Last night I "finished" wiring the first of my KT88 monoblocs. After double checking all component values (ticking them off on my component list) and double checking all connections (ticking them off on the schematic as I stepped through it from the input) I decided to turn it on.
Following my standard procedure for turning on anything electric/electronic that I've built myself I connected it to a heavy-duty (15A) extension cord plugged in to a switched socket across the room. Standing across the room I flipped on the switch. No squeal from the cheap speaker attached, no bang, no fire not even a wisp of magic smoke! In a moment the filaments began to glow and appeared normal on all three tubes.
However... after 30 to 40 seconds the 2A slo-blo fuse blew. Checking all components revealed nothing was overheated, no smoking or burning anywhere. I checked all component values again and all connections again. I tightened down my chassis ground post, removed all the valves and blew out the tube sockets with compressed air in case a bit of wire got caught in there. I replaced the fuse and turned it on again with the same result.
Nothing was connected to the input in either case. I did have an 8 ohm speaker connected at the output.
I am using tube rectification (5U4G) so it seems that the fuse is blowing after the rectifier begins passing current. I've attached a Tubepad drawing of my components and how I've grounded things. The first cap is 40uF, the choke is 8 henries, the second cap is 100uF. PSUD II says this should work for 400 VDC, my target B+.
Any suggestions on where to start? My current ideas are-
1- disconnect the rectified high voltage from the CLC filter and check the filament voltages and see if the fuse blows with only the filaments connected
2- check the rectified voltage before the caps (with it still not connected to the CLC filter)
3- reconnect the rectified high voltage and check its value
4- ????
Any help greatly appreciated!
Following my standard procedure for turning on anything electric/electronic that I've built myself I connected it to a heavy-duty (15A) extension cord plugged in to a switched socket across the room. Standing across the room I flipped on the switch. No squeal from the cheap speaker attached, no bang, no fire not even a wisp of magic smoke! In a moment the filaments began to glow and appeared normal on all three tubes.
However... after 30 to 40 seconds the 2A slo-blo fuse blew. Checking all components revealed nothing was overheated, no smoking or burning anywhere. I checked all component values again and all connections again. I tightened down my chassis ground post, removed all the valves and blew out the tube sockets with compressed air in case a bit of wire got caught in there. I replaced the fuse and turned it on again with the same result.
Nothing was connected to the input in either case. I did have an 8 ohm speaker connected at the output.
I am using tube rectification (5U4G) so it seems that the fuse is blowing after the rectifier begins passing current. I've attached a Tubepad drawing of my components and how I've grounded things. The first cap is 40uF, the choke is 8 henries, the second cap is 100uF. PSUD II says this should work for 400 VDC, my target B+.
Any suggestions on where to start? My current ideas are-
1- disconnect the rectified high voltage from the CLC filter and check the filament voltages and see if the fuse blows with only the filaments connected
2- check the rectified voltage before the caps (with it still not connected to the CLC filter)
3- reconnect the rectified high voltage and check its value
4- ????
Any help greatly appreciated!
Attachments
Let's do some basic troubleshooting. Disconnect the signal circuitry from the power supply- substitute a power resistor as a dummy load, maybe 5Kohm at 50W. See if the fuse still blows. If not, your problem is in the signal cicuitry. If so, your problem is in the power supply. And either way, you've cut the possibilities in half.
SY said:
Thanks for that advice. I'll have to see what power resistors are in my bin. I'm sure I don't have 50W but I do have a handful of 500 ohm at 25W. I could put some in series to get 5K.
SY said:
Actually the bypass caps are non-polar electolytics so orientation there shouldn't be a problem.
Jax said:
You are right! The original schematic had a 100K pot between the input and the 6N1P. When I decided to do monoblocs I just put a 100K resistor in line with the input and forgot about putting one to ground.
What value would you recommend there? 500K or higher? I checked a schematic for a different SE amp and the designer recommended replacing the 100K pot in that design with a 100K resistor in series between input and grid and a 1M to ground.
Update
OK,
I soldered in a 4.7K grid stop and put in a 2A fast blo fuse. Turning it on caused the fuse to blow immediately (fast blo vs. slo blo which took maybe 30 seconds).
So on a hunch and before anything else I removed the filament CT from ground and taped off the end, installed another 2A fast blo fuse and turned it on. (I also had my DVM connected to B+). Voltage quickly climbed to 520 but then began dropping as the circuit stabilized. It leveled off at about 430 (my target B+ is 400 so that wasn't too bad). After a minute everything was still working so I shut it down.
