HF AC DHT heating

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Alex Kitic
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Post by Alex Kitic » Wed Jun 11, 2014 6:06 am

Paul Barker wrote: Yes maybe, but doubt it was the only factor, and I doubt it would have been a complete failure to pass any voltage, which was what happened. It didn't do anything, just shut down, despight being connected to supply.

We have always found that there are mechanisms in them which frustrate our efforts at times. But once we get them working they always do work. Any which don't work we bin.
The electronic transformer shuts down although connected to supply in two cases:

1) Not enough current draw: oscillation cannot be sustained. Maybe with very little output, a 10W bulb might have the filament slightly reddish instead of dark. On tubes this would look like not working at all. Operation should resume once an adequate load is applied (touching the wires with a 10W automotive bulb makes both bulbs light up).

2) Too hot (due to drawing too much current, like my units rectified for DC operation on 813, almost 9A current draw, which is on the upper side for a 105W unit). Operation will resume once it has cooled down!

Of course in either case your unit might have sustained terminal damage, but that is improbable. I hope you have not thrown it in the bin?

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Paul Barker
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Post by Paul Barker » Wed Jun 11, 2014 6:24 am

Its not worth anything to me, I wouldn't waste my time farting about with it.

There is a scenario which I had considered, and if it is that then the present attempt will also crash and burn.

We encountered this with smps power supplies at times. Filaments can offer almost a short circuit from cold then as they heat up their resistance increases. So the smps safety device sees overcurrent and doesn't allow itself to start. In that scenario we used to start it with a resistor in series with the filament then short out the resistor after filament heated up.

I doubt this is the scenario but it can't be ignored as an outside posiibility.

Most likey is that because of all my pratting around with it I broke it.

This one I am leaving intact and doing my pratting about off board.
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Alex Kitic
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Post by Alex Kitic » Wed Jun 11, 2014 8:50 am

Paul Barker wrote: We encountered this with smps power supplies at times. Filaments can offer almost a short circuit from cold then as they heat up their resistance increases. So the smps safety device sees overcurrent and doesn't allow itself to start.
The difference between SMPS and Half Wave is that the latter is the "basics" that allows SMPS to work. After the half wave circuit, and the output transformer - there is nothing, while in the SMPS there is additional circuitry.

This additional circuitry contains the overcurrent protection (just like linear regulators do, for instance most 1.5A devices like LM317 will not start on a cold 2E22 which draws 1.5A when heated).

There is no overcurrent protection in Half Wave circuitry - it will not start, or work intermittently (flashing light) if there is not enough current. If there is too much current being drawn, the task is on the power transistors which usually do not contain any safety devices (and can be destroyed). Usually this does not happen since before a transistor is destroyed it overheats... and that is where higher quality units excel, at having self-resettable protection, probably in the form of thermistors.
Paul Barker wrote: Most likey is that because of all my pratting around with it I broke it.

This one I am leaving intact and doing my pratting about off board.
If it did not work at all afterwards, you most probably broke (tiny fracture, perhaps) the toroid. Actually, ferrite core material is rather easy to break, and can break if "overloaded" as well (but in your case there was no overload).

*******

I am still in the planning phase: I would like to use 5 turns per secondary, but the resulting voltage will be too high even with regulation. To overcome the input/output voltage differential of a 7805 or LM317, I must have less than 30V ripple and for proper operation set it at some voltage below the minimum peak after the first cap. It seems that either way it will be near 300V, which is still a long way from the 250V I think I would need.

On the other hand, it seems that HF toroid transformers do not suffer particularly from a volts per turn problem (decreasing the number of primary windings leads to more volts per turn) but rather from minimum required number of turns for operation... I will have to check the operating basics and than decide whether it makes sense to remove some of the primary (I would rather not do that... but you cannot have everything tailor made and buy it in a shop... you must do something to modify it to your needs).

Alex Kitic
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Post by Alex Kitic » Wed Jun 11, 2014 12:18 pm

I have just blown what seems to be a fuse (marked on PCB as F1) and all 4 diodes in the bridge after connecting the cap (330uF)!

