Neptune
New Member
- Joined
- Mar 6, 2022
- Messages
- 8
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- 8
Hi guys
Dad offered help (much needed) and we spent the entire morning on the 4104: first of all we repeated the test with the tubes in place, the "darkened" JJ was obviously put in the "affected socket", and we saw that after a minute or so the insides of that tube start to glow very bright, which as you all know is not a good thing: in fact, with the multimeter on, we witnessed the failure of the new fuse that I had put in (not a big problem, I had bought a pack of 10). So, here's what we did after:
No bright glow inside the JJ connected to the affected socket, no fuse blowing, no burning smell---no nothing. And pin 5 measured fine on both sockets.
So, what is/was it, then?
I mean, what is the probability of having a vintage tube (with 35 years of honorable service on its shoulders) fail dramatically, buying a matched pair of new tubes and having the new one connected to the same socket shorted out of the box???
It's beyond absurd, but since everything tested fine inside the amp and now, with different tubes, it seems to work properly I can't think of anything else. Any other ideas?
P.S.: now the plan is to order another matched pair of tubes, putting them in and rebiasing; then, plug a guitar in and see what happens as far as electronics and sound are concerned.
Dad offered help (much needed) and we spent the entire morning on the 4104: first of all we repeated the test with the tubes in place, the "darkened" JJ was obviously put in the "affected socket", and we saw that after a minute or so the insides of that tube start to glow very bright, which as you all know is not a good thing: in fact, with the multimeter on, we witnessed the failure of the new fuse that I had put in (not a big problem, I had bought a pack of 10). So, here's what we did after:
- we checked all the pins of both power tube sockets without the tubes in place and measurements were all fine. Then we put the tubes back (and a new fuse, obviously) and tested them all again, flipping the bypass switch on and off to avoid tube melting and fuse blowing while we were measuring all values in both sockets and we found something strange: with the tubes in place, pin 5 (control grid) of the unaffected socket tested fine (minus 40something) while pin 5 of the affected socket tested zero (!)
- we freed the pcb from the chassis and checked all the capacitors in the power section (first from the soldered pins below the pcb, then we desoldered them one by one and checked them again, then resoldered them): as strange as it may seem, particularly for some of them, even if all are stock from 1986 they were all in perfect working conditions;
- we checked the output transformer and the total output was fine, and equal on both sides going to the sockets;
- we found a resistor that seemed a bit ruined on the D.I. input/output jack (that I never used) but it tested fine;
- we checked the resistors of the power section and they were all good;
No bright glow inside the JJ connected to the affected socket, no fuse blowing, no burning smell---no nothing. And pin 5 measured fine on both sockets.
So, what is/was it, then?
I mean, what is the probability of having a vintage tube (with 35 years of honorable service on its shoulders) fail dramatically, buying a matched pair of new tubes and having the new one connected to the same socket shorted out of the box???
It's beyond absurd, but since everything tested fine inside the amp and now, with different tubes, it seems to work properly I can't think of anything else. Any other ideas?
P.S.: now the plan is to order another matched pair of tubes, putting them in and rebiasing; then, plug a guitar in and see what happens as far as electronics and sound are concerned.