First time here, peeps. Lurked quite a while, finally decided to jump in!
I'm working on a Line 6 / Bogner DT50 that has hum as soon as you power it up with B+ applied to the center tap of the output transformer.
I've checked the rectifier diodes and replaced the main filter cap on the B+ but that didn't change anything.
I'm wondering about the health of the output transformer, so here's a basic question: I noticed a difference in the DC resistance of the two output windings. So, measuring from center tap (Red) to the two windings (Blue and Brown) there is a 20 percent difference, approximately 40 and 50 ohms.
I know impedance is what matters, but does that sound suspicious?
Hey, thanks for reading and any ideas.
I'm working on a Line 6 / Bogner DT50 that has hum as soon as you power it up with B+ applied to the center tap of the output transformer.
I've checked the rectifier diodes and replaced the main filter cap on the B+ but that didn't change anything.
I'm wondering about the health of the output transformer, so here's a basic question: I noticed a difference in the DC resistance of the two output windings. So, measuring from center tap (Red) to the two windings (Blue and Brown) there is a 20 percent difference, approximately 40 and 50 ohms.
I know impedance is what matters, but does that sound suspicious?
Hey, thanks for reading and any ideas.
Can you describe the hum? 60Hz, 120Hz, noisy etc. Sounds more like a grounding issue TBH. And do you own an oscilloscope. It wouldn't hurt to 'see' how clean the B+ is.
Sounds buzzier than 60Hz, more like 120.
Yeah, I'll look at it with the scope and maybe post a picture.
In checking out the output xformer, I unbolted it from the chassis and isolated the output wires and tested resistance with a swing meter (d'arsonval)
Didn't see any leakage to the xformer case.
Yeah, I'll look at it with the scope and maybe post a picture.
In checking out the output xformer, I unbolted it from the chassis and isolated the output wires and tested resistance with a swing meter (d'arsonval)
Didn't see any leakage to the xformer case.
Don't know about your case, but Buzz is usually bad earthing.
Humm is either imbalance somewhere, or a plain fault allowing hum to creep in.
If it comes on immediately after switch on, a fault is more likely, since the tubes are not even warmed up to pass any current.
Does the hum go away at any stage?
Have you tried pulling tubes out to check to see if it makes a difference?
Humm is either imbalance somewhere, or a plain fault allowing hum to creep in.
If it comes on immediately after switch on, a fault is more likely, since the tubes are not even warmed up to pass any current.
Does the hum go away at any stage?
Have you tried pulling tubes out to check to see if it makes a difference?
Relocated to the Instruments and Amps forum per forum policy..Someone here will likely have the solution..
The way transformers are wound explains why the secondaries are different DC resistance. The first half is wound closer to the bobbin, so it has smaller diameter turns. When the second half is wound on top of the first after the center tap has been brought out, its turns have a longer diameter since it is wound on top of the first, so it takes longer length of wire to maintain the same number of turns as the first half. Longer wire naturally has more DC resistance.
As for the 120 cycle hum, it is associated with rectified AC. When there is full wave rectification, the 60 Hz. power is changed to 120 Hz. pulsating DC giving it a more buzzy sound instead of the lower hum of 60 cycles. Usually associated with filtering in the power supply and the grounding associated with it.
As for the 120 cycle hum, it is associated with rectified AC. When there is full wave rectification, the 60 Hz. power is changed to 120 Hz. pulsating DC giving it a more buzzy sound instead of the lower hum of 60 cycles. Usually associated with filtering in the power supply and the grounding associated with it.
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Thanks for the transformer info boobtube, that was interesting.
In answer to Ken, the hum occurs even with ALL tubes pulled. The hum never goes away, and there is NO output signal.
There is a mode switch to select Class A or Class A/B, and it's much worse in Class A where the B+ voltage is lower. Ripple is 10V peak to peak in Class A mode.
I am currently in communication with a tech at Line 6 and will let you know how it goes.
If you have any familiarity with this chassis please chime in. Thanks all for reading.
In answer to Ken, the hum occurs even with ALL tubes pulled. The hum never goes away, and there is NO output signal.
There is a mode switch to select Class A or Class A/B, and it's much worse in Class A where the B+ voltage is lower. Ripple is 10V peak to peak in Class A mode.
I am currently in communication with a tech at Line 6 and will let you know how it goes.
If you have any familiarity with this chassis please chime in. Thanks all for reading.
Did you tried to isolate the mains transformer from the rest of the amp (remove plug H2, H6, H13; only the mains connected)?
1. Still hum?
The transformer is probably bad.
2. No hum?
Reconnect the plug H2 (Tube Heaters).
3. Hum?
That’s something to have a closer look at later.
4. No hum?
Reconnect H13 (Bias voltage and power to the main board)
5. Hum?
That’s something to have a closer look at later.
6. No hum?
Reconnect H6 (B+ voltage)
7. Hum?
That’s something to have a closer look at later.
8. No hum?
Practically impossible…
So it’s either 1 to 7.
Let us know
1. Still hum?
The transformer is probably bad.
2. No hum?
Reconnect the plug H2 (Tube Heaters).
3. Hum?
That’s something to have a closer look at later.
4. No hum?
Reconnect H13 (Bias voltage and power to the main board)
5. Hum?
That’s something to have a closer look at later.
6. No hum?
Reconnect H6 (B+ voltage)
7. Hum?
That’s something to have a closer look at later.
8. No hum?
Practically impossible…
So it’s either 1 to 7.
Let us know
The answer to 7 is yes. Disconnecting H12 to center tap of output Xformer kills the hum. B+ and SB+ still connected to output circuitry.
If you have the schematic, I have isolated D1, D2, D5, D6, D7 and C2 and all those components test good out of circuit.
If you have the schematic, I have isolated D1, D2, D5, D6, D7 and C2 and all those components test good out of circuit.
Hum from the speakers even with power tubes removed means something is pulling current through the output transformer primary. Look at the power tube socket plate pins. With power off, measure for low resistance to ground. If it is there, then my first suspect is a shorted flyback diode.
Thanks Enzo, it was the nudge I needed. I saw a short between the plate and grid number 2 pins on one output (EL34 pins 3 and 4) that turned out to be a bad relay that selects pentode/triode operation.
Ready for something amazing? I had the EXACT part in stock!!!
Thanks all.
Ready for something amazing? I had the EXACT part in stock!!!
Thanks all.
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