/diy/ - Do It Yourself

Hello /ohm/

I bought a cheap, handbuild guitar amplifier (schematic: pic related).
It basically works, except at high distortion levels (aka 'gain' or 'preamp volume') the sound cuts out in an unpleasant way. The distortion "vanishes" and the output volume level drops for as long as there is *no* new note being played. So it is somehow like an inversed compressor: When playing the guitar loud and hard, the sound is there, but as the note decays the sound suddenly drops by a big margin. There is no smooth transition between clean and distorted sound.
I don't know how to describe the problem in any better way and that is probably why I have not found anything on the googles.

What I have done in the process of trying to fix that damn thing without success is:
* rebuilt the whole amp and used new components except: power transformer, output transformer, choke, powertubes and filtercaps
* swapped preamp tubes
* used crocodile clips to insert a temporary post-phase-inverter-master-volume. I used the most basic one that I could think of: right behind the 22n capacitor after V3 I inserted a potentiometer that, when turned counterclockwise, cancels the signal and thereby reduces the output volume. This does not change the sound that much, but the feel of the amp. However, the problem of dropping from distortion into almost not hearable persists. I therefore conclude that the problem does not lie within output transformer and powertubes.

Maybe someone could help me google the problem, that would be of big help to me, or maybe someone knows this kind of problem and a fix.

hello /ohm/

I bought a cheap old guitar amp from the 80ies (a Marshall JCM800 model 2204). I got it really cheap, way below usual prices, so it does not basically not matter what I have to invest in new parts. Transformers, tubes and chassis are in a very good condition.

First of all: the amp work seems to work. There is sound, it is loud, maybe bit noisy.
However, studying the schematics (pic related), I found that mine has a strange power supply. The setup for this amp should be: 50u+50u reservoir cap
to outputtransformer and choke, choke to to 50u cap, powertubes and dropping resistor, dropping resistor to 50u cap, phase inverter and another dropping resistor, that one to another 50u cap, second preamp tube and a last dropping resistor, and form that last dropping resistor to a last 50u cap and the first tube. (see schematic)

But mine is strange: 50u+50u cap to OT and choke, choke to 50u+50u, powertubes and dropping resistor, dropping resistor to 50u, phase inverter tube and a dropping resistor, and that dropping resistor goes to a last 50u cap which supplies both V1 and V2.

I figure that this basically raises plate voltage on the first tubes while lowering the overall filtering of their power supply.

Are there any other side effects of this? Why would anyone change it to this configuration? Does this change its overdrive characterists? (in a negative way, which means LESS overdrive, as that is what this amp was built for: overdriving the preamp)

Should I convert it back to original specs? I guess this change was made when the amp was recapped, but for what reason? Is this some kind of common modification to tube amps?

I am really new to tube amps, any help is highly appreciated.

Marshall JCM 800 Lead 50w version

this

Marshall JCM 800 2204 50W