Recently there was a debate between our fellows at our local DIY forum.
One guy came up with a typical PP schematic without filter choke on B+ line and claimed that besides being the output device, each half of the OPT also serves as B+ filter for its power tube. He even brought up an old local technical book with similar statement, however without any further explanation.
This is clearly incorrect for me, but I couldnt find some clear and persuasive silencers to point out that the OPT does not have any filter action but the hum/noise cancelling exists only because this is a PP stage.
Can you guys help?
Great thanks,
Duong
One guy came up with a typical PP schematic without filter choke on B+ line and claimed that besides being the output device, each half of the OPT also serves as B+ filter for its power tube. He even brought up an old local technical book with similar statement, however without any further explanation.
This is clearly incorrect for me, but I couldnt find some clear and persuasive silencers to point out that the OPT does not have any filter action but the hum/noise cancelling exists only because this is a PP stage.
Can you guys help?
Great thanks,
Duong
Yes, a valve PP output gives some ripple cancellation because it is PP but the ripple is common-mode. This is not because the OPT acts as a smoothing choke!
Your friend may be confused by hum cancellation tricks sometimes used in domestic radio receivers. A tap on the OPT was used to reduce hum.
Your friend may be confused by hum cancellation tricks sometimes used in domestic radio receivers. A tap on the OPT was used to reduce hum.
i have lots of Poor quality PA output transformers used with PP KT66.
Wondered if you could use them as chokes.?
Phil
Wondered if you could use them as chokes.?
Phil
@DF96: and from the tap on the OPT it goes on to power the low-power circuitry (IF, mixer) in these receivers. In a PP amp the transformer is only the plate load of the output tubes, nothing else. There is no filtering action from the OPT. Filtering is done by the push-pull action of the circuit. The OPT trick might work in a cheapskate SE amp 😀
Tjerk, 9ZZ
Tjerk, 9ZZ
Can you enlighten me on this? I'm not so familiar with this hum reduce trick.
Regarding using the KT66 PP OPT as filter choke; I think the answer is yes but since the OPT has no air gap, it should be used in a way so that each half supplies for a channel. As long as current comsumptions are equal/similar for both channel, this method can give adequate result. However since the transformer was used for KT66, current rating wont be high.
Duong
You can take measurement of ripple value (in mV ac) at B+ and Plate for 2 cases in which both power tubes run and one of them runs (remove one power tube). So I think you get the answer.
Note: doing this if 2 power tubes run with separate cathode resistor.
Will you do ?
Note: doing this if 2 power tubes run with separate cathode resistor.
Will you do ?
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Hi "bác sĩ", nice to see you here!
This wont do because if you pull out one powertubes, the core of th OPT will be saturated by the DC current of the other tube and the measurements wont make sense.
However I intend to do some measurements on a SE circuit, this will remove the hum cancelling effect which happens on PP and make the results clearer.
Any other ideas?
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