Hello,
Few weeks back I completed my first DIY project a pair of WE91 300B Monoblocks.
They are based on exact design of Joe Roberts's Sound Practices article.
The amplifiers were working very fine for three weeks.
Yesterday while listening I noticed that sound from left mono is absent.
So no tubes glowing on left mono and everything cold. So checked the fuse and it was blown. Then I put new fuse and started again and no sound. So I checked fuse it was also blown. Then I noticed that one of the speaker wires was loose and had come out , so looks like the amplifier was running without speaker load. This was because my unawareness. I had connected speakers directly without binding posts. I had cheap brass binding posts from Audiophonics.fr and I saw that they only gave half output so I directly connected bare wires to speakers with electrical insulation tape. It seems that somebody in home stumbled on speaker wires and pulled on them so one wire got loose near amplifier. So it looks like the amplifier ran quite a long time without speaker load.
I opened the amplifer and removed the power transformer checked the resistance of primary and continuity and resistance is around 5 Ohms and continuity test gives beep . If I apply 230 volts to transformer primary I get only 6 volts at the rectifier secondary instead of 375V. Looks like power transformer primary is shorted.
I checked Output Transformer (OT) primary it shows 70 Ohms resistance and no beep on continuity test.
I read on internet that with open sec or no lad on OT or running the amplifier without speaker load should damage OT and not Main power transformer?
My questions to 300B SET experienced masters are..
1. Is it possible that Power transformer primary shorted because no load on OT?
2. Which other parts like Resistors and Capacitors should I check for possible damage?
3. Do I need to check the OT and 10H Choke also for possible damage?
I was very sad about my un- awareness about connected speakers without binding post....
Your experienced input in valued ..
Thanking you😕
Few weeks back I completed my first DIY project a pair of WE91 300B Monoblocks.
They are based on exact design of Joe Roberts's Sound Practices article.
The amplifiers were working very fine for three weeks.
Yesterday while listening I noticed that sound from left mono is absent.
So no tubes glowing on left mono and everything cold. So checked the fuse and it was blown. Then I put new fuse and started again and no sound. So I checked fuse it was also blown. Then I noticed that one of the speaker wires was loose and had come out , so looks like the amplifier was running without speaker load. This was because my unawareness. I had connected speakers directly without binding posts. I had cheap brass binding posts from Audiophonics.fr and I saw that they only gave half output so I directly connected bare wires to speakers with electrical insulation tape. It seems that somebody in home stumbled on speaker wires and pulled on them so one wire got loose near amplifier. So it looks like the amplifier ran quite a long time without speaker load.
I opened the amplifer and removed the power transformer checked the resistance of primary and continuity and resistance is around 5 Ohms and continuity test gives beep . If I apply 230 volts to transformer primary I get only 6 volts at the rectifier secondary instead of 375V. Looks like power transformer primary is shorted.
I checked Output Transformer (OT) primary it shows 70 Ohms resistance and no beep on continuity test.
I read on internet that with open sec or no lad on OT or running the amplifier without speaker load should damage OT and not Main power transformer?
My questions to 300B SET experienced masters are..
1. Is it possible that Power transformer primary shorted because no load on OT?
2. Which other parts like Resistors and Capacitors should I check for possible damage?
3. Do I need to check the OT and 10H Choke also for possible damage?
I was very sad about my un- awareness about connected speakers without binding post....
Your experienced input in valued ..
Thanking you😕
A triode output rarely blows a transformer with open circuit, more probably with load shorted. A pentode, all contrary. Then, start removing all tubes and replace the fuse. Power the amp via a filamentary lamp wired in series with one of the primary (Preferably in the live wire), the lamp any in the order of 300W, I believe it is sufficient. If the lamps brights full, then some of the components not removed is shorted. Start with the electrolytic caps, the power traffo, etc, until you find that the lamp no more brights.
Then replace part(s) affected.
If lamps doesn't brigth at full, then one of the tubes, or the associated circuitry is damaged. Start with rectifier, pre's, power, etc. Perhaps a tube itself is shorted or very gassy.
Then replace part(s) affected.
If lamps doesn't brigth at full, then one of the tubes, or the associated circuitry is damaged. Start with rectifier, pre's, power, etc. Perhaps a tube itself is shorted or very gassy.
Thank you Osvaldo. It turned out that Secondary of Power Transformer is defective 2X375 V rectifier plate secondary. How it happened I dont know 🙂
It may have 3 (or more causes). One: a shorted rectifying mechanism (Tube or SS) or severe overload, or two, a defective winding procedure (Bad or defective varnished wire), or three simply, it catched too humidity.
Hi Osvaldo, Thank you very much for your analysis of the situation. THe transformer manufacturer also said that he does not know how the HT sec got defective.
One more question you you don't mind my WE91 based mono-blocks get very hot around the rectifier tube and power transformer is also quite hot. Is it common with this design? Because there is 880ohm 50 w Resistor and also 50 K 25 W resistors directly soldered on Aluminum plate. The sound is amazing. Somebody said that it draws almost 120 Watts power out of which only 12 Watts is used rest is dissipated in heat, is this the reason? This circuit is based on Joe Roberts design from Sound practices. What currents and voltages should I measure with multimeter?
One more question you you don't mind my WE91 based mono-blocks get very hot around the rectifier tube and power transformer is also quite hot. Is it common with this design? Because there is 880ohm 50 w Resistor and also 50 K 25 W resistors directly soldered on Aluminum plate. The sound is amazing. Somebody said that it draws almost 120 Watts power out of which only 12 Watts is used rest is dissipated in heat, is this the reason? This circuit is based on Joe Roberts design from Sound practices. What currents and voltages should I measure with multimeter?
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