Before that, I was operating the el.tr. with a 55W automotive bulb (wow, splendor of light!) without any problems or heating... (the unit is 160W, after all). Voltage was measured at 25V approx. on my DMM, and 5.2-5.3V on the toy analog meter mentioned before in the thread. Expecting voltage above 10V (11.5V probably) the readings make a lot of sense.

But once the cap entered into play, I blew the fuse (I guess that was a fuse) as well as the fuse of the mains socket... and all 4 diodes.

Strangely, these diodes were not 1N4007 as previously found, but 1N5399. They seem to be very similar, though, so I am a little bit surprised? What could have happened? Cold start, overvoltage, over-current?

I am going to replace the diodes with BY255 and try again, first without cap. I don't know about the fuse, though...

Any ideas? I was suprised that this has happened, but if the cap has exceeded the current limit of the diode, why did it not happen previously with the same type cap on the units that I am using?

I'm inclined to say because that was not a sh... Chinese cheapo unit.

160W transformer used

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Paul Barker
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Post by Paul Barker » Wed Jun 11, 2014 2:54 pm

I suppose get better diodes.

The units look a lot easier to adapt. Shame about the diode issue.

What if you change to a pi filter with say 80uF then 300ohm power resistor then 220uF. Give the diodes easier life but also give you new output voltage to discover and adapt to.
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Alex Kitic
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Post by Alex Kitic » Wed Jun 11, 2014 5:18 pm

Indeed, the diodes were not good. It is quite possible that 1N5399 would be just fine for the task, but the specimens installed are sub-quality.

I had to enlarge the holes, and have installed BY255 instead: 1300 PIV and 3A continuous current. No problems with the diodes since, with or without cap.

I had to externalize the fuse - but I did blow several fuses. In the end I understood that the issue was with the way I started it - without switch, by inserting the plug into the socket. I expect that with a switch this would not happen... but it seems that a fuse is always a good idea.

Modification: the original unit had 67 primary and 7 secondary windings. I chose to avoid messing with the primary and concentrated on the secondary.
First tests have shown the usual predictable results - very bright light from the 55W bulb, and even brighter light with the cap. The increase in DC voltage was basically from 208 to 312V (330uF cap), and the increase in measured HF AC (with my DMM) was 1.8x. Original 24.5-25V, with cap 45V approximately.

Assuming 11.5V or maybe slightly more, I have decided to go for 2x4 turns, expecting something between 9.6 and 10V per secondary. The wire I intended to use was too thick (insulation, probably), thus I used some (Litz) wire 1mm^2, AWG 17 approximately. AWG16 is recommended for 0-10ft in critical applications, thus I expect that at maybe 1/2ft AWG17 Litz would be fine for HF AC). I wound one brown and one blue wire parallel to each other (minimizing stray capacitance between the windings, and improving efficiency) into two separate windings which are basically spread across almost 300 degrees - at first with 5 turns.

The result, each with 55W bulb, without cap, was rather dimmed light (11.5/7*5=8.22V) but with cap the light was again very bright (11.5x1.48/7*5=12.15V (It measured 26.5V on my DMM).

Since it is rather easy to remove one turn (just extract one turn, I do not even need to unsolder anything), it came back to my original intention, 4 turns per secondary. With the cap, lights were dimmed, but somewhere between standard light and 5 turns without cap. DMM measurement almost 23V, and basically in accordance with my expectation: 11.5x1.48/7*4=9.72. Measured with the toy meter, interestingly 4.9, just slightly lower than 5 (which is what I measure with the units I am using in the RH813). With 5 turns, at estimated 12V, the toy meter was at 6V. It seems that if current draw is approx. 5A, it is going to show approximately 1/2 of the expected RMS reading.

Finally, the 55W bulb is probably drawing a little more current than the heater of the 813. All measurements and testing was performed with my standard 223V home mains, while once I try them in the RH813, they will operate at 230V (boost transformer). Thus I expect that the voltage per secondary will be slightly lower than the units I am using, and probably ranging between 9.7 and 10V. Being in the 5% tolerance (on the lower side), I guess that is totally acceptable.

