6AK6-PP Weird Duck Project

SerpentRuss

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The genesis of this project is kind of convoluted. It started with a spur of the moment addition to a parts order. I was buying some things from Parts Express and added eight, 29 cent full-range TV speakers as a lark (link below)


Then, starting about a month ago, my son starting messing with what we called "Lo-Fi" speaker experiments. There are all kinds of small, unused speakers floating around our place and we started testing them with a head I had just finished. I posted earlier about some experiments we did with exciters that were pretty interesting. So, I remembered these speakers and I made a temporary cardboard cabinet using 4 of them in series/parallel to get back to 8 ohms. Short story, he asked me to build a real cabinet because he liked the sound.

I started thinking about doing that and then it dawned on me that I could put a cheap tube amp circuit in that cabinet and make it a combo. I would include a 1/4 inch jack to bypass the amp and make it simply a speaker cabinet. One of the things on my bucket list to try was a push-pull 6AK6. Another thing I wanted to try was a 6AK5 as a pre-amp tube. Also, I wanted to try a new 100-volt line transformer as an OT in a push-pull amplifier. So, why not try all of that at once?

So that's the road that I'm going down. Keeping it cheap because it's likely to disappoint in multiple ways, but still fit the bill of making a speaker cabinet.
I plan to use all 8 speakers in a 16 ohm array. I'm using an ANTEK 160 volt, 50VA PT that I still can't remember why I purchased over a year ago. I'm using the 140 volt tap with a voltage doubling power supply (why do anything simple). I'm shedding the extra voltage that creates via balanced filtering to get back to near 300 volts on the 6AK6 plates. This project is very close to making an amp from the contents of my junk drawer.

Initially, I was going to use an LTP phase inverter using a 6J6, but I couldn't get the gain under control. It just doesn't take much to drive a pair of 6AK6 power tubes. When I switched my design to a concertina PI, the amp was a bit weak because of the tone control between the 6AK5 preamp and the PI. So, my solution was to put in an LND150 input buffer and move the tone stack before the 6AK5. Though the gain of the LND150 as configured is about 8, coming out of the TS, it's just about 2 which the 6AK5 has no trouble handling.

Keeping all the tubes 7 pin. I chose a 6AB4 for the PI. This is a 7 pin, one triode version of the 12AT7.

I'm attaching an LTSPICE screen shot as the working schematic. I am doing the layout now and I have the chassis complete.
 

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printer2

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I do not know why the 6AK6 is not more populare around here. It can make a 1W amp in SE and 3-4W in P-P. It is easy to drive and takes little heater current.
 

mountainhick

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Watching! I'm also considering 6AK6 PP right now, quite different, but looking forward to seeing where this goes!

Can you share the details of the line/OT transformer you are using? What impedance are you putting on the plates, and how are you getting that with the line transformer you are using?

I've been reading up on them and still remain a bit confused about their performance in terms of power handling/saturation vs the demand placed on them with differing ratios.
 

printer2

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I used as 4W line transformer on mine using the lowest wattage tap (can't remember the wattage). Run it at 230V.
 

SerpentRuss

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If you look at this chart I made for the transformer in question, the .625 watt tap has approximately twice the turns (referenced to common) as the 2.5 watt tap. That means the number of turns from 0.625 to 2.5 is roughly the same as 2.5 to common. So I will be putting power in at the 2.5 watt tap and the plates will be connected to the .625 watt tap and common.

The speaker will be connected to the 8 ohm taps. If you look at the chart for a 16 ohm speaker wired to the 8 ohm tap, using those taps should give a plate to plate reflected impedance around 26k. 20k would be close perfect, but I've run the curves and I think it will be close enough. Using a higher impedance makes the 6AK6 tubes a bit easier to swing because it lays the curve down closer to horizontal. If it doesn't work, I have a Hammond universal as a fallback part.


If you dig into many of these fixed-voltage, line transformers that have wattage taps with power relationships of 4x from each other, you'll find they can work the same way. On this one, notice that you could also use a tap combination of the 1.25 watt, 5 watt, and common for a push-pull amp.

