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Old 19th August 2006, 10:01 PM   #1
zobsky is offline zobsky  India
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Default DHT heater voltage

Hi, ..
measured at the base of the pins , with a 46 in it, I get 3.4 volts instead of 2.5 v. I'm using a seperate 2.5V filament transformer for each channel.

Is this okay? I suspect not.
Any suggestions for fixing this ? Would using the same filament transformer for both channels work (could it be that the filament transformer doesn't have a good enough load regulation and isn't loaded down enough by a single tube)?

Thanks
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Old 19th August 2006, 11:14 PM   #2
Fuling is offline Fuling  Sweden
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3,4V seems way too much for a 2,5V tube.
Put some resistance in series with the filaments.
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Old 19th August 2006, 11:59 PM   #3
kevinkr is offline kevinkr  United States
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Hi Zobsky,
Who made your filament transformers and what is the secondary current rating.. Lightly loading some filament transformers, umm Hammond comes to mind amongst others, may give this outcome.

If you are using a center tapped filament winding (using center tap) then a small pair of ~ 0.27 ohm 2W resistors would be about right if this is not the case then a single 0.47 ohm 5W resistor would be about right. The idea is to drop about 0.9V with a load current of 1.75A..
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Old 20th August 2006, 12:28 AM   #4
zobsky is offline zobsky  India
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Quote:
Originally posted by kevinkr
Hi Zobsky,
Who made your filament transformers and what is the secondary current rating.. Lightly loading some filament transformers, umm Hammond comes to mind amongst others, may give this outcome.

If you are using a center tapped filament winding (using center tap) then a small pair of ~ 0.27 ohm 2W resistors would be about right if this is not the case then a single 0.47 ohm 5W resistor would be about right. The idea is to drop about 0.9V with a load current of 1.75A..

These are some ancient vintage surplus filamnent trannys (White-Rodgers, model S81-288). I couldn't find any info on them (or their current rating), though they are rated for 2.5V secondary (I'm using A/C heaters).

They don't have a center tap. I guess I'll have to burn off some voltage. I only had the circuit on for a minute, so I doubt that any harm has been done.

Thanks
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Old 20th August 2006, 12:33 AM   #5
kevinkr is offline kevinkr  United States
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Hi Zobsky,
Do these transformers have more than one primary tap? I have run across old transformers that had taps at 105, 110, and 117V.. Also if they are rated at in excess of 5 - 10A they may produce excessive voltage when this lightly loaded.

I agree it's unlikely that this short exposure caused any harm..
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Old 20th August 2006, 02:50 AM   #6
zobsky is offline zobsky  India
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No kevin, .. they have only one primary tap. I know what you mean about some trannys having multiple primaries but not these.

Before trying some inline resistors, i'm thinking of attach both channels to one filament transformer to load it down some more.
1. is this okay (the tranny doesn't have any load specs on it)?
2. would this have a negative effect on the sound?


I'll try and post some pics on this (long overdue) project sometime soon.

thanks.
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Old 20th August 2006, 04:16 AM   #7
poobah is offline poobah  United States
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zobsky,

Loading the trans should not make more than a 5% difference maybe 10%. When you do this, you are only moving the heat dissipation from a resistor to the trans. Now.... that's OK, but you need more than 10%.

It would be worth your time to investigate all these voltages. It seems the error is greater than it should be... are you measuring the 3.4 volts on the 200 volt setting of your meter? Check the little things carefully.

Resistors are probably your best bet here...

Also, how you reduce the voltage will not affect your sound... issues of hum aside. You either have hum or you don't... usually not. Don't worry about it...

But you need some resistors anyway. You need to create a voltage divider across your heaters with 2 resistors, so that you can ground, or reference, your heater supplies. They cannot be left "floating"... it makes makes noise and sometimes other mysterious problems (feedback).





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Old 20th August 2006, 07:02 PM   #8
zobsky is offline zobsky  India
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Quote:
Originally posted by poobah
zobsky,

Resistors are probably your best bet here...

Also, how you reduce the voltage will not affect your sound... issues of hum aside. You either have hum or you don't... usually not. Don't worry about it...

But you need some resistors anyway. You need to create a voltage divider across your heaters with 2 resistors, so that you can ground, or reference, your heater supplies. They cannot be left "floating"... it makes makes noise and sometimes other mysterious problems (feedback).


Yes, .. I'm measuring using the 200V setting on the multimeter.

I do have a hum reducing potentiometer across the two ends of the filament but adjusting that shouldn't affect the voltage across the heater. See the filament wiring scheme on the referenced link (not my schematic, but similar filament wiring and cathode biasing scheme).

http://www.angela.com/catalog/how-to/SE.2A3.html
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Old 20th August 2006, 07:08 PM   #9
poobah is offline poobah  United States
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OK then,

Repeat your measurements using the 20 Volt scale... you should always use the lowest scale possible. You may have less of a problem than you think.

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Old 20th August 2006, 07:17 PM   #10
zobsky is offline zobsky  India
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Quote:
Originally posted by poobah
OK then,

Repeat your measurements using the 20 Volt scale... you should always use the lowest scale possible. You may have less of a problem than you think.


I don't have a 20V scale on the A/C measurement scale of my multimeter, .but I tried measuring again with my secondary (autoranging) multimeter with the same results ie. voltages around 3.5 V AC or so.
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