Transformer for tubes

baffled77

Member
Hi all,

Just a quick question.

I am a building 6N16B Sub-Miniature SRPP circuit for an output stage of CD player.

B+ is 200VDC.

Transformer -> BR1 -> CRC -> 200VDC.
6.3VAC for heaters also (no hum at low current).
230V - 140VAC roughly equates to 200VDC.
Happy with the power supply scheme.

Very low current draw.

What if my transformer was either:

- 100VAC secondary (too low after converted to DC still)
Or
- 300VAC (too high to start with)

How would you scale up or down?

Or would you go with the 140VAC to BR1 route as simpler from the start?

Thanks,
Luke.
 
Last edited:
depending on the secondary arrangement and available current you can change the rectifier setup. A center tap full wave will give lower voltage than a bridge but only works if you have a center tap. A voltage doubler works well if you've got the current but too low an AC voltage. You can also use a regulator to drop the voltage.

Need specifics on current requirements and the transformer.
 
2.5mA at 200VDC and 0.2A at 6.3VAC required approximately.
May need more for the 6.3VDC in terms of current, maybe double for 2 tubes, but this should be fine on most transformers.

I didn't go for the 140V, 6.3V transformer as was a bit big for inside CD player.

Another transformer, smaller:

-Primary 230 V
-Secondary 1: 6.3 V - 1A
-Secondary 2: 100V - 0.1 A

I assume voltage doubler would work in this case, then through Bridge and CRC?
 
Or:

TRANSFORMER 2 X 24V 12W
Power Rating : 12W
Isolation Transformer Primary Voltages : 70V to 140V
Secondary Voltages : 2 x 24V
Current Rating : 500mA
Plug Type : -
Product Range : -
Core Size : E16
Inductance : 1.8mH
Transformer Type : Flyback

Run this in reverse with pulsed DC, get 140V out. Then BR1-> CRC -> Anode 200VDC
 
Transformers in reverse tend to run less efficiency. I'd expect closer to 120vac output than 140vac.

If you've got a 100vac secondary, full wave rectified would give about 140 vdc. A voltage doubler would be somewhere around 280 vdc. If you want to fool around in PSUD (google, its a free program) you can calculate exactly what its going to do, but those numbers should be in the ballpark.
 
Hi all,

Just an update. Firstly, built the SRPP sub-miniature 6N16B tube pre-amp board for CD player. Only tested the heaters so far. Have the transformer, it's actually 6.3VAC and 150VAC outputs, so I can hopefully work with the latter 150VAC, full wave rectified for around 200VDC. Ideally should have been around 145V in, but hey.

1) Contribution: I am therefore posting a picture of the SRPP board glowing, pending testing fully - for all to see.

2) My question: Power supply board - once this is working, I can hook up the transformer, have a play around, and then continue to test the SRPP board. The power supply board is not really functioning well, hence the post.

- I am posting a photo of the strip/veroboard layout top and bottom. Before the caps, the full wave rectifier seemed to be working. After the caps, nothing - bridge overheats if anything. Even the CRC for the 6.3VAC line, nothing measured on first leg of resistor in line.

- Am I missing something? Probably something very simple.

3) Contribution: Posting a mock up pic inside CD player (just put in place for show). CD player: already tapped direct to VOUT on TDA1549 DAC and built A/B test stock output vs. tube via selector and output via Russian PIO caps. Should be nice.

IMG_5385.jpeg IMG_6130.jpeg IMG_6131.jpeg IMG_6132.jpeg IMG_6133.jpeg
 
A voltage doubler works well if you've got the current but too low an AC voltage. You can also use a regulator to drop the voltage.

Just a note on doublers.

Voltage doublers are problematic because the configuration is a dual half-wave, so the winding halves are separately used.

Ignoring the regulation issues, the form-factor for each secondary half is identical to a half-wave. To avoid overheating and winding damage the transformer must be designed for such pulse loads.

Also, the pulse from a half-wave tends to stimulate the tanks created by the transformer's parasitics, so the transformer will ring. Snubbers can fix that, of course.
 
What are your board's wire connections to and from? And what is the power supply supposed to be electrically?
 
What are your board's wire connections to and from? And what is the power supply supposed to be electrically?

Hi,

From topside board left, power in. The top golden cap CRC power rail has an input of 150VAC. Output will be just over 200VDC for tube anodes in parallel.

The bottom cap power rail is 6.3VAC in and just a CRC filter out for AC which will be fine here. Parallel to the heaters on the tube.

The bridge worked fine (tested prior to caps and resistor).

After caps and resistors, both rails have the same problem - no output. Top rail the bridge overheats with the caps and resistor.

Power supply - > cap in parallel -> resistor in series (positive line) -> cap in parallel -> output
 
so the 6.3 vac one, you can't run a polarized electrolytic cap across an AC line like that. They're meant for primarily DC loads. Honestly using big caps across a heater supply like that isn't much use anyway. If you're looking to filter HF trash out of the heater supply, an 0.1uf film cap will do that very effectively.

