OTL Heater Questions

[desperateaudio]

Originally Posted by piano3

The 25 volt supply only knows that it has a certain total resistance across its terminals, will obey Ohm's law and a certain current will flow. They voltage drop due to this current across the 3 different valves will also be determined by Ohm's law and in each case will simply be the product of the current flowing and the heater resistance of the individual valve. Series heater connection is tricky enough even with identical valves. Personally I would never recommend it be attempted with the very different valves you are using.

so u agree that if it were 2 - 12ax7s with their heaters in series to each oother than no issues.

[piano3]

Yes; also don't forget to respect the maximum heater-cathode potential for each valve. This is given on the data sheet.

[woody]

Personaly I would just go for a 6v transformer for all 3 tubes sure would be simplier.

[palmas]

Originally Posted by piano3

Only if all 3 require the same current

Even then it is very risky as impedance varies a lot when heating up, and different tubes can take different times to heat up.
Do not do it!

[DF96]

Series heater connection is OK provided that all valves take the same current and either:
1. they are all similar valves (e.g. all 6.3V 300mA) and there are only a few of them (say up to four), or
2. they are specifically designed for series heater connection, such as the P-prefix 300mA TV valves in Europe or the A-suffix controlled warmup tubes in the USA, or
3. you use a constant current supply.

In other situations you may get voltage hogging, where the hotter valve gets hotter and the rest get cooler.

[desperateaudio]

ok but what about this series heater circuit? V4-V11 are 6LF6 tubes with 6.3v heaters drawing 2 amps each. V2 and V3 are 12ax7's ran at 12.6v 150mA of current each. The V1 is a 12au7 ran at 12.6v drawing 150mA of current. The R57 is a 82 ohm 5 watt resistor which I suspect is replicating the resistance of the missing tube and the C23 is a 0.1uf cap to ground. So why the different tubes with varied current here? Thoughts?

[DF96]

No ground reference. Heaters should always have a DC reference.

[cnpope]

Originally Posted by desperateaudio

ok but what about this series heater circuit? V4-V11 are 6LF6 tubes with 6.3v heaters drawing 2 amps each. V2 and V3 are 12ax7's ran at 12.6v 150mA of current each. The V1 is a 12au7 ran at 12.6v drawing 150mA of current. The R57 is a 82 ohm 5 watt resistor which I suspect is replicating the resistance of the missing tube and the C23 is a 0.1uf cap to ground. So why the different tubes with varied current here? Thoughts?

It looks basically OK. Each individual chain has heaters drawing the same current, and R57 correctly drops the extra voltage in that chain.

The problem of inequivalent tubes with the same nominal heater current but different warm-up rates being in a single chain was mentioned a while back in this thread. (The ones that heat up fastest can get an excessive voltage when the power is first turned on.) I encountered this problem in a recent project. A way to mitigate it is to connect a suitably chosen power zener diode (if it is DC chain) or series-connected back-to-back zeners (if it is an AC chain) across the heater of each faster-warming tube, to limit the voltage it can receive while the chain is warming up.

This will work very effectively with a DC heater chain, since the zener can be chosen so that it will limit the voltage to just slightly above the desired operating voltage for that heater. In an AC chain it is not so ideal, since the back-to-back zeners must be chosen so that they conduct if the *peak* of the desired heater voltage is exceeded (for example 12.6 * sqrt2 ~ 18V for a 12.6V heater). This means that while warming up, the fast-warming tube might still receive an overvoltage in the form of something approximating an 18V square-wave initially. But still, it does put an ultimate limit on how much of an overvoltage the tube can receive. I'm using this technique in an AC heater chain that has hefty 6082 power tubes, a 6SN7, and a pair of parallel-connected 12AX7 tubes, with back-to-back zeners across the 6SN7 and the 12AX7 tubes. They do still get a bit brighter at first and then dimmer agaiin, but it is much less pronounced than before I added the zeners. And nothing untoward has happened so far.

There would be no such problem with a DC chain, and the zeners should do a more or less perfact job of protecting the fast-warming tubes.

Chris

[desperateaudio]

Originally Posted by cnpope

It looks basically OK. Each individual chain has heaters drawing the same current, and R57 correctly drops the extra voltage in that chain.

The problem of inequivalent tubes with the same nominal heater current but different warm-up rates being in a single chain was mentioned a while back in this thread. (The ones that heat up fastest can get an excessive voltage when the power is first turned on.) I encountered this problem in a recent project. A way to mitigate it is to connect a suitably chosen power zener diode (if it is DC chain) or series-connected back-to-back zeners (if it is an AC chain) across the heater of each faster-warming tube, to limit the voltage it can receive while the chain is warming up.

This will work very effectively with a DC heater chain, since the zener can be chosen so that it will limit the voltage to just slightly above the desired operating voltage for that heater. In an AC chain it is not so ideal, since the back-to-back zeners must be chosen so that they conduct if the *peak* of the desired heater voltage is exceeded (for example 12.6 * sqrt2 ~ 18V for a 12.6V heater). This means that while warming up, the fast-warming tube might still receive an overvoltage in the form of something approximating an 18V square-wave initially. But still, it does put an ultimate limit on how much of an overvoltage the tube can receive. I'm using this technique in an AC heater chain that has hefty 6082 power tubes, a 6SN7, and a pair of parallel-connected 12AX7 tubes, with back-to-back zeners across the 6SN7 and the 12AX7 tubes. They do still get a bit brighter at first and then dimmer agaiin, but it is much less pronounced than before I added the zeners. And nothing untoward has happened so far.

There would be no such problem with a DC chain, and the zeners should do a more or less perfact job of protecting the fast-warming tubes.

Chris

Thx, may I ask your opinion on the DC ground reference in the thread put forth by DF96

[cnpope]

"Thx, may I ask your opinion on the DC ground reference in the thread put forth by DF96"

Well, I found his comment a little "telegraphic," but I think he may have been saying that the heaters ought to be referenced to some definite voltage, and not just left "floating." And there can be arguments favouring making that reference voltage greater than ground.

There can also be issues about not exceeding heater/cathode breakdown voltages. If the cathodes of some tubes are at significantly different voltages from others, then it may not be possible to stay within all heater/cathode breakdown limits if all the tubes share a common heater supply. But that is a different issue from what DF96 was referring to, I think.

Chris