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Old 24th June 2007, 10:40 AM   #1
jarthel is offline jarthel  Australia
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Default DHT output tubes and DC heating

Can I:

1. connect their heaters in parallel?
2. same as no. 1 but in series
3. I guess that if nos. 1 and 2 aren't possible, does that mean I must have a separate winding for each tube?

Thank you very much
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Old 24th June 2007, 01:30 PM   #2
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Is this SE or PP? Output tubes or...?

For SE, you will need a separate winding for each channel and each stage.

For PP it is ok to parallel up the heaters of the tubes in a PP pair.
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Old 24th June 2007, 02:43 PM   #3
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Quote:
1. connect their heaters in parallel?
This is possible. You can connect the heaters in parallel even in a SE amp, if one side of the heaters is grounded. This requires fixed bias, and some other means to monitor plate current. I use a 10 ohm resistor in series with the plate. I use one filament winding, rectifier, filter, and regulator chip to power both channels.

It is also possible on a push pull amp if both tubes share a common cathode resistor. A seperate winding for each channel would be required.

I have seen (in a name brand amp) one filament winding powering the heaters in both channels of a SE amp. The two channels shared a common cathode resistor and relied on a big electrolytic bypass cap for channel separation. Not what I would call optimum.

Wiring the two heaters in series would guarantee a different bias level for each tube. This could be compensated for with fixed bias, but again not optimum.
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Old 24th June 2007, 08:11 PM   #4
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One thing to bear in mind is whether you are likely to pull one tube out when you run two tubes off one DC supply. If you do then you will find the following:

2 tubes in series, voltage regulation - OK - remaining tube does not light since circuit is broken

2 tubes in parallel, voltage regulation - OK - remaining tube gets same voltage

2 tubes in series, current regulation - OK - remaining tube does not light since circuit is broken

2 tubes in parallel, current regulation - NO!!! - remaining tube gets double the current!
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Old 24th June 2007, 11:15 PM   #5
jarthel is offline jarthel  Australia
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so what is the recommended solution?

separate heaters for each output tubes?

for example in push-pull: Can I construct 2x DC supply out 1 secondary for each channel? Is this okay?

Thanks again
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Old 18th February 2013, 11:49 AM   #6
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Quote:
Originally Posted by tubelab.com View Post
This is possible. You can connect the heaters in parallel even in a SE amp, if one side of the heaters is grounded. This requires fixed bias, and some other means to monitor plate current. I use a 10 ohm resistor in series with the plate. I use one filament winding, rectifier, filter, and regulator chip to power both channels.

It is also possible on a push pull amp if both tubes share a common cathode resistor. A seperate winding for each channel would be required.

I have seen (in a name brand amp) one filament winding powering the heaters in both channels of a SE amp. The two channels shared a common cathode resistor and relied on a big electrolytic bypass cap for channel separation. Not what I would call optimum.

Wiring the two heaters in series would guarantee a different bias level for each tube. This could be compensated for with fixed bias, but again not optimum.
If using LED bias on a center-tapped heater, would having both channels of an SE amp to the same floating filament supply result in channel mixing?
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Old 18th February 2013, 12:00 PM   #7
Pano is offline Pano  United States
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It does not in my 4P1L SE amp. At least as far as I can hear and measure.
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Old 18th February 2013, 04:05 PM   #8
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Then I suppose running the cathodes of both channels through the same LED would give the same result (and get me more current through the LED with no mismatch in voltage drop between channels). Just need to build it and listen with input playing only on one channel at a time.
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Old 6th March 2013, 12:16 PM   #9
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Default Some crosstalk present

Just a follow-up: running heaters of both channels from the same battery does create some crosstalk, meaning that some signal travels through the heaters to the other side and this occurs even when heater center tap (cathode) of one side is disconnected from ground altogether. Can't really measure the signal level leaking to the other channel but it's definitely audible.
On the positive note, having both channels biased via shared LED does not further increase the crosstalk but does have some (subjective?) feeling of increased resolution and improved soundstage compared to separate heaters + separate cathodes.
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