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 have bought myself an oscilloscope recently, so I will be interested in how you get up to speed with it ;-) There seems to be a lot of potential to ruin it with ... err .. a lot of potential :-/
I'm sure there is, yes.
Okay - I've removed my relay switching circuit and its associated PSU from the circuit and powered it up directly from the mains IEC connector. The hum is just the same.
Okay - I've removed my relay switching circuit and its associated PSU from the circuit and powered it up directly from the mains IEC connector. The hum is just the same.
Along the lines of 'divide and conquer', are you easily able to split off one channel, so only a single channel is being evaluated? I.e. seperate the inputs, power supply, speaker grounds - anything in common.
I could remove the power to one channel but it would bump up the voltage on the other and no doubt throw the bias off too. The inputs and speaker outs are separate anyway.
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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Okay. Is that because you think some interaction between the two amps channels may be the cause of the hum?
Possibly a ground loop between the two amp modules.
The only other difference mine had is I kept the valve rectifier instead of going solid state in order to retain the simple delayed/soft start up.
The only other difference mine had is I kept the valve rectifier instead of going solid state in order to retain the simple delayed/soft start up.
I see. But won't disconnecting one amp board make the voltages to the other too high? I'm more than happy to try it, I just don't want to damage it.
The soft start up on this one is provided by the two NTC resistors. I wonder whether they could also be a source of hum?
The soft start up on this one is provided by the two NTC resistors. I wonder whether they could also be a source of hum?
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.
Okay, so if I pull the HT from one channel is that enough? Or should I pull the heater feeds too?
I was going to say you could power the heaters of one side of the amp (EF86 and ECC83) off 4 AA alkaline cells tape together. See if the hum goes.
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!
The ECC83s are Mullards, but the EF86s are Russian military spec tubes.
All this is going on with the inputs grounded - makes no odds to the 50Hz hum. I get a little more 100Hz with the inputs ungrounded, but the 50Hz is unaffected.
All this is going on with the inputs grounded - makes no odds to the 50Hz hum. I get a little more 100Hz with the inputs ungrounded, but the 50Hz is unaffected.
Well I said baodouin0's idea of running one channel's EF86 and ECC83 heaters from a battery pack was a good idea - and it was. The hum totally disappears from that channel. So why is the heater feed introducing so much noise to V1 and V2?
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?