Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ELS122 on March 11, 2020, 10:23:08 am
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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.
can you help me, please?
I honestly think this amp has been cursed.
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Your test signal input should be about 1Vrms (2.828Vpp). I don't see any clues with the PI voltages. However, did you really measure 61v and 59v on the grids? What kind of meter?
Grid voltages for the output tube are suspicious, although not your problem. You show a bias voltage of -46v but only -38v on the grids. The grids should also be -46v.
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Your meter itself is probably causing the wonky bias DC readings.
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ok I was really dumb, when I lowered the bias voltage by A LOT, it's now about -18V it now works, I guess it's the old tubes I have in there.
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Sorry I overlooked the 6P14P tubes. Those should be biased somewhere between 12 and 18 volts.