Pultec MEQ-5 relative output level = -3.5dB even in bypass?

Discussion in 'Graphic / Parametric EQ' started by OlympicPhil, Jun 11, 2007.

  1. OlympicPhil

    OlympicPhil Guest

    Can this be right?

    It's just been pointed out to us that our Pultec MEQ-5 drops the signal level by almost 4 dB, which we've confirmed in our lab with a signal generator / analyser.

    After changing ALL the valves the relative o/p level is now -3.5dB but I don't know if this particular unit is supposed to be "zero in, zero out" or if this signal drop is normal.

    Any idea anyone?

    Phil
    Techie, Olympic Studios
     
  2. Boswell

    Boswell Moderator Distinguished Member

    The passive EQ section of a Pultec has an insertion loss of 16dB. In the MEQ-5, this is supposed to be made up to 0dB by the gain in the output stages. However, it is an open-loop design, and hence not accurately gain-controlled. Check the anode and cathode-cathode resistors of the first triode stages and the cathode by-pass capacitors of the output triodes.
     
  3. Thomas W. Bethel

    Thomas W. Bethel Well-Known Member

    Also how are you running the unit? Balanced in and out, unbalanced in and out or a combination? When was the last time the tubes were checked and were they checked for gain (mu on some tube testers). Do you have the right tubes installed. A 12AU7 looks the same as a 12AX7 and will work the same in a circuit but has less gain.

    Here is a simple way to check on the gain of your tube(s) http://www.rru.com/~meo/Guitar/Tubes/t3.pl
     
  4. OlympicPhil

    OlympicPhil Guest

    Balanced in and out.

    The tubes that are in there now are all brand new - just arrived this morning, and are all direct replacements for the ones that were in there before I changed them (I'm assuming they're the correct ones and that someone hasn't put the wrong valves in there previously).

    They are: a 6X4, an ECC82(12AU7) and an EL90(6AQ5).
     
  5. OlympicPhil

    OlympicPhil Guest

    Yeah, it's time to open it up, isn't it :)

    Thanks for your help, both of you guys.
     
  6. Boswell

    Boswell Moderator Distinguished Member

    I've actually had a look at a UA Pultec circuit, and I had mis-remembered the output configuration on those types. The output stages are a closed-loop design via a feedback winding on the output transformer that differentially drives the first cathodes. So if you are getting a no-load overall gain of around -4dB it means either that the passive input stages (including the input and interstage transformers) are attenuating more than 16dB, or the open-loop gain of the output stages is insufficient to make up the 16dB when the transformer feedback is applied. The fact that you see a gain difference when you try different valves would point to the latter.

    Tom's link to the gain check site is interesting, but not easy to apply to circuits that have transformer-coupled feedback.

    BTW, the gear list at Olympic Studios is impressive!
     
  7. Kev

    Kev Well-Known Member

    what lab with a signal generator / analyser are you using ?

    have you confirmed that it will hold output level ... when terminated with Transformer input stages like the MEQ-5

    beware the diferences between bridging input and their relatively high impedance
    and
    old styled 600 ohm ... and lower ... transformered inputs

    I think people around here already know I like to use passive VU meters at both input and output, to confirm these sorts of measurements
     
  8. OlympicPhil

    OlympicPhil Guest

    It's a Neutrik A1 "Audio test and service system"
     
  9. OlympicPhil

    OlympicPhil Guest

    Did I mention that it also drops the level when the "bypass" switch is applied?
     
  10. OlympicPhil

    OlympicPhil Guest

    right - I found a supposed 1.6k resistor that was measuring 1.85k... swapped it out and now the relative level is -3.3dB.

    Getting closer :)
     
  11. OlympicPhil

    OlympicPhil Guest

    OK - I've stumbled across something...

    while testing these resistors, I was unsoldering one leg of each component in turn for testing, then resoldering afterwards.

    after all the obvious resistors, I unsoldered one leg of what I had thought was a resistor before unsoldering, which turned out to be an inductor, however when that one leg was free, the whole thing dropped out of the circuit, so it must've had a dry joint on the other leg. (promising sign).

    The problem is, because I was concentrating on the leg I was unsoldering, I hadn't taken any notice of where the other leg was attached.

    Does anyone have a schematic that they can either let me have a copy of, or could anyone that has a schematic just inform me of which terminals this inductor should be soldered to?
     
  12. OlympicPhil

    OlympicPhil Guest

    Right - I just powered it up without the inductor in circuit and I'm getting the same result... 3.3dB down... which means that is very likely to've been the problem.

    Can anyone let me know where it's supposed to go? (y)

    P.S. for the record, the rest of Olympic's maintenance guys are much better than me at this sort of thing - I'm the "computer guy" :p
     
  13. Kev

    Kev Well-Known Member

    never mind ... "computer guy"
    this is how you learn

    did you actually find a schematic ?

    try a search at http://www.groupdiy.com
     
  14. Boswell

    Boswell Moderator Distinguished Member

    Jakob Erland's site Gyraf Audio http://gyraf.dk/ has some schematics from which you can piece together most of the circuit of the MEQ-5. Go to the Resources link under Technical Documents and scroll down to Equalizers. The MEQ-5 EQ section is there, and the output stages can be taken from the PE1A schematic.

    If you are experiencing gain loss in bypass mode, the problem is unlikely to be due to poor signal inductor connections. However, my memory of those Pultecs is that all the signal inductors are tapped and so are not 2-leg devices.
     

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