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Old 4th October 2012, 03:02 PM   #11
disco is offline disco  Netherlands
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Something fishy crept in. On high levels there was a sharp edge to the midrage, formerly not heard. The last change was the Tamura B-6004 interstage so I opened the manual: 30Hz to 15KHz within 4dB, output power level 25dBm. I understood the power level having to do with the bandwith, but it seems to do with the onset of distortion. Lacking a professional power meter I exchanged the interstage for other models and finally for a coil and coupling capacitor. The sharp edge was gone (those C-cores are spaced for 50mA) but every coil or interstage brought their own characteristic "sound". Although I could not stand the distortion of the B-6004, I missed that lush low end. So much that I reinstalled and only used its primary. Coupled with an oil cap I have the impression I'm near finishing. The reproduction is at a level I wish everyone could enjoy, a feast to the ears.

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Old 6th October 2012, 06:52 PM   #12
disco is offline disco  Netherlands
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Just added the Coleman Filament Supply to the PX4 so the only unregulated power source now is the 228 volt HT. If you're not familiar with DC heating DHTs with Rods' fruit of labor, then you're in for a shock installing one. One? Buy 'm at a dozen because they're a steal!

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What may you expect, crossing over from the dark side?
From the first seconds I missed that 'romantic' feel about the music, leaving room for imagination. Now small details are on top of the bigger sounds, making the music easy to absorb, very precisely. Thinking about a parallel experience, changing the bed sheets comes to mind (every odd year)
Chapeau sir, wel done!

Disco's off to the woods now, drawing crosses on ancient oaks.
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Old 6th October 2012, 09:09 PM   #13
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Jaap, Thank you - a poetic response is the best possible way to enjoy Direct Heated Triodes!
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Old 6th October 2012, 09:53 PM   #14
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I would second Andy's suggestion of trying 'filament bias' - which is simply another way of employing a Thévenin equivalent circuit.

Then you can say bye-bye to the cathode by-pass.
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Old 7th October 2012, 08:41 PM   #15
disco is offline disco  Netherlands
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The Triode Guild® was a great inspirator for poetry

Well, to get rid of that one pesky electrolytic cap I'd burn an extra 20 watt, if my calculation is right:

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Rb= 24V/1,040A= 23 ohm
P= 29 watt

In the actual setup the filament supply has a rendement of 4.0W over 8.5W, that's a figure I can live with
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Old 12th October 2012, 05:29 PM   #16
disco is offline disco  Netherlands
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I noticed the two loops in the center glow darker than the outer two. Can't remember having seen that before...

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Old 16th October 2012, 12:12 PM   #17
disco is offline disco  Netherlands
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Default chop chop, a new case, a new built.

The PS was built round one salvaged Gardner transformer, offering better regulation and higher supply voltage. Feeding the PX4 filament took a few extra turns of 1mm wire. Unfortunately the PCB of the HV reg needs to be revised for extending with kelvin connections. So it feeds a 30cm remote load via shielded wire, not an ideal situation but compromises had to be made. I found that tinning the PCB traces had a positive influence on temperature dependency. It sets and holds (at 60mA) as a charm.

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The idea was to squeeze all in to a small box, to be acceptable for permanent use in the living room. It measures 31x21cm and is made from solid, fine sanded wood (Finnish birch -Betula pubescens- and Indies Rosewood -Sonokling-) adding on to the delightfull aroma of ancient tubes. The bottom is open for ventilation, the lower front is made of 3mm copper to cool the semi's of the LV regulator. The meter is an indication for HV because the mains has its ups and downs. I could not get rid of that very ugly elektrolyte cap (a 350V Cerafine 2x 220uF, also decoupling the PX4 cathode) because the OPT is taking up most space underneath. The lead cells are to come under the case, in a seperate plinth.

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When under development I wandered between two thoughts: a buffered voltage amplificator versus a 'normal' two stage anode follower. For practical reasons (swing at the PX4) I opted for the last, leaving the cathode follower to be examined perhaps on a later time -it certainly offers its own charms. The KC1 triode multiplies the theoretical 0.7Vrms output level of a CD-player eleven times while the #26 in this setup amplifies three times. Full throttle the PX4 receives circa 30Vrms, a little more than the targetted 60Vpp.

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The first coupling cap reads 0.1Uf, it should be 1.0uF


Reading here and there about problems asociated with the use of type 26 (rattle and humm, microphonic or ringing noise) I experience none at all on the twenty nine 26 tubes I own, from well used thirties globes to unused fifties ST. Without restraint I hold the lead cell accountable for this feat as my circuit is probably like yours.
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Old 16th October 2012, 12:43 PM   #18
iko is offline iko  Canada
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Nice project, as always from you. Incidentally, I also haven't had one problem with any of my 26 tubes. Only heard them in the speakers when I dinged them, but that's to be expected.
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Old 16th October 2012, 12:52 PM   #19
disco is offline disco  Netherlands
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What's your I/O voltage ratio on the 26?
I can't hear them at all, even dinged unrespectfully.
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Old 16th October 2012, 01:01 PM   #20
Cassiel is offline Cassiel  Libya
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Nice.
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