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:
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
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:
An externally hosted image should be here but it was not working when we last tested it.
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
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...
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...
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.
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.
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:
Here's the result:
An externally hosted image should be here but it was not working when we last tested it.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.