Would that be testing pins 1/8 (they're tied together) to ground? I get 10 ohms there. (Ground connection is via auto-bias board.)
probably not for the heaters. The original Dynaco design had the CT at AC ground through two capacitors.
Would it make sense to one-at-a-time remove things from the star ground and see if the hum changes? (Other than the wire that goes to the power cord ground?)
OK, this is probably a dumb question: How many ohms should I get from the positive speaker terminal to ground?
Pins 2 and 7 are the heater pins on EL34. Pin 1 is Grid 3 and 8 the Cathode. You would expect 10 Ohms between the Cathode and ground due to the sensing resistors on the auto-bias.
If you are talking about the resistance between the 0 and 8 Ohm taps on the output you will have a fraction of an Ohm.
ok, that's the cathode....so where would I test for heater ground? (I doubt that's the issue but I'll take all suggestions right now!)
Indeed I have 0.4-0.5 ohm between the 8-ohm tap and ground.
I am just at a loss as to what to try now.
Ross,
I believe @nerdorama and @KeesB meant the AC grounding of the twee heater coils via two capacitors. Nerdorama spotted a potential missing connection as he explained in post #25.
Please check the resistance from your PCB ground to that star ground AND to the chassis. It appears that you have a new stainless steel chassis, and those had been known to be problematic in making a good ground connection with.
OK, this will be very challenging because of how things are attached to the amp right now but maybe I have to find a way. Are the capacitors you're describing the film ones on the main board rather than electrolytics?
The two green caps where the filament center tap wires are connected. Upper right in the picture I remember.
Hi there,
So those two capacitors are connected to the terminal strip so that one leg of each is connected to an outer terminal and the other leg of each (thus two legs) are connected to the center terminal. The brown with stripe is connected to one of the outside terminals and the green with stripe is connected to the other outside terminal. No wire is attached to the center. Are we talking about the same thing...just below the little rectangular PCB by the left channel RCA and speaker posts?
Not yet. Need to make sure I understand which caps to remove. And so far after multiple checks I have not found any wiring error as far as connecting particular wires to wrong pins of tubes but I'll check again.
It’s odd you are having so much trouble. I’ve done a couple of VTA based St70’s for friends and they were dead quiet on 105-107dB horns.
Oh, it's not a new chassis. It's from the 1960s! Just in great condition.
When I test, for example, ground from a circuit board to the ground pin of the plug that goes into the wall, I get no resistance on the ohmmeter so I think my ground connection on the chassis is OK.
man, i know. I really didn't expect this problem. I haven't built an amp before but I built a phono preamp that was nearly this complicated (with fewer wires though because tubes were all on PCBs) and it worked perfectly the first time.
I'm really at a loss. No idea what to try. I spent almost a month building this slowly and carefully and clearly messed something up (or have a bad part, but that seems less likely) and I'm unbelievably frustrated.
OK, so Francois told me to try something and I got what might be an interesting result:
So I don't know if this is an important datapoint but removing the center 6SN7 tube eliminates the hum. Does this give any of you experts ideas of what I should be looking for at this point? Does this datapoint eliminate things like ungrounded power tube heaters as possible hum sources, and many of the other things we've been looking at?
What do you all recommend I look at now?
p.s. I also tried swapping tubes to make sure it wasn't a problem with the actual tube that was in that spot and it made no difference...no hum without center tube.
So I don't know if this is an important datapoint but removing the center 6SN7 tube eliminates the hum. Does this give any of you experts ideas of what I should be looking for at this point? Does this datapoint eliminate things like ungrounded power tube heaters as possible hum sources, and many of the other things we've been looking at?
What do you all recommend I look at now?
p.s. I also tried swapping tubes to make sure it wasn't a problem with the actual tube that was in that spot and it made no difference...no hum without center tube.
Pulling the center tube breaks the feedback path. You might still have a phasing issue with the output transformer connections or the phase inverter connections to the output tubes. I had the same issue with one of the auto bias boards in my Mk2 Dynacos. The wiring diagram has the connections from the autobias board to the output tubes reversed and they were out of phase.