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Old 12th March 2013, 02:12 PM   #2571
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Quote:
Yep, two tubes, one box. I think all these wonderful modules that are available (Rod's filament boards, Tomchr's regulator, K&K CCS boards, Salas, Tentlabs, etc.) have made it so much easier to get great performance without resorting to ponderous construction projects.
How did you wire Rod's boards to the 26? It looks like they are in series. But how do you bias them in that case?

Last edited by Bas Horneman; 12th March 2013 at 02:19 PM.
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Old 12th March 2013, 03:41 PM   #2572
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I used battery bias on the grids with Rod's filament supply floating. I hacked up Kevin's schematic a little for this drawing:

Edit: I have the battery drawn backwards, oops.
Attached Images
File Type: gif 26 bias.GIF (3.9 KB, 328 views)

Last edited by 12B4A; 12th March 2013 at 03:43 PM. Reason: Battery wrong way
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Old 12th March 2013, 03:43 PM   #2573
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Thanks for that!
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Old 12th March 2013, 03:56 PM   #2574
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O, that's interesting. I would expect that a 1.5V alkaline cell would suffice to balance them approximately, since the shift in bias equals 1x Vf (filament voltage). In case of #26 this is 1.5V..

Of course, this assumes that the 26s draw the same current at equal bias, which may not be so.
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Old 12th March 2013, 04:09 PM   #2575
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A simple question;
Has someone tried LIFEPO4 battery for filament supply?
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Old 13th March 2013, 12:39 AM   #2576
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Originally Posted by Rod Coleman View Post
O, that's interesting. I would expect that a 1.5V alkaline cell would suffice to balance them approximately, since the shift in bias equals 1x Vf (filament voltage). In case of #26 this is 1.5V..

Of course, this assumes that the 26s draw the same current at equal bias, which may not be so.
There's no imbalance, I just didn't draw the input circuitry for the other channel's tube. Each tube has its own battery. I lucked out with heater resistance matching since each tube develops 1.52-1.54V across its own filament. While there is a 1.5V difference in voltage referenced from the grid to each end of the cathode, isn't the effective bias voltage the lower potential in this case?
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Old 14th March 2013, 02:31 PM   #2577
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Originally Posted by DIYspaceW View Post
A simple question;
Has someone tried LIFEPO4 battery for filament supply?
It looks like no one...Need to try...
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Old 14th March 2013, 02:41 PM   #2578
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I'm sure many of us have at one time or another tried batteries for filaments. You'll probably find that most have given up on batteries. I frankly didn't find that they sounded any better than a choke input supply with one of Rod's regulators. So the added grief of charging batteries etc was simply not worth it. And even if batteries were favoured for a DHT, this would be more appropriate for the lower current ones rather than the 26 which is rather hungry. Even worse for filament bias.
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Old 14th March 2013, 02:51 PM   #2579
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Quote:
Originally Posted by DIYspaceW View Post
A simple question;
Has someone tried LIFEPO4 battery for filament supply?
I don't believe there are any good reasons to try any battery for a filament supply!

In the case of LiFePO4 cells, the terminal voltage varies from 3,5V or more, down to 2,0V. The terminal voltage varies with temperature, too.

Meanwhile, for the 26 filament, the specification for the filament is 1,5V. To get the proper lifetime of the 26, the long-term voltage should be toleranced: +0, -4%. In other words the supply to the filament should keep within 1,44V to 1,50V - for most of the time, anyway. Running above the rated 1,5V is crazy - the lifetime will degrade very fast.

so to use a LiFePO4 cell on a 26 will require a regulator, unless you don't care about the life of the DHT.

Also, there is no guarantee that the cell will be quiet - other chemistries like lead-acid are in fact notoriously noisy.

Next, you will need a circuit to detect that the battery is low on voltage, and switch off the B+.

You can see from many reports in this thread that you can achieve complete silence with a 26, by using good B+ filtering or regulation, and my filament regulator kits.

With the complexity of charging, and low-voltage detection, the battery solution will cost much more, and still be a nuisance when you forget to recharge etc. And the sound is certainly not improved, nor is the noise!
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Old 14th March 2013, 02:59 PM   #2580
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Originally Posted by Rod Coleman View Post
I don't believe there are any good reasons to try any battery for a filament supply!

In the case of LiFePO4 cells, the terminal voltage varies from 3,5V or more, down to 2,0V. The terminal voltage varies with temperature, too.

Meanwhile, for the 26 filament, the specification for the filament is 1,5V. To get the proper lifetime of the 26, the long-term voltage should be toleranced: +0, -4%. In other words the supply to the filament should keep within 1,44V to 1,50V - for most of the time, anyway. Running above the rated 1,5V is crazy - the lifetime will degrade very fast.

so to use a LiFePO4 cell on a 26 will require a regulator, unless you don't care about the life of the DHT.

Also, there is no guarantee that the cell will be quiet - other chemistries like lead-acid are in fact notoriously noisy.

Next, you will need a circuit to detect that the battery is low on voltage, and switch off the B+.

You can see from many reports in this thread that you can achieve complete silence with a 26, by using good B+ filtering or regulation, and my filament regulator kits.

With the complexity of charging, and low-voltage detection, the battery solution will cost much more, and still be a nuisance when you forget to recharge etc. And the sound is certainly not improved, nor is the noise!
Thank you Rod,
I see...
I had your filament supply in 71A tube amp and was very happy,
now I didint yet decided what to use for 26.
I want to do it in separate case, so if I would use your filament supply
then the transformers would be in the other case, so the CLC filtering of DC...
How long can the wires be from one case to another (from DC basic filtering to filament boards)???
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