This circuit might appear in other, earlier tube manuals but someone has put it on the internet as coming from RC-23. You should notice from leafing through the Circuits section of an RCA tube manual that they often publish "circuit chunks" which aren't complete systems on their own. Like a Phono Preamp, but a different circuit is needed to incorporate tone controls, and yet a different circuit needed for a power output stage with a driver and phase inverter.
Sometimes RCA tells you what output you get for a specified input, but they didn't do that here. So we have to look at the circuit.
1st stage is a cathode follower so you'll get no voltage gain there (actually a slight loss). Last stage is also a cathode follower, so again no gain there. In between, you have a gain stage, tone controls, gain stage and volume control.
RCA doesn't give pin voltages, but quick plotting of Rk and Rl on a set of 12AU7 curves suggests low-current operation of the 2nd stage (~2mA) and a
data sheet indicates internal plate resistance will be ~12.5kΩ at that current for ~70v plate-to-cathode (supply voltage 270v - 200v across 100kΩ plate load). The tone controls after the 2nd stage look more or less like a 298kΩ load to the 2nd stage, in parallel with the 100kΩ plate load (~75kΩ total). Data sheet curves on page 3 suggests Amplification Factor may only be 16.5-17 for this relatively low-voltage, low-current condition. These numbers don't account for voltage drop in the power supply due to the 1st, 3rd or 4th stage plate currents (sorry, but this process is recursive enough already).
Gain for 2nd stage will be about μ*[Rl/(Rl+ra)] = 16.5*[75kΩ*/(75kΩ+12.5kΩ)] = ~14. Cathode resistor is fully bypassed, so this estimated gain is not reduced by local negative feedback.
Source impedance to tone circuit is around 11kΩ considering 12.5k internal plate resistance in parallel with 100kΩ (maybe a tiny bit higher because I haven't factored in the effect of the cathode resistor). Applying circuit values and calculated source impedance to the James model in the Duncan Tone Stack Calculator, mid-band loss at 1kHz is -19dB with controls set mid-way, though bass has a significant emphasis. Regardless, that mid-band loss should be the basis for calculation.
-19dB converts to about a 1:9 loss. So 2nd stage gain is ~14, loss of ~1/9th in the tone circuit (midband, controls set halfway), overall gain is only 5. Really, the 2nd stage is only keeping gain near unity or a mild loss after you consider sweeping the controls below their mid-point.
3rd stage plate current appears similar to what I (incompletely) calculated for the 2nd stage, so Amplification Factor and internal plate resistance should be similar. Plate load is 100kΩ, in parallel with a 100kΩ Volume control for 50kΩ overall load.
Gain for 2nd stage will be about μ*[Rl/(Rl+ra)] = 16.5*[50kΩ*/(50kΩ+12.5kΩ)] = ~13. Cathode resistor is fully bypassed, so this estimated gain is not reduced by local negative feedback.
But an audio taper pot set mid-way is a 1/10th loss, so the 3rd gain stage leaves overall signal level just above unity here as well.
Even if you max'd the Volume control, you should only count on an overall gain of 5 * 13 = 65. Not accounting for cathode follower losses, you'd need 16.5vac / 65 = 250mV of input signal. That might be a bit much, and the overall amp is likely to never distort with a guitar input (and may not be fully driven, either).
I won't re-calculate yet for dropping 12AX7's in the circuit; however, you should know that while the amplification factor is a lot higher, the loads in the circuit (tone controls, volume control) are small enough to seriously drag-down a 12AX7's gain, giving much less amplification than what you'd expect.
Overall, this looks a lot like the typical RCA pattern I noted at the start: this is a circuit to add tone controls and a volume control to an existing complete preamp/power amp while adding minimal additional signal loss. If you want to swipe this circuit, you'll need additional preamplification and possibly a driver circuit for your output tube after the final cathode follower.
Topic: preamp gain ? (Read 5318 times)