I honestly can't work out what you've got going on with the bridge arrangement. The hole pattern just isn't obvious to me what part is going which direction. Any chance you could draw in the diodes on the bottom side? It almost looks like you've got one side of the AC input line tied straight through to the filter caps.
 
so the 6.3 vac one, you can't run a polarized electrolytic cap across an AC line like that. They're meant for primarily DC loads. Honestly using big caps across a heater supply like that isn't much use anyway. If you're looking to filter HF trash out of the heater supply, an 0.1uf film cap will do that very effectively.

I honestly can't work out what you've got going on with the bridge arrangement. The hole pattern just isn't obvious to me what part is going which direction. Any chance you could draw in the diodes on the bottom side? It almost looks like you've got one side of the AC input line tied straight through to the filter caps.

Seems you're right on the AC line for the diode bridge. The plus output of the bridge feeds into the CRC, but the strip hasn't been cut, so it's seeing the AC on that line also. My error. I've tested the bridge prior to the CRC installation and seems fine. Seems the overheating is down to this. But then we're at the same situation as below, because it's a mirror setup.

What is strange is that the bottom one, I haven't tested with AC, only DC. No voltage is measurable. I simple used a 6V lantern battery and as soon as you put a test probe on the resistor in series, nothing is reading... So to make this clearer. I test the battery, fine, 6v, then test voltage at first cap, fine, same. Then test voltage at the first leg of resistor, nothing.
 
You can't pass DC through a capacitor, so that makes sense. Normally caps are used across a rectified DC line to flatten out the wave. Essentially its passing the AC part to ground but not the DC part. When used between amplifier stages, you'll pass the AC but not the DC.

Caps do have a fair bit of impedance though, so if you put a capacitor between your AC voltage source and your heater load, a lot of voltage will drop across that cap, which will create a lot of heat in the capacitor. I'd honestly skip this entirely.
 
You can't pass DC through a capacitor, so that makes sense. Normally caps are used across a rectified DC line to flatten out the wave. Essentially its passing the AC part to ground but not the DC part. When used between amplifier stages, you'll pass the AC but not the DC.

Caps do have a fair bit of impedance though, so if you put a capacitor between your AC voltage source and your heater load, a lot of voltage will drop across that cap, which will create a lot of heat in the capacitor. I'd honestly skip this entirely.

Just for illustration the top side is something like this.
Bottom is same minus bridge.

309601.image2.jpg
 
yeah, you can't pass an AC heater supply through that sort of filter network. It will not work correctly. If you rectify it and provide DC for heaters, then it works.
 
yeah, you can't pass an AC heater supply through that sort of filter network. It will not work correctly. If you rectify it and provide DC for heaters, then it works.

Agreed. I can modify the supply or just provide DC 6V from battery - or via rectifier into CRC then out to heaters. Or maybe just AC with small filter.

The issue is I tested the CRC with DC and by the end of the rail there is no voltage.
 
It should pass DC without issue if your resistor is not an excessively high value and everything has a good connection. If the caps are backwards, they will act more or less like a short, and you'd see a lot of voltage drop across the resistor.
 
It should pass DC without issue if your resistor is not an excessively high value and everything has a good connection. If the caps are backwards, they will act more or less like a short, and you'd see a lot of voltage drop across the resistor.

That's what I thought!

Anode DC Power Supply:
  • 1 x Bridge Rectifier for Anode 1A 400V - have 4 x 1N4007 (1A 1000V) Rectifying Diodes
  • 2 x Nichicon 120uF 400V Capacitors for CRC
  • 1 x Tenta 0.68uF MKP 200 - 600v - to bypass 2nd Nichicon
  • 1 x Resistor 10k 1 watt - try 3.3k for 200V anode
Heaters AC Power Supply:
  • 2 x Nichicon 10,000uF 16V Capacitors for CRC - Nichicon Standard Audio Grade caps
  • 1 x Resistor 1 ohm 2 watt
  • BRIDGE! Or DC source
Resistors not excessive by any means.
 
Hi everyone, long time since my last reply.

In any case, power board works fine now.

Anode power supply: Bridge was fine, AC lines isolated from downstream circuit on the board (good spot)!
Heater supply: AC 7.7 VAC to 1 ohm 2 watt resistor and parallel heaters yields 6.3VAC at heaters. That's good.
Generally: Some poor solder joints also found and corrected!

The transformer is rated 150 VAC and 6.3 VAC but AC line voltage in the UK has meant 7.7 VAC (sorted) and 188 VAC (not sorted yet) is seen on the output windings.

I need a 200 VDC B+ voltage. I've used PSUD2 software as previously mentioned. I have played around with values. Roughly 250VDC comes out of regulator with 120uF cap and R 3.3k 1 watt, so I changed to 300 ohm 1 watt and it is now saying 194 VDC or there abouts, which would be fine. I can fine tune that also.

Question: Do you think this method of value swap for resistor would be acceptable? From 3.3k to 300 ohm, both 1 watt?
 
I think exploring further, potentially my 188 VAC may be ok. With 3.3k in series and 470 ohm cathode resistor, that's say 3.8k. Voltage peak is about 196 VDC with mean around 180 ish. Would you say that sounds reasonable?

Could I check like this:

TR1 -> BR1 -> CRC (3.3k in there - series) -> 470 ohm load (parallel resistor - check voltage across this) and if 200 VDC or thereabouts, proceed to test on tube, with the 470 ohm as cathode R?
 
Back
Top Bottom