Help with mods

[Amled87]

I've recently been working on an old Marshall AVT 50. It wasn't working when I got it, but I resoldered the input and it's came back to life. While I got it apart, I figured I'd mess with it a little. I'm planning too

1. Add a 0.68uf cathode bypass capacitor to R29.
2. Replace R32 with a 100k resistor.
These two swaps would increase gain from the 12AX7 .

I've also picked up all the Dijon caps from mojotone to replace all the caps in the signal Path. Including the few electrolytics that I need.

There is some clipping LED's labeled LED 2, LED 3, LED 5 & LED 6. I was wondering, would it be ok to remove LED 2&3 and replace with jumpers and then replace LED 6 with 1N4007 diode. Then, I could replace LED 5 with wires going to a On/off/on toggle with a different diode arrangement on the switch?

My main question, would be could I remove LED 2 & LED and replace with jumpers to get rid of some the harsh distortion from the clipping circuit?

For the record, I am aware amps contain high voltage. I am safe, and take all precautions while working inside the amp. I have provided a schematic of the preamp section for reference

 

[musipol]

You can remove led2,3 but the possible clipping from that opamp might sound worse than what you are getting now. You wouldn’t want to replace with jumpers as you stated above.

 

[Amled87]

Why would the op amp be clipping as I'm not changing anything in that circuit but removing the LED's?

 

[grid_stopper]

It seems like those LED's are there to prevent the opamp from getting clipped at the input. The clipping diodes in this amp are LED 5+6

 

[Amled87]

It seems like those LED's are there to prevent the opamp from getting clipped at the input. The clipping diodes in this amp are LED 5+6

I may be wrong, but if they were there to prevent the op amp from clipping wouldn't they be on both inputs of the op amp? They have them connected to pin 6 and the output of the op amp and the o/d gain control it appears?

 

[ccaudle]

would it be ok to remove LED 2&3 and replace with jumpers

Those diodes allow the signal gain to be set by the resistor and variable resistor, but then reduce the gain when the signal hits around 1.5V peak.
If you replace with jumpers you have a direct connection from output to inverting input on an inverting amp topology, which will not amplify at all. If you want to use that op-amp as amplification without clipping remove the diodes, then adjust the resistor values for the gain you want, but do not replace the diodes with a jumper.

 

[Amled87]

Those diodes allow the signal gain to be set by the resistor and variable resistor, but then reduce the gain when the signal hits around 1.5V peak.
If you replace with jumpers you have a direct connection from output to inverting input on an inverting amp topology, which will not amplify at all. If you want to use that op-amp as amplification without clipping remove the diodes, then adjust the resistor values for the gain you want, but do not replace the diodes with a jumper.

So LED 5 & 6 are the clippers then? Could I remove those? I know this thing has clipping diodes in it that can be removed... I want to get rid of some of the diode clipping distortion that makes it harsh sounding.

 

[ruffrecords]

LEDs 5 and 6 are definitely the clipping LEDs.

Cheers

Ian

 

[Amled87]

LEDs 5 and 6 are definitely the clipping LEDs.

Cheers

Ian

Thank you Ian! I'll remove those since I can't remove LED 2 & 3. I've talked to you before a long time ago on here with a different username. You helped with alot of things I was working on at the time. Would you say 10uf would be about right for the cathode bypass capacitor?

 

[grid_stopper]

I may be wrong, but if they were there to prevent the op amp from clipping wouldn't they be on both inputs of the op amp? They have them connected to pin 6 and the output of the op amp and the o/d gain control it appears?

No, the negative terminal is just the NFB loop, so any signal its seeing has already come in through the positive terminal

 

[Amled87]

No, the negative terminal is just the NFB loop, so any signal its seeing has already come in through the positive terminal

Thank you gtid_stopper, you have been a lot of help. So if I'm to remove LED's it appears it'll be 5&6.

 

[grid_stopper]

Thank you gtid_stopper, you have been a lot of help. So if I'm to remove LED's it appears it'll be 5&6.

No problem. The other thing you can try, instead of no clipping diodes is experimenting with different diode types. Looks like theres 3mm LED's in there right now(although i cant say for sure, the text is a bit blurry), which in my experience can sound kind of edgy. sometimes that works, sometimes it doesnt. Germanium, silicone, ect- all worth trying.

 

[Amled87]

No problem. The other thing you can try, instead of no clipping diodes is experimenting with different diode types. Looks like theres 3mm LED's in there right now(although i cant say for sure, the text is a bit blurry), which in my experience can sound kind of edgy. sometimes that works, sometimes it doesnt. Germanium, silicone, ect- all worth trying.

What I'm going to do is change the tone stack, by changing R72 to 33-40k to increase low mids. Also changing C79 from 15nf to 22nf.

Change R30 to 100k, add a 10uf cathode bypass capacitor to R29 and R33 to increase gain from the 12ax7.

I'm then going to replace all of the caps in the audio path except for the electrolytics with Mojotone Dijon film caps, the few electrolytics will be replaced with Elna Simlic electrolytics.

I still want to remove the clipping circuit, this amp has so much gain, I feel the clipping isn't needed.

 

[Amled87]

Also they are in fact red 3mm LED's.

 

[grid_stopper]

If youve got enough gain that the clipping diodes arent needed, then yeah. probably best just to cut them out entirely. let us know how it goes!

 

[Amled87]

If youve got enough gain that the clipping diodes arent needed, then yeah. probably best just to cut them out entirely. let us know how it goes!

Well, I hated up LED 5 & LED 6 and removed them, but now I get no sound whatsoever.

 

[musipol]

Check the resistance between the junction of r34 and r35 and ground. Is it shorted?

 

[Amled87]

Would it be possible for me to hook a preamp pedal to the input of CN3 in this schematic? My idea would be to delete the preamp of this amp all togethe, and use a preamp pedal direct to the input of the power amp input at CN3. The preamp pedal I have in mind, has its own power supply as well.

 

[Khron]

Would it be possible for me to hook a preamp pedal to the input of CN3 in this schematic? My idea would be to delete the preamp of this amp all togethe, and use a preamp pedal direct to the input of the power amp input at CN3. The preamp pedal I have in mind, has its own power supply as well.

That would be pretty much the same as plugging the output of that pedal straight into the amp's Return input.

 

[Amled87]

So if I plug it into the input of the return, the preamp is bypassed? Possible to just remove the PCB all together and maybe use the space for a different PCB circuit of some kind?