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Resistors on DHT Cathode
Resistors on DHT Cathode
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Old 2nd October 2006, 09:58 AM   #1
dsavitsk is offline dsavitsk  United States
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Default Resistors on DHT Cathode

My understanding is that on a DHT it is a good idea to put a resistor on each heater connection (below the heater power supply), to join these resistors, and to then bias from this joined point.

First off, is this actually a good idea (in this case for an 01A) and second, what value should these resistors be? It seems they need to be large enough that the heater current goes though the heater and not the resistors, but small enough so as to not interfere with biasing. This second part is important here as i was going to try this as an ultrapath and I assume that the cap connects below the par of resistors but above the bias resistor? Thoughts? Am I looking in the 2R range, 20R, higher?
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Old 2nd October 2006, 02:49 PM   #2
ivegotmono is offline ivegotmono  United States
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I'm really not an expert on this AT ALL but I've ben looking at 2a3 SE schematics lately and JE Labs uses a 60 to 75 ohm pot between the heaters like this. Google JE Labs and see those.
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Old 8th October 2006, 04:43 AM   #3
Graeme is offline Graeme
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Just completed the first of a pair of monoblocks - confounds me - as it is totally hum/hiss free....intend to share my conclusions elsewhere, but I used a 5v DC filament supply (just ~ 3mv ripple) and right at the tube socket used 12 ohm/2watt carbon resistors to provide a centre-tap for bias. (Adjacent to the socket I also used a large toroidal common-mode choke) The resistor-centre-tap was indeed the "source" for the bias resistor and the 2 x 47ufd capacitors....the 16mfd cap from the HT - "ultra-path connection??) was also made here.

I'm a happy chappie....

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Old 10th October 2006, 10:45 AM   #4
Eusebius is offline Eusebius  United Kingdom
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I was looking at the data sheet for the 3A5 DHT which is for DC operation. Pin 1 is positive, pin 7 is negative and pin 4 is centre tap (it has a common cathode). Says

A shunting resistor must be connected between pins 1 and 4 for series filament operation, it's value should be such that the voltage across the shunted section is equal to the voltage between pins 4 and 7. An additional shunting resistor may be necessary between pins 1 and 7...etc

When I use two DHTs in a differential pair (something like 26 or 30), I connect the filaments in series as above, but just put the CCS on the cathode to the centre point in the filaments.

I'm wondering if I should use a pot and put the CCS on the wiper, and use the pot to equalise the voltages across each filament
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Old 10th October 2006, 11:05 AM   #5
panos29 is offline panos29  Greece
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It seems they need to be large enough that the heater current goes though the heater and not the resistors, but small enough so as to not interfere with biasing.
As far as there is enough current to supply heaters + resistors the heaters will always have the same voltage as the resistors because are connected in parallel, thus the voltage will be exactly the same. Practicaly, for the 01A tube you can use 33-47 Ohm resistors or even better a 1-2W potentiometer around 47-100Ohm.
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