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?
Amp with power on. Not much glow. Lit room.
Amp with power on. Un-lit room.
Thanks in advance 🙂
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Amp with power on. Not much glow. Lit room.
An externally hosted image should be here but it was not working when we last tested it.
Amp with power on. Un-lit room.
Thanks in advance 🙂
Any 12AX7, 7025, ECC83 will do. The number on the Marshall tube is their stock or part number. Is the original one bad?
Craig
Craig
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.
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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.
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.
Thank you for the answers! It might just be a bad amp in general. I will take it to a tech, though.
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?
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?
Hi, this thread was a life saver. R223 had gone to about 4k. The fans still turned which threw me but the STANDBY was not being released. I also noticed an undocumented TR105 (J174) which acts as a mute for the A amplifier input only. This FET circuit divides R216 from C212 into two parts 1.5k and 330 ohm. It pulls the junction of these to ground unless there is sufficient voltage on the standby circuit.
Glad you have it working again. Marshall document things better than most, even though they do have a few design failure s like this.