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I've been having a little hum on the phono stage. I replaced the quad filter cap with an exact replacement. Improvement, but still a slight hum. I read the archives for more information on the subject.The only thing left to do was to replace the selenium and associated caps. I was weary of doing this for the sake of originality, and because the selenium in the heater circuit is not as critical as in the bias supply of the Stereo 70.
Well, I took the plunge and installed 1N4007 diodes and new 2,200 uF caps per Curcio's instructions.
Fired up the system and at first I detected a softening of the sound. mmmm. CD and SACD through the line level sounded a bit lower in level. Line stage was silent but the slight hum remained on the phono section.
I played some records and they sounded nice. Good balance from top to bottom.
Curiously, now my LPs have improved sonically, but CDs and SACDs seem to sound softer. What's going on here?
I had thought that replacing the selenium and the increase in heater voltage would make the sound hotter, not softer!
With the old selenium the midrange was fleshed out even with CDs and SACDs. For LPs perhaps was overkill, but I was growing fond of this sound.
Well, I am not going back to the selenium for obvious reasons, but I am quite surprised with the overall change in system sound.
Regards,
Follow Ups:
Well, when I replaced the filament voltage doubler with 6A2 6-amp 200 PIV diodes. I also use two 4700UF caps with 1.5 ohms resistance @ 10-watts installed in the transformer lead that goes to the center position of the two caps in series.
This resistance lowers the voltage to 12.6 volts as solid-state replacement rectifiers over-voltaged the tubes with my modification. Another benefit is lowering the high peak to average current. This is easier on the transformer & reduces noise as well. My preamp produced no audio hum after modifications.
So, what's the practical benefit of using larger caps? further reduction in hum? I am using 2,200 uF replacements. I was thinking of sticking to those, as long as the heater voltages are okay.
Well, I had the caps in my parts bin. The caps have a little better filtering, but most likely unnecessary.
Make that resistor .5 ohms. I may add all series caps were replaced with AuriCap. Huge improvement, it is a new preamp.Stock PAS? Not a chance of having a place in my tube audio gear inventory.
Line stage has the usual hiss at maximum volume but no discernible hum.Phono has a 120 Hz constant hum that increases with volume. It can be heard faintly between cuts or during low passages in a record. Not terribly distracting but it's there.
Hum is still present with the phono cartridge disconnected.
Filter caps are new, as well as the heater rectifier circuit.
This PAS was factory wired and all leads appear to be well dressed.
Any ideas on how to get rid of the hum?
Regards,
I put lead tube shields on my phono stage tubes on my preamp and it Really helped.... but there are so many variables to consider.
Makes sense, and that's perhaps why tube shields were used in the McIntosh and Marantz preamps of the period.
Also.if you live near any radio/TV towers as I do.....its a no- brainer to shield those phono-stage tubes IMO.
They REALLY worked....I was lucky to find some vintage ones from a tube dealer, but I bet you could make your own from thin lead sheeting.
BTW the modern tube shields...non lead...did not work very well.
There could be grounding or shielding issues, or this noise is riding on your B+. The phono stage is amplifying many times more than the line stage which will bring out the noise. This noise is likely present in your line stage as well, it is just not amplified as much.You could try dropping in a Choke in the power supply, upping the capacitor values or removing the transformer to a separate box and using an umbilical.
Use a small (0.25 - 0.5 ohm) resistor to drop your heater voltage back to spec. You should not run with heaters over 10% above center point. Your sound should return. Making tubes suffer is never the road to sonic bliss.
If they are above the 11 volt spec, I will add the series resistor. 11 volts was chosen for a reason. For one thing, tubes would last much longer and the Telefunkens I have in there are quite valuable.Regards,
Of course your ears and equipment might sound different than mine but to me the PAS was far better when the heater voltage was over 12 volts than it was at 11 volts. I assume you know that the tubes are in series and what come out of the heater supply is closer to 22-24 volts. I'd also tell you to remove the on/off light bulb as it really draws a lot of current and loads that heater supply too much. These things really helped the soft vauge sound. Removing the tone controls is a must. Also radio shack has a 100K pot made by Alps that is a great dirt cheap replacment for the volume control.
12.6 volts was superior to 11 volts for sonics. Best tubes were 1962 GE 12AX7 17mm tall flat grey plate in phono section & late 1950s Tung-Sol 12AX7 in main section.
I replaced the volume control with the 100K pot from Radio Shack. Excellent results. Blasting was gone and tracking was improved. Disabled the loudness and tone controls, again an improvement in sonics.Replaced the quad cap with an exact NOS replacement. Significant reduction in hum, specially in the line stage.
The selenium replacement regrettably has made my PAS sound slow, dark and with limited resolution. Perhaps this is the way the PAS should sound. The fleshed out midrange and increased clarity was an abnormal mode of operation caused by the old parts. Incredible!
I can tell you I am very happy with Joe Curicos upgrade boards in my PAS. It isn't cheap but the results are worth it. If you buy the bare boards and obtain your own parts cost isn't too bad. Also get a new selector switch and RCA kit/PCB.Try disabling the loudness tap on the new volume control. I felt mine was rather dark sounding with it in place.
But I am going to measure voltages just in case. It should read 22 volts at the output of the silicon bridge.You say that higher voltages are preferrable. If so, isn't tube life compromised? It's a well known fact that filaments/heaters that run below rated voltages will last longer.
The well known fact you make note of is open to dispute. I think if you carefully research the matter you will find that longer life is accomplished with heater voltages that are a couple of tenths lower....not a volt and a half lower. And a 12AX7 is intended for 12.6 volts on the heater. So I'd say 12.3 is about as low as I'd like to go. But like I said, YMMV in your equipment.
But just to make sure, I will measure the heater voltages with my Fluke digital VOM. If they are within the 25.2 volt limit (series connection, 12.6 volts for each tube), I will leave it alone.Given your reasoning, it is then safe to say that those Telefunkens would have lasted as long even with a higher heater voltage than the 11 volts specified by Hafler right?
Regards,
Look those tubes would last the longest if we never applied power to them at all. So the question becomes one of not absolute longest life but instead longest life while still performing up to specs. And I would submit to you that at 11 volts on those tubes they never ever performed up to spec and hence the sound was always compromised.IMHO any measure of increased tube life has to be done at conditions where the tube still functions normally and I am sure if you put most tubes in a tester and slowly decreased heater voltage somewhere around the 90% mark emissions would start falling off.
It may interest you to know that once upon a time the army, because they used a lot of tubes in important roles, conducted a study and published a guide to tube life under proper conservative operating conditions. In that publication (IIRC) a good quality 12AX7 was estimated to last about 70 years when operated 8 hours a day. By conservative operation I mean keeping the current the tube dissapates at the middle of it's ratings. The current the tube operates is the biggest determining factor of tube life.
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