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Old 1st February 2007, 04:56 PM   #11
angelfj is offline angelfj  United States
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First, please accept my apology for this long post and to those (I know you are out there, who see all of this as very elementary) please be patient with me.
I am trying to establish the operating point as suggested in previous posts. However, if I go to the tube characteristics for the 304TL, as published by Eimac, they are not the same as the typical representation for a triode. I found an excellent reference on the Internet, and this guy did a wonderful job of explaining, step by step, how to establish the operating point.

see: http://www.valveheart.com/

In order to follow the example, I seem to be missing some critical information, for the 304TL, which would normally be included in the characteristics for other tubes/manufacturers.

1. Rp = plate resistance
It was suggested that for triodes , the load resistance, Rl, also referred to as the output transformer primary impedance, can be calculated as 2 - 6 x's Rp. So, if I do not know what is the Rp, I can not calculate Rs, which is critical for calculation or a graphical check for the operating point.

2. The example uses a graphical representation of the tube operation. The horizontal axis represents plate voltage, and the vertical axis represents plate curent. Superimposed on this graph are curves for several values of grid bias. The Eimac data is very different. There are two types of curves. The first is a graph of power output (watts) horizontal axis VS grid driving power (watts)) vertical axis , for a fixed value of plate voltage (Eb). Each of these graphs show several curves for different values of Ec (cathode voltage???). The second type of graph, and there is only one, has plate voltage on the horizontal axis and grid volts on the vertical axis. Several curves are drawn for both different levels of plate current and grid current. So, it seems that these graphs are not suitable for the graphical determination of the operating point.
Can someone offer a little help? I have to believe that establishing the operating point fpr a 304TL, to be used in a SE, class A amp has been done before. In the spirit of not re-inventing the wheel, please induldge by curiosity. My only limitations are the voltage and capacity of my plate transformer, which will be limited to approx. 2000 vdc @ 500 ma.

Thanks in advance, Frank
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Old 2nd February 2007, 03:38 AM   #12
angelfj is offline angelfj  United States
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To all:

Two questions regarding tube filament requirements:

1. which is better ac or dc?

2. how critical is the voltage?

For example the 304TL requires 10 volts @12.5 amps. I have located a transformer rated 12 volts @ 25 amps. $30 each , not center-tapped. Would 2 more volts kill the filaments? I was thinking of taking the transformers apart and removing turns, until I got 10 volts with a load to simulate the tube filaments. Or putting a variac in the primary. What do you think? What about not having a center tap. Is this a game stopper?

Thanks, Frank
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Old 2nd February 2007, 03:58 AM   #13
wfmali is offline wfmali  Europe
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Hello,
I would tentativly go DC, in order to avoid hum - and you have to use the right filament voltage - I would not even try 20% overvoltage short term. If I would have to light this beast, I would go SMPS right away. You could use a standard 9V supply and adjust to max. voltage. The only problem could be that hiccup supplies might not start on cold filaments, current limiting would be better. Or, use a 12V supply, adjust to min. voltage and drop some .5V or so in a resistor. That will work with hiccup overcurrent protection as well.
Marcus
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Old 2nd February 2007, 12:27 PM   #14
nhuwar is offline nhuwar  United States
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On most eimac tubes the filament voltage at beam on state is not to vary more then 5% also you should not apply full filament voltage at turn on.
There are maximum current limits on the filament and they can only be exceeded applying full voltage to a cold filament.

There is also a minimum heating time before plate voltage can be applied usually 3 to 5 minuates for transmitter tube.

Nick
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Old 3rd February 2007, 12:04 AM   #15
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I don't think you'll be happy with AC because of the very high mu.

Also, the 304TL is essentially four 75TL's in one bottle, all connected in parallel. Well, not everything is always in parallel. The 10V@12.5A connection puts one pair of filaments in series with the other pair. The consequence is that with DC two of the sections will be running with 5V more/less bias than the other two. Using the 5V@25A connection puts all of the filaments in parallel so that all four sections bias up the same.

-- Dave
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Old 3rd February 2007, 01:19 AM   #16
rdf is offline rdf  Canada
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I didn't realize that about the 304TL filaments. The obvious question: is it possible to feed each section with an independent winding in opposite phase to reduce hum?
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Old 3rd February 2007, 05:12 AM   #17
Gluca is offline Gluca  Italy
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Default huuuumm

304TL has 2 filaments to be connected in parallel @ 5V (or series @ 10V) ... if you connect two 5VAC secondary windings 180deg out of phase you get 0V, don't you?

Ciao
Gianluca
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Old 3rd February 2007, 06:24 AM   #18
rdf is offline rdf  Canada
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The data sheet shows two isolated, completely independent filaments. I was thinking along the lines of a centre-tapped 5 volt transformer to each filament, each filament transformer's centre tap tied to ground.
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Old 3rd February 2007, 08:21 AM   #19
Gluca is offline Gluca  Italy
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mmmm ... that makes sense to me. Anyone out there is running 304TLs and would care to give it a try??

Gianluca
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Old 3rd February 2007, 12:22 PM   #20
angelfj is offline angelfj  United States
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It seems that most people are recommending DC filament supply only for SE. 304TL in class A. The reason they say is that ac filament supplies introduce too much noise. I am certainly no expert, but this seems logical.

Regarding independent filaments, I believe that these 5-volt filaments are electrically isolated. I had plan to connect them in series and us a 10 volt DC supply. This will be based on a salvaged welding transformer that can easily deliver 30 -40 amps at 12 volts. However, I don't believe that the tube cares how the cathode is heated.
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