When my headphone amp has been operating for some time, the power transformer gets warm, not hot, I can easily physically put my hand directly on the power transformer for a minute or so and not get burned by it.
Hammond claims their products can output more than the rated power. But, you must consider Rectifier Efficiency. When using a bridge rectifier you multiply the rated current by 0.62 to get the effective current, in this case about 1.2A. The BR doesn't use the whole sine wave, some of the power is lost.
Obviously it works, for now, but something to think about. I don't believe DC is required to heat this amp, if the heater supply is properly elevated a good 60-75v. Fortunately it's yours so you can tinker!
Obviously it works, for now, but something to think about. I don't believe DC is required to heat this amp, if the heater supply is properly elevated a good 60-75v. Fortunately it's yours so you can tinker!
I did some calculations before putting the 220k/2W 5% bleeder resistor across the +HT supply, if we do some basic Ohms Law calculations, we can calculate how much current is flowing through the 220k/2W 5% resistor.
Okay so my measured +HT is +306V DC.
I=E/R
I=306V/220k
I= 0.00139090909091
or
Approximately 1.4mA
The HT Secondary of the power transformer I used is capable of supplying up to 115mA, so, the power transformer should have plenty of current supply capacity to not be overloaded, I might do some voltage measurements tomorrow, and will list all the voltages if I do.
Okay so my measured +HT is +306V DC.
I=E/R
I=306V/220k
I= 0.00139090909091
or
Approximately 1.4mA
The HT Secondary of the power transformer I used is capable of supplying up to 115mA, so, the power transformer should have plenty of current supply capacity to not be overloaded, I might do some voltage measurements tomorrow, and will list all the voltages if I do.
You have more than enough High Voltage supply. I'm worried you are overloading the low voltage windings that supply the heaters. Two different windings inside the power transformer.
There's a good reason why the heaters in my headphone amp are powered by DC rather than AC, and that is explained in the original article I used as a build guide for my build, here's the link to the article again: https://headwizememorial.wordpress.com/2018/03/20/the-6n1p-otl-headphone-amplifier/
In the article, the writer, Bruce Bender states that "The first version of the power supply used A.C. straight out of the transformer to heat the filaments. The amp sounded fine, but had a very loud hum that didn’t go away until the filament circuit was rectified as shown in the schematic."
I have attached a pic of the full power supply schematic from the original article:
This is exactly how I built the power supply in my build, except that I used 1N4007 power diodes in the HT power supply, and 1N5404 diodes in the +DC heater supply, they seem to work fine.
In the article, the writer, Bruce Bender states that "The first version of the power supply used A.C. straight out of the transformer to heat the filaments. The amp sounded fine, but had a very loud hum that didn’t go away until the filament circuit was rectified as shown in the schematic."
I have attached a pic of the full power supply schematic from the original article:
This is exactly how I built the power supply in my build, except that I used 1N4007 power diodes in the HT power supply, and 1N5404 diodes in the +DC heater supply, they seem to work fine.
I wouldn't worry about "overloading" the heater winding. At rated load, the heater winding dissipates about 1 watt, at 150%, it will be about 2 watts - spread around the outside winding on the transformer - no big deal. May be below 6.3V, but if the DC voltage is good, who cares? And B+ at 30 mA for the 2X MJ circuit is well below the transformer rating.
As I said before, I suspect the hum was due to improper elevation. No matter, you have 1.8A worth of tungsten to heat up, but your power transformer is not rated to provide that much rectified power. Don't be surprised if the transformer has a short lifespan.
And if you've never seen heaters elevated with just a capacitor, then you haven't seen a Dynaco?
Very nice DrNomis.
That's a "classical" among the OTL for headphone. As for me it was one of the early headphone amp I built.
That's a "classical" among the OTL for headphone. As for me it was one of the early headphone amp I built.
Well, as far as I can see, the cathodes of all three of the Svetlana 6N1P Tubes I used do seem to all be glowing at about the right brightness, as are the four blue Leds, I did some more calculations.
The power transformer I used is a Hammond 369AX I ordered online from Evatco, the label on it states that the 6.3-0-6.3V AC secondary can supply up to 2.0A, each of the three 6N1P Tubes requires 6.3V at 600mA to power the heaters.
Each of the four Blue Leds have a 2k2 current limiting resistor in series with them.
So for each Led the current draw is 6.3V/2200=0.0028636363636364, or roughly 3mA rounded-up.
So then the total current draw by the heaters and Leds is going to be:
1.8A for the Heaters.
And 12mA for the Leds, which adds up to a total current draw of:
1.8+0.012= 1.812A
Which is still within the current supply capability of the 6.3-0-6.3 Heater secondary.
The power transformer I used is a Hammond 369AX I ordered online from Evatco, the label on it states that the 6.3-0-6.3V AC secondary can supply up to 2.0A, each of the three 6N1P Tubes requires 6.3V at 600mA to power the heaters.
Each of the four Blue Leds have a 2k2 current limiting resistor in series with them.
So for each Led the current draw is 6.3V/2200=0.0028636363636364, or roughly 3mA rounded-up.
