I decided to upgrade my 30-year old (and until now properly working) DIY phono/line preamplifier. Basically it is a 3-stage phono preamplifier and a 4th stage line amplifier, each stage is an SRPP circuit using 6N23P tubes, plus a relay input selector matrix. I decided to make some improvements, keeping the basic construction, and now the time arrived to realize them.
First of all, I separated the signal ground from the chassis ground, being it an IEC Class I device, chassis connected to mains PE. I installed separate ground breakers (25A diode bridge and 12R resistor) per channel.
Second, I replaced the mains toroidal transformer. The new transformer has separate HT windings for the L and R channel, instead of a common winding before. The bridge rectifier and the filter capacitors were already separate. I reduced the HT from 320V to 280V. The new transformer has 8 heater winding for the 8 tubes. The AC for the heating is rectified and regulated per each tube, and elevated to 180V. Before that I used one single bridge rectifier and 8 regulators without elevation, i.e. one side of the heating was simply grounded.
Now I get big hum, especially in the phono part. What did I wrong? I can do partial rollback, but I don't know where to start troubleshooting. Any advice appreciated. Pictures will follow.
First of all, I separated the signal ground from the chassis ground, being it an IEC Class I device, chassis connected to mains PE. I installed separate ground breakers (25A diode bridge and 12R resistor) per channel.
Second, I replaced the mains toroidal transformer. The new transformer has separate HT windings for the L and R channel, instead of a common winding before. The bridge rectifier and the filter capacitors were already separate. I reduced the HT from 320V to 280V. The new transformer has 8 heater winding for the 8 tubes. The AC for the heating is rectified and regulated per each tube, and elevated to 180V. Before that I used one single bridge rectifier and 8 regulators without elevation, i.e. one side of the heating was simply grounded.
Now I get big hum, especially in the phono part. What did I wrong? I can do partial rollback, but I don't know where to start troubleshooting. Any advice appreciated. Pictures will follow.
I don’t know exactly what’s causing the hum, but 180V elevation is too much. I would expect it to be around 40 Volts. Please check max. heater - cathode voltage for your particular circuits.
Regards, Gerrit
Regards, Gerrit
Indeed, the heater of the lower half tube sees 180V elevation but the upper half sees only 40V referred to the cathode. I will try grounding the heater again (I mean grounding the common of the LM317T regulator).
Shouldn't it be the other way round:
- the *lower* half tube in a SRPP elevated to *40V*
- the *upper* half elevated to *180v* ?
Although I do not believe that this will cure the hum problem.
I suspect the ground breaker ... just a gut feeling ... try to remove it or put a short across it, in other words connect signal gnd to PE again and see what happens ...
what if either your signal source or your power amp has a grounding issue which was "fixed" by PE grounding of the pre. And now with the ground breaker in place this "fix" is no longer there ... thats why I proposed to bridge the breaker again ... just a theory to be confirmed ... or dismissed.
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You're right, but I connected the two halves of the tube in piggyback. Lower tube anode to upper tube g1. I wanted to elevate the heating to more positive than either cathode. The upper tube cathode sits at about half B+ (140V), that is the reason I set the heating even more positive. But I will try grounding the heating.
Some photos here:
Preamplifier - Google Photos
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FWIW I've built many designs with the top cathode sitting at 140V - 160V. Never elevated a heater, never had a problem.
Is your hum 50Hz or 100Hz? Does it go away if you short the inputs?
Did you really get a transformer with 8 separate heater windings, one for each tube? Seems unnecessary, no? I use one 12V supply for all the tubes in the amp (pairs wired in series)
Like Sorento, I suggest you short your ground breaker and see how that goes. All of my preamp/ amplifier designs tie audio ground to electrical ground. That way I can disconnect an RCA without having 60Hz at full blast while I do it and if there IS a ground loop, I i'll use isolation xfmrs for the audio rather than breaking the ground. TY-250P by Triad is impressive for it's cost.
Koda
Is your hum 50Hz or 100Hz? Does it go away if you short the inputs?
Did you really get a transformer with 8 separate heater windings, one for each tube? Seems unnecessary, no? I use one 12V supply for all the tubes in the amp (pairs wired in series)
Like Sorento, I suggest you short your ground breaker and see how that goes. All of my preamp/ amplifier designs tie audio ground to electrical ground. That way I can disconnect an RCA without having 60Hz at full blast while I do it and if there IS a ground loop, I i'll use isolation xfmrs for the audio rather than breaking the ground. TY-250P by Triad is impressive for it's cost.
Koda
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