Electronics > Beginners
really bad output on 2 EL84 amp (whistling, barelly any sound)
ELS122:
so I built a copy of the "Matchless DC30" amp but I used only 2 6P14P tubes (Russian EL84's) and used the output transformer form a Rigonda 102 and connected the taps for screen grids to the screen grids of the pentodes (I connected it like an ultra-linear output)
it also used 6N2P preamp tubes (Russian 12ax7's) I don't have the 2nd channel since the only 6J32P (also Russian, EF86) seems to be dead, while the heater measures 8Ohms it doesn't light up whatever I try.
anyway,
all the parts I used are pretty much scrap parts since I'm so cheap ;D but I used mostly ceramic or tubular caps and the only (I think) oil cap I have is the coupling cap to the PI cause I didn't have any other that was that value, but I never seen those Russian oil (I think) caps fail catastrophically i.e I only have seen a few of them be leaky in a sense that the next stage is over biased.
the only way I've seen them fail "catastrophically" is when they go pretty much open.
so that might be the case.
I'll try finding a good cap that value and change it if I find any.
I measure 293v on the output transformer for the output and 148v for the preamp tubes (but I have more dropping resistors dropping the voltage after that)
so it may be too low voltage on the preamp tubes, I'll connect it to a sooner point on the PSU.
I'll link a sound sample I recorded.
I used a small 16Ohm resistor to load the output and they used a 47nF cap (in case there's any dc) straight to my sound card.
and I have the amp cranked AND I have super-hot pickups and I am smacking the strings as hard as I can.
also at the start, I just touched the tip of the input jack thus inducing some 50Hz hum into it.
I have a 1Meg volume pot just before the PI.
untitled.mp3 (1100.32 kB - downloaded 44 times.)
EDIT: actually it's 148v for the output transformer and 293v for the preamp tubes. so I'll fix that.
bob91343:
Sounds like a bias problem, likely due to a leaky coupling capacitor to the output stage.
ELS122:
--- Quote from: bob91343 on February 08, 2020, 10:50:09 pm ---Sounds like a bias problem, likely due to a leaky coupling capacitor to the output stage.
--- End quote ---
well, ceramic capacitors don't leak like ever.
so I fixed the problem with the 148v on the output stage, now there's like 330v
but now when I touch the tip of the input jack the output transformer squeals loudly, I've heard this before on a SE amp.
but now it has some output at least.
but still, the output sounds really shit.
now I'm pretty sure it's actually that single oil (or whatever, doesn't look like one) cap before the PI.
I'm gonna replace it with like a 47nF cause that's the only one I have handy.
and I know that ceramic caps are microphonic but it's not THAT big of a deal.
also I have a pot for the bias on the PI (cause I didn't have the needed cathode resistor) and also a pot for one of the preamp stages. also all the volume/tone pots could be dirty as well. I'll clean them the best I can. The only thing is that all of them are sealed so it will be a pain getting to them but I'll see what I can do.
ELS122:
well, I pre-adjusted all the pots that were in the place of resistors and changed the last oil cap to a 15nF ceramic, but now it's oscillating. going down in frequency and getting louder. It sounds like it goes from about 2-3khz and goes down to about 200-400Hz.
also checked all the tone/volume pots and they are good.
have 350v at the output transformer when the tubes are cold and drops to 290 when they warm up. they are after a power choke.
I didn't measure the voltage for the preamp but it's after a 5.6k resistor after the source for the output stage.
when the preamp tubes are cold it doesn't oscillate at all.
the amp does not have ANY feedback anywhere, like in the original design.
Le_Bassiste:
without the schematics a shot in the dark: did you check for correct phase of the feedback winding (if any)?
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