Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hokenpoke on February 13, 2012, 04:10:05 pm
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I'm building a stock 5F6A and soldered in my heater string before (duh) noticing the recommendation for 18ga to the power tubes and 20ga to the pre-amp tubes. This made me a little nervous until I checked the specs on the PT (6.3V/4A) and the wire (22ga is good for 7A in chassis). Seems like there's plenty of (if you will) headroom in this application. Any reason to be concerned or to rewire the whole dang thing?
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I've used 20 gauge in 5F6A's with no issues.18 gauge is a PITA to hook up to heaters and IMHO unnecessary.
22 gauge is too light i would think.I would not trust it.
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I wouldn't worry about 22 gauge on preamp tubes, but I wouldn't recommend it on power tubes.
j.
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Thanks for the input. Just so I understand - Is this because the current may sometimes exceed 4A? Wouldn't that tend to kak the PT before running over the 7A that 22ga can carry? Or is there another reason?
In steady state, with 3x 12A*7 and 2x 6L6 the heater current should be under 3A. Obviously if there's a short all bets are off but a 2x factor seems like it ought to be enough. Of course more copper rarely hurts . . . I just don't want to have to redo the thing.
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Just replace the run to and between the power tubes and leave the 9 pins sockets as is. Jim
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The current flow along the heater string is not the same
Each tube adds to the current flow
22ga would probably be just fine for the first couple pre amp tubes
As you go down the line, you can switch up to 20 gauge
If you have 4 power tubes, I would probably switch up to 18 gauge for the power tube sockets
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> 22ga is good for 7A in chassis
It may be safe (won't melt).
Will it sag?
You don't want 6.3V at PT and 5.7V at tubes. Preferably well over 6.0V at tubes.
#22 is 16.5 ohms per 1,000 feet.
At 4 Amps, 1,000 feet will drop 66 Volts.
1 foot will drop 0.066V.
Worst-case might be power tubes one foot from PT. That is _2_ feet of total run. So the drop is 0.132V.
6.3V at PT gives 6.168V at tube. 2% low. Not bad at all.
If your run is less than a foot, don't fret.
Note that current may be 4X higher for the first couple seconds. Not long enough to hurt the PT. Probably not long enough to hurt the wire.
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I still think the 6.3v and 5v taps on a transformer are used in resistive loads. In other words, the power draw is almost constant. The wire is not resistive, the load is.
Light bulbs are nearly-resistive, once at operating temperature. They are much-smaller "resistors" when cold. That means there is a current surge at switch-on much higher than at steady state after they warm up.
Hewlett-Packard made use of this feature to stabilize the output of their patented audio oscillators (their first product), starting in 1939. A light bulb was used in place of a cathode resistor in part of the oscillator circuit, with the voltage and current well below the normal operating range of the bulb. The bulb acted as a non-linear resistance, because it was not operated at full, normal operating temperature.
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Will it sag?
Worst-case might be power tubes one foot from PT. That is _2_ feet of total run. So the drop is 0.132V.
Physical sag or voltage drop due to resistance? Your calculations seem to indicate that the latter is not a problem and if my rudimentary math is correct, the heater string would be dissipating less than .5W which ain't diddly. So I guess there's no real concern however you slice your sag.
And now another question (or two) - because of the physical location of everything in the chassis, I ran the heater string from the lamp, to the power tubes and then to the 100ohm center tap resistors which are on the board. The preamp and inverter tubes are last in the string AFTER the CT resistors which - in most layout schematics are located before the power tubes. I know that what I have created is electrically equivalent - is there any reason to not do it this way?
Also, just to confirm - if I am using the resistors to create an artificial CT, then I should not connect the real CT to anything? Just cap it and leave it out of the way in the chassis?
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Also, just to confirm - if I am using the resistors to create an artificial CT, then I should not connect the real CT to anything? Just cap it and leave it out of the way in the chassis?
Yes.
Don't use the heater CT wire with an artificial CT, it's 1 or the other, _ not _ both.
If you have a heater CT wire on the PT, why are you bothering to use an artificial CT?
Brad :icon_biggrin:
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If you have a heater CT wire on the PT, why are you bothering to use an artificial CT?
My perusal of this board seems to indicate a preference for the artificial CT - I think I read about lower noise and certainly read that the resistors can serve as (imperfect) fuses in the event of a kerfuffle. I guess, thought, that the real reason I'm using them is that I soldered them onto the board before I (ordered and) realized that the PT had its own CT.
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Yeah I had a BF SR for years and it had the 2, 100R's for the heater CT and 1 time 1 of the R's fried and might have saved the PT. :dontknow:
I just don't like having taped/caped off PT wires. I know it's fine, it just bugs me. :laugh:
Gonna be a great amp when your done.
Brad :icon_biggrin: