5vac rectifier heater voltage from a 6.3vac.

[pjd3]

Hi Folks, wondering if you think this is how it can be done.

I may be adding a GZ34 to lower B+ a bit. (my PT has no 5vac but has an extra 6.3vac secondary heater coil) I tried to figured this out and just wanted to see if you thought this would work to create 5vac rectifier heater voltage from a 6.3vac secondary coil.

*GZ34 draws 1.9 amps of heater current.
*Add a resistor in series with the filament so that it will see 1.3 vac across it (the remaining 5vac with be across the GZ34 filament.
* V / I = R so, 1.3 / 1.9 = .68 ohms
* P = V x I so, 1.3 x 1.9 = 2.47 watts.

So, if I haven't missed anything, then run .68 ohms of resistance in series with the GZ34 filament that is rated for say, at least 5 watts, maybe better make it 10 watts. The 6.3vac should divide up properly with the GZ34 filament and the resistor.

If this is correct, I noticed that Ohmite has .33 ohm 5 watt resistors. 2 of them in series would be around .66 ohms. Perhaps that would be close enough?

Thank for helping to double check this with me! And have a great Labor day weekend if you can.
Best,
Phil D.

[martin manning]

Your math is correct, and you should be able to find a 0.68 ohm 5W, like this: https://www.mouser.com/ProductDetail/Xi ... wZz2A8c%3D

[R.G.]

Martin's right. 0.68 is a stock value.

However, there are some gotchas lurking in the wings. First, the winding will be riding on B+. Is it well enough insulated for that? Probably, as 500Vdc isn't all that big a voltage, but still the 6.3Vac in a trannie was probably designed to run down near ground.

The second one is that 6.3Vac is a nominal rating. What actual voltage does yours put out? I had one that was 7vac nominal, and sagged down to 6.5 under load. I'm sure some of them are a bit low, too. If you're tuning it with a resistor, you might want to do a first tuning, then measure the AC on it under load, then correct

Another way to wipe some voltage off is with reverse-paralleled diode pairs. These subtract 0.7 to 1V in both directions. 6.3Vac (if it really is 6.3, not 6.1 or 6.9) has a peak voltage of 8.9V A diode pair drops that to peaks of 7.9 to 8.2, or an AC equivalent of 5.6Vac. Two diode pairs give 4.95Vac. So four 3A to 6A diodes will drop you right down to 5Vac without much calculation. The diode withstanding voltages don't matter at all, as a reverse-paralleled pair gives each diode a maximum reverse voltage of only diode drop.

Mouser has 6A/1kV diodes for as low as $0.19 each, 15A 1kV FREDs for 0.50 each.

[pjd3]

Thanks R.G. and Martin, sure helps fill in the picture some more.

I actually did discuss using the reverse-parallel diode scheme with a fellow tech in work. From what I recall, we thought that 2 series diodes in each reverse-parallel branch "might" drop the voltage to a good place Thats if each diode is dropping approx 0.7 volts for around a total 1.4 volt drop, only off by a tenth of a volt according to math. I surely thought that is worth a try and see how it looks under load.

Perhaps I can have a handful of different resistor values that are just plus and minus of the mathematical nominal of what I'd suspect, and then do some parallel or series addition tweaks to more closely hone it in for the rectifier filament. I should still see 5 volts across the GZ34 filament even though its riding along with the HV, correct?

You know, even after building 2 tube amps I still don't have it clear in my head whats happening with the 5 volts riding along with the rectified DC output. (pin 8 I think).
I really need to get that straight! Its sorta driving me crazy. I totally get the idea of ac riding on dc, I see that and deal with it all the time in work but, this 5 volt on the high voltage is bewildering me. Probably making it more complicated than it is in the head.

Just to clarify, I was going to use a pair of KT88's and diode rectification since the KT88's don't mind and would probably benefit from a somewhat higher plate voltage.
But, the head of EE in work brought in a pair of real St Petersburg EL34's that were pretty much never used. I decided these will be fine for this next project, that the KT88's were a little overkill anyways. So, being that the PT I have is 335-0-335, the EL34's would have been seeing too high a plate voltage. With a GZ34 I expect them to see something around 430vdc on the plates as opposed to around 460vdc with SS rectification.

thanks guys!
best, Phil D

[R.G.]

Just as a point of reference, tube rectifiers always conduct from what would be the plate (if it had a control grid...) to the cathode. In rectifier tubes, there are actually two plates and one giant cathode. The cathode is what has to be heated so as to boil off electrons, which is what actually carries the current to the plates. But the electrons are negative charges, so the >current< flows from plate to joined cathode.

In more possibly useful info, tube diodes actually have very little "forward voltage drop" like solid state diodes, but they have a large internal resistance. That is what accounts for the 30-50V loss across the diodes and the lower output voltage of a tube rectifier compared to simple solid state diodes. But you really can fake this out by putting a resistor in series with solid state diodes. This makes a very, very close approximation to a tube rectifier. Weber, the transformer makers, also make solid state replacements for tube rectifiers. These are - yep, you guessed - two diodes and a power resistor, maybe a NTC thermistor. There are web pages that recommend resistor values.

In any case, get some of those 1kV 6A diodes. You can put two in series with each transformer HV lead before it goes into the tube rectifier. If you do this, a shorted tube rectifier can't kill your filter caps, output tubes, output transformer, etc.

It's a good idea to parallel any SS diodes that you put in series with 1M or so resistors so reverse voltages are divided equally and a weak one won't die, then let the other one die.

ACK!!! You got me going on power supply designs...

[martin manning]

Just as a point of reference, tube rectifiers always conduct from what would be the plate (if it had a control grid...) to the cathode. In rectifier tubes, there are actually two plates and one giant cathode. The cathode is what has to be heated so as to boil off electrons, which is what actually carries the current to the plates. But the electrons are negative charges, so the >current< flows from plate to joined cathode.

In vacuum rectifiers there are (physically) two separate plate structures, and each has its own cathode. In rectifiers with indirectly heated cathodes (the more modern 5AR4/GZ34) the cathodes are essentially one, electrically speaking. In rectifiers with directly heated cathodes (5U4, 5Y3) not so much. Below I've drawn up equivalent circuits for the two types. Generally speaking, the heaters are referenced to B+ to minimize transformer winding and heater-cathode insulation requirements.

Indirectly heated type
- Heaters are paralleled
- The B+ connection is at the joined cathodes (pin 8) to avoid dragging the HT current through the heater.

Indirectly heated type
- Heaters are series connected (in the diagram below I've connected the cathodes at the mid point of the heater resistance to represent the average cathode voltage).
- It does not matter which heater connection is used for the B+, as in either case the B+ alternately flows through different parts of the heater resistance. Back in the directly-heated era, there was sometimes a center tap on the heater winding for the B+ connection to equalize the current pulses.