The Amp Garage

Is it normal, that treble pot has a little effect compared to my jcm 800:s treble pot wich changes higer frequences strongly?

Yes maybe i must play with it for a while. I agree that treble caps do sound diffrent. I first put there silver micas but i like ceramic disc better. With micas the higer frequenses comes tight and sharp. With ceramic discs highs come moore ear frendly bit softer and mushier. Best would be something...

What is the meaning of output grid resistors? Early marshalls do not have those.
What happends if i leave them off?

I might find the broblem. It was bad solderjoint. There is alot variations of plexis circuit. I like it this way coupling caps 0.022 after first stage 4,7n bright cap on volume pot 0,68yF V2 bypass cap 47k NFB resistor 0.68yF cap on the presence control Filtering 40+40 40+40 32+32 Thanks all for help!

Oh I see.. what is missing in your amp's tone then? I am sorry if I miss lead you. It is difficult to explain exactly what i mean. i do not know the right word for it. Sound should be like a wall that is balanced. Mine sounds like some frequences (mostly high frequences) is coming out of the speake...

I have Celestion G12M speakers in Harley Benton 4X12 cab. The cabinet may not be the best. It is made of quite thin polywood, maybe the cabinet with thicker wood can be better choice.

I will try that 56k slope resistor. That boxy/chesty sound is that i like.

Here is the layout wich is with same part values as mine. I have tried various diffrent tubes. Best that i was found is GT12ax7 m V1 tung-sol V2 JJ ecc803 V3 and power tubes jj el34. I must measure the B+ voltage i don´t remember it. Power tubes pin 3 has a 433V Can i drop the B+ voltage with out ch...

Hi! I have built this amp and it sounds little different like orginal. Orginal sound is little bigger and more 3-dimensional. This is more rounder, dull and "on your face". I have done lots of experimenting different caps and resistor values but nothig helps this broplem. I have classictone transfor...

Finally i found the source that oscillation! OT:s secondary ground wire was too close OT primary wires. Now i can turn all knobs at noon and amp is quiet.

i tried swap those wires put they where right. There is no effect if i push the primary wires down. Here is couple pictures of it.

I will try swap the primary wires. I recall that they first was wrong way and then i changed them. I will post some pictures tomorrow. I tried diffrent phase inverter tube and it not squeeled until other pots where up. Could really michrophonic tube squeels in that place? That is really sad that all...

Now my express squeels, even at all controlls zero but precense! I have been moving vires etc but it is still there. I dont know what to do next. I changed my OT while ago and first it was normal, exept now.

Thank you!
This was wery helpful, now i will try these with my own amp.

Hi! I've found it helpful to think of the lead dress in terms of what is negative feedback and what is positive feedback. The plate is out of phase with the cathode and grid so if you run those parallel, you're stabilizing the amp. Avoid positive feedback by crossing at 90 degree angles, etc. I am a...

When testing oscillation with amp dimed is the guitar plugged? Or is it just the amp without guitar cable and guitar?