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Thread: Mesa Boogie Nomad 55 OT

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    Member Wombaticus's Avatar
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    Mesa Boogie Nomad 55 OT

    Hi All,

    Iíve got a Mesa Nomad 55 on the bench with a dead OT. Iíll be dammed if Iíll pay MM $170 for their drop in replacement. Presumably this is a fairly standard 50W class PP OT. Itís a standup style. The amp can take either 6L6s or EL34s. Mesa part # is 562051 - I think they use it on several other amps. Anyway, does anyone have a line on a replacement that has the right mounting screw dimensions (apologies, I forgot to measure these before heading home)? And does anyone know what the primary impedance might be?

    Thanks,

    Paul

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    Lifetime Member Enzo's Avatar
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    What does Mesa want for one?

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    You could probably find a generic Fender OT that will do the job.
    Check the specs of the expensive replacement and find one similiar at a reasonable price.

    I have an old hybrid Peavey Classic with a bad OT.

    Found a few that will do the job without costing too much.

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    Supporting Member The Dude's Avatar
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    ^^^^^^ Yep. You can buy a lot of drill bits for $170.

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    Last edited by The Dude; 10-09-2019 at 04:42 AM.
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    Old Timer Leo_Gnardo's Avatar
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    What Enzo said. ^^^ Call Mesa for a price. They've always come through for me, and kept prices very reasonable. Shipping - I can't help you with. FDX & UPS have the world by the short hairs on that one. OW!

    Just don't call on a Friday. They're all out surfing.

    707 789 3324 Richard Duvall - Service Department - I hope he's still there, I haven't had to call for a couple years.

    If you can't score the OT from Mesa, this ClassicTone oughta do very nicely:

    https://shop.amppartsdirect.com/Proj...1-40-18091.htm

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    Member Wombaticus's Avatar
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    Leo,

    Spoke with Richard -- good advice, and thanks for the number! Unfortunately they want $130 + shipping for the (fairly dinky) mesa OT.

    So... trying again here to get answers to the very specific questions I care about.. does:

    1) anyone happen to know the actual primary impedance of the Mesa transformer?
    2) Does anyone know the mounting hole dimensions? (Yes, I'm trying to save myself a drive down to the workshop).

    Cheers,

    Paul

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Wombaticus View Post
    Leo,

    Spoke with Richard -- good advice, and thanks for the number! Unfortunately they want $130 + shipping for the (fairly dinky) mesa OT.

    So... trying again here to get answers to the very specific questions I care about.. does:

    1) anyone happen to know the actual primary impedance of the Mesa transformer?
    2) Does anyone know the mounting hole dimensions? (Yes, I'm trying to save myself a drive down to the workshop).

    Cheers,

    Paul
    Primary impedance - you should have asked when you called Mesa. Know? I don't, but if it's compatible with both 6L6 and EL34, you could look at other OT's that are recommended for use with those tubes and take an educated guess from them. FWIW I have on occasion installed EL34 in Fender amps, no change of OT, they ran fine and in most cases gave a boost in power around 20%. Maybe someone has an exact answer for you. But I'd say, don't sweat it, find an OT that fits and go with it.

    Mounting dimensions - again, Mesa would have the answer. Unless someone here happens to have a Nomad ready to hand.

    If Nomad's OT is "dinky" as you mention, it's possible the OT I recommended won't fit. Oh well, the search goes on.

    Not many cheap deals available when it comes to transformers, that's the way it is.

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    4k primary impedance would be a good compromise between typical/optimal values for EL34 and 6L6 at 400V to 500V plate voltage.

    But I would like to check with the circuit: Please post schematic.

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    Last edited by Helmholtz; 10-10-2019 at 03:38 PM.
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    Supporting Member Jazz P Bass's Avatar
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    Quote Originally Posted by Helmholtz View Post
    4k primary impedance would be a compromise between typical/optimal values for EL34 and 6L6 at 400V to 500V plate voltage.

    But I would like to check with the circuit: Please post schemattic.
    boogie_nomad55.pdf

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    Unfortunately the link doesn't work.

    I would like to mention that in very rare cases a differently constructed OT having different primary self-capacitance and leakage inductance can cause HF instability at certain amp settings. So I would be willing to pay a little more for the original.

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    Lifetime Member Enzo's Avatar
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    Just thinking about it from the other direction, how much do you think a new OT ought to cost?

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    Quote Originally Posted by Enzo View Post
    Just thinking about it from the other direction, how much do you think a new OT ought to cost?
    You're not asking me?

    (I'd probably order the drop-in Mesa OT and explain to the customer that a cheaper generic OT bears some risk. I've measured Mesa OTs and found them to be good quality. Don't understand what "dinky" means in this context.)

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Helmholtz View Post
    You're not asking me?

    (I'd probably order the drop-in Mesa OT and explain to the customer that a cheaper generic OT bears some risk. I've measured Mesa OTs and found them to be good quality. Don't understand what "dinky" means in this context.)
    Enzo's is a very good question and I'm sure directed at Wombaticus. "Dinky," apparently Mesa's OT looks compact compared to other OT's claiming similar power handling. My view, if there's not enough space on the chassis to fit a larger replacement OT then your only choice is to buy from Mesa.

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    Quote Originally Posted by Helmholtz View Post
    But I would like to check with the circuit: Please post schematic.
    Boogie_nomad55-1.zip

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    Senior Member Pedro Vecino's Avatar
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    It has a somewhat complicated location and is fixed only at two points, but I would measure it to find a substitute.

    Like this for example:

    https://www.hammfg.com/files/parts/pdf/1750M.pdf

    Click image for larger version. 

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ID:	55492

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    Last edited by Pedro Vecino; 10-10-2019 at 08:41 PM.

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    Quote Originally Posted by Dave H View Post
    Thanks! The power stage looks straight forward, even has some HF lag compensation. So I have nothing to add.
    Interesting: For EL34 operation an additional screen filter/dropping resistor is switched in to relieve the pentode screens and increase/match output impedance.

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    Quote Originally Posted by Pedro Vecino View Post
    It has a somewhat complicated location and is fixed only at two points, but I would measure it to find a substitute.

    Like this for example:

    https://www.hammfg.com/files/parts/pdf/1750M.pdf

    Click image for larger version. 

Name:	nom50.JPG 
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    The OT in the picture doesn't look as if it could put out more than 40W before clipping/saturation (similar to a Fender Vibrolux OT).

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  18. #18
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Helmholtz View Post
    The OT in the picture doesn't look as if it could put out more than 40W before clipping/saturation (similar to a Fender Vibrolux OT).
    I agree. Either $155 or 130 is too much for that kind of OT, unless it can't be had elsewhere.

    Back to Amp Parts Direct, we're looking at any of the following ClassicTone models: 40-18006, 40-18105 , 40-18088 all under $60. I've bought iron from them before, it's always worked out fine.

    Mesa makes a habit of boasting about their output power, they must measure at 20-30% distortion. I doubt the Nomad made 55W before clipping, even if you drove it off a cliff, with the wind at its back. That's what I used to say about my 62 Volkswagen, with a speedo that read up to 85 mph. Expect more like 40.

    https://shop.amppartsdirect.com/Transformers_c2.htm

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