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Old 11th December 2009, 10:32 AM   #261
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I used a transformer with dual secondary windings of each 5V/1A>>

I've known hum to happen when there was not sufficient capacity in the transformer. For a 1A filament I would be looking at a 2A secondary, or as close as possible to that.

May not be relevant, but I'd try a different filament transformer with bigger capacity and see if that changes anything.

andy
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Old 11th December 2009, 10:50 AM   #262
martyh is offline martyh  United States
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Speaking of filament supplies… I will have my current regulator section finished in the next couple of days and will be ready to test the filament supplies from end to end. I plan on checking voltage and current at the tubes which I guess is pretty basic. Beyond that, is there anything else I can look for using a 15 MHz dual trace scope that might give me an indication of trouble or verify proper design and layout?

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Old 11th December 2009, 03:10 PM   #263
disco is offline disco  Netherlands
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Lightbulb Voltage source for the 26 filament

About supplies... here are mine.

At the left the LV, made from a 8.6V & 8.6V toroid (diy ) giving two times 8.3Vdc. It has Schottky diodes, 2200μF/25, 20mH oil filled choke (3A), 3300μF/25 and a shielded connection cable to the amp.

Ripple is down to 65mV. What's important: the wave shape resembles a sinus. If you just put a big cap after the diodes the waveform will be more triangular, very hard to flatten out afterwards.

In the amplifier lays the big trick for today, invented by the illustrious Salas. The third picture shows the final ripple on the 26 filament, down to an extreme low 1mV level.
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File Type: jpg 26pre-foto5.jpg (97.3 KB, 1056 views)
File Type: jpg 26pre-foto4.jpg (92.9 KB, 1008 views)
File Type: jpg 26pre-foto3.jpg (94.7 KB, 978 views)
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Old 11th December 2009, 03:38 PM   #264
iko is online now iko  Canada
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I don't understand what generated the third picture.
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Old 11th December 2009, 04:44 PM   #265
disco is offline disco  Netherlands
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Hi Iko,
It's a residual I'm picking up, probably from the circuit itself. 1m cables, antique tubes prone to oscillation... who tells. Who does?
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File Type: jpg 26pre-foto8.jpg (94.0 KB, 975 views)
File Type: jpg 26pre-foto9.jpg (93.9 KB, 534 views)
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Old 11th December 2009, 04:49 PM   #266
iko is online now iko  Canada
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What I meant is nothing so subtle, rather this: you talked about 65mV ripple, then you showed 1mV ripple. Something is responsible for that improvement. I was asking, what part of the circuit, possibly not shown, is the one responsible for going from 65mV ripple to 1mV ripple. Hope I'm not confusing the issue again.

Is is that "big trick for today" that did it?
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Old 11th December 2009, 05:12 PM   #267
Salas is offline Salas  Greece
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Its the grand total noise on the notoriously sensitive #26 filament in a fully wired working preamp with everything on it, including what it can pick up from any field, and its own imbalance, plus decoupling technique. Final 0.35mV RMS AC noise on such a filament which is in circuit by being the cathode, is great, can't bother any system at all, and is skipping a nightmare that only DHT practicing DIYers can fully appreciate Iko.
Bravo Disco, you hit the nail on the head, upon first try! Without anything more dramatic than CLC and a small board with few components! The question boils down to fundamental heating strategy only, now. Is a hum free voltage source with extended and flat rudimentary impedance better than battery, tonally preferable to best Isource there is? To be or not to be (Vsource), that is the question.
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Old 11th December 2009, 05:18 PM   #268
Salas is offline Salas  Greece
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Quote:
Originally Posted by ikoflexer View Post
Is is that "big trick for today" that did it?
He is happy, thus humoring. He means a remote sensed simplistic shunt reg that we got it to work good enough down to 1.5V only. He can share the schematic and all application info for all #26 guys now it is a sure success as far as I am concerned.

Don't know if he will deem it better than a current source on sonic grounds in the end, but its a cheap, easy and buzz free alternative at least.

Only if you see the measures people take for a hum&buzz noise free #26 preamp...expensive and copious non fully hum free horror stories abound.
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Old 11th December 2009, 05:30 PM   #269
disco is offline disco  Netherlands
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Quote:
Originally Posted by ikoflexer View Post
What I meant is nothing so subtle, rather this: you talked about 65mV ripple, then you showed 1mV ripple. Something is responsible for that improvement. I was asking, what part of the circuit, possibly not shown, is the one responsible for going from 65mV ripple to 1mV ripple. Hope I'm not confusing the issue again.

Is is that "big trick for today" that did it?
Ah, comme ça.

In the third picture you see two blue boards. The left is a Simplistic HV Shunt (V1.0), the right a noval Simplistic LV Shunt.

After discussing the pros and cons of voltage versus current source heating the DHT filament, Nick modeled a discrete parallel regulated 1.5V voltage source which is put in action here for the first time.

It consists of a 1.35A constant current source, isolating the 8V power supply and offering low output impedance. 0.3A is fed into a voltage source which maintains perfect smooth 1.5V on the filament. Remote sensing of the filament voltage is incorporated so an arm length of wire won't hurt.

First impressions are positive, measurements too but -hey- what a heatsink In the weeks to come I'll investigate the results of giving more current to the sink and compare it against some known excellent competitors.
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Last edited by disco; 11th December 2009 at 05:33 PM.
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Old 16th December 2009, 04:19 PM   #270
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Default high frequency hum

Finally I managed to control the slight high frequency hum which troubled my listening session of 26 pre for quite some time. I found this excellent article from the site Do it Yourself - Reducing Hum about hum reduction which help me to find the problem. Funnily the issue was a filament wire crossing the plate b+ supply which inject the hum to the signal path. Moving away the wire from plate supply, got rid of the issue plus add a whole new dimension to the sound which is now more clearer and detailed than before. No wonder why people are trying so desperately to reduce hum in 26. . However, whatever the effort you put to refine 26 circuit, the end result worth all the pain all the time. Long live 26.
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