Is it possible to just power up the amplifier with the relay shorted and the PSU off, just to eliminate this? Since this is a departure from the original kit, it would seem to me to more likely to be the culprit than some poor design in the amp.
There must be 100's of these amplifiers out there, so it must be a specific issue in this particular build.
I agree that t would be nice to be absolutely sure it's not the cause of the noise. As you say, there must be 100s of these out there and surely don't all hum like mine or nobody would build them! So I'll have a go at bypassing it to eliminate it from the debugging exercise.
On the upside, I've just bought an oscilloscope on eBay which I'm picking up this afternoon.
On the upside, I've just bought an oscilloscope on eBay which I'm picking up this afternoon.
I built this amp as a monobloc with autobias and in point to point form earlier in the year. I had no mains hum from it at idle with no input. I'm still using a derivative of it now as I rebuilt it using a 6CG7 as phase splitter and putting EF86 back into pentode operation.
I would disconnect one of the boards and see how things behave with just one half operating.
I would disconnect one of the boards and see how things behave with just one half operating.
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I wouldn't have thought the NTC's are responsible.
Voltage wise, you may have issues with C8 if it is only a 450V rated capacitor. I had no issues as mine are all 630V rated. I had no trouble running mine without any output valves, which caused B+ to exceed 520V.
Elektor fit a zener to limit the voltage around EF86 anyway.
Voltage wise, you may have issues with C8 if it is only a 450V rated capacitor. I had no issues as mine are all 630V rated. I had no trouble running mine without any output valves, which caused B+ to exceed 520V.
Elektor fit a zener to limit the voltage around EF86 anyway.
If these EF86 and ECC83 are tubes that were produced exactly according to the original Philips/Mullard designs, or are NOS preferably, you won't see significant improvement on your hum issue when DC heating them. According to the original designs, both deliberately featured bifilarly wound heaters to suppress hum that might be introduced by the heater AC. This doesn't hold true with US 12AX7's and most probably with others, though.
Btw, the 7025 also matches the European ECC83.
What does your amp do if you short both inputs each?
Best regards!
Btw, the 7025 also matches the European ECC83.
What does your amp do if you short both inputs each?
Best regards!
This is the next thing to try then. My only worry is that this will increase the heater current, and the power trans is already at the limit of its capabilities here. So I may have to go to DC heaters instead.
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6,3V/200R=31,5mA per valve extra, I would not worry.
These notes on EF86 is from R-Type:
Hum. When used as a normal voltage amplifier with a line voltage of 250 V and an anode load of 100 kΩ and a grid resistor of 470 kΩ the maximum hum level of the valve alone is 5 μV, the average value being about 3 μV when operated with one side of the heater earthed. This can be further reduced by centre tapping the heater to earth. Under these conditions the maximum hum level is 1.5 μV. The low level of hum attained with this valve can be completely masked by that due to an unsuitable valve-holder, in which excessive leakage and capacitive coupling between pins will introduce considerable hum.
Noise. The low-frequency noise generated by a valve is most conveniently specified as an equivalent voltage on the control grid for a specific bandwidth. For the EF86 under normal conditions, I.e. line voltage of 250 V and an anode load of 100 kΩ, the equivalent noise voltage is approximately 2 μV for the frequency range 25-10,000 Hz.
Microphony. Care in the design of the valve to ensure that the electrode structure and its mounting are as rigid as possible has reduced the microphony of the EF86 to a very low level. There are no appreciable internal resonances at frequencies below 1 kHz. At higher frequencies the effect of vibration is usually negligible on account of the damping provided by the chassis and the valve holder. In high gain applications such as tape recording care should be taken in siting the valve, particularly when a loudspeaker is present in the same cabinet or where a motor is mounted on the same chassis. In such cases a flexible mounting for the valve-holder or a separated weighted sub-chassis is advisable.
These notes on EF86 is from R-Type:
Hum. When used as a normal voltage amplifier with a line voltage of 250 V and an anode load of 100 kΩ and a grid resistor of 470 kΩ the maximum hum level of the valve alone is 5 μV, the average value being about 3 μV when operated with one side of the heater earthed. This can be further reduced by centre tapping the heater to earth. Under these conditions the maximum hum level is 1.5 μV. The low level of hum attained with this valve can be completely masked by that due to an unsuitable valve-holder, in which excessive leakage and capacitive coupling between pins will introduce considerable hum.
Noise. The low-frequency noise generated by a valve is most conveniently specified as an equivalent voltage on the control grid for a specific bandwidth. For the EF86 under normal conditions, I.e. line voltage of 250 V and an anode load of 100 kΩ, the equivalent noise voltage is approximately 2 μV for the frequency range 25-10,000 Hz.
Microphony. Care in the design of the valve to ensure that the electrode structure and its mounting are as rigid as possible has reduced the microphony of the EF86 to a very low level. There are no appreciable internal resonances at frequencies below 1 kHz. At higher frequencies the effect of vibration is usually negligible on account of the damping provided by the chassis and the valve holder. In high gain applications such as tape recording care should be taken in siting the valve, particularly when a loudspeaker is present in the same cabinet or where a motor is mounted on the same chassis. In such cases a flexible mounting for the valve-holder or a separated weighted sub-chassis is advisable.
Sadly, adding two 100Ω resistors to ground from either side of the 6.3v feed to the front two valves has had no effect on the hum either. Which I suppose explains why Elektor didn't spec them in the first place.
But it leaves me with the same problem, although at least I now know where the problem is. Does anyone have any ideas for my next trick?
But it leaves me with the same problem, although at least I now know where the problem is. Does anyone have any ideas for my next trick?