolutly cursed tube amp? |
| (1/2) > >> |
| ELS122:
I am making a guitar tube amp but I am already stuck at the PI part, I've tried changing like every component, shorting the 4.7k resistor temporarily to check if it may be going open, replaced all the resistors on the cathode side, swapped the preamp tube 5 times, even tried to put in a 6N1P not a 6N2P but nothing changed, behaved exactly the same, resoldered all the connections, ptp wired like every part now, changed the output tubes, both of them, did like everything I can think off. I even asked a discord server and they didn't have a clue to what's going on. I drew up a schematic of it as detailed as I could: this amp (this output transformer) has worked only once, and it worked great, but I used the cathodyne PI and didn't like the really low gain and wanted to change it for an LTP PI. I am 100% sure that the output transformer is fine and not blown. what is the amp doing?: well, when I turn it on it doesn't make any sound. until I crank up my volume to 11 when my sound card is like outputting 2V ptp it starts to output some pops of the original sound. when I turn it off, the power, it wakes up and sounds really great until the plate voltage drops below like 50V. when I turn it off all voltages go down about equally, nothing really that would change the behavior of it. if I switch it on a short while after it's gone silent it starts to sound ok for a moment and then again goes back to square one. if I switch it on and off fast it works how it should, even tho the heaters are like pitch black, but I guess they're still glowing inside if it's working. and before you start screaming "HOLY MOLY THAT'S A LOT OF VOLTS" my multimeter battery is pretty discharged and it's measuring voltage bit higher, the plate voltage on the 6P14P's was 450V when I measured that before the battery was dead. and yes that's still a lot but I'll just stick with it. anyway, the voltages should be precise relative to each other. I've changed: the PT tail resistor the cathode resistor the input grid leak resistor 2 times the 82k plate resistor the input coupling capacitor 2 times can you help me, please? I honestly think this amp has been cursed. |
| greenpossum:
Hmm, the 6P14P is supposed to be equivalent to the EL84 right? -46V for the grid seems a bit extreme. Where do you get that value? Such a negative voltage would mean only large peaks of the music would be able to turn on the tube a bit. If you look at the transfer curves for the EL84, typically the grid is not so negative: https://www.jj-electronic.com/en/el84-6bq5 So where did you get the circuit with so negative biasing? And come to think of it, 550V for the anode seems high too. I remember I ran EL84s at 350V max, or maybe that was even the higher rated 7189. Are you sure the circuit isn't for a different power tube? |
| TheUnnamedNewbie:
--- Quote from: ELS122 on March 11, 2020, 03:17:33 pm --- and before you start screaming "HOLY MOLY THAT'S A LOT OF VOLTS" my multimeter battery is pretty discharged and it's measuring voltage bit higher, the plate voltage on the 6P14P's was 450V when I measured that before the battery was dead. and yes that's still a lot but I'll just stick with it. --- End quote --- How do you plan on making any meaningfull statements on what is going on if your meter might be measuring 20% or more off? Is it that much to go to buy a new multimeter battery? |
| ELS122:
well, I tried turning the bias up to even -10V and nothing changed. well, the tube was over biased but nothing else changed. also the real bias it like 33V since my multimeter messured a bit high and also the bias it the grid voltage, not the voltage going to the grid leaks. also yes the 6P14P is equivalent to EL84, 6N2P is equivalent to 12ax7, 6N1P equivalent to 12au7. also with so much plate voltage even -46V is not really that extreme. --- Quote from: TheUnnamedNewbie on March 11, 2020, 03:50:20 pm --- --- Quote from: ELS122 on March 11, 2020, 03:17:33 pm --- and before you start screaming "HOLY MOLY THAT'S A LOT OF VOLTS" my multimeter battery is pretty discharged and it's measuring voltage bit higher, the plate voltage on the 6P14P's was 450V when I measured that before the battery was dead. and yes that's still a lot but I'll just stick with it. --- End quote --- How do you plan on making any meaningfull statements on what is going on if your meter might be measuring 20% or more off? Is it that much to go to buy a new multimeter battery? --- End quote --- I mean the relative voltages is what matters really, I'm not wanting to make a hifi amp, but a guitar amp, slightly diffrent voltages wont make it not work at all. I pretty much messured them BECAUSE I wanted to see where the voltages are relitive to each other and are they doing some weird stuff, and I found out that NO, all the voltages are about where they need to be, but the bias is really low, there's only like -1V on the grids relative to the cathode, this is not right but it would still work afaik with only -1V on the grid, |
| ELS122:
SHIT, I was so dumb, --- Quote from: greenpossum on March 11, 2020, 03:43:43 pm --- -46V for the grid seems a bit extreme. Where do you get that value? Such a negative voltage would mean only large peaks of the music would be able to turn on the tube a bit. --- End quote --- thank's for giving me the idea to turn the bias up even more, I turned it to -18V and it started working. I guess my old salvaged tubes were so used up they just didn't conduct correctly at -38V even tho the plate voltage was high |
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