OK. Yesterday, i had some free time in the studio so i wanted to go a bit deep on the 5687 problems i´ve had with all four tubes in. After i put the dual regulated LM350K power supply, i wanted to put all tubes back (at first, i thought that my problem was underheated tubes). Then, i adjusted heaters voltage to 6.3v with all tubes in. No luck. The PM is still behaving weird with the 4 tubes. Weird ? i mean, interaction and VU dancing when moving controls.
Sorry for my english, i´ll try to explain what i did as clearly as possible, maybe this can help anyone with similar problems :
So, i read somewhere in this topic, to adjust a bit the 136v supply to make it more stable....no luck. I raised the 136 rail a bit (it was a couple volts low) with no luck. I lowered the voltage with no luck....so, probably, my problem isn´t related to the 136v PSU and/or the 6BC8´s. Now what ? as i haven´t any more 5687 tubes to test, i decided to measure the 5687 plate voltages, in both halves of T4 transformer, with 2 tubes on, and 4 tubes on, and when 4 tubes on, i measured both when the VU reacted to any change in the controls, and when the VU is stable and in 0VU.
Now, with only 2 5687 tubes on (the way my PM it´s working quite nice), i have 245 volts on the center tap of the transformer, and the same voltage on both taps.
With 4 5687 tubes on, and the VU somewhat stable, i have 245 volts on the center tap, and the other taps are measuring something like 245-230. Fiddling with the controls, i found a position where the VU´s are dancing around -12 or so, then i measured again and again. inverting taps on T4 give me somewhat different voltage readings, as well as greater variations, but one thing remains common. It is always one of the two taps feeding the plates, which is showing a big voltage unbalance from 245v. The other tap is always close th 245v.
So i think i must have extremely mismatched 5687 tubes, right ? Since i got my PM working with only 2 5687 tubes, i suspected from these tubes. I´ll have to wait some time to get more (and hopefully matched) 5687´s and try again. And i´m still very happy with my comp. Right now, i´m working on a "sidechain protoboard" idea i´m veroboarding.
The idea is to have easy access to different RC combinations, and the idea is that i can easily change resistors and caps, as in a protoboard. I´ll post some pics when it´s done, installed and working.
BTW, i think my power cabling on the PM is under-rated. Im redoing this, too.
Best regards.