Obviously, this is something you wouldn't want to roll out on a high-dollar, high-power project, but for a 2-watt amplifier it should be fine.
 

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mountainhick

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I used as 4W line transformer on mine using the lowest wattage tap (can't remember the wattage). Run it at 230V.
I don't want to sidetrack too much, but I got that Bogen 4W line TF. I don;t think it will service more than about a watt at 16-22K. Working on figuring it out. This Visaton is 100V, 10W and burlier though, potentially better power handling.
If you look at this chart I made for the transformer in question, the .625 watt tap has approximately twice the turns (referenced to common) as the 2.5 watt tap. That means the number of turns from 0.625 to 2.5 is roughly the same as 2.5 to common. So I will be putting power in at the 2.5 watt tap and the plates will be connected to the .625 watt tap and common.

The speaker will be connected to the 8 ohm taps. If you look at the chart for a 16 ohm speaker wired to the 8 ohm tap, using those taps should give a plate to plate reflected impedance around 26k. 20k would be close perfect, but I've run the curves and I think it will be close enough. Using a higher impedance makes the 6AK6 tubes a bit easier to swing because it lays the curve down closer to horizontal. If it doesn't work, I have a Hammond universal as a fallback part.


If you dig into many of these fixed-voltage, line transformers that have wattage taps with power relationships of 4x from each other, you'll find they can work the same way. On this one, notice that you could also use a tap combination of the 1.25 watt, 5 watt, and common for a push-pull amp.

Obviously, this is something you wouldn't want to roll out on a high-dollar, high-power project, but for a 2-watt amplifier it should be fine.
Thanks! I was under the mis-impression that the impedance numbers would represent the split.
 

printer2

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I don't want to sidetrack too much, but I got that Bogen 4W line TF. I don;t think it will service more than about a watt at 16-22K. Working on figuring it out. This Visaton is 100V, 10W and burlier though, potentially better power handling.

Thanks! I was under the mis-impression that the impedance numbers would represent the split.
I am pretty sure I got more than a watt out of it, as long as the tubes are balanced there will be minimal dc in the core so the transformer should do reasonably close to the rating.
 

SerpentRuss

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This project is going to drag out a bit. I have lots to do around the house that is not tube amp related and takes priority. I have continued to tweak the schematic and I have put together the core of the voltage doubler and filter. I decided to make a support board for the main filter caps so I could orient them vertically. It's essentially two pieces of phenolic floor laminate separated by standoffs. In the photo you can see some of the connections have not been soldered because they'll tie in with incoming and outgoing wire. I will attach sub assembly to the main board when I start putting all the rest of components together.

Since I have the LND150 to mount, I am going to use a project board and also place some of the lower current components on the board with it. I'm working on that layout now.

I'm attaching my revised schematic, though I don't expect anyone to follow in my footsteps here, LOL.



6AK6_PP_V3.JPG



20220604_182921.jpg
 

2L man

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If I understand right your power transformer secondary is 191VAC? Using bridge rectifier it has potential for almost 270VDC and I would not use voltage doubler circuit. If current is 27mA there comes over 100V loss to four 1k series resistors!

Interesting that you are going to use individual carhode resistor/capacitor for both tubes!

Thats how I have mounted radial electrolyts!!!
 

SerpentRuss

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If I understand right your power transformer secondary is 191VAC? Using bridge rectifier it has potential for almost 270VDC and I would not use voltage doubler circuit. If current is 27mA there comes over 100V loss to four 1k series resistors!

Interesting that you are going to use individual carhode resistor/capacitor for both tubes!

Thats how I have mounted radial electrolyts!!!
My power transformer has two taps, 160 or 140, I'm using the 140 volt. This is a transformer that I mistakenly ordered and this is a way to utilize it.

The sine wave function on LT spice is peak to peak, not RMS, so I've already adjusted the 140 volt RMS output of the PT to an actual 197 volts peak to peak for modeling. The reason for the resistors is to shed that excess voltage and to enhance filtering. They'll also provide some sag. It all models correctly and the modeled voltages are in blue on the schematic.

Individual resistors on the tubes to keep the required wattage down and I have room for them.
 




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