Earth tweed amp PI help

[goldstache]

Having some issues biasing the grounded base PI on this Earth amp.
It’s the 50w version identical to the 100w schematic attached.


So here’s the scenario.
Q6/Q7 bias voltages are all similar on the transistor pins when energized.
Results in no pass condition.


When I dangle my DMM in resistance mode on the lower volt HT side of the 10k base biasing rail, collector voltage rises and amp passes signal. (Red to supply side, black to chassis)


Aside from soldering in my DMM and calling it done. Lol
What options do I have and why does it bias with a leak path there???


All of PI has been rebuilt, actives and passives. So I don’t think it’s a component issue.
Any insight? Mega thanks.
-joe

[shooter]

guessing you don't have a good connection somewhere, cold solder joint, sorta thing, probably at a "common" tie point for multiple parts.


lots of quality time with a wood stick, poking all the parts along both the HT and ground side of the "path" might yield something, play happy music, just in case it was a wasted effort   

[goldstache]

Thanks for chiming in.
To no avail, yet……..


The issue was the same before I rebuilt the PI. So I’m pretty sure it’s my understanding of the grounded base referenced PI (AC reference) that’s also faulty.
How does this dang thing work????


Some notes:


When PI transistors are not passing all pins are around the same voltage 20 or so volts.


When I hang the magic amp starter (dmm in resistance setting, in this case). Emitter/Base bias about 10v apart. And Collector starts soaring upward on its way to higher voltage and headroom.


This is with the dmm red probe on the supply rail side of the 10K’s heading to base of transistors, black probe to chassis.


None of the chassis points, 0v points in the amp/circuit show more that .6ohm R.


When it isn’t passing.
Oddly, the wiper of the master volume pot has no signal (it’s E cap isolated, minus - to wiper of pot!?!?)


When the dmm is a probing’, I have signal at that spot. (Amp passes)


All actives/passives are fresh from new master volume pot>grid R’s of power tubes.


I’m betting it’s a DC biasing issue. But know very little about this SS PI topology, I’m at a loss.


Any thoughts, fire away!

[sluckey]

Looks like Gibson engineers were hired to draw that schematic.

I think it's a bad idea to poke a live amp with ohm meter probes!

[goldstache]

Yeah it translates, in the chassis, to a RVT era gibson as well. Lol


RE: Ohm meter in a live amp.
Can’t argue there.


I wasn’t just plopping the test leads any old place. I was primarily interested in the base voltage of the first transistor, and was gatorclipping in resistance to ground in the form of a divider to shift stage bias. Trying to see if I could get the setup to pass signal, as it seemed biased off by the voltages at the pins.


https://music-electronics-forum.com/filedata/fetch?id=840360


This Peavey has a similar setup going on. Though simpler maybe?