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Old 10th November 2013, 05:29 PM   #1
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Join Date: Nov 2013
Default Marshall AVT20 repair and mods

As I mentioned in my 'hello' post, I came across this forum when searching for a schematic for the Marshall AVT20 transistor combo. The schematic is attached to this thread.

I bought the amp with the reverb known not to be working. Having seen some gut shots, I thought one of the push-fit connectors might have come loose, but a check with a meter showed that the output transducer had gone open circuit. It is on the left in the picture:

Click the image to open in full size.

My next job is to drill out the rivets securing the spring tank so that I can get in close and see if I can locate and fix the broken wire. Failing that, the tank is an Accutronics, coded 8DB2E1D, the closest match that I can source is the 8DB2C1D. This has the same input and output impedances (2nd and 3rd characters) and will fit within the chassis.

The amp has the typical Marshall sound. I have left the back of the cab off, after removing it to disconnect the speaker, as I prefer the sound.

I also plan to try a 12AT7 in place of the 12AX7 fitted, and try removing the diode pair from the second op-amp gain stage. This should give me a more usable gain range - I don't need face-melting distortion!

I'll keep y'all posted
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Old 14th November 2013, 12:05 PM   #2
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Join Date: Nov 2013
Well I removed the clipping diodes, a pair of red LEDS, but the overdrive was still a bit mushy-sounding. I swapped out the Marshall-badged 12AX7 for a JJ 12AT7 and this has made a huge difference. Distortion still becomes audible at about the same point in the gain range, but increasing the gain further produces a sound with a much more solid body and less top-end mush. Very pleased! Just waiting for the new reverb tank to arrive now...
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Old 15th November 2013, 02:55 AM   #3
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Location: Philadelphia, PA
does that amp have PCB mounted tube sockets? I think a good "mod" for durability and longevity would be to replace that/them with a/several chassis mounted tube socket(s). It wouldn't be too difficult if you had a chassis punch and the space, and could make a world's difference in the longevity of the amp...
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Old 15th November 2013, 08:40 AM   #4
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The amp is mainly solid state, with just one tube in the pre-amp section. All the pots (6) and sockets (5) are board-mounted, but the board is also supported by 3 metal stand-offs, one of which is quite close to the tube socket. Having changed the 12AX7 (ECC83) for a 12AT7 (ECC81) I am unlikely to change that tube again unless it dies on me. Also I picked up the amp for 49 ($79) mainly for home and jam use, so reliability is not a major worry!

You're right though, tube sockets on circuit boards are a recipe for failure - swapping tubes requires a fair amount of force, flexing and stressing the PCB, and sooner or later one of the traces will develop a hard-to-find hairline crack.
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Old 17th November 2013, 08:53 PM   #5
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Join Date: Nov 2013
New reverb tank fitted, sounds lush! The guts are the same as the original, but it's mounted in an earthed metal channel on 4 shock-absorbing springs, and the chassis bolts go through squishy rubber grommets. I 'locked' the nuts with heat shrink tubing.

Here's the result:

Click the image to open in full size.
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