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Current drive for 6922 filaments

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I cannot think of a good reason to have a separate regulator for each heater. What was your reason?

At least three reasons:

- As I mentioned just now, three different bias levels were used for the filaments, so at least three separate regulators were required for each channel;

- Allen's preference was for current regulation, and 6922s are not designed for totem-pole series connection of their filaments.

- LM317s only cost about 40p, so why not? That's one of the joys of DIY...

Alex
 
Perhaps I didn't make myself clear - the heater current appears to be consistent for a given brand, with the EH 6922s appearing to be working at around 365mA, and the Sylvania ones at 300mA.

I think this is a very important point that should be made more widely known. It's a very telling reflection on the maker/distributor of these tubes. Did the retailer specify the heater current for the EH tubes and put a BIG NOTICE that they deviate from common 6922 specs?

Perhaps I didn't make myself clear - the heater current appears to be consistent for a given brand, with the EH 6922s appearing to be working at around 365mA, and the Sylvania ones at 300mA.

My tube substitution manuals denote these types as parallel heater supply subs only. With no other explanation. Surely means two different heater currents.
 
Alex M said:
Allen's preference was for current regulation
Why? Voltage control of heaters is better, as it works with the heater temperature response instead of against it. The only downside of voltage feeding is the initial current surge, but a little series resistance can fix that.


Modern versions of ECC88/6DJ8 and E88CC/6922 do not distinguish between these two similar but different valves. You need to buy NOS to know what you are getting.
 
Valves intended for a voltage heater supply should be fed voltage, especially when near-equivalents are likely to have different current draw. Current supply is OK for valves designed for that use, or valves which you know are all identical and you are not too bothered about correct heating.
Well expressed !
In addition, any of the suggested "improvements" of current-feed in this thread
will render the tube positions needing adjustment whenever a tube is replaced !


Repeat after me : " 6922 has 6.3Volt filament "
 
I think this is a very important point that should be made more widely known. It's a very telling reflection on the maker/distributor of these tubes. Did the retailer specify the heater current for the EH tubes and put a BIG NOTICE that they deviate from common 6922 specs?

I could name names, but I won't until I have put 6.3VDC across a handful of valves and measured the current in situ.

Alex
 
Why? Voltage control of heaters is better, as it works with the heater temperature response instead of against it. The only downside of voltage feeding is the initial current surge, but a little series resistance can fix that.

Allen isn't around any more to confirm one way or another, but I recall him saying it was partly because of the low stress at power-up, and partly because he was convinced current regulation sounded better.

At the time I followed his designs to the letter, but now I can certainly see the advantages of voltage drive.

Alex
 
Yes, of course - a 100R pot across the resistors I have now would do this quite neatly. I am just reluctant to do too much tuning for individual valves.

For now, my plan is to use fixed resistors to increase the current to the EH 6922s so the voltages are closer to the data sheet value, and then in due course to change to voltage regulation.

Alex

I was surprised at the way AC, DC voltage regulation and DC current regulation sounded different with indirect heated tubes. Preferring current regulation, but adjusting for specified voltage across the heater. Even with the same type of tube the heater current will vary for a fixed voltage, and vice-versa.

I suspect that with 12 pcs 6922 'type' tuning will be necessary unless you move to voltage regulation. You could try DC voltage regulation first, see how it sounds to you, you may even prefer it and it would then run all of the different types of that tube at the same voltage.

With adjustable current regulation you can not only run all of the 6922 types and variants at correct heater voltage, but also the 7volt 'equivalents'. Once the heater voltage is adjusted, it will stay correct for the life of the tube. Very simple, exacting yet flexible.

Depending on how much AC voltage headroom you have at the secondary of the transformer, you could try a small value pair of resistors in common mode to help limit current charging pulses from the rectifier into the first filter cap (between the rectifier and the first cap).

HK
 
Perhaps I didn't make myself clear - the heater current appears to be consistent for a given brand, with the EH 6922s appearing to be working at around 365mA, and the Sylvania ones at 300mA.

6922EH heater current

(310 +/- 25) mA

https://www.newsensor.com/pdf/electro-harmonix/6922eh.pdf

As I said earlier, I plan to change from current to voltage regulation for the heaters, since this is far better for tube rolling.

