Hi there,
I decided to try my hand in at implementing UL in my 807 parafeed amp. It uses mains toroidal transformers as output transformers and the parafeed cap is on the cathode leg of the transformer, and so it sits at a 300V potential. I thought I would try the UL but had worries about the DC in the toroidal causing saturation.
I hooked the screen up to the 110V tap of the toroidal (50%) through a 1K resistor. The good news is that it worked and plays a lot louder than before (it was triode strapped originally). There is no audable sign of saturation even with very heavy bass. I will get the scope out and see if there is anything visable. The sound is a lot cleaner than the triode mode, though the tone has moved away from the soft warmth of triode.
I tried it with only a 100R grid resistor, but found the sound a bit strident with to much attack. I also tried a 4K7 resistor, which gave a much softer sound but when driven hard there was significant distortion in the bass.
I'am surprised that this worked at all. One thing in the 807's favour is that it draws significantly less screen current than most other tetrode valves. I would guess that I am probably getting double the usable output power with a cleaner overall presentation and no hint of loss of bass.
Just though this might interest those playing with parafeed amps.
Shoog
I decided to try my hand in at implementing UL in my 807 parafeed amp. It uses mains toroidal transformers as output transformers and the parafeed cap is on the cathode leg of the transformer, and so it sits at a 300V potential. I thought I would try the UL but had worries about the DC in the toroidal causing saturation.
I hooked the screen up to the 110V tap of the toroidal (50%) through a 1K resistor. The good news is that it worked and plays a lot louder than before (it was triode strapped originally). There is no audable sign of saturation even with very heavy bass. I will get the scope out and see if there is anything visable. The sound is a lot cleaner than the triode mode, though the tone has moved away from the soft warmth of triode.
I tried it with only a 100R grid resistor, but found the sound a bit strident with to much attack. I also tried a 4K7 resistor, which gave a much softer sound but when driven hard there was significant distortion in the bass.
I'am surprised that this worked at all. One thing in the 807's favour is that it draws significantly less screen current than most other tetrode valves. I would guess that I am probably getting double the usable output power with a cleaner overall presentation and no hint of loss of bass.
Just though this might interest those playing with parafeed amps.
Shoog
Shoog said:
Hi there,
807's brilliant tubes ,..HF equiv to 6L6 class.
Your analysis of the the distortion and sound is tricky subjectively but perhaps for others some brevity explained to how the transformer reacts.
Intermodulation and harmonic distortion in the output stage due to overloading at LF equals too low primary inductance .. Basically effective load Z is reduced and becomes reactive to LF response. Graphically, the anode current and volts become mismatched and the load curve goes from elliptical to more of a circle.
In other ways, the same effect can be produced by non linearity of mag field v.s current in the transformer....solution keep Bmax down.
Harmonic thd can also be produced by too high primary resistance.
Take your pick ! .....however as you suspect LF thd is pretty high, perhaps alot of 2nd harmonic which may give the impression of a fuller bass. This is used alot in the music industry to make smaller L Systems sound fuller......but makes sound more artificial. I allow myself to be corrected on this.
As you know this output tranny business is a murderous conflict of many parameters.
Using a toroidal mains tranny as an o/p tranny, I would assume the leakage inductance between windings to be reasonable, as the winding machine would make some attempt to properly equalise all windings round the core. The primary would be the winding that would cause the trouble as it isn't sandwiched between secondaries and far more turns is required often resulting in overlap and leakage flux spew out. The main problem is the leakage capacitance, this would show up on a square wave test at 10Khz. Do it and observe. I would like to know what you find. The amp might go unstable with ringing.
Standard mains toroids don't bother about leakage capacitance and no neutralisation of the windings is done. The exception is HV types where excess leakage inductance could occur resulting in a resonance condition with the leakage capacitance..result very high voltages.
Enough!
richj
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