Well I don't know this, as I haven't completed the amp and tested it, but I can see this is the way it is made and I GUESS that it is the reason why they made it that way
And, as I wrote in the text in the same post, I belive the reason why they made it like that is to be able to have equal value anode resistors in the two halves of the 12bh7's.
In your schematic, the "left" tube of LTP has negative voltage on grid, the right tube's grid DC grounded.
In this case the anode current difference is significant, and gain of LTP parts is different.
The "classical" LTP has connected grids (via large resistor) and right side is AC grounded via capacitor.
As noted, I am not the Guy that is constructing this, I only reconstruct what Mactone has done. On the other hand, I still think the reason for the (small) negative grid voltage on on of the grids on the long tailed phasesplitter is to avoid the otherwise necessary different anode resistor. I haven't completed the amp, so I can't test it yet, but eventually I will do that and report back. I can find no other reason for applying the negative voltage to the input grid of the LTP.
Take a look at the schematic in post 24 , here the grids in the LTP are at the same potential, and then it is necessary to make the two anode load resistors different.
In the Mactone 300B the anode resistors are equal, but at the same time , they add a small negative voltage to one grid. I do not yet know if that is a better solution, but it could be that the balance in the LTP is better this way , although it complicates the circuit .
The same holds true with the negative voltage as return path for the common chatode resistor in the LTP, it complicates things and requires a capacitor between first stage and LTP, but the voltage swing to drive the 300B is greater, as far as I can see.
Testing the amp will tell if it is a good choice.
Take a look at the schematic in post 24 , here the grids in the LTP are at the same potential, and then it is necessary to make the two anode load resistors different.
In the Mactone 300B the anode resistors are equal, but at the same time , they add a small negative voltage to one grid. I do not yet know if that is a better solution, but it could be that the balance in the LTP is better this way , although it complicates the circuit .
The same holds true with the negative voltage as return path for the common chatode resistor in the LTP, it complicates things and requires a capacitor between first stage and LTP, but the voltage swing to drive the 300B is greater, as far as I can see.
Testing the amp will tell if it is a good choice.
"the (small) negative grid voltage on on of the grids on the long tailed phasesplitter"
Hmmmm... 14.7V is anything but small.
Hmmmm... 14.7V is anything but small.
I am not sure about the value of the common cathode resistor. This is taken from the picture of the MA-300B and it is not very clear.
I even think that there has maybe been tampered with values of the resistors in the - bias circuit. I can see that the guy , who took out the components in this amp, has put in some wrong values and wrong places in the filament/cathode circuit for the 300B´s. Not even the same in the 2 channels. What I have put into the schematic is from the MA-300B picture and not what was in this amp. If you look at the pictures of this amp, you will see how wrong it is.
where did you get the -85 v as bias2? 70V ac is about 99V dc and there is a 350 ohm resistor in series with bias 2. If the current in the 2 two 12BH7 is 26 ma there is 52 mA going through the 350 Ohm resulting in a 18,2 V drop. That is 80,8 V not 85V and probably lower as I don't calculate with losses in the rectifiers. Will that change anything in the simulation?
Dear friends, there is a problem in this part. If I have seen correctly, the resistors of both ecc82 triods are in the irposition of connecting to the negative!!!Is there a clearer picture?
Essentially no.
The rough DC imbalance (due to the left grid large negative and right grid zero DC voltage) exists in the same way.
Is the first stage AC coupled to the second (LTP)?
If yes, the first stage's DC behaviour does not affect the LTP balance.
BTW in the wreck the PSUs are possible functional, could be measured the LTP grid voltages even without tubes.
If yes, the first stage's DC behaviour does not affect the LTP balance.
BTW in the wreck the PSUs are possible functional, could be measured the LTP grid voltages even without tubes.
No the cathode resistors from ECC82 goes to the middle of the three terminal strip, this is GND and the 2 blue wires that comes from the selector switch goes also to the cathodes of ECC82.
The left of the three terminal strip are the bottom of the two potmeters and this goes through the 2 KOhm resistor to - bias circuit. The anode resistor is as you show.