I'm trying to fix hum coming from both channels of my diyhifisupply 300B Joplin. The multimeter gives readings of 2.3mV each side and hum is audible through the speakers at 14 feet. Hum should be around 0.9mV
The hum level doesn't change if I turn the volume control nor does it stop if I remove the 6072 and 5687 pre-amp signal tubes. The hum is present with any one of the 300Bs inserted.
I'm using short 25g pure silver teflon coated hook up wire for the audio grounds but not twisted or shielded. Have checked all connections and they're good. The ground from the volume control runs parallel with the HT PCB but about 2 inches clear. Rerouting it just makes the hum worse.
Can anyone please suggest what I could do to reduce the hum? Would twisting or covering the ground wires with tubular braiding be a good idea?
The hum level doesn't change if I turn the volume control nor does it stop if I remove the 6072 and 5687 pre-amp signal tubes. The hum is present with any one of the 300Bs inserted.
I'm using short 25g pure silver teflon coated hook up wire for the audio grounds but not twisted or shielded. Have checked all connections and they're good. The ground from the volume control runs parallel with the HT PCB but about 2 inches clear. Rerouting it just makes the hum worse.
Can anyone please suggest what I could do to reduce the hum? Would twisting or covering the ground wires with tubular braiding be a good idea?
Hello,
The heater supply can cause hum. How is your wiring? Does the PT give you a CT for the heaters? This ct should be grounded or a vertual ct can be made with two 100 ohm resistors to earth from the heater supply. This is if the heater supply is at fault.... anyway its something to check out. Other than that check your earth connections.
Best of luck
Mario.
The heater supply can cause hum. How is your wiring? Does the PT give you a CT for the heaters? This ct should be grounded or a vertual ct can be made with two 100 ohm resistors to earth from the heater supply. This is if the heater supply is at fault.... anyway its something to check out. Other than that check your earth connections.
Best of luck
Mario.
I was also having humming issues with a 5687 based tube preamp. I managed to minimised the hum considerably by using DC heater voltage. There is however some humming left (not affected by volume control) which I suspect, is caused by B+ ripple.
Will add another stage of choke and/filter stage to see if it helps.
Will add another stage of choke and/filter stage to see if it helps.
Hi, User68
What frequency is your hum: 50Hz(60Hz) or 100Hz(120Hz)?
Can you show schematic of output stage (better full amp schematic)?
Do you use linear pots for filament hum level eliminating?
Give us some more information and we will certainly find a solution!
Kind Regards, Konstantin.
Thanks for all your suggestions. I have attached schematics.
@ levelfive: it sounds more like a constant 100hz hum with an intermittent higher pitch crackling buzz. Have a 47R hum balance pot for each power tube and 22K signal balance pot between each pair.
@ levelfive: it sounds more like a constant 100hz hum with an intermittent higher pitch crackling buzz. Have a 47R hum balance pot for each power tube and 22K signal balance pot between each pair.
Attachments
Hi User,
Menno van der Veen describes a procedure on his site to adjust an output stage. see: Menno van der Veen, audio electronic research & consultancy, section 'Modify the UL40-S2 into a Super-Triode amplifier.'
he describes an interesting find:
Thus from that we could see that if both side of your amplifier do not have equal plate resistance (a slight imbalance between the tubes) then this might lead to some hum.!
Well just a thought.
In my own 300BC project I am working on, I get a hum from the rectifier block - while when I use a tube rectifier (5R4), there is no hum. Annoying. I can trace the hum all the way on the gound plane ( a solid wire) from the first capacitor till the point where it is grounded on the chassis (at the entry point) and there the hum is lowest, or absent. I can hear it on the speaker (very faint) so it is no artifact from my equipment ground wires. It's the commutation spike.
albert
Menno van der Veen describes a procedure on his site to adjust an output stage. see: Menno van der Veen, audio electronic research & consultancy, section 'Modify the UL40-S2 into a Super-Triode amplifier.'
he describes an interesting find:
Thus from that we could see that if both side of your amplifier do not have equal plate resistance (a slight imbalance between the tubes) then this might lead to some hum.!
Well just a thought.
