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Matamp Microphonic?


Billy Apple
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My GT200 has been making a high-pitched noise of late, and when I turn it off there is a crack through the cab. I spoke to Andrew yesterday at the factory and he thought it maybe a pre-amp valve that has gone microphonic, and the crack is it discharging when turning off. He talked me through the process of check for microphonic valves, so that's what I've been up to today. Thought I'd post up the process and my findings...

Firstly get the head out from the sleeve. Matamp recommend hanging one side over a bench and undoing the two bolts underneath....



Turn head round, do the other side and ease the amp out holding handles at the front...



Once amp is out, take a few snaps for BC!..



Check underside for rodent and insect infestation..



Take the pre-amp valve shields off by pushing down and turning to the left. Then hook up your cab and plug in and switch on. Andrew recomends to take to vol half way and the gain down. No need to plug the bass in.. Then take a moment to check the valve-glow..



Once we are up and running, one by one gently tap the pre-amp valves. If any are microphonic then you will hear an echoing sound through the cab..




It was at this point I discovered that non of my valves are affected, and no sound came through the cab. However I did find a noise coming from the transformer on the right as you look at the front. It was also warmer than the one on the left as it ran for a while. Don't know if this is significant, but sound, tone and volume are unaffected. Just the noise and the crack when shut down.

[u][b]TOP TIP[/b][/u]

Use the opportunity for having the amp out the sleeve to give it a good clean, as it's so blinkin' easy to handle!




I'm heading up North on Monday, so will call by the factory and see what they think.

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[quote name='umph' timestamp='1381155101' post='2235043']
Take it to the factory, sounds likesomething is leaking dc somewhere!
[/quote]

I spoke to Jeff yesterday and told him about the noisy transformer. He was offering advice about how to check it, but it was already in the back of my mates car going up the M1. My pal should have it there Tuesday morning, and I should get it back later in the week as I'm up there in business. Hopefully will be resolved quickly, but once again Matamp have been extremely helpful (even on a Sunday).

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The amp spent the today in the factory. Matamp could not really pin point the crack I could hear, and they thought the humming noise from the transformer nothing to be concerned about. They replaced the power switch and re-soldered a few bits and pieces they thought looked dry. It was only pushing 160 watts and seeing as the power valves are knocking on 3 years there was talk of replacing them. But, each power valve was tested individually and found to be OK. The drop in power was traced to a duff pre-amp valve which was replaced. The power is back up and I've been saved a new quartet of KT88's for the time being.

Anyway, I'm collecting from my mates on Thursday, and hopefully that's it for the time being.

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[quote name='Billy Apple' timestamp='1381263996' post='2236784']
The amp spent the today in the factory. Matamp could not really pin point the crack I could hear, and they thought the humming noise from the transformer nothing to be concerned about. They replaced the power switch and re-soldered a few bits and pieces they thought looked dry. It was only pushing 160 watts and seeing as the power valves are knocking on 3 years there was talk of replacing them. But, each power valve was tested individually and found to be OK. The drop in power was traced to a duff pre-amp valve which was replaced. The power is back up and I've been saved a new quartet of KT88's for the time being.

Anyway, I'm collecting from my mates on Thursday, and hopefully that's it for the time being.
[/quote]

they only do about 156watts anyway

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[quote name='umph' timestamp='1381445922' post='2239408']
had them on zee benchh, it's the same as the orange 200watter. They don't have the proper drive or OT's
[/quote]

Thought the 82 or 81 could push the current to sort the power, and the Orange problem was down to using 12AX7 for everything because cheaper.

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[quote name='umph' timestamp='1381445922' post='2239408']
had them on zee benchh, it's the same as the orange 200watter. They don't have the proper drive or OT's
[/quote]

The last time I was at the factory they showed me the meter reading on mine which showed it was running 200 watts. This time it was at 160 until they changed the pre amp valve and now it is at 180. The 180 was attributed to the power valves being 3 years old. Maybe the one you had one the bench had old valves too.

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[quote name='Mr. Foxen' timestamp='1381497806' post='2239900']
At what distortion level is the thing. Valves are easy enough to check if they are giving the right power or not.
[/quote]

They always (with me) check at number 1 drive (out of 4). Hence the 200 watts clean claim.

BTW Matamp are no longer using JJ Tesla KT88's in as power valves due to quality issues. Now they're installing Gold Lion.

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