What remains to be done is to test it on the amp. That will of course have to wait a little bit. I am interested to find out: 1) voltage values compared to currently used units; 2) sound: hum/noise/buzz issues (if any), and of course how does it sound, i.e. whether there are any differences.

Although this unit is declared as operating at "just" 30kHz, I do not expect to be able to hear any hum/noise/buzz, since I am not hearing any with the units I am currently using. As for the sound, I guess one HF AC unit is equal to another.. or is it? This is going to be a test of a two secondary unit, but I cannot imagine that using separate transformers for each filament should lead to particularly audible differences...

Anyway, if everything goes well, one transformer and one cap cost at least 2x less than two transformers... and you can always keep the second unit as a spare :) Not to mention the space, since these units are rather small anyway.

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Paul Barker
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Post by Paul Barker » Wed Jun 11, 2014 5:43 pm

yes the space saving is the practical winner that and the low cost. Which just leaves the sound.

If time permits I need to build an amp with only dht output valve. Heat it with this and with Doc Andrew's special dht heater supply and see what the panel think. At present negotiations sounds like that will be from September to November. Also depends on my situation at the time.
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Alex Kitic
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Post by Alex Kitic » Wed Jun 11, 2014 6:30 pm

There is a couple important things we have learned from this so far:

1) It is very difficult to work with a small transformer: I was not able to use the wire I was planning to - fortunately enough, the wire used for the secondary can be rather thin as compared to our expectations.

2) The wire used should be either relatively thin, or multistranded (thin) wire. This because of the frequency. Nothing above AWG24 is adequate (in solid wire).

3) If modifying some cheap unit, it could be a good idea to change diodes (or bridge rectifier): they are probably sub-standard quality.

I would go as far as to suggest modifying higher quality units as a better alternative. Take the Philips Primaline shown in that video: besides having all the protections, it is also equipped with toroid transformer.

If working with a smaller unit, it might be a good idea to wind your own ferrite toroid transformer - ferrite cores should be rather cheap, and the number of turns is feasibly low (on my unit only 67 turns).

Now we need to assess the sound quality...

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UPDATE ON NEWLY TWEAKED UNIT

Post by Alex Kitic » Thu Jun 12, 2014 8:23 am

I have just tried the new unit in the real amplifier, with promising results.

1) There has been no problem with the fuse - it seems that having a switch helps establishing a fast contact (unlike plugging into a socket).

2) No appreciable difference in hum/noise/buzz without preamp on. Maybe a little bit more, but having double the current must mean more ripple with the same cap. Anyway, once the preamp is on, everything drains in the hiss of the tubes (and since it is a low hiss indeed, it just confirms the good result).

Very important - no caps or filtering at the output (completely separated secondary windings), but no relevant noise on the output (RF, oscillation, or anything).

3) Output voltage on the heaters - as you clearly put it, an assumption. Just like I assumed it will be slightly below 10V (and the other units are slightly above 10V) - the slightly dimmer glow confirms it. The glow reminds me very much of the 9.7V DC set with linear regulators.

I am measuring it at 16.2V on my DMM, though (it should be more?), but the toy meter reads it at 5.4V ... no certainty there. A good instrument would be very helpful, but I am very much confident that the voltage is between 9.6 and 9.9V -- all of which should be OK in Paul's experience.

As it is at the moment, the only external element is the capacitor (and the fuse, of course, but that can be arranged differently). If listening tests prove disappointing (perceived as underpowered, i.e. too low heater voltage), I intend to try the "regulated" solution. Basically, put back the 1 turn (easy) to return to 5 turns per secondary - but reduce the DC voltage that operates the circuit. In other words, one 100uF-150uF cap followed by a regulator of sorts (like a 7805 up on 250V zener string), to get a regulated output of 255-260V approximately (with any mains voltage). This should give 10V at the output of each 5 turn secondary, plus ripple will not be 7-10V (at the input of the half wave) but mV or even micro-volts... with predicted total inaudibility of any noise or hum or whatever. Unfortunately, these few elements would have to be placed externally to the unit, on a small separate PCB.

