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Interesting....I am going to measure heater voltages

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crooner

(A)

In Reply to: Re: Sonic Observations - Replaced Selenium on PAS posted by Chris O on September 17, 2004 at 05:36:14:

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,
crooner

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Disagree - Russ57 10:05:21 09/17/04 (7)
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.

Russ
- Re: I agree with Disagree - Jimmy 17:44:45 09/17/04 (0)
  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.
- Selenium replacement..first mod that actually did not help the sound... - crooner 11:21:16 09/17/04 (1)
  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!
  - Now if you want the next step up - Russ57 12:08:05 09/17/04 (0)
    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.
    
    Russ
- Yes, indeed. They are series connected.. - crooner 11:16:27 09/17/04 (3)
  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.
  - Actually... - Russ57 12:03:26 09/17/04 (2)
    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.
    
    Russ
    - I am going to follow your suggestion... - crooner 18:43:40 09/17/04 (1)
      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,
      crooner
      - About lasting as long - Russ57 07:36:50 09/18/04 (0)
        
        In Reply to: I am going to follow your suggestion... posted by crooner on September 17, 2004 at 18:43:40:
        
        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.
        
        Russ

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