1. We will be in Read Only mode (no new threads, replies, registration) starting at 9:00 EDT for a number of hours as we migrate the forums to upgraded software.

Broken diode in my guitar amp?

Discussion in 'General Electronics Chat' started by tmac, Oct 28, 2008.

  1. tmac

    Thread Starter Member

    Oct 28, 2008
    Hi! I have a malfunctioning tube guitar amplifier, and I think I have found a broken diode. I have little experience with practical electronics, so I was hoping to get som answers here.

    First question: would you agree that this is a damaged diode?:


    I did try the diode-measuring setting on my multimeter without disconnecting the diode from the board, and got a fluctuating result 10-100 both ways.

    Secondly, I have tried to read off the text on the diode (there are three lines of text on it), but may be missing a letter/number in the second line:


    (The last visible letter in the second row could also be an E)

    Does anyone know for sure if I am missing a letter/number in the second row of the description? Will I need to get an identical diode, or would other types also work as a drop-in replacement? (I haven't found this exact one when browsing the net)

    I would really appreciate any useful information.

    Best regards,
    Torquil Sørensen
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    It's a zener diode, so diode checks will look funny. The rating is 120 volts with 5 watts dissipation. With power on, you should read 120 volts across it, with the polarity reversed (positive voltage on the cathode of the diode). The 1N5380B is the device descriptor

    It's probably there to regulate your B+ on a couple of tubes. If you have a 100 Hz roar in the output, look for an open filter capacitor.
  4. tmac

    Thread Starter Member

    Oct 28, 2008
    Thanks guys, this was really helpful. I will read up a bit on the usage of zener diodes in the context of B+/tube amps before I try to get a replacement. I didn't see any broken capacitors, so I am hoping that just replacing this will make it work.

    For the interested: this is a Mesa Boogie Express 5:25 amp which works fine in class A/B (30W) mode, but not in class A (5W) mode. Hopefully this diode is only related to the 5W mode and is the reason for it malfunctioning.

    It was already damaged when I got it so I don't know how it happened, maybe a power tube broke donw or something. The tubes that are in it now are not the originals. In 5W mode, the sound from the amp is extremely weak, and quite thin with little bass. The sound coming from the amp is also wrong in other ways but it is not easy to describe. But in the 30W class A/B mode it is great.

    Thanks again for your help!
  5. SgtWookie


    Jul 17, 2007
    You'll probably need your amp to be re-biased after you install the new Zener.

    Zener diodes aren't all equal, and tubes age. The proper bias can be determined by examining the datasheets for the tubes that are in use. Frequently, this is done by inserting a 1-Ohm resistor between the tube's cathode and ground, and measuring the voltage across the resistor to determine current while adjusting.
  6. tmac

    Thread Starter Member

    Oct 28, 2008
    Unfortunately, Mesa Boogie amplifiers don't have an adjustable bias. To change it, one has to change resistors inside... Do you think that the resistance of the Zener-diode is important in this regard?

    I'm assuming that the resistance values for Zener diodes is the resistance for the current that flows the "wrong" way, when the external voltage is higher than the "voltage barrier"?

    Since I am having some problems in getting hold of that particular Zener, and I'm considering connecting two in series to get the correct voltage value. But then I will get a much too low resistance value, much lower than 170 Ohm, which I think is the "resistance" in the original Zener. So that's no good if the resistance value is important for the operation of the amp.
  7. kubeek


    Sep 20, 2005
    A schematic would be helpful to determine the function of the zener inthe circuit.
  8. SgtWookie


    Jul 17, 2007
    Mouser has those diodes in stock for $0.32/ea.
    You might as well get 10 of them; you may very well fry several until you discover the actual part that went bad.

    OK, I downloaded the Express series user manual from Mesa Boogie, from this page:
    Direct link: http://www.mesaboogie.com/manuals/ExpressSers.pdf
    Diagnostics begins on page 14.
    Bias adjustment notes begins on page 16. I found it quite interesting and helpful.

    After reading that section, I suggest that you should consider getting a new set of matched tubes from Mesa Boogie, along with a replacement Zener. Something caused excessive current through the Zener which blew it up. Out-of-spec tubes could've caused that. It might've just been a surge.