Hi everyone. I've just powered on my amp project for the first time, and have a devastating hum problem. The design is the JC Morrison 300B from diyaudioprojects, which it appears at least a few people in this community have built successfully. The schematic is here https://diyaudioprojects.com/Tubes/300B-SET-Amplifier/
As soon as I power up the amp, both channels start to hum at 50 Hz. The hum is very loud, and doesn't change if the input is attached or shorted. I initially had everything star grounded, but have changed the grounding scheme in one channel with absolutely no change in the hum. I've attached a picture of the current layout. Any input about what to try next would be greatly appreciated.
As soon as I power up the amp, both channels start to hum at 50 Hz. The hum is very loud, and doesn't change if the input is attached or shorted. I initially had everything star grounded, but have changed the grounding scheme in one channel with absolutely no change in the hum. I've attached a picture of the current layout. Any input about what to try next would be greatly appreciated.
Attachments
Can you post a picture of the top side as well? Are the two anode chokes for the driver? The JC Morrison 300B in this thread does not have any chokes:
JC Morrison 300B
Can you post your schematic?
Sorry about the dud schematic link. This one should work DIY 300B Single-Ended-Triode (SET) Hi-Fi Amplifier Project.
Andyjeavans, the two chokes visible are the 200H chokes to the 6SN7s. They should have very little AC through them.
Korneluk, I'll post a photo of the top in the next post.
Palustris, you're right, I don't have my safety earth and circuit ground connected. Is that important?
Alasikata, instead of the hum pot, I have a pair of matched resistors of half the value to form the equivalent voltage divider.
Thank you all for your help. Really hope I can get this working.
Andyjeavans, the two chokes visible are the 200H chokes to the 6SN7s. They should have very little AC through them.
Korneluk, I'll post a photo of the top in the next post.
Palustris, you're right, I don't have my safety earth and circuit ground connected. Is that important?
Alasikata, instead of the hum pot, I have a pair of matched resistors of half the value to form the equivalent voltage divider.
Thank you all for your help. Really hope I can get this working.
No, it probably won't work. You need a pot to balance the hum out across the filament, but it may not be exactly 50% on each half.
Its the chokes next to the speaker jacks. See if this affects it. Unbolt them and move them away .
I would also buy a pair of tent labs heater boards for the 300b heaters. Mine is dead silent even with almost 100db spl speakers. Those boards they sell are simply awesome. I have a 300b set amp myself. if you need any more help let me know.
I would also buy a pair of tent labs heater boards for the 300b heaters. Mine is dead silent even with almost 100db spl speakers. Those boards they sell are simply awesome. I have a 300b set amp myself. if you need any more help let me know.
The chokes are mounted at the opposite end of the chassis with leads running down to the 6SN7s - definitely would move them.
Also, since there're only two chokes which resistor are they replacing R2 on first stage or R4 on the second stage?
6SN7 heater supply is floating ?
According to
The Valve Wizard
"Rule number one is that the heater supply must have a DC connection to audio ground."
According to
The Valve Wizard
"Rule number one is that the heater supply must have a DC connection to audio ground."
Hi Everyone. Thanks for your feedback and ideas. I've gone and tried each of these things to no avail (tying safety earth to circuit ground, replacing the paired resistors on the 300b with a hum pot, taking the 200H choke out of the circuit, moving the DC supplies away from the PT, and adding a virtual centre tap to ground on the 6SN7 heater supply).
I realise it's hard to troubleshoot somebody else's problem from a couple of photos. Is anybody able to point me to a guide to troubleshooting hum for people who don't own oscilloscopes?
I realise it's hard to troubleshoot somebody else's problem from a couple of photos. Is anybody able to point me to a guide to troubleshooting hum for people who don't own oscilloscopes?
...lol....took me a while to figure out.
Ok mine is 300B Push-Pull, so it's a little different.
All heaters are AC (300B, 6SN7, 12AXt)
Ground house got hum if connect to main ground so I tied it to the chassis crew by itself. Ground House --- Chassis
Create a center main ground, connect to the chassis with 10-22 ohm 5w (do not connect ground house here, will get hum). GND ---22ohm5w---Chassis
6SN7 and 12AX7 on the same AC, so I added 100 ohm on each AC leg, then connect both resistors to the ground.
My amp is dead quiet now. Barely hear much even put my ear at the speakers.
Ok mine is 300B Push-Pull, so it's a little different.
All heaters are AC (300B, 6SN7, 12AXt)
Ground house got hum if connect to main ground so I tied it to the chassis crew by itself. Ground House --- Chassis
Create a center main ground, connect to the chassis with 10-22 ohm 5w (do not connect ground house here, will get hum). GND ---22ohm5w---Chassis
6SN7 and 12AX7 on the same AC, so I added 100 ohm on each AC leg, then connect both resistors to the ground.
My amp is dead quiet now. Barely hear much even put my ear at the speakers.