Hi,
I test my anode voltages with a DMM set to DC and I see the voltage 'swing' from in my case 183v to 179 or so over maybe 10s or so on a slow fluctuation back and forth. Should this be like this or could it be indicative of a more fundamental fault. Other voltages in the amp don't move like this really.
Sorry if this is a dumb question?
Thanks,
I test my anode voltages with a DMM set to DC and I see the voltage 'swing' from in my case 183v to 179 or so over maybe 10s or so on a slow fluctuation back and forth. Should this be like this or could it be indicative of a more fundamental fault. Other voltages in the amp don't move like this really.
Sorry if this is a dumb question?
Thanks,
Could be a low frequency instability. Can you post the complete schematic?
Usually a coupling capacitor is too large. Not at all dumb, a very good observation.
Is the behavior the same in both channels, or does it differ a little?
This is more common than you would think. Once in an industrial control system,
I found a low frequency instability at around 0.015Hz.
Usually a coupling capacitor is too large. Not at all dumb, a very good observation.
Is the behavior the same in both channels, or does it differ a little?
This is more common than you would think. Once in an industrial control system,
I found a low frequency instability at around 0.015Hz.
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this is the circuit - same both sides, and on both v4 and v5 (160-163 or so).
https://www.arcdb.ws/Database/SP8/ARC_SP8_manual.pdf
mine is Rev E at the end of the manual.
https://www.arcdb.ws/Database/SP8/ARC_SP8_manual.pdf
mine is Rev E at the end of the manual.
Is there variation at the power supply B+1 also?
If B+1 is stable, the C9 may be too large. To see, try directly grounding both grids of V5
(that is, short both R24) and see if it is gone.
If B+1 is stable, the C9 may be too large. To see, try directly grounding both grids of V5
(that is, short both R24) and see if it is gone.
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no the B+ is pretty stable, I will have a look at c9 - it is the standard spec at 0.47 uF in parallel with a little one. It measures 0.48 uF in circuit with a DMM both channels.
It still may be too large by design. Perhaps ARC thought that since the variation will be filtered out
by the main output capacitor, it was not important.
by the main output capacitor, it was not important.
the other thing I have done which I am hoping does not cause in itself problems is to physically remove the optocoupler from the circuit, this was an attempt to find the elusive hum. I sounded a lot better like that so I did not replace it.
Won't be a problem in this regard, but that could allow turn-on and turn-off transients
to reach the power amplifier and speakers.
Those are fairly high Z circuits. Poking around with DMM leads can cause small fluctuations. Connect a 10Mohm digital scope probe to each point and don't touch it during measurement. Then take a picture of the scope trace and post it here.
If a modern DMM causes loading on a circuit then something is seriously wrong. We aren't measuring nanovolts here.
Edit to my post #14: And set the input selector to an input that is shorted, to prevent any signal or noise.
Maybe. I see several possibilities where his measurement could cause these observations. Also, it would be helpful to see a trace of the voltage. May offer clues.
Actually, the user needs to be very careful with DMM.
As an inexperienced Tube tinkerer, I once decided to switch to the mV scale on my DMM to measure rail voltage fluctuations that were under 1 V.
big mistake
The input impedance, depends totally on the voltage range being measured.
Sometimes trying to 'zoom in ' on a measurement, is not advisable.
As someone said, take a 10x or 100x scope probe and measure using that and a scope.
Or get a BNC to 4mm adapter, use the same 10x or 100x probe, and measure the AC mV. Scale up. Done.
FWIW, this is a quality, reputable DMM with AC bandwidth useful for measuring AF noise.
I was not aware of that. Can you show the spec sheet for a decent quality DMM where that is true?
No....I can't be bothered to prove what I have found in RL....but it is readily available.
Fluke 28II
Fluke 187
Input Z is far lower on 200mV AC range (or is it DC ranges?), than the 2V or 20V range, for example
Either way, that 'phenomenon' and/or contact between leads, poor connection, tip oxidation, or simply an accidentally floating or unnoticed OC lead, can be possible causes.
A DMM certainly does not have the high Z input of Meter with a Valve front end.
Fluke 28II
Fluke 187
Input Z is far lower on 200mV AC range (or is it DC ranges?), than the 2V or 20V range, for example
Either way, that 'phenomenon' and/or contact between leads, poor connection, tip oxidation, or simply an accidentally floating or unnoticed OC lead, can be possible causes.
A DMM certainly does not have the high Z input of Meter with a Valve front end.
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