Hi Ivanman - these kinds of projects can be really, really frustrating. :cussing: And I have done this more than once...
Because there are things that just don't seem to make sense--what might make the most sense is to go back to the fundamentals of wire routing and checking very closely ALL the soldering that has been done. For example, just two things that look suspicious are solder joints like the one on the 1K 5W screen resistor on pin 4 of V4 and the wire routing to the control grids of V4 and V5. Also having the main filter cap hanging over the input components/V1--probably the most sensitive and vulnerable area of the amp in terms of signal--can contribute to extra noise that could be avoided. You might shorten the leads on the bright cap as it kind of acts as a antenna for noise. With all due respect, you might find that a couple of hours invested just re-routing wires and reflowing all solder joints will eliminate a lot of gremlins--as well as ones laying in wait. (Check out dummyload's Boilermaker as it looks similar in layout)
I could be wrong (and according to my bride, I often am :icon_biggrin: ) but I have found that when it comes to amps, good layout beats good luck nearly every time. Regards
dennis
:laugh: I use either all red, or all yellow for tube hook-ups
If your big filter cap is above V1's parts, I'd un-bolt it, GENTLY hang it outside the chassis, see if that changes anything, very doubtful, but easy.
Done but no result. I try again to trace the input signal in all the preamp stages, but it seems good. At the output of PI the two signals are symmetric (good sinewave, same peak values).
In any case, when the power tubes are not redplating, the sound is quite good, very "Marshallish" and with no particular noises.
Hi Ivanman - these kinds of projects can be really, really frustrating. :cussing: And I have done this more than once...
Because there are things that just don't seem to make sense--what might make the most sense is to go back to the fundamentals of wire routing and checking very closely ALL the soldering that has been done. For example, just two things that look suspicious are solder joints like the one on the 1K 5W screen resistor on pin 4 of V4 and the wire routing to the control grids of V4 and V5. Also having the main filter cap hanging over the input components/V1--probably the most sensitive and vulnerable area of the amp in terms of signal--can contribute to extra noise that could be avoided. You might shorten the leads on the bright cap as it kind of acts as a antenna for noise. With all due respect, you might find that a couple of hours invested just re-routing wires and reflowing all solder joints will eliminate a lot of gremlins--as well as ones laying in wait. (Check out dummyload's Boilermaker as it looks similar in layout)
I could be wrong (and according to my bride, I often am :icon_biggrin: ) but I have found that when it comes to amps, good layout beats good luck nearly every time. Regards
dennis
Today I checked again all the solders, replaced R24-R25-R31-R32 (with this I replaced the whole bias circuit and the grid resistors) and cable to the tubes (with AWG20, solid core) but no results.
I replaced also C23 (the first filter cap) because I had a quite high ripple (4V at idle, 20V at max power). Ripple is eliminated but no changes in redplating.
Measuring at ends of the 1 Ohm resistor I use for biasing, with a signal applied at input (I know is not a real measure, but I use it as an indicator) the redplating starts when the value is 120mA and growth if I raise the input signal (no matter if this is obtained raising the input signal at High input jack, or rotating CW the gain or the master volume).
When the value goes beyond 120mA the tube starts redplating a little at cross of plates, and much more if raise the input up to 200-220mA (max value I measured).
In the mean time I searching on the web about redplating and find this very interesting thread on Metropoulos forum: http://forum.metropoulos.net/viewtopic.php?t=30036
There are a lot of things I haven't the knowledge to understand exactly , but there a couple of comments of Randall Aiken about the too high voltage we apply to the power tubes and the fact that this situation force the tube to work in a critical area where redplating could happen more easily. In fact observe that all my voltages on B+ are quite higher than the original Marshall ones (see attachment).
Thare are also a couple of suggestions in that thread:
- try to test with a load of 16 Ohm but with the selector on 8 Ohm --> no results, redplating still the same
- try to reduce to around 400Vdc the B+ value (at the moment I've around 460Vdc).
What do you think about? Can you suggest a simply way to make this test (it's enough a resistor in series)? Thanks to everybody for your comments and helps.