Hello again … .. I have a problem that I noticed these days, I do not know if I had it or now I saw it ,,, first I have a marshall jcm800 clone where I recently did some mods .. so what is the problem .. without having a cable at the input. when I raise the master volume it makes a hum. not possible but noticeably understandable and the stronger the master volume, the stronger it becomes ..I tried and removed a tube from the pre amp section and it sounds absolutely nothing ,,, this means that the problem has to do with the preamp and not with power section α I also noticed that of the 3 tubes of the preamplifier, one does not light well enough (in the filaments) ie while the other two seem to light up strongly, this one does not light well enough)I change the position of tubes and the poor light have the same tube .. .. and something else .. if I connect the guitar and turn up the master volume while I have turned off the guitar potentiometer, again the buzzer sounds loudly ..to emphasize that the mod I did was to put cathode bypass capacitor to cold clipper .. these ,, I just said to ask for your help before I open it once more ..and something else . I can’t find 12ax7 to test other tubes , here in Greece because you know the Ukraine war ..thank you
Attachments
Today , before starting my work . I put off the chassis from the head cabinet and the power valves out off sockets , with my audio probe I starting to tracing with nothing plug-in … from the beginning of the signal did not make any noise. I tried to go upside down, from the master volume. and from pin 1 of the MV POT I could hear the noise α I continued backwards .. the noise is heard in all the pots of the tone stack ,, I did not have time for another tracking… I will continue in the afternoon ..
New update ..without plugin , I put only v1 and v2 ( without phase valve ) . I tracing the signal with audio probe and the noise start to pin 1 of v2 … I put off the v1 and the noise gone ! It’s weird ! Because the v1 have NO noise on all the pins and the circuit the same ,,,I make lot of changes with valves but the noise keep the same …I thinking now maybe it’s the hi-lo input switch . I make this amp without 2 inputs ( hi and lo ) and I make a switch … here’s the photos
Attachments
And new update : with v1 and v2 on socket when I hit with pencil the v1 , listen the microphonic hit , and when I hit the v2 , nothing , but if reverse v1 and v2 the microphonic issue is on the v1 socket again ! So my theory is : the problem start from input , but I can’t listen with audio probe from v1 and when the signal goes to v2 and amplified them I listen ,,,, any ideas ?
Good morning ,,, I forgot to telling I don’t use shielded wires for input ( the schematic states that shielded wires are needed) one from the input goes to pin 7 v1 and other from pin 2 v1 goes to preamp pot … because I could not find such a cable in Greece and I had to order. but I could not wait for it to come, I made this type of cable, of course from what I understood it does not work like shielded, because as soon as I shake it it sounds like a microphone ,,,, so now I'm sure that's probably the problem… I will remove the hi-lo switch and rebuild the wiring with hi input only… I will try to pass these 2 wires carefully without having but wires next to them and the circuit also…
It’s just the same hum or plug-in Jack or not ,, maybe some noise pass through my anshielded wires and after amplifiered,,,,,
It’s not exactly hum , it’s like radio frequencies / noise ,, .. look , with audio probe this signal start to hearing from the pin 1 of valve 2 … the valve 1 has no noise at all pins ( 1-2-3 and 6-7-8 ) but have microphonic issue . I.E. when I hit with my finger I hear huge noise on audio probe , but the valve 2 has no microphonic issue .. but if I reverse the valves the Noise will always be heard in valve socket number one,,,,
Not sure I undwrstand. Do you say that the noise starts at pin 1 of valve (socket) 2, but if you change the valves, the noise starts at valve (socket) 1?
The noise follows the valve?
Jan
The noise follows the valve?
Jan
Not sure I undwrstand. Do you say that the noise starts at pin 1 of valve (socket) 2, but if you change the valves, the noise starts at valve (socket) 1?
ΝΟ .. with audio probe the noise ( on audio probe start to listen on v2 pin 1 ) but the microphonic issues stay only to the socket 1 , whatever valve if I change ,,,
New update 🙂 ….I replace the socket 1 ( pins 2-6 ) with shielded wires with bare wire close to Jack and preamp pot . I remove the hi-lo switch and I make the circuit only for hi input .. and now without plug-in , i turn the master volume and nothing I hearing 😀 , only after 3-4 o’clock something I hear .. but it’s fine … but the microphonic issue on socket 1 remains … now something other problem appear . On the cold clipping switch ( 10K and 2k2 ) when it’s on the 10k with full preamp pot , it’s fine .. but when I switch to the 2k2 it’s like very very over distortion sound with radio frequencies .. but this solved with 4k7 instead 2k2 ,,,, tomorrow I try to make lot space between those cables …
Ok last update … I found all problems and I solve it … first of all the microphonic issue is from cold clipper cathode capacitor .. when I remove it , the microphonics gone ..I use 680 NF CAP but lower maybe has no problem , but for now I’m thinking not use it 🙂 .. second : I return the cathode cold clipper to 10k but I’m think to use 4k7 . and the last one must be use on these amps shielded cables on the first gain stages of 1st valve ,, it’s important … really I’m so happy with my amp and I happy why I solve the problems and I understand that ….