In my own 300BC project I am working on, I get a hum from the rectifier block - while when I use a tube rectifier (5R4), there is no hum. Annoying. I can trace the hum all the way on the gound plane ( a solid wire) from the first capacitor till the point where it is grounded on the chassis (at the entry point) and there the hum is lowest, or absent. I can hear it on the speaker (very faint) so it is no artifact from my equipment ground wires. It's the commutation spike.
albert
As triode_al has suggested, phasing could be a problem (if you have one filament wired the wrong way, the noise will double instead of canceling itself - just reverse leads on one filament conenction to try it out).
If filaments are wired up correctly so that PP action cancels any common mode hum the only thing left is the adjustment of "hum potentiometers" (the ones between filament supply connections). These need to be balanced so that any noise filament cancels itself out between both tubes. I'm confident you should be able to eliminate all the hum this way.
If you don't feel like tweaking the potentiometers there's always the option of going with DC filament supply. This has been discussed in details here before, there seems to be consensus that 300B sounds best with CCS filament supply (avoiding power-up shocks aside I can't imagine why, I don't subscribe to this theory but I'd use CCS for a different reason).
If filaments are wired up correctly so that PP action cancels any common mode hum the only thing left is the adjustment of "hum potentiometers" (the ones between filament supply connections). These need to be balanced so that any noise filament cancels itself out between both tubes. I'm confident you should be able to eliminate all the hum this way.
If you don't feel like tweaking the potentiometers there's always the option of going with DC filament supply. This has been discussed in details here before, there seems to be consensus that 300B sounds best with CCS filament supply (avoiding power-up shocks aside I can't imagine why, I don't subscribe to this theory but I'd use CCS for a different reason).
Ok, this is NOT filament hum - filament hum is always in the 50Hz(60Hz) range.
Another question: Is your level volume pot (W1 50K) grounded?
At what point exactly?
You should always solder a wire from the grounded pot connections back to the common local ground of the stage the pot is used in.
The pot cases will be grounded via the mounting nut!
LevelFive,
When I balanced the residual hum on my 300BC set yesterday, the minimum position showed that it 'balanced' into 100Hz. Turning one side lopped towards one phase of 50Hz, mid a residual 100, and then to the other side again into the other 50 Hz.
I can see the residual hum appear on the B+, and I really mean appear, because that changes with the setting. I have a 47 muF mylar on the B+ separately for the output stage. But I don't see the 100Hz on the output, it effectively is filtered out.
On the output I see some 50Hz and some spikes ('yikes').
albert
Hi, Albert
Thermal hum? The result of non-infinitely filaments feed with 50Hz AC?
Hmm...
Yes, I see it also on Level Five output. But no 100Hz freq hum.
What are the grounding arrangements around the rectifier and first (reservoir) capacitor? Are you injecting charging spikes into a signal ground, or inducing them magnetically?
If you built it exactly as shown in the PSU diagram (with the rectifier -ve wire going to the capacitor, then a connection from there to ground) then it should be OK. If, on the other hand, the rectifier wire goes to the star ground then you will be injecting noise here.
If you built it exactly as shown in the PSU diagram (with the rectifier -ve wire going to the capacitor, then a connection from there to ground) then it should be OK. If, on the other hand, the rectifier wire goes to the star ground then you will be injecting noise here.
User68,
Interesting sound. As if there are bugs around your place!
Well, I think this is mains noise, and expect that the HT jumps up and down. This should be visible on the scope in the downstream HT voltages. This might be completely in phase with the sound.
On my PC I can't hear hum of any kind. Neither do I hear rectifier bridge spikes.
What would happen if you use an electronic choke? I have put it in my own circuit. I used a FET as source follower. This introduces a high frequency roll off that only passes some of the slow mains related garbage (i.e. the sub 1 Hz sounds that come from mains noise because the power company introduces some kind of signaling in it.) But the latter is now less than 15 mV.
albert
I've been able to narrow things down a bit. There are two types of hum. There's the background hum coming from the power tubes which I think I'll have to live with unless I use DC heaters. The other hum, which is the louder bug-like sound, ceases when I remove the 6072 tube. I've swopped this tube with another 6072 and a couple of ECC81s but the hum's still there, so I don't think it's a defective tube. It must be something around the pre-amp circuit.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.