Hopefully some listening tonight, to evaluate the sonic results. Frankly, I am not expecting any relevant difference, except for heater voltage induced (i.e. maybe I like it better at 10.2V than at 9.7V).

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Re: UPDATE ON NEWLY TWEAKED UNIT

Post by Paul Barker » Thu Jun 12, 2014 9:01 am

Alex Kitic wrote:(i.e. maybe I like it better at 10.2V than at 9.7V).
That is mot likely, but the valves will last longer. If the sound is about the same I would be very happy with 9.7v.

I am sure I am not telling you anything you don't know, that TT filaments life is not a linear relationship with voltage, life expires at a more rapid rate during over voltage. Miniscule undervoltage like 3% preserves life and has no ill effects, too low a voltage can cause what Phil aluded to when he cut and pasted a statement about under voltage filaments, but it isn't going to happen at 9.7v. Just output power therefore volume therefore sounds better. Same volume level and it probably sounds the same, but life of valve difference between 9.7 and 10.2 tangible.

The simple test is heat one with known voltage heat same one with your assumptions. tune to exact same kathode current.
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Re: UPDATE ON NEWLY TWEAKED UNIT

Post by Alex Kitic » Thu Jun 12, 2014 9:29 am

Paul Barker wrote: That is mot likely, but the valves will last longer. If the sound is about the same I would be very happy with 9.7v.
That is extremely reassuring :D because in this case voltage seems to be below 10V...
Paul Barker wrote: The simple test is heat one with known voltage heat same one with your assumptions. tune to exact same kathode current.
When you say cathode current, I guess you mean heater current (both tubes cathode current is already set to equal with LM317 regulators...)?

Do you think I could measure HF AC current accurately with my DMM (I would have to use the "not fused" setting)?

Furthermore, do you think that HF AC current can be accurately measured with an AC clamp meter? Maybe you've got one and can try?

In this case, measuring 5A would mean exact voltage, 5.5A would probably mean overvoltage, etc...

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Post by Paul Barker » Thu Jun 12, 2014 10:18 am

I forgot you used a CCS on cathode..

Then adjust for identical anode voltage.
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Post by Alex Kitic » Thu Jun 12, 2014 12:33 pm

Actually, there is no difference in anode voltage when using one device (which is above 10V) and the other device (which is below 10V)... probably due to the fixed current draw.

The two tubes currently in use differ in voltage across the tube and bias (cathode) voltage at equal current draw, but that is because the two tubes are not identical.

BTW, what size is the ferrite core that you are using in your multiple-secondary project? I found some ferrite toroid cores available locally
toroid coated ferrite core -- it is 31.5mm external diameter, 19.5mm internal dia. and 12mm high.

I have the impression this core is a total overkill, it would probably be good for 200W?! Or maybe not... but the 19mm of inside diameter seem a good idea when attempting to wind my own primary and secondaries...

Alternatively I could get a smaller core, just 14mm of inside diameter, 7mm high -- this one might be small enough to fit inside the small cheap 60W unit I intend to modify...

I was also considering the use of NTC devices in series to limit the current inrush. Any experience with that?

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Post by pre65 » Thu Jun 12, 2014 1:07 pm

Alex Kitic wrote: I was also considering the use of NTC devices in series to limit the current inrush. Any experience with that?
I use inrush current limiters (thermistors) on most of my amps (that have toroidal mains transformers) on the mains inlet.

So, slow build up of power, and using 6D22s damper diodes in the HT rectification gives all the valves 30 seconds of heat before the HT rises. :)
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Post by Alex Kitic » Thu Jun 12, 2014 1:55 pm

pre65 wrote: I use inrush current limiters (thermistors) on most of my amps (that have toroidal mains transformers) on the mains inlet.
Any practical advice, what values (resistance at 25 ohms) and what max. current values do you use most often?

Current inrush after the insertion of a 330uF cap is 30A on the diode bridge at the input of the electronic transformer. After stabilization, the transformer circuitry draws for instance 100W, which equates to approximately 450mA. What value would you use?

I am, of course, limited with what I can find locally. I was thinking whether 16 ohms (R25) and 2.9A current limit would be adequate, or a higher resistor value would be a better option?

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