I connected my newly completed pre-amp and a cheap CD player and fired everything back up. The amp didn't come on, the fuse was blown. It blew when I powered down! I replaced the fuse and everything started working. Voltage stabilized again at 430 so I started the CD player and turned up the volume on the pre-amp. Music came from the speaker!! And it sounded good (though too attenuated).
I think I need to rethink my use of a 100K resistor in series with the input. I'll have to check but I think my pre-amp has an output impedance of 430 ohms.
In powering up and down a few times I found the 2A fast blo fuses blowing on shutdown about 1/3 or 1/2 the time. So either I haven't totally solved the issue or I need to put in a slo-blo.
OK,
I soldered in a 4.7K grid stop and put in a 2A fast blo fuse. Turning it on caused the fuse to blow immediately (fast blo vs. slo blo which took maybe 30 seconds).
So on a hunch and before anything else I removed the filament CT from ground and taped off the end, installed another 2A fast blo fuse and turned it on. (I also had my DVM connected to B+). Voltage quickly climbed to 520 but then began dropping as the circuit stabilized. It leveled off at about 430 (my target B+ is 400 so that wasn't too bad). After a minute everything was still working so I shut it down.
I connected my newly completed pre-amp and a cheap CD player and fired everything back up. The amp didn't come on, the fuse was blown. It blew when I powered down! I replaced the fuse and everything started working. Voltage stabilized again at 430 so I started the CD player and turned up the volume on the pre-amp. Music came from the speaker!! And it sounded good (though too attenuated).
I think I need to rethink my use of a 100K resistor in series with the input. I'll have to check but I think my pre-amp has an output impedance of 430 ohms.
In powering up and down a few times I found the 2A fast blo fuses blowing on shutdown about 1/3 or 1/2 the time. So either I haven't totally solved the issue or I need to put in a slo-blo.
Three suggestions:
The 100K resistor should go between the input and ground, not in series with the input. In series, you want the 4.7K grid-stopper.
You do want a slow-blo fuse.
You might investigate using a zero-crossing relay to control power-up: this is one that fires as the power line waveform crosses zero. Standard Mouser or Digikey item.
The 100K resistor should go between the input and ground, not in series with the input. In series, you want the 4.7K grid-stopper.
You do want a slow-blo fuse.
You might investigate using a zero-crossing relay to control power-up: this is one that fires as the power line waveform crosses zero. Standard Mouser or Digikey item.
SY said:
Thanks Sy!!
That makes sense. I'm going to change the resistors around now. I'll also investigate the relay if fuses continue to be a problem when I change it to slo-blo.
I'll report back, probably tomorrow, after the changes. I have to teach a class tonight so I don't have much more time to work on it today. (And no, the class isn't "tube audio"!!!
)
SY said:
First an update. I changed the resistors (now 4.7K in series with the input and 100K to ground). I also triple checked everything for shorts, bad solder etc. and hooked it back up and turned it on. No blown fuse, same 430V B+, so far so good.
Now give it some input from the preamp and wow! This is why I built it! I wish I had a decent speaker in my basement workshop. Even with the cheapies it sounds pretty darn good. The only detectable hum is what is injected by the preamp (which is currently using AC for filaments but may soon be DC). With the preamp off or disconnected there is no sound whatever from the speaker.
Now I just have to get in gear and finish the other one!
As for teaching... tonight it is "Turf Management" (I have a University degree in Ornamental Horticulture). I've also been asked to design a course in photography and I've been approached about teaching information systems.
I'm a bit of a jack of all trades. Degree in Horticulture, 16 years in information systems, 30 years of serious photography as a hobby and now 2+ years of DIY HiFi.
reaction said:
I don't have a base made for it yet it is now just a 1/8" aluminum top plate but I'll post some pics this morning. I may make a base before tackling the other monobloc.
SY said:
My lawn could use me!
You know the old saying about the cobbler's shoes always having holes in the soles. Similar thing applies to probably every profession!Thanks for your help with my amp issues. As mentioned above I'll post photos this morning.
SY said:
SY,
I seemed to have solved the fuse-blowing problem (haven't blown a fuse since last post). However I'd still like to investigate a zero-crossing relay. However a search for zero crossing at Mouser or Newark turns up no match. Do you have a part number or manufacturer I can look up?
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