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-   -   Valve for Marshall AVT275 amp (http://www.diyaudio.com/forums/tubes-valves/157931-valve-marshall-avt275-amp.html)

eyrepm 4th January 2010 07:04 PM

Valve for Marshall AVT275 amp
 
I have this transistor amplifier, Marshall AVT275, with a valve pre-amp. I bought it used over 5 years ago, and I think it's time to sell the amp, and to replace the valve. But, I'm having trouble getting info about it. What I do know is that it is a 12ax7, dual triode. The valve itself says "Marshall VLVE-00041 WK14". Can I get just any 12ax7?

http://bildr.no/image/560777.jpeg
http://bildr.no/image/560776.jpeg

http://bildr.no/image/560775.jpeg
Amp with power on. Not much glow. Lit room.

http://bildr.no/image/560774.jpeg
Amp with power on. Un-lit room.

Thanks in advance :)

llwhtt 4th January 2010 07:13 PM

Any 12AX7, 7025, ECC83 will do. The number on the Marshall tube is their stock or part number. Is the original one bad?

Craig

eyrepm 4th January 2010 07:17 PM

I think the amp sounds bad... It's tiresome to play on. And it's hard to get any satisfying sounds on it. Which is why I'm selling it, but if a new tube is cheap enough, it will hopefully be easier to sell :) I am green as can be with these things, but I've read pre-amp valves last for about 4 years. Other places say just one. They have almost no glow, either. As you can see.

Rod Coleman 4th January 2010 07:39 PM

The AVT275 runs these gently, and I never found one that sounded better with a new 12AX7 (=ECC83). But they are cheap enough to be worth trying.

More important, there is a frequent failure on AVTs that has turned up here regularly. Two resistors R202 & R223 (both 2.2 Ohm, 1 Watt parts) are given to failing high value, or open. This happens a lot if the fans get obstructed (like parking the amp on a pub seat with the backrest blocking the vents).

The heater goes dim or even out when this happens. check for 12v across valve pins 4 to 5.

It's a 15-minute fix if this happens to yours, and the parts cost less than a drink. Take it to a tech, though - it may take a while just to find the parts on the back-PCB, if you don't know where to start.

If you do know, follow the wires from the transformer to the back PCB. The two that go to W106 & W107 will lead you to the resistors, which can be checked in situ for value, or just changed anyhow.

eyrepm 4th January 2010 07:44 PM

Thank you for the answers! It might just be a bad amp in general. I will take it to a tech, though.

sengineer 12th June 2012 11:02 AM

Hi eyrepm!

I have an AVT 275 that I love playing. The OD's of course do not compare to a Full valve amp but I love the clean sound. I play it in combination with a Mesa V-twin and a Boss-MT2 pedal for the High-Gain sounds.
I try to have the input signal low, the channel volume low and the Master Volume up to 2 o'clock, that makes the Amp sound great to my ears.

I just serviced the fans because these seem to be a weak point with the amp. I replaced both fans with high-grade fans and just ordered new tung sol tubes. I will post you what the new tube sounds to me.

What do you recon after changing the tube?


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