Hi
I have a tube output stage in my DAC which is humming. It's not a lot of hum I guess, but I can hear it in the background when my amp is turned to normal listening levels.
The tubes are a pair of ECC99s and a 6X4 in the power supply. All three tubes are AC heated. I ran the high voltage power supply through PSU Designer and it seemed pretty clean so I'm concluding it must be the AC heating that's causing the hum.
Is it possible to heat tubes with AC and eliminate hum entirely? or do I have to live with some hum?
What can I do to reduce/eliminate the hum?
I have a tube output stage in my DAC which is humming. It's not a lot of hum I guess, but I can hear it in the background when my amp is turned to normal listening levels.
The tubes are a pair of ECC99s and a 6X4 in the power supply. All three tubes are AC heated. I ran the high voltage power supply through PSU Designer and it seemed pretty clean so I'm concluding it must be the AC heating that's causing the hum.
Is it possible to heat tubes with AC and eliminate hum entirely? or do I have to live with some hum?
What can I do to reduce/eliminate the hum?
I am not a tube expert but I know from doing pro sound and dealing with guitar amps that maybe you have a ground issue either inside the amp or from the wall.....I would try another outlet that is on a different circuit first and if that does not cure your problem then try relocating your ground in the amp...another question arises that you did not say whether your amp is grounded to the mains or are you feeding it just the Hot and neutral wire>????
Are your tube heater wires tightly twisted and far away from all signal circuitry? If any signal wires cross it, do they cross at right angles? And here's a trick that you can use with tubes like the ECC99 which might reduce the hum - build a voltage divider from B+ to ground such that the 'center point' is at about 40V, and connect this to the heater. You'll need to make sure the AC supply (i.e. the transformer secondary) is floating, and you'll probably also need to add a capacitor from the heater to ground. This should help with the hum.
Disclaimer: if you can't visualize what I'm trying to say, please don't try it. Also, this may not be an option if your tubes are set up as a mu-follower or SRPP or a direct-connected cascade or something like that. In those cases you probably already have a voltage divider raising the heater's potential.
Disclaimer: if you can't visualize what I'm trying to say, please don't try it. Also, this may not be an option if your tubes are set up as a mu-follower or SRPP or a direct-connected cascade or something like that. In those cases you probably already have a voltage divider raising the heater's potential.
Thanks for the replies..
Joe: I've even tried my DAC at someone else's house and it has the same issues. The DAC is grounded to the mains ground through a termistor.
fragman: I have not tried this but it sounds like a good idea.
wd40: i've considered DC heating but I don't have any more space for the parts required to do this!
saurav: no, my heater wires are not twisted. I guess I should do that. They don't cross any signal wires but they are about 5-10cm away from them..
I think I know what you're talking about but I can't really visualize it. I'm not sure what components to use for that.
Joe: I've even tried my DAC at someone else's house and it has the same issues. The DAC is grounded to the mains ground through a termistor.
fragman: I have not tried this but it sounds like a good idea.
wd40: i've considered DC heating but I don't have any more space for the parts required to do this!
saurav: no, my heater wires are not twisted. I guess I should do that. They don't cross any signal wires but they are about 5-10cm away from them..
I think I know what you're talking about but I can't really visualize it. I'm not sure what components to use for that.
Hi,
10/10 for WD.
The voltage levels in a DAC are too low to eliminate hum entirely.
If hum is audible at normal listening levels then that's a lot of hum in my book.
While you're at it, regulate the heater feed and set it for at least 6V or 12V using a 78xx or similar or use a LM317 and set the output to the correct voltage.
Either way no more or less than 10% of the required voltage.
For the ECC99 I'd pt for the 12.6V heater arrangement which eases the current carrying demand.
Cheers,
10/10 for WD.
The voltage levels in a DAC are too low to eliminate hum entirely.
If hum is audible at normal listening levels then that's a lot of hum in my book.
While you're at it, regulate the heater feed and set it for at least 6V or 12V using a 78xx or similar or use a LM317 and set the output to the correct voltage.
Either way no more or less than 10% of the required voltage.
For the ECC99 I'd pt for the 12.6V heater arrangement which eases the current carrying demand.