Good idea, a valve designed for constant voltage must be fed by constant voltage.
 
That sould be true if heaters were fixed resistors.

For constant voltage

V = I R = constant

Tungsten, often used in heaters, has a positive temperature coefficient, i.e. its resistance increases with temperature. Clearly an increase on temperature yields an increase on resistance, which in turn yields a decrease on current, which in turn yields a decrease on heat, which in turn yields a decrease on temperature, the heater is self protected.

For constant current

I = V / R = constant

Clearly an increase on temperature yields an increase on resistance, which in turn yields an increase on voltage, which in turn yields an increase on temperature which in turn yields an increase on resistance, which in turn yields an increase on voltage (voltage hogging)
 
For constant current

I = V / R = constant

Clearly an increase on temperature yields an increase on resistance, which in turn yields an increase on voltage, which in turn yields an increase on temperature which in turn yields an increase on resistance, which in turn yields an increase on voltage (voltage hogging)


All grey theory IMHO. Zillions of TV sets were produced in Europe without power transformers, but a 300 mA series chain of P and E tubes' heaters instead, fed through a series resistor or capacitor directly from the - then - 220 Vac mains. Heater failure rather was a minor cause of a broken TV. Voltage drop across each heater used to be according to the datasheets within close tolerances.
Btw, never heard of sonic differences between series and parallel heating. Where should they result from?
Best regards!
 
Good idea, a valve designed for constant voltage must be fed by constant voltage.


There are zillions :D of European tube types designed for 6.3V parallel heating as well as 0.3A series heating. In other regions the current might be replaced by 0.45 or 0.6 A. Now, who tells the heater whether it is fed in parallel or in series?
Out of curiosity I've tried it with multiple different tube types (we've yet had this discussion several times here :rolleyes:), and I've never observed heater voltage runaway under feeding from a 0.3 A CCS. The measured heater voltage drops remained well within the datasheet specs. I think the heater simply controls itself by it's thermal radiation.
Best regards!
 
That sould be true if heaters were fixed resistors.

For constant voltage

V = I R = constant

Tungsten, often used in heaters, has a positive temperature coefficient, i.e. its resistance increases with temperature. Clearly an increase on temperature yields an increase on resistance, which in turn yields a decrease on current, which in turn yields a decrease on heat, which in turn yields a decrease on temperature, the heater is self protected.

For constant current

I = V / R = constant

Clearly an increase on temperature yields an increase on resistance, which in turn yields an increase on voltage, which in turn yields an increase on temperature which in turn yields an increase on resistance, which in turn yields an increase on voltage (voltage hogging)

We need to first make clear the difference between a filamentary tube, and one where the cathode is separate from the heater. DHT have filaments, IDHTs have heaters. Thoriated tungsten filaments are different from oxide coated filaments, and both emit electrons which are drawn to/from the plate, depending on your view of electron flow, and there are two to choose from, to the same end.

Heaters are different, they are not carrying plate current. And we are talking to the OP, 6922.

Resistance increases with the increase of the heat provided to the heater within the valve. Once heated to the rated value, the resistance is constant, provided that the current through that (now) fixed resistance is constant, so will the voltage also be if current regulated.

The fully heated resistance of the heater will, and does vary from sample to sample - even with tubes of the same type/brand/batch.

Speaking for an individual IDHT, once heated to rated voltage,the resistance does NOT vary and constant current will develop (slow start up which is of benefit with a sufficiently delayed B supply) a constant/regulated voltage across the heater. Which is another way of saying what was just said, but I feel necessary considering your reply.

To say otherwise is incorrect.
 
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Alex M said:
Allen isn't around any more to confirm one way or another, but I recall him saying it was partly because of the low stress at power-up, and partly because he was convinced current regulation sounded better.
I find it hard to believe that the sound can possibly be affected by the form of heater regulation, especially as the valves were designed to run from an unregulated AC voltage. The thermal mass of the cathode means that you would need a catastrophically poor heater power source with huge 1/f noise in order to hear any change.

These valves were designed to have their start-up current limited by the series resistance of a transformer secondary. You can mimic that by using a voltage supply with a few ohms in series.
 
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