PLEASE, can you help me with this absolutly cursed tube amp? - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: March 11, 2020, 03:17:33 pm
content: 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:[ Attachment Invalid Or Does Not Exist ]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 resistorthe cathode resistorthe input grid leak resistor 2 timesthe 82k plate resistorthe input coupling capacitor 2 timescan you help me, please?I honestly think this amp has been cursed.

author: greenpossum
author_rank: Frequent Contributor
timestamp: March 11, 2020, 03:43:43 pm
content: 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/el84-6bq5So 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?

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: March 11, 2020, 03:50:20 pm
content: Quote from: ELS122 on March 11, 2020, 03:17:33 pmHow 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?

author: greenpossum
author_rank: Frequent Contributor
timestamp: March 11, 2020, 04:08:31 pm
content: Quote from: ELS122 on March 11, 2020, 03:54:07 pmWell if you look at the transfer characteristics of a pentode, increasing the anode voltage has little effect on the current when the plateau is reached, which is the desirable thing about a pentode compared to a triode, but such a negative grid voltage will mean the tube is non-conducting for most of the cycle, instead of half the cycle, in a class B configuration.