Changing Output Tubes.
As mentioned on the first page of this article Tim wanted to use 7591A tubes in the amplifier but that ran him over budget. Instead he used Russian 6P3S-E tubes. All test data to this point has been taken with these tubes in the amplifier. But now we are going to change them to type 7591A.But there is a small problem. The pinout of the 7591A is different from that of the 6P3S-E.
Pinout of 6P3S-E and 7591.
Element 6P3S-E 7591A Heater 2 & 7 2 & 7 Cathode & Grid 3 8 5 Grid 1 5 6 Grid 2 4 4 & 8 Plate 3 3 Rather than rewire the sockets I am going to make adapters. I have blank octal tube bases and sockets so I don't have to buy anything. Also Tim recommends changing the 15 k ohm resistor that connects between ground and the ends of all 4 bias pots. The new value is 3.9 k ohms. It is right at the bottom a little left of center on the power supply schematic.
Tim advised me to change the feedback resistor to 3.6 k ohms but to my sorrow I forgot to do it. Everything seemed alright but I found that it went into oscillation when driven above 10 watts. The change was made.
Measured loop gain (AB) with 2.4 k ohm resistor = 12.75.
AB = 12.75, A = 12.75/B,
B = Rk / (Rk + Rf) = 180 / (180 + 2400) = 0.0698
A = 12.75/0.0698 = 183.
To set AB = 9 we have
AB = 9
B = 9/A = 9/183 = 0.0492.
B = Rk / (Rk + Rf)
Solving for Rf,
B(Rk + Rf) = Rk
B Rk + B Rf = Rk
B Rf = Rk - B Rk
Rf = (Rk - B Rk) / B
Factoring out Rk from the numerator gives,
Rf = Rk(1 - B) / B
Rf = 180(1 - 0.0492) / 0.0492 = 3.48 k ohms.
Let's go with Tim's recommendation of 3.6 k ohms.
Clead = 1 / (2 pi 3600 x 94.4 kHz) = 468 pf.
New value of cap in parallel with feedback resistor: 470 pf.
The cathode current of each tube was set at 46 mA in accordance with a data sheet downloaded from Franks Electron Tube Pages.
Frequency response -3 dB from 1000 Hz.
Less than 10 Hz to 21,000 Hz.Power before visible distortion of peak.
18.6 watts weakest channel, one channel driven.
Let's call it a 16 watt amplifier. That makes a nice binary sequence for power levels.THD
THD at 1000 Hz
Power (W) THD (%) (L) THD (%) (R) 16 0.70 0.60 8 0.48 0.34 4 0.195 0.185 2 0.079 0.088 1 0.037 0.044 Distortion at various frequencies for 8 watts
Note: I would like to use a lower power for this but my history has locked me in to half of rated power. I think a more realistic power level would be 10% to 25% of rated power. I will likely change this in the future.
Frequency (Hz)THD (%) (L) THD (%) (R) 20 0.88 0.44 30 0.43 0.295 40 0.315 0.26 50 0.295 0.235 100 0.290 0.230 1,000 0.57 0.35 2,000 0.96 0.525 5,000 1.05 0.86 7,500 1.75 1.25 10,000 2.10 2.78 15,000 3.9 3.2 20,000 7.6 4.6 I think it is time to present verification of my instrumentation. It is as much for me as it is for you. If at some future date I have doubts about the distortion readings of the audio oscillator I can verify them against this permanent record.
Readings with Generator Connected Directly to THD Meter.
Frequency (Hz) THD (%) 20 0.054 30 0.04 40 0.036 50 0.034 100 0.028 1,000 0.03 1,000 * 0.026 * 2,000 0.026 5,000 0.026 7,500 0.052 10,000 0.056 15,000 0.058 20,000 0.058 * Generator on X100 range dial on 10. Quieter Meter on Analyzer.
IMD
IMD with 60 and 7,000 Hz, 4 to 1 amplitude ratio.
Power (W) IM (%) (L) IM (%) (R) 8 11 3.85 4 1.45 1.20 2 0.64 0.635 1 0.33 0.32 Conclusion.
It seems pretty clear that the much maligned Russian 6P3S-E tubes out performed the highly rated 7591AEH. Both Tim and I are at a loss to account for the difference. If you decide to build this amplifier we, Tim and I, don't recommend changing the tubes. After all, it was designed with the 6P3S-E tubes and they give the best performance.Introduction, Circuit, and Specifications.
Distortion Testing.
Changing Output Tubes. You are here.
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This page last updated July 11, 2013.