Cheers,
HeadSh0T said:
Yes. You need to do the heater wiring with tightly twisted (4-5 turns per inch) solid core wire, it needs to be pushed into the corners of the (earthed) metal chassis and arrive at each valve on a radial line. The order from the transformer must be valve with highest signal voltage first, least signal voltage last. Never make a loop between the conductors that encloses a valve base, keep the conductors twisted together at all times. The heater winding needs to have a centre tap to chassis. If you do all that perfectly, you can make an RIAA stage that doesn't hum using AC heaters.
And what's a rough rule-of-thumb for raising the heater potential above the cathode to saturate/reverse-bias the cathode-heater 'diode'? Obviously I don't want to go beyond the maximum allowed heater-cathode voltage difference. But assuming I don't have an SRPP or something else that requires one (or both) cathode(s) to be at elevated voltages, how much should the heater be raised above the cathode? I have mine at about 40V now, but I'm not sure if that's too high.
Hi,
Saurav,
Just keep in mind that this is independent of the fact whether the heaters are fed by AC or DC.
In every tube datasheet you'll find heater t cathode insulation limits, add the AC signal voltage swing of your stage to the cathode.
Lift the cathode out of that danger zone and provide some safety margin.
If you don't do this you may hear a very unpleasant squealing high frequency oscillation that can blow tweeters in a fraction of a second.
Cheers,
Saurav,
Just keep in mind that this is independent of the fact whether the heaters are fed by AC or DC.
In every tube datasheet you'll find heater t cathode insulation limits, add the AC signal voltage swing of your stage to the cathode.
Lift the cathode out of that danger zone and provide some safety margin.
If you don't do this you may hear a very unpleasant squealing high frequency oscillation that can blow tweeters in a fraction of a second.
Cheers,
Now you lost me. Did you mean lift the cathode or lift the heater? I thought the idea was to lift the heater above the cathode, so that electrons in the heater aren't attracted to the cathode (and end up adding hum). And the AC signal you mention - this is the swing at the cathode, not the plate, right?
If I understand you right, you're saying that I should lift the heater high enough above the cathode so that it stays positive w.r.t. the cathode even when the cathode is moving up and down with the signal. Which in my case would be the peak plate current * cathode bias resistor. And the oscillation... ok, if the cathode's peak excursion makes it positive w.r.t. the heater, then the heater-cathode 'diode' will be switching on and off with the signal, and that's what'll cause the oscillation.
Did I get that right, or do I have it all backwards?
If I understand you right, you're saying that I should lift the heater high enough above the cathode so that it stays positive w.r.t. the cathode even when the cathode is moving up and down with the signal. Which in my case would be the peak plate current * cathode bias resistor. And the oscillation... ok, if the cathode's peak excursion makes it positive w.r.t. the heater, then the heater-cathode 'diode' will be switching on and off with the signal, and that's what'll cause the oscillation.
Did I get that right, or do I have it all backwards?
Hi,
Bad phrasing on my part...you actually lift the heater voltage and by the same token you get the cathode out of the dangerzone.
So what you need to do if heater to cathode insulation risks to be exceeded is inject a portion of the B+ into the heater circuit and take into account that voltage you're swinging is also partially seen by the cathode.
If you don't respect this, not only do you risk rectification of the AC signal by the diodic effect but you also risk a "sticky" heater; the heater to cathode insulation will break down rendering the tube useless.
And no, we won't have a directly heated tube after that happens...
So, yes you do have the theory right and in the worst case scenario the tube will oscillate at HF even without any signal present...it only happened to me once with a dodgy tube in a SRPP stage and it cost me a ribbon tweetie.
Cheers,
Bad phrasing on my part...you actually lift the heater voltage and by the same token you get the cathode out of the dangerzone.
So what you need to do if heater to cathode insulation risks to be exceeded is inject a portion of the B+ into the heater circuit and take into account that voltage you're swinging is also partially seen by the cathode.
If you don't respect this, not only do you risk rectification of the AC signal by the diodic effect but you also risk a "sticky" heater; the heater to cathode insulation will break down rendering the tube useless.
And no, we won't have a directly heated tube after that happens...
So, yes you do have the theory right and in the worst case scenario the tube will oscillate at HF even without any signal present...it only happened to me once with a dodgy tube in a SRPP stage and it cost me a ribbon tweetie.
Cheers,
OK, I think I understand now. The only datasheet I could find on the ECC99 was on the JJ website, and I didn't see any heater-cathode insulation rating. I could have missed it, of course. Anyway, 40V (which is 36V H-K since I have 4V at the cathode) should be safe enough, I would think. Most voltage limits I've seen are up in the 80-90V range.
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