Electronics > Projects, Designs, and Technical Stuff
Voltage sag problems in tube amp (SS rectifier)
ELS122:
I have a terrible sag in volume after I play something through the amp and it "flubs out" basically, I measured the voltages and found that:
the bias supply is ok
all the B+ supplies are sagging a LOT and even before the choke.
so could the 4uF capacitor before the choke be of too small value? I don't have an LCR meter and I've heard that if you don't know the value of the choke it should better be left alone (the capacitors left alone)
so should I increase the value? and to what value?
here's the schematic of the power supply
maybe the rectifier diodes are dead? I used the only ones I could find and they were some Russian 1N*something* and put 4 in series. since I couldn't stand to wait shipping ;D
and also it will decrease the pretty high B+. I could use a tube rectifier like a 5Z4S, but I've heard that for 2 6L6's or 6P3S's in this case, it's underpowered and will steal a lot of output power. and also I just don't really want a tube rectifier, I don't really see the point. plus my power transformer got uncomfortably warm when I had the tube rectifier instead of the SS one.
greenpossum:
I wonder if your diodes can really handle the current without causing a lot of drop. Do they get warm? Is the 4uF capacitor the original value? It seems a bit low. Pity you don't have a scope to look at the waveform after the diodes. If the diodes or capacitors are inadequate there will be a lot of ripple.
ELS122:
diodes are ice cold. the 4uF and the 20uF caps before and after are the original value. choke is the same too.
although it originally had the 6P3S's cathode biased, so it wasn't designed for this power with fixed bias. but the OPT by the look should easily handle ~40W or power, and it's also ice cold.
what seems weird is that into an 8 Ohm dummy load it outputs at the absolute max about 10W, but into a 12Ohm speaker it goes up to 48W. maybe my multimeter is measuring some transients or something and coming up with all that voltage (~24VAC) but it doesn't seem like it.
the OPT is 24:1 so 8Ohms should output more power than 12Ohms.
T3sl4co1l:
What is the choke value, what is the nominal minimum and maximum load current, and what is the design B+ voltage?
Tim
ELS122:
--- Quote from: T3sl4co1l on June 03, 2020, 04:30:18 pm ---What is the choke value, what is the nominal minimum and maximum load current, and what is the design B+ voltage?
Tim
--- End quote ---
don't know the value, don't have an LCR meter. haven't calculated the load current, and don't have any schematic of the original amp to know the designed B+ voltage, I'll remeasure the AC voltage before the diodes and calculate the theoretical voltage output.
and then calculate all the current draw.
the AC voltage out of the transformer is
380V-0-380V.
so if I calculate the DC voltage by 380V * 1.4 = 532V and the diodes are about 1V voltage drop so, like 528V
I checked and the diodes are 'D226B'
they're rated for 300mA (ummm.. I think my amp is right up there :-[ ) and 400V reverse voltage (that's why I put 4 in series)
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