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OK, sorry it's been awhile but I've now tried 2 methods on another amp, which uses p-p 6L6 into 4k2:8 ohm OT @ static VB+ of abpout 425V (GZ34 rectifier, 47uF reservoir cap), no global NFB.
Using the method in post#1 (run amp at low level 4Vrms into an open circuit, then load it until the output voltage drops to 2V), it comes to ~ 90ohms.
Then using 'compare output into 8 ohm with output into 7 ohms' method. The 7 ohms was achieved by using a 56 ohm resistor in parallel with the regular 8 ohm load.
I checked it at 3 signal levels:
4Vrms (both 6L6 conducting for full cycle) ~ 104 ohms
18Vrms (max clean output before hitting the rails) ~ 51 ohms (the 7 ohm load didn't result in saturation)
EDIT 13Vrms (21 watts@8 ohms ~ half power) ~ 51 ohms
25Vrms (max square wave) ~ 25 ohms
Quite interesting and unexpected! I thought that the output impedance at low levels (ie when both tubes were conducting for the full 360, the 'class A' bit of 'class AB') would be lower than when they would be in cut off some some of the cycle (ie the 'class B' part of 'class AB); but it's the other way around.
I guess that the '25 ohms output impedance at max square wave output' is related to the power supply impedance + OT primary impedance + 6L6 plate impedance at saturation? It's lower than I expected.
Does anyone care to give an analysis of what's going on here?
Thanks - Pete
Using the method in post#1 (run amp at low level 4Vrms into an open circuit, then load it until the output voltage drops to 2V), it comes to ~ 90ohms.
Then using 'compare output into 8 ohm with output into 7 ohms' method. The 7 ohms was achieved by using a 56 ohm resistor in parallel with the regular 8 ohm load.
I checked it at 3 signal levels:
4Vrms (both 6L6 conducting for full cycle) ~ 104 ohms
18Vrms (max clean output before hitting the rails) ~ 51 ohms (the 7 ohm load didn't result in saturation)
EDIT 13Vrms (21 watts@8 ohms ~ half power) ~ 51 ohms
25Vrms (max square wave) ~ 25 ohms
Quite interesting and unexpected! I thought that the output impedance at low levels (ie when both tubes were conducting for the full 360, the 'class A' bit of 'class AB') would be lower than when they would be in cut off some some of the cycle (ie the 'class B' part of 'class AB); but it's the other way around.
I guess that the '25 ohms output impedance at max square wave output' is related to the power supply impedance + OT primary impedance + 6L6 plate impedance at saturation? It's lower than I expected.
Does anyone care to give an analysis of what's going on here?
Thanks - Pete
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