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Old 29th April 2012, 01:32 PM   #11
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Quote:
Originally Posted by nazaroo View Post

Click the image to open in full size.
Altough I count in meters, I think it meant to say per inch
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Old 29th April 2012, 02:43 PM   #12
nazaroo is offline nazaroo  Canada
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Originally Posted by costis_n View Post
Altough I count in meters, I think it meant to say per inch
The picture here for illustration was taken from a motor wiring page...so maybe for their purposes 6 turns per foot was adequate to prevent bleeding of servo-motor signals into motor power line...
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Old 29th April 2012, 05:26 PM   #13
kevinkr is offline kevinkr  United States
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I thought this thread was worthy of being made a sticky - lots of really good filament wiring information here. Several fellow mods agreed. This is an area where a lot of newbies, and not so new(bies) get into trouble.
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Old 29th April 2012, 08:20 PM   #14
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this is a very good post.
thank you very much for this useful information!

a little question,
if i convert the heater supply to d.c, it should eliminate the RF antenna effect?
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Old 29th April 2012, 08:59 PM   #15
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Altough I count in meters, I think it meant to say per inch
I don't think so. All vintage wire and some modern wire can't be twisted 6 times per inch then subjected to thermal stress. Teflon is an obvious exception but even the stranded copper wire will object to such a tight twist. The smaller the gauge, the tighter it can be twisted.

I tend to twist the wire tighter for short runs and about 1 to 2 twists per inch maximum for longer runs.

Some of what is accepted practice in the tube audio world is in direct conflict with some of the concepts I have learned in 40 years of building two way radio equipment for extreme duty applications.

If you are building a box that sits on a shelf for years at a time and rarely gets moved, you can get away with long twisted runs and components suspended by their leads only.

If you are building a box that goes in the trunk of a black cop car in Arizona or Minnesota in January, or the dash of a Hummer in the desert, or filled full of hot tubes sitting on the top of a stack of speakers on a stage, you can't.
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Old 29th April 2012, 09:58 PM   #16
nazaroo is offline nazaroo  Canada
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Originally Posted by zivhackman View Post
this is a very good post.
thank you very much for this useful information!

a little question,
if i convert the heater supply to d.c, it should eliminate the RF antenna effect?
Great question.

A power supply regulator and caps cannot stop an antenna arrangement from being one. But it will block and absorb transmission and effectively completely alter and destroy the function of it.

Any 'antenna' effects still coming in on the heater lines would have to enter through the open bottom (or top) of a 3/4 closure like a guitar amp chassis. The final cure is to install a finishing plate, which should have been provided by the maker.
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Old 30th April 2012, 01:44 AM   #17
rongon is offline rongon  United States
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Really helpful, thank you!

Can you explain how the typical cathode bias resistor of a DHT would interact with the hum-balancing network in the "Power Supply for a large direct-heated triode"?

Click the image to open in full size.

Would the (usually capacitor bypassed) cathode bias resistor simply go from the junction of the two .002uF capacitors to ground, effectively floating the whole filament secondary?

Or would one use a center-tapped secondary and attach the cathode bias resistor in series with the center tap to ground, in the usual way? Wouldn't that bypass this hum-cancelling network too?

I'm probably missing something that's obvious to most...

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Old 30th April 2012, 03:25 AM   #18
nazaroo is offline nazaroo  Canada
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Lets have a look at your 'typical' circuit (I'm using your description here):


Click the image to open in full size.

The design suffers from a few potential problems:

(1) The Winding of the transformer and its total reactance (AC/DC) should be closely balanced. If it isn't there is no obvious adjustment to fix this as it stands.

(2) The wear in the tube should be even along with emission currents from both ends of the heater element. Typically however, tube emission varies unpredictably over the tube-life.

(3) There should be no current through the grid, and the resistance path to ground should be relatively low, to keep the bias stable and well-defined.


Now lets talk about why the circuit cannot deliver:

(1) The winding for a heater-transformer will not be wound the way a Push-Pull Output xformer is. In those, the windings and current must be balanced, to cancel the magnetization and core-saturation. A typical heater winding is a single coil, not a bifilar winding, and so, balancing it has no meaning in regard to D.C. flow. It WILL ALWAYS magnetize the transformer core, and use up precious core-flux. That is why it is better for the other windings to leave unneeded windings unused. Nothing is free.

The only issue then is to balance the D.C. flow for the purpose of even wear on the cathode emitter. But this also will fluctuate with tube-wear and is uncontrollable from outside of the tube. At best, you can have a compensating circuit which monitors D.C. idle current through both terminals, and adjusts itself.

(2) There is a real danger here that any attempts at adjustment of current-flow will exascerbate tube element wear unevenness (and also applied voltage). I leave that to your imagination.

(3) In many cases, Direct-Heated Triodes have relatively low Bias-points, and conduct some grid current during a cycle. In this case, Cathode-Bias technique may be less stable than desired.

Last edited by nazaroo; 30th April 2012 at 03:41 AM.
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Old 30th April 2012, 04:56 AM   #19
rongon is offline rongon  United States
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Thanks, this is great stuff!

OK, so I can only think of two relatively simple solutions to propose.

1) DC filament supply, or

2) Fixed bias

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Quote:
The only issue then is to balance the D.C. flow for the purpose of even wear on the cathode emitter.
Could one heat the e.g. 6.3V filament with a well-filtered or regulated +3.15V and -3.15V supply? Would having the voltages fixed be at least a partial solution, allowing cathode bias to work better?

Or would regulating the current drawn by the filament (say at 1A for a 6B4G, for instance) be a better solution?

--

I should warn you that I'm at the limit of my understanding here. Thanks for the great drawings, they help a lot. Do you teach by any chance?
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Old 30th April 2012, 04:56 PM   #20
kevinkr is offline kevinkr  United States
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Fixed bias and DC filament supplies are always a good idea with DHTs, just make sure that the filament supply is very well filtered - there is nothing worse sounding than a touch of 120Hz sawtooth in the filament DC.

AC DHT heating also adds intermodulation distortion products at multiples of the line frequency between the heating supply and the audio you are trying to amplifier. George (tubelab) has commented on this in the past.

My experience in dht power amplifiers is that anything over 2.5V is best DC heated to avoid buzz issues, and the IMD issue mentioned previously.

Constant current heating of dht filaments is pretty near ideal IMLE, controlled warm up limiting inrush current to the rated design current of the filament should be good for filament life, and the rejection of noise on the filament supply is usually quite good. (10yrs of using CCS for filament heating and I have never had a filament failure in that time. Previous to this they were rare but did happen.) In addition with some topologies the filament raw power supply can be completely excluded from the audio path.
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