I just breadboarded a KT88/6550 PP using a Talema 120VA 115+115 primary and 2x12V secondary (secondary windings were paralleled). (this mains transformer will give reflected primary Z of 2.9K, I can't find a 120VA with 9V secondary).
The driver is a 6n1p and phase inversion is accomplished using a small autoformer or center tapped choke. (just like the Seth amp at magnequest).
The two KT88/6550 shares a cathode resistor of 280R and bypassed with 220uf electrolytic cap. The power tubes are triode strapped since I don't have UL taps on the toroids.
This amp sound so good that I am having my winder wind me a ptx for a stereo (I was using the ptx for my SE KT88).
I am also planning to remove some secondary windings so that I will have a reflected primary Z of 5K (this might further improve the sound).
Any ideas/thoughts/or criticism of this topology?
The driver is a 6n1p and phase inversion is accomplished using a small autoformer or center tapped choke. (just like the Seth amp at magnequest).
The two KT88/6550 shares a cathode resistor of 280R and bypassed with 220uf electrolytic cap. The power tubes are triode strapped since I don't have UL taps on the toroids.
This amp sound so good that I am having my winder wind me a ptx for a stereo (I was using the ptx for my SE KT88).
I am also planning to remove some secondary windings so that I will have a reflected primary Z of 5K (this might further improve the sound).
Any ideas/thoughts/or criticism of this topology?
This is the sort of setup I have, but with 6080's as the outputs. I went from toroidal interstage phase splitting to center tapped and found that an improvement.
It is so easy to unwind some of the windings on the outputs. Just strip off the plastic cover, hook it up to the mains (preferably via an isolation transformer) and check the voltage, take off 10 winding and measure the voltage again. I found with mine that each 10 windings represented about 1.2V Carry on stripping until you get your nine volts.
Shoog
It is so easy to unwind some of the windings on the outputs. Just strip off the plastic cover, hook it up to the mains (preferably via an isolation transformer) and check the voltage, take off 10 winding and measure the voltage again. I found with mine that each 10 windings represented about 1.2V Carry on stripping until you get your nine volts.
Shoog
Hi Shoog, what is the correct way of determining the size of the toroidal transformer used for PP OPT?
I read somewhere that it should be 4X the power (so if the power output of the amp is 20 watts, the toroidal should be around 80VA). In another thread, I read that it should be around 10X.
Am using 120VA for a PP KT88 with expected power output of 20 watts. Should I get 200VA or my 120VA enough?
Although the sound I am getting now is excellent, I sense some flabbiness in the bass (typical of KT88 power tubes?). It might also be a sign that I need to balance the current draw of the power tubes (LM317 like you have implemented is already out of the question, cathode voltage hovers around 39VDC, LM317 does not like anything over 40VDC).
Thanks for your work Shoog, I am enjoying my parafeed 6080 using toroidal OPT immensely.
I read somewhere that it should be 4X the power (so if the power output of the amp is 20 watts, the toroidal should be around 80VA). In another thread, I read that it should be around 10X.
Am using 120VA for a PP KT88 with expected power output of 20 watts. Should I get 200VA or my 120VA enough?
Although the sound I am getting now is excellent, I sense some flabbiness in the bass (typical of KT88 power tubes?). It might also be a sign that I need to balance the current draw of the power tubes (LM317 like you have implemented is already out of the question, cathode voltage hovers around 39VDC, LM317 does not like anything over 40VDC).
Thanks for your work Shoog, I am enjoying my parafeed 6080 using toroidal OPT immensely.
I think the minimum sizing on the outputs is about 4x. All of my builds so far have been more like 10x. I have encountered absolutely no penalty with top end extension on going so high. I have also found my transformers to be surprisingly tolerant of DC (which surprised me). Still I absolutely recommend balancing DC to below 1mA, It is possible to extend the voltage of the Lm317 and Broskie tells you how in his original article (its either a series resistor or a zener).
In my latest project I had to use the ring of two CCS, as used in the Baby Huey, because of voltage issues. Using a MJE340 allows you to cope with 300V on the cathode. This is a really simple circuit to build though heat is the real issue when you go much above 20V.
I would say that your bass will tighten up if you go over to CCS bias.
Shoog
In my latest project I had to use the ring of two CCS, as used in the Baby Huey, because of voltage issues. Using a MJE340 allows you to cope with 300V on the cathode. This is a really simple circuit to build though heat is the real issue when you go much above 20V.
I would say that your bass will tighten up if you go over to CCS bias.
Shoog
I paralleled the two triodes of the 6n1p and WOW! Bass is a lot tighter!
Quite hard to describe, but the 120VA toroidal mains transformer that I used as PP OPT sounded better than an Acrosound 330 PP OPT.
I am using a center tapped choke as phase inverter. Since the toroidal's primary does not have UL taps, I triode strapped the KT88.
It sounded more like my SE KT88 than a PP KT88.
Quite hard to describe, but the 120VA toroidal mains transformer that I used as PP OPT sounded better than an Acrosound 330 PP OPT.
I am using a center tapped choke as phase inverter. Since the toroidal's primary does not have UL taps, I triode strapped the KT88.
It sounded more like my SE KT88 than a PP KT88.
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