So then the total current draw by the heaters and Leds is going to be:
1.8A for the Heaters.
And 12mA for the Leds, which adds up to a total current draw of:
1.8+0.012= 1.812A
Which is still within the current supply capability of the 6.3-0-6.3 Heater secondary.
You lose power when you convert AC to DC. If you were heating the tubes with AC your total draw would be 1.812A, but you are using a bridge rectifier to convert that AC to DC, and when you do that, you lose power. The heater supply draws more than 1.812A from the transformer to deliver that much DC power. I figure you're running at about 150% rated capacity.
Since the HV secondary is not heavily loaded, the transformer has power reserves for that second secondary to heat the tubes. If the wire is thick enough that it doesn't get too hot, it won't hurt. He should measure the AC voltage at full load (as it is) so if it is not below 6.3V then everything is fine. If it is less than 6.3VAC, the wire is not thick enough for the increased current.
That 1000uF capacitor is not enough to iron out the voltage, so the heating is some DC plus strong ripple current of 120Hz. I would try AC heating with a CT lifted from the ground to 1/4 HV. There are other methods to control hum with AC heating. AC should be better because the diode bridge creates a bunch of interference, and there is not enough capacity to iron it out. He need to check the voltages, because the heating secondary is probably not 6.3-0-6.3V (as written and drawn) but 6,3V CT (3,15-0-3,15). 6.3-0-6.3V would be 12.6V CT. Otherwise, according to attached schematic, the heating voltage is much higher than 6.3V.
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I used some red and black silicone rubber insulated wire that's rated to 7.5A for all the heater wiring to pins 4 and 5 of each of the three 6N1P tubes, the wire therefore has plenty of current-carrying capability for the heater supply, and should not overheat at all.
All the heaters and leds are all powered off the same 6.3V secondary since the transformer I used has only one 6,3V CT secondary.
I'm just about to get things all set-up on my test bench so I can take screenshots of the output waveforms of the two left and right outputs of my OTL Tube Headphone amplifier build, I will be making a little stereo dummy load for the amp, which will consist of a standard 1/4 inch, or 6.5mm Tip/Ring/Sleeve plug and two 33 Ohm/1% Metal Film Resistors to roughly simulate a pair of 33 Ohm headphones, it may not be a totally accurate simulation but as I'm only going to be using a 1kHz Sinewave as a test signal, I think it'll be "close enough for government work".
I finally figured out how to create and export screenshots from my new FNIRSI 100MHz Digital Scope, so, once they're done, I'll post them here for you to check out....stay tuned!!!!
I finally figured out how to create and export screenshots from my new FNIRSI 100MHz Digital Scope, so, once they're done, I'll post them here for you to check out....stay tuned!!!!
Here we go, I managed to get it all set up so I could show both the left and right channel outputs of the OTL Headphone amplifier, the yellow trace is the left-hand channel going into a 33 Ohm resistive load, the pink trace is the right-hand channel going into a 33 Ohm resistive load, I adjusted the volume control for the highest output level with the least amount of clipping, there is some soft-clipping on the bottom peak of the waveform though.
Attachments
My 3 cents (inflation upped it from 2 cents).
It is tubes we are working with here which by nature are quite forgiving (unless dropped).
All the theory looks right but the proof is in the pudding.
If the heater winding is providing ~6.0V - 6.5V (I don't know 6N1P heater variation but it's probably somewhere around that voltage range) to the tubes then voltage should be fine.
If after ~ an hour you can leave your hand on the power transformer for ~ 5 seconds without skin from your palm fusing to it then you are good to go.
If the transformer failed before the hour then its overloaded😎
It is tubes we are working with here which by nature are quite forgiving (unless dropped).
All the theory looks right but the proof is in the pudding.
If the heater winding is providing ~6.0V - 6.5V (I don't know 6N1P heater variation but it's probably somewhere around that voltage range) to the tubes then voltage should be fine.
If after ~ an hour you can leave your hand on the power transformer for ~ 5 seconds without skin from your palm fusing to it then you are good to go.
If the transformer failed before the hour then its overloaded😎
Andrewbee, I actually measured the +DC heater supply voltage, it was about +5.7V DC, I can put either of my hands directly on the power transformer for well-over a minute or so without getting burnt at all, the power transformer does get quite warm after the amp has been operating for an hour or so, even after extended listening sessions, the power transformer is still not too hot to put my hand/s directly on it for more than a minute, so, I'm considering it as perfectly normal, if the transformer was getting excessively hot, that would indicate the presence of a short-circuit somewhere, I read somewhere that a power transformer is about 80% efficient, which means 20% of the energy is lost in the form of heat.
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I just looked at the 6N1P specs and you are within limits 6.3V +/- 0.6V https://shop.ehx.com/catalog/addimages/6n1p-svetlana.pdf
You have enough voltage to heat the cathode and get the electrons dancing.
You still have your palms intact.
Enjoy the music.
You have enough voltage to heat the cathode and get the electrons dancing.
You still have your palms intact.
Enjoy the music.