Signal generator.
Hi Saurav,
Burn a test signal CD and play it on your cd player through your older amp ( or build a small chip one ) and use the speaker output with a protective resistor - say 100 ohms - in series with the output. You have a readymade signal generator in your bedroom.
Cheers.
Hi Saurav,
Burn a test signal CD and play it on your cd player through your older amp ( or build a small chip one ) and use the speaker output with a protective resistor - say 100 ohms - in series with the output. You have a readymade signal generator in your bedroom.
Cheers.
> switch from an 8 ohm to a 4 ohm load resistor
I like this trick best.
Personally, I use no-load and rated-load. Measure the 8Ω tap, well below clipping, open-circuit and then with an 8Ω resistor.
However as noted, some tube amps act real different (or are unstable) with no-load. 16/8Ω on the 8Ω tap should be good, although it forces you to measure a small difference.
I have not seen a 8Ω resistor as inductive as the average SET transformer, but carbon or non-inductive would be needed for high precision.
Do run a sweep: 30Hz, 100Hz, 1KHz, 5KHz, 10KHz, 15KHz; and see how flat the impedance is. For most SETs the impedance will droop in deep-bass and rise/wobble at the top of the audio band.
In calculation: look-up Rp or estimate it from the plate curves. Add the tranny primary resistance. Divide by the tranny impedance ratio. Add the tranny secondary resistance. That's the mid-band output impedance (without feedback; divide by feedback ratio for estimate Zout with feedback).
If you have not picked the transformer yet: for Hi-Fi response the DC resistance on each winding will be at least 5% of the rated impedance, and in SET design it could be 10%. Primary and secondary DC resistance tend to be about equal (when adjusted for transformer impedance ratio).
So a "5K:8Ω" tranny could have 250 to 500Ω DC in the primary, 0.4 to 0.8Ω DCR in the secondary.
The higher DCR, in good SET design, go with improved bass response. Yes, the deeper the bass the more power is "wasted". No free lunch.
Note that this can mean that: even if you had a triode with ZERO plate resistance, the Damping Factor might be only 10 or even 5.
I like this trick best.
Personally, I use no-load and rated-load. Measure the 8Ω tap, well below clipping, open-circuit and then with an 8Ω resistor.
However as noted, some tube amps act real different (or are unstable) with no-load. 16/8Ω on the 8Ω tap should be good, although it forces you to measure a small difference.
I have not seen a 8Ω resistor as inductive as the average SET transformer, but carbon or non-inductive would be needed for high precision.
Do run a sweep: 30Hz, 100Hz, 1KHz, 5KHz, 10KHz, 15KHz; and see how flat the impedance is. For most SETs the impedance will droop in deep-bass and rise/wobble at the top of the audio band.
In calculation: look-up Rp or estimate it from the plate curves. Add the tranny primary resistance. Divide by the tranny impedance ratio. Add the tranny secondary resistance. That's the mid-band output impedance (without feedback; divide by feedback ratio for estimate Zout with feedback).
If you have not picked the transformer yet: for Hi-Fi response the DC resistance on each winding will be at least 5% of the rated impedance, and in SET design it could be 10%. Primary and secondary DC resistance tend to be about equal (when adjusted for transformer impedance ratio).
So a "5K:8Ω" tranny could have 250 to 500Ω DC in the primary, 0.4 to 0.8Ω DCR in the secondary.
The higher DCR, in good SET design, go with improved bass response. Yes, the deeper the bass the more power is "wasted". No free lunch.
Note that this can mean that: even if you had a triode with ZERO plate resistance, the Damping Factor might be only 10 or even 5.
PRR said:
Which is a good reason to use pentodes and add 20dB or so of NFB afterwards; thus you are also freed of the nonlinear NFB inherent in triodes which produces more distortion for a given amount of NFB.
Tim
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