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Questions about tubes

Discussion in 'Amps and Cabs [BG]' started by marclom, Mar 13, 2005.


  1. marclom

    marclom

    Mar 4, 2005
    I bought an Ampeg Portaflex B15N from Blanky's Amps about 2 week ago. It came {supposedly} cleaned, thoroughly bench tested, and retubed (some NOS), with a 90day warranty. The sound of the amp was truely atonishing because of it ability to keep the treble unharsh and smooth without it becomming muddy or blurred. Well, the warranty came in handy when both the 5ar4 rectifier tube, {I think it was the rectifier - it was the last in the chain}, and another unusual tube,{the first in the chain} failed this weekend.
    Coincedentally, the tubes that failed were all NOS. The 5AR4 was a RCA Blackbase, and was much taller than the new Electro Harmonix 5AR4 that Blanky used as a replacement.(he said he was out of NOS 5AR4) Immediately I noticed that the charming and detailed upper end became muddy. I asked Blanky" does this tube have the same headroom, because it sounds different to me?" He said " Yes, same specs and headroom" so I I thought it was just my imagination. However, I did notice that he had to adjust a screw that said hum adjust on the back of the amp to get rid of a hum { also a new problem after the rectifier tube was changed from RCA to Electro Harmonix } Is this adjustment a variable bais control? If so, then would the need for the bias adjustment be evidence that the repacment recifier is not the same as the NOS RCA blackbase, and consequently has a different sound? I also thought the change in sound could be due to the other tube that partially failed {the tube in the begining of the chain} Blanky switched this tube to a different position in the chain where it seemed to be less important, untill he could order a replacement.
    I have noticed that NOS RCA 5AR4's are available, but are expensive. However, If what I think I heard in the change of the Ampeg's sound is true, the extra expense would be justified. And since the amp came with supposedly fresh tubes, and was warrantied, I could insist that the tubes that were NOS, be repaeced with NOS, or a least that Blanky pay a portion of the cost of the relacement NOS tubes.
    Do you think that Blanky's NOS are somehow, defective, or worse that they are salvaged used tubes? Do you think this failure rate is common with NOS tubes, ( an NOS 12AX7 also failed in Victoria's Vibro Champ,{again the first in the chain}), or is somthing else going on here? Are the tubes at the begining and end of the chain more vulneralbe to failure? Wendy's vintage P base, that really sounded great thru the Ampeg, had a bad jack and crackled a bit, could this have dammaged the tubes?
    I could really use some objective advice on these question, before Blanky orders me any more new replacement tubes.
    Thanks,
    Marc
     
  2. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Well, first off, there's some people that try to pass off "other" tubes as 5AR4's. The 5Y3's are most common, there are some Soviet knockoffs that "aren't" real 5AR4's. So that's one thing to watch out for. IME, the JJ 5AR4's, are pretty good, they have decent specs and they sound pretty good. The main difference is probably the "longevity" of the tube, in other words, the old tubes would last just about "forever", that's why they command such a high price on the secondary market. With the new ones, it's probably too early to tell. I haven't had a JJ fail on me yet, but it's only been a little over a year since I've been using them, so it remains to be seen what their longer term performance is. In terms of the rectifier changing your sound, it "might", but if it's the right kind of tube, it shouldn't. The rectifier should be the "least" influential tube in your sound chain. Except if it somehow isn't the right kind of tube. You might want to check into the JJ's (if you haven't already), my take is "so far so good". :)