Hi guys,
I have this 20 years old 300b single ended amp. It works perfectly in the right side but the left is completely dead. I have tried to find the error but no luck so far. I have found an irregularity in the place marked with blue in the schematic. The working channel has apron. 150V as supposed but the silent channel has apron. -50V.
I cannot figure out why the voltage both are dropped and reversed polarity. Do any one have some tips on where this fault might be located?
Regards Daniel
I have this 20 years old 300b single ended amp. It works perfectly in the right side but the left is completely dead. I have tried to find the error but no luck so far. I have found an irregularity in the place marked with blue in the schematic. The working channel has apron. 150V as supposed but the silent channel has apron. -50V.
I cannot figure out why the voltage both are dropped and reversed polarity. Do any one have some tips on where this fault might be located?
Regards Daniel
Firstly pull out V2 (ECC88).
Measure voltage on R28/R38 (220R 10W).
If both of equal to 19V (I hope that in schematic it is 19V) the 300B tubes are survived.
The -52V is the 300B tubes negative grid voltage.
If it is on the R11-C9 point, one of the coupling capacitor (C12, C13, C18, C19) possibly shorted.
Measure its and change bad parts (some old type capacitor hate the heat in the amplifier, and tend to shorting).
The ECC88 possibly dead, otherwise this is "pull up" voltage to +150V (bias voltage equipment has high resistance=very low current).
p.s.
Be sure that ECC88 pins and socket are not dirty. I may occurs contact failure, so tube maybe "not working".
Turn off the amplifier, clean tube pins with alcohol and push/pull tube to socket 8-10 times. Wait while alcohol evaporates, then push tube to the socket and turn on the amplifier.
Measure voltage on R28/R38 (220R 10W).
If both of equal to 19V (I hope that in schematic it is 19V) the 300B tubes are survived.
The -52V is the 300B tubes negative grid voltage.
If it is on the R11-C9 point, one of the coupling capacitor (C12, C13, C18, C19) possibly shorted.
Measure its and change bad parts (some old type capacitor hate the heat in the amplifier, and tend to shorting).
The ECC88 possibly dead, otherwise this is "pull up" voltage to +150V (bias voltage equipment has high resistance=very low current).
p.s.
Be sure that ECC88 pins and socket are not dirty. I may occurs contact failure, so tube maybe "not working".
Turn off the amplifier, clean tube pins with alcohol and push/pull tube to socket 8-10 times. Wait while alcohol evaporates, then push tube to the socket and turn on the amplifier.
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thanks for the reply. I tried your advise.
Both are equal to 19v. I tried cleaning the pins and socket on all tubes. I had 4 new caps so i tried replacing c12+13+18+19 but with no luck. I will try and poke around some more.
I wonder what is the difference in the cap values when it says m or M? i can't remember to have seen that before.
Both are equal to 19v. I tried cleaning the pins and socket on all tubes. I had 4 new caps so i tried replacing c12+13+18+19 but with no luck. I will try and poke around some more.
I wonder what is the difference in the cap values when it says m or M? i can't remember to have seen that before.
The capacitor values are 220nF (0.22uF), 630V.
IMO the paralleled 220nF capacitors (sum: 440nF) per tube a little bit exaggeration, some 300B amp working well even with 100nF.
As I wrote, ECC88 possible dead. Is it's heater lighting?
Try to change it.
If you pull out -temporarily- ECC88 from working channel and push it to now non-working one, may it check if it was the cause of the problem.
Without ECC88 the power part is working (but no input signal), and first tube (ECC83) VAS stage also, so the voltages are mostly similar ones as in working amp (except the ECC88 part).
IMO the paralleled 220nF capacitors (sum: 440nF) per tube a little bit exaggeration, some 300B amp working well even with 100nF.
As I wrote, ECC88 possible dead. Is it's heater lighting?
Try to change it.
If you pull out -temporarily- ECC88 from working channel and push it to now non-working one, may it check if it was the cause of the problem.
Without ECC88 the power part is working (but no input signal), and first tube (ECC83) VAS stage also, so the voltages are mostly similar ones as in working amp (except the ECC88 part).
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