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08-16-2019, 01:19 PM
Hi
You have an early printing of TUT5 - the two 30k1's for each of Q10,11 were replaced by a single 30k1 from C to B on each BJT.Also note that the NPNs drawn earlier was a mistake and that these should be PNPs with C to V- and E to the bias pots. Everything works as it should.
Relay sticking can be caused by various things, including the contact welding you mentioned. What have you done on the coil side?
The coil voltage is zero when not energised, with both ends sitting at the supply voltage. Upon closing a switch for the switched end of the coil, the supply is across the coil and the relay contacts have switched. When you open the coil switch, there will be a very high transient voltage on the free end of the coil caused by inductive flyback, which for a 12V coil could easily be 80Vpk.
Normally a diode is placed across the coil to absorb this energy and clamp the voltage across the coil. However, even though this is the most common snubber method used, it is far from the best as it restricts how fast the contacts can move. Using a zener diode across the coil control element allows one to allow a specific transient height. For example, a 36V zener would allow a 24V transient given a 12V supply.
One can also use a cap or series-RC as a snubber across the control element, but these do not work as well as the zener.
Regarding Rk and Ck: Rk has to be sized for the average heat over the audio cycle. An unbypassed Rk will have transient heat higher than its rating but that is okay if the avrage is within the rating. Personally, i use Rk values that set the idle power to much less than the full rating of the tubes and TUT5 maintains that protocol except when one tube of the pair is pulled, then the remaining tube will run at its full rating. DO NOT TRY THIS IN OEM CATHODE BIASED AMPS UNLESS YOU INCREASE Rk AS IS DONE HERE !!
Ck is there to keep bias stable, and as TUT4 informs, is the first form of "fixed bias".
have fun
You have an early printing of TUT5 - the two 30k1's for each of Q10,11 were replaced by a single 30k1 from C to B on each BJT.Also note that the NPNs drawn earlier was a mistake and that these should be PNPs with C to V- and E to the bias pots. Everything works as it should.
Relay sticking can be caused by various things, including the contact welding you mentioned. What have you done on the coil side?
The coil voltage is zero when not energised, with both ends sitting at the supply voltage. Upon closing a switch for the switched end of the coil, the supply is across the coil and the relay contacts have switched. When you open the coil switch, there will be a very high transient voltage on the free end of the coil caused by inductive flyback, which for a 12V coil could easily be 80Vpk.
Normally a diode is placed across the coil to absorb this energy and clamp the voltage across the coil. However, even though this is the most common snubber method used, it is far from the best as it restricts how fast the contacts can move. Using a zener diode across the coil control element allows one to allow a specific transient height. For example, a 36V zener would allow a 24V transient given a 12V supply.
One can also use a cap or series-RC as a snubber across the control element, but these do not work as well as the zener.
Regarding Rk and Ck: Rk has to be sized for the average heat over the audio cycle. An unbypassed Rk will have transient heat higher than its rating but that is okay if the avrage is within the rating. Personally, i use Rk values that set the idle power to much less than the full rating of the tubes and TUT5 maintains that protocol except when one tube of the pair is pulled, then the remaining tube will run at its full rating. DO NOT TRY THIS IN OEM CATHODE BIASED AMPS UNLESS YOU INCREASE Rk AS IS DONE HERE !!
Ck is there to keep bias stable, and as TUT4 informs, is the first form of "fixed bias".
have fun
