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Thread: Deluxe Reverb Build Problem!

  1. #36
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    Quote Originally Posted by Enzo View Post
    The 10 meg resistor is there for only one thing, it prevents the neon bulb circuit from "ticking". Noise from the bulb flash can leak into the audio without it.
    Thank you for the clarification, Enzo.

    I've been playing the amp quite a bit at all volumes. I can't get any grit out of it until the volume is at "9" which seems unusual to me. That even applies to when I'm running the amp with no negative feedback! It's almost behaving like a Twin Reverb. It also sounds a bit thinner and brighter than a normal Deluxe Reverb.

    I've heard clips of this Mojotone kit and it seems to overdrive with ease. The transformers are all stock. My bias and voltages are where they should be. Thoughts?

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  2. #37
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    Please post your new readings for bias voltages.

    When it's turned up, is it as loud as you'd expect? If possible measure the output. Maybe you have a build mistake, so check the components that you have in your own layout against the drawings to make sure you haven't got parts transposed. It's easy to overlook something when you've been staring at it for long enough. You say the voltages are where they should be - have you measured the critical voltages through the preamp - that is, plate and cathode for each stage?

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  3. #38
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    To check the bias suitability, I suggest that 1 ohm cathode resistors are installed between the power tube cathodes and their 0V / chassis return
    Click image for larger version. 

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    and measure the mV across each (ie 0V to cathode) at idle.
    The voltage across them will correspond to cathode current, eg 10mV indicates 10mA is flowing.
    If they're below ~10mV the amp may seem a bit flat and dead, >25mV and tube life may suffer and the amp may seem to lack dynamics.
    The resistors don't need to be the 3W type shown, anything >1/4W is fine. Metal film 1% is preferred.

    The particular setting of the control is usually immaterial. BF and RI BF Fenders use a ~35% audio taper on their vol and treb controls.
    You have probably fitted regular 10 or 15% audio taper pots.
    Hence the settings won't correspond.

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    Last edited by pdf64; 10-23-2017 at 05:05 PM.

  4. #39
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    Quote Originally Posted by pdf64 View Post
    To check the bias suitability, I suggest that 1 ohm cathode resistors are installed between the power tube cathodes and their 0V / chassis return
    Click image for larger version. 

Name:	bias.gif 
Views:	59 
Size:	6.3 KB 
ID:	45392
    Click image for larger version. 

Name:	Pic-12-Cathode-1-Ohm-resistors.jpg 
Views:	48 
Size:	424.4 KB 
ID:	45393
    and measure the mV across each (ie 0V to cathode) at idle.
    The voltage across them will correspond to cathode current, eg 10mV indicates 10mA is flowing.
    If they're below ~10mV the amp may seem a bit flat and dead, >25mV and tube life may suffer and the amp may seem to lack dynamics.
    The resistors don't need to be the 3W type shown, anything >1/4W is fine. Metal film 1% is preferred.

    The particular setting of the control is usually immaterial. BF and RI BF Fenders use a ~35% audio taper on their vol and treb controls.
    You have probably fitted regular 10 or 15% audio taper pots.
    Hence the settings won't correspond.
    Thank you for the explanation, pdf64.

    I will have to install those resistors tonight and take some measurements. I noticed a slight red glow on the plates so I backed off until it disappeared. However, even when the plates were glowing it acted as if the bias was too cold.

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  5. #40
    Supporting Member eschertron's Avatar
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    Quote Originally Posted by Penguin View Post
    Thank you for the clarification, Enzo.

    I've been playing the amp quite a bit at all volumes. I can't get any grit out of it until the volume is at "9" which seems unusual to me. That even applies to when I'm running the amp with no negative feedback! It's almost behaving like a Twin Reverb. It also sounds a bit thinner and brighter than a normal Deluxe Reverb.

    I've heard clips of this Mojotone kit and it seems to overdrive with ease. The transformers are all stock. My bias and voltages are where they should be. Thoughts?
    I had a problem with a guitar pickup that sounded thin and weak when it shouldn't. Turns out a bad solder joint was responsible. Apparently high resistance but not completely open. Since you describe your sound with similar terms, I'd suggest that while you are in the chassis to check DC voltages, trace a signal as well. There may be some unexpected attenuation due to a cold joint. Or just 'check all' solder connections.

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    Last edited by eschertron; 10-24-2017 at 01:16 AM. Reason: typo
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


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    late: didn.t saw you allready solved, sorry

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    Last edited by catalin gramada; 10-23-2017 at 08:08 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  7. #42
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    I havenít had a chance to check the bias yet but will do so soon. However, I have a question. When I tap on most of the capacitors I can hear the tapping loudly and clearly through the speaker. Is that unusual?

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  8. #43
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    I decided to elevate the eyelet board a bit and removed the insulating board from underneath it. That cured most of the sound generated when I tapped on the capacitors.

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