Phase Inverter Resistors

Circuits

In <3gbfo7$k--(at)--ewsbf02.news.aol.com>, omarnep--(at)--ol.com (OmarNepo) writes:
>In a 59 Tweed Bassman, why are the resistors feeding the plates of the
>phase inverter not equal (ie both 100k)? One is 82k and the other is 100k.
>Is this due to the difference in the transformer windings of side of the
>push-pull. Gerald Weber mentioned something about the difference in his
>article on x-formers.
>
>Later
>Omar

Transformer are different for Class A, but that is not the reason for the
plate resistor difference. For a phase inverter of this type,
the half driven from the tone stack sees an input signal equivalent to the
tone stack output minus the cathode resistor feedback. The other half sees
just the signal from the cathode feedback, which is a slightly smaller
signal, about 20% less (its inverted as well, since the signal is on the
cathode rather than the grid). This makes the current swing in the driven
half 20% larger. Since the current through the tube sections is relatively
a function of the input and independent of the plate resistors, the plate
resistor values are adjusted so the the voltage swing in each section is
the same.

The unbalanced output effect of a phase inverter is greater with a small
cathode feedback resistor, which the Bassman's 10k ohm is. Some Vox amps
have a large 47k ohm cathode feedback resistor. The cathode feedback is
high enough to neglect the unbalance, so the plate resistors are
equal (100k). In fact one input drives one side of the inverter, the other
input drives the other. The high amount of cathode feedback does limit the
voltage gain available in the inverter.

Regards,
Max

Index:

AB crossover	Amp Classes Defined	Amp Classes Explained	Amp Mods Harmful
Bass Cutoff Frequencies	Bias adjust safeguard	Bias Cathode Resistors	Bias Shift
Biasing methods	Biasing	Channel Switching LDRs	Choke Input Filter
Circuit Memory	circuits	Drop B+	Eyelet board
Fender Response Curves	FET source follower	Filament Cathode Ground	Fixed to Cathode Bias
FM Transmitter Voltages	Grid Blocking	Identify Power Trans Leads	Load Impedance
LV No PC boards	Output impedances	PC Boards	PCB amp problems
Phase Inverter Resistors	Pilot Light Tone	Rectifier voltage drops	Resistor Design
Resistor Max Voltage	Resistor noise	Reverb Tank Types	Reverb tanks
Screen Grids	Standby Switch Ratings	Standby switches	TAME Digest
Tone Circuits	Transformer Design Book	Transformer Info	Transformer Placement
Zener Voltage Drop