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Old 9th August 2009, 04:22 AM   #1
bobbyq is offline bobbyq  United States
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Default Heater-Cathode Voltage

Hello all,

This is my first post, although I've been a member now for a while. As a hobby,I've been studying "electronics" and "tube electronics" for a couple of years now. My goal is to be able to design and build great sounding components for my enjoyment and that of my family and friends. I am still far from my goal. I'm afraid I'm seriously mathematically disinclined, and I'm really stubborn about asking for help.

I've recently built Fred Nachbaur's phono-pre and I've just finished "Frank's 6SN7" line level pre. Both of these were built with about 97% salvage materials, in the interest of minimizing the cost of learning. I did make a walnut and aluminum chassis for the new pre though.

Since Frank's pre schematic doesn't really include a power supply, I'm a little concerned about the heater-cathode voltage since I cobbled together the power supply from on-hand components.

I've searched high and low, and maybe I'm not using good search terms, but I can't find a direct description of how to measure "heater cathode voltage". I mean, seemingly all tubes have a maximum "heater-cathode voltage" which may be X volts positive or negative. Everybody must already know how to measure this or else there would be some better references on the web. I however, do not.

Would someone please point me to a reference of how to measure this parameter or provide their own description?

Thanks.
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Old 9th August 2009, 05:55 AM   #2
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Default I don't quite get what you mean

the max h-k voltage is a parameter of the tube you are using, and you design your heater supply to address that if required. In the case of your 'sn7, a quick glance at the datasheet shows a max h-k of 100v.

Since the 'sn7 cathodes in Franks design are both at different voltages it may be best to "float" the heater supply.
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Old 9th August 2009, 12:48 PM   #3
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I am not sure what you really mean by "how to measure" h-k voltage.

If you literally mean how to measure it in an existing amp, then you measure it the same way you measure any voltage, one meter probe on cathode pin, the other on either heater pin. Dont worry about which heater pin, the difference of a few volts is insignificant.

But you probably already know that.

If you mean how to determine what the h-k voltage would be by looking at a schematic, do 2 things.

1) look at the transformer winding. If one or the other winding, or a center tap, is tied to ground then the k-h voltage is just the cathode voltage ( notice I said k-h this time, note the polarity ).
Or if the center tap is connected to ground through a pot, or some resistors. All virtually the same thing.

If the winding or center tap is connected to a dc voltage, then that will be added to, or subtracted from, the cathode voltage, paying attention to the polarity.

2) look at the cathode circuit. figure out the cathode to ground voltage.

Now add/subtract the cathode - ground voltage to the heater-ground voltage, paying attention to the polarity.

It takes longer to say than to do, I think so anyway.
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Old 9th August 2009, 02:18 PM   #4
danzup is offline danzup  Romania
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Default Re: Heater-Cathode Voltage

Quote:
Originally posted by bobbyq
Hello all,

This is my first post, although I've been a member now for a while. As a hobby,I've been studying "electronics" and "tube electronics" for a couple of years now. My goal is to be able to design and build great sounding components for my enjoyment and that of my family and friends. I am still far from my goal. I'm afraid I'm seriously mathematically disinclined, and I'm really stubborn about asking for help.

I've recently built Fred Nachbaur's phono-pre and I've just finished "Frank's 6SN7" line level pre. Both of these were built with about 97% salvage materials, in the interest of minimizing the cost of learning. I did make a walnut and aluminum chassis for the new pre though.

Since Frank's pre schematic doesn't really include a power supply, I'm a little concerned about the heater-cathode voltage since I cobbled together the power supply from on-hand components.

I've searched high and low, and maybe I'm not using good search terms, but I can't find a direct description of how to measure "heater cathode voltage". I mean, seemingly all tubes have a maximum "heater-cathode voltage" which may be X volts positive or negative. Everybody must already know how to measure this or else there would be some better references on the web. I however, do not.

Would someone please point me to a reference of how to measure this parameter or provide their own description?

Thanks.
Very simple : you put an programmable DC at least 300V max source between heater and cathode of that tube and increase the voltage from 0V till they do a short circuit and the tube are not usable !
This parameter may be measured only by destroying the tube !
Did you understand ?
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Old 9th August 2009, 06:09 PM   #5
bobbyq is offline bobbyq  United States
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Thank you both for your replies. This helps, but I still need a little further clarification. Robert, your initial description is what I was looking for.

To float the heater supply means to not connect the center tap to ground, correct?

When I don't connect the center tap to ground, I get -10vdc to cathode on pin 3 and +14vdc to cathode on pin 6. This is measured with the common or ground VOM lead on the heater and positive lead to the cathode..

When I connect the center tap I get +3.8vdc on 3 and +128vdc on 6, using the same VOM lead connections. If I reverse the lead connections then the measurements change to negative DC.

How do I know which is the correct method for connecting the leads? The 6SN7 permits 100vdc "heater positive with respect to cathode" and 200vdc "heater negative with respect to cathode". I find these terms confusing.

I would prefer to have the heater winding center tap grounded, as this reduces hum, although the hum is already pretty low. However, if I am measuring the voltage correctly, it seems I'm exceeding the h-k voltage spec. On the other hand, if I'm measuring it backwards, then I'm ok. So which way should I connect the measurement leads?
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Old 9th August 2009, 10:38 PM   #6
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Default so don't float the heaters - elevate them

connect the centre tap to an elevated voltage source - it doesn't require current, just a voltage reference. Typically, you would aim to be at around 66% of the h-k max, but as long as you are within the range in this situation its not critical.

One trick is to reference the heaters to the cathode itself take the centre tap back to the junction of your bias and load resistors in the cathode of the cathode follower. Again, through a high value resistor, since we don't want current from the heaters entering this circuit!

Sorry about the messy, confusing diagram - ask if you need explanation. Others, feel free to criticise and comment.
Attached Files
File Type: pdf heaters.pdf (9.4 KB, 317 views)
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Old 9th August 2009, 10:47 PM   #7
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Default btw

your voltage readings look about right - thats what you would expect to see.

If you tap to an elevated supply as I suggested, the voltage readings should change by the value of the reference voltage.
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Old 10th August 2009, 03:56 AM   #8
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In addition to the cathode trick above, you can also use a simple voltage divider from your B+ to reference your heater center tap. Use high value resistors (since you don't want to draw any meaningful current from the power supply) and select the R values to get the heater voltage raised to be comfortably within the h-k rating for the tube.

The basic idea is show in post #173 of this thread for the Aikido heater bias on Bas Horneman's PS PCB:

Poll..anyone interested in an Aikido linestage PCB group buy?
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Old 11th August 2009, 12:03 AM   #9
bobbyq is offline bobbyq  United States
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Again, thanks all. I'm about to give the B+ voltage divider a try.
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Old 11th August 2009, 12:16 AM   #10
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Make sure to bypass the tap on the voltage divider with a 10uF cap to ground, to minimize hum.
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