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Old 22nd July 2012, 04:21 PM   #1
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Default 300b filament

hi i am looking for ways to rectify the filaments of the AC-DC 300B
But with the output transformer with 5voltios could be done?
someone has to do a circuit and has 5 volts stable
The 300B I have are about JJ.
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Old 22nd July 2012, 04:39 PM   #2
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Quote:
Originally Posted by miguelnoda View Post
hi i am looking for ways to rectify the filaments of the AC-DC 300B
But with the output transformer with 5voltios could be done?
someone has to do a circuit and has 5 volts stable
The 300B I have are about JJ.
You probably want to use DC filaments on the preamp and driver tubes.
DC filaments are not usually considered necessary for the output stage.
If Hum or AC ripple shows up in the output, it is usually the result of AC filaments on the preamp tubes, not the output tubes.
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Old 22nd July 2012, 05:27 PM   #3
disco is offline disco  Netherlands
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AC heated DH output triodes are prone to hum on high efficient loudspeakers. If you're experiencing hum for this reason and good filement supplies are unobtainable, you might consider building a current source from the LM317/LM350 as a cheap alternative.
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Last edited by disco; 22nd July 2012 at 05:30 PM.
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Old 23rd July 2012, 03:01 AM   #4
zelgall is offline zelgall  Canada
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Are you using the 300b SE or PP? If single ended AC is the way to go for sound quality. DC can be a bit brighter sounding. Some people say "washed out", I think brighter is a better description. With push pull it tends to be less of an issue.
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Old 24th July 2012, 09:41 PM   #5
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I think it fails the AC filament 300b, the voltage margin is
5.5 volts + -. ou can adjust the voltage of 5 volts some way?
thanks
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Old 24th July 2012, 10:03 PM   #6
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Voltage tolerance for 300B is +/- 5% [4,75 .. 5,25V] at constant (long term) measurement. Lifetime is reduced outside of this range. 5,5V, or +10% is not acceptable, the lifetime will be badly degraded.

This presents an immediate problem for ac-heating: the mains tolerance. For instance, here in the UK the mains tolerance is 230V +/- 10%.

The second problem with ac-heating is that the music is modulated by the ac-heating current. It does not matter whether the hum is nulled (using a hum-pot). The intermodulation is 50/60Hz AND 100/120Hz cross-products.

This effect is easily audible as muddy, inarticulate sound. Despite its reputation, ac-heating of DHTs sounds bad.

The reason that ac-heat has a good reputation is that carelessly implemented dc heating sounds even worse! Plain rectified dc is the worst. In this case, some of the rectifier recovery current ( up to 3A for 300B) passes into the filament, where it mixes with 2 .. 50mA of music signal. No wonder that this is not hi-fi.

3-terminal regulators are only slightly better. These have high noise, and their feedback loop cannot be re-compensated to suit the needs of filament heating.

The problem can be solved though - Tentlabs offer a designed solution, ready built & tested, or, if you don't mind assembling some components into a PCB - I offer a self-assembly kit that solves all the problems in filament heating, and allows the filament voltage to be finely adjusted. Seach for 'Rod Coleman regulator', or send me some DIYAudio-email.
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Old 24th July 2012, 10:44 PM   #7
popilin is offline popilin  Argentina
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Try this, has low voltage drop.
R4-R5 can be replaced by a trimpot.

Ufil = [(R4+R5)/R5)] Ureff

Ureff = 1.2V (LM385 1.2)
Ureff = 2.5V (LM385 2.5)
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Old 24th July 2012, 11:10 PM   #8
zelgall is offline zelgall  Canada
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[QUOTE=

This effect is easily audible as muddy, inarticulate sound. Despite its reputation, ac-heating of DHTs sounds bad.
[/QUOTE]

I haven't heard a DC filament amp that didn't sound "brightened". Perhaps they were poor implementations or I'm just a tin ear. AC heating provides a more relaxed and natural sound to me and many others just as many people can't get past a small amount of hum when they have their ear pressed up to the speaker. Different strokes and all that.
I'm sure that you have a great deal more experience and a more valid opinion than I do but on the other hand, I'm not selling anything.
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Old 25th July 2012, 07:40 AM   #9
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The usual (bad) dc implementations can sound 'Forward' (too close) which may be the same thing that you're hearing.

ac heating may sound better than these.

But ac-heating is a huge compromise, not just because of the hum, but the intermodulation. Steve Bench has independently recorded measurements for this IM:

Effects of AC Heating Power Applied to Directly Heated Triodes

High IM is certainly not Hi-fi.

The problems can be fixed, using low-noise, low gain discrete transistor regulator design.

I do sell kits for DHT filament regulation, but only after many folks asked me to productionise the design I published here on DIYaudio:

New DHT heater
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Old 25th July 2012, 12:44 PM   #10
disco is offline disco  Netherlands
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True indeed, however the topic starter might be served by 'the inbetween' solution when his goal is not state-of-the-art or if it's meant for a cramped space like in a cheap factory build.

I was looking for a schematic in your tread. I'd like to test it hardwired. Is it there or must I order a board ?

Furthermore, what's the bandwith of your regulator? I guess it must be beneficial for noise limitation to have reduced bandwith -possibly twice the audio bandwith?
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