heater supply questions

[jarthel]

general heater supply questions.

I have tranformer has 3 secondaries. 1 5V for the rectifier and 2x 6.3V for other tubes. All secondaries has a CT.

question 1:
Should I use the transformer CT or use a derive one?
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question 2:
I've read a reply from a person (the person is a member here but the reply was from AA. Also but do not if I can qoute his name so I left it out) that said:
"Referencing the heater CT (whether a real CT or derived from a pair of 100 ohm resistors) is a smart idea. Making the heaters more positive than the cathodes (say ~20v or so) prevents the heaters themselves from emitting and can reduce hum."

how do I reference the CT? maybe something similar to this? http://headwize.com/images2/ciuff_nc3.gif (look at E182CC heater supply)

The circuit mentioned above uses derived CT as the Lundahl transformer he uses doesn't have CT. What if the transformer has a CT? how do I reference it to something else? maybe similar to the E182CC above?

The circuit above uses 1/2 B+ (I have limited knowledge so I could be wrong) as a reference. Is there a usual value to use? I've seen a blog from tubecad.com that uses 1/4 B+.
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question 3:
Do I need to reference all heater supply?

scenario:
I have 1 tranformer secondary connected to the heater of 2
6922s.

The heater supplies below uses a separate 100VA transformer (2 secondaries: 0-9V 4.17A).

I also have a regulated DC supply that is connected to a pair of 8CG7 (8.4V 450mA heater requirement). I needed this regulated supply it's hard to find 8.4V heater secondary unless you have it custom-made.

I have another regulated DC supply supplying 1 6AS7G (6.3V 2.5A heater requirement). I needed this supply because the secondary supplying the 6922s can't supply enough current for the 2 6922s and the 6AS7G.

Thanks for the help

[tubetvr]

Hi,

I think there are some different views on this but here are my answers to your questions.

Should I use the transformer CT or use a derive one?

If you are sensitive about hum levels use a low ohmic pot in parallell of the heaters, then you can adjust the pot for min hum, NOTE! this adjustment only works if the amplifier from the beginning has very low hum, i.e. is well designed, then it will be possible to tune the humpot to a minima.

"Referencing the heater CT (whether a real CT or derived from a pair of 100 ohm resistors) is a smart idea.

Yes it is a good idea to put the reference level, i.e. the wiper of the pot to a positive voltage and in some cases this has a very noticeable effect on the hum, recommended DC voltage of 15 - 40V over the cathode potential.

Do I need to reference all heater supply?

Yes, it is necessary in order to keep hum at a low and constant level.

In my experience DC heaters are only necessary for Phono stages, not for line amplifiers and certainly not for power amplifiers. If you use twisted wires for the heaters and route them correctly, (close to the chassis and away from sensitive inputs), the hum level from the heaters and heater wiring will then be lower or at level with other hum sources, say typically -100dB or so in a power amp. For me it is a bad design, (A bodge I think they call it in England) when you need DC heaters to reduce hum to a reasonable level in anything exept a phono amplifier.

Regards Hans

[jarthel]

Originally posted by tubetvr


If you are sensitive about hum levels use a low ohmic pot in parallell of the heaters, then you can adjust the pot for min hum, NOTE! this adjustment only works if the amplifier from the beginning has very low hum, i.e. is well designed, then it will be possible to tune the humpot to a minima.

what pot value and wattage would you suggest?

Originally posted by tubetvr
For me it is a bad design, (A bodge I think they call it in England) when you need DC heaters to reduce hum to a reasonable level in anything exept a phono amplifier.

Regards Hans

I have a feeling you didn't read my reason for using a DC heater :|


but thanks for the reply

[tubetvr]

what pot value and wattage would you suggest?

100ohm is common, for 6.3V 1W is enough.

I have a feeling you didn't read my reason for using a DC heater :|

You are correct but now when I read it more carefully I still don't see a reason for using DC, a 9V standard AC transformer and a resistor in series works as well, is much cheaper and have lower total power dissipation. For the 6AS7 there is no reason to not use a 6.3VAC transformer.

Regards Hans

[jarthel]

Originally posted by tubetvr


I still don't see a reason for using DC, a 9V standard AC transformer and a resistor in series works as well, is much cheaper and have lower total power dissipation.

no really cheaper. Since there are very few single secondaries available locally. and if they are available, they are the same price as dual secondaries.

I need at least 25VA on 1 secondary to be useful.

Originally posted by tubetvr

For the 6AS7 there is no reason to not use a 6.3VAC transformer.

Regards Hans

There are no 6.3VAC available locally.

ps. The extra cost in making the DC supply is neglible since I have most of the parts already from a previous project.

[thevoice]

Originally posted by jarthel


There are no 6.3VAC available locally.

Common as muck, actually, if you really are in Australia:

Jaycar stock a few like this

All the electronics retailers have these, and others like it with different ratings.

[jarthel]

Originally posted by thevoice



Common as muck, actually, if you really are in Australia:

Jaycar stock a few like this

All the electronics retailers have these, and others like it with different ratings.

heh not enough for a 6AS7 + 6SL7.

From what I can see most transformers are single secondaries. I would still need another transformer for the 8CG7.

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request: could we please get back on topic? This thread is not about cons/pros of AC/DC heating nor is it about where to buy transformers in Australia thanks

[lndm]

I happen to like using a small series resistance for reducing the inrush current to my heaters. For this I need a slightly overrated or overvoltage winding.

[EC8010]

Originally posted by tubetvr
(A bodge I think they call it in England)

We do indeed.

[SHiFTY]

All heater supplies must be referenced to something, even if you just connect one side of the heaters to ground (which works just fine for output valves).

For small signal valves, you need to be VERY careful not to exceed the heater-cathode voltage limit, especially if you are using a SRPP / cathode follower or other design. Check the datasheets for this value.

To reference the heaters to a high voltage, just put a couple of large value resistors from B+ to ground, and connect one side of the heater winding to the junction. Again, be careful not to exceed the heater-cathode voltage limit.