Caps coming out of phase inverter value

[danotron]

The capacitors coming out of the phase inverter into the power tubes on 183 are 0.05uF and on 102 are 0.01uF.....what difference does this make? Tone? Function?

[10thTx]

Post LTPI caps are .1 not .01.

Larger caps will allow different frequencies more bass to pass thru signal chain.

With respect, 10thtx

[tubeswell]

Each coupling cap forms a C/R divider filter in conjunction with the grid leak resistor of the following output tube, to produce a hi-pass filter, where the frequency rolloff (at the rate of 20dB/decade) is determined by f= 1/(2Pii x R x C) where 2Pii = 44/7, R = resistance (in Ohms), C = capacitance (in Farads).

Say you had a 0.1uF cap and 220k grid leak: 1/[(44/7) x 220,000 x 0.0000001] = 7.2Hz. This passes all but the lowest of the low frequencies. Whereas if you used 0.01uF, the rolloff would occur below 72Hz

But that is not all. This C/R filter acts in conjunction with the miller capacitance of the following (output) stage to result in an overall bandpass filter effect. If the following stage is a pentode or tetrode, this doesn't make much difference (to the preceding hi-pass filter), because the miller capacitance is very low due to the effect of the screen grid. But if you are using a triode output stage (or a pentode or tetrode operating in 'triode mode'), the additional roll-off of higher frequencies is more noticeable.

[talbany]

Yes changing the output couplers from .1 to .05 will raise the -3db knee, however, the main reason is to prevent the output tube grids from being driven to the point of conduction. (Blocking Distortion).Which EL-34's are susceptible too. If you don't plan on running the amp at high volume levels you can generally get away with the lower value couplers just know that when you reach the point where the amp begins to "fart out" that's usually the onset of blocking distortion!. Negative feedback can also make the problem worse. When the output tube goes into cutoff, the negative feedback loop opens up, and the gain of the phase inverter stage increases. This increase in gain pushes the amp even farther into cutoff.
This is why the later Plexi's loaded with 34's went with the .02's to help speed up the time constant and of course, keeps the amp tight at high volumes due to the raised knee

BTW. Sometimes raising the value of the grid blockers can also help
Tony

[danotron]

Thanks, guys...great info....great explanations...I feel more informed now...

[Roe]

Vox went as low as 10nf on the ac30/4