MT20 Cutting out even with 16ohm speaker

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jarthur
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MT20 Cutting out even with 16ohm speaker

Post by jarthur » Sun Dec 30, 2018 4:33 pm

I got this 1st gen MT20 from someone who gave up on it. Before I do as well, I thought I'd ask here. I know tube amps fairly well and have repaired and build a few. But I'm not familiar with these transistor types. I have tried this in various speaker configurations (4, 8, 16ohm). It works on low volumes/gains, but cuts out immediately if over a certain output. (i can't even get above noon on both dials unless I strum very lightly). Headphone output works great.

I'm guessing something is overheating, but I'm not sure why. I am going to order some thermal paste and clean/reapply to the heat sink (some of it seamed to have "melted" down along the pcb). But if anyone has any ideas on other possible solutions, I would really be grateful.

Thanks in advance,
John

jontheid
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Re: MT20 Cutting out even with 16ohm speaker

Post by jontheid » Sun Dec 30, 2018 11:39 pm

What power supply are you using with it? The original supply?

Jondog
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Re: MT20 Cutting out even with 16ohm speaker

Post by Jondog » Mon Dec 31, 2018 3:14 am

jontheid wrote:
Sun Dec 30, 2018 11:39 pm
What power supply are you using with it? The original supply?
Wasn’t there an issue with the originals that couldn’t supply enough current?
Image

jarthur
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Re: MT20 Cutting out even with 16ohm speaker

Post by jarthur » Mon Dec 31, 2018 5:20 am

jontheid wrote:
Sun Dec 30, 2018 11:39 pm
What power supply are you using with it? The original supply?
I believe I have the original judging from the photos on amazon here: https://www.amazon.com/Orange-Amplifica ... merReviews

I ordered a 2nd cheap charger off amazon that is 19v 2.37a. I had read that the difference would be fine, but that did't work well at all (kept cutting off and on). Is there a better supply to get the oem one? Didn't want to risk $40 on something that won't make a difference.

Rlw59
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Re: MT20 Cutting out even with 16ohm speaker

Post by Rlw59 » Wed Jan 02, 2019 4:05 pm

The problem with the stock power supply still exists. 15V/2A = 30W, which is marginal for a 20WRMS amp with a tube filament and Class AB conversion loss.

It only manifests itself with the amp dimed into a 4Ω load. And even then, your style of playing and ambient temp may let you play with no cutting out (if you leave space in your playing where the PS gets chances to cool down).

Unless the PS has malfunctioned it should have no problems when the amp is driving an 8 or 16Ω load.

(They "fixed" the problem by relabeling the amp's output jack as "8Ω minimum", with a note in the manual that you can use a 4Ω load with the risk of cutting out.)
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Pretty sure the problem is in the amp.

The STA540 output chip is inexpensive and readily available from lots of sources. I'd probably start by replacing it.
Last edited by Rlw59 on Thu Jan 03, 2019 1:42 pm, edited 1 time in total.

jarthur
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Re: MT20 Cutting out even with 16ohm speaker

Post by jarthur » Thu Jan 03, 2019 5:35 am

Good idea, I'll start with the STA540 replacement. Thanks!

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