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6SN7 push pull flea amplifier project
6SN7 push pull flea amplifier project
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Old 16th February 2017, 03:09 AM   #1
Lingwendil is offline Lingwendil  United States
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6SN7 push pull flea amplifier project
Default 6SN7 push pull flea amplifier project

Finally got a few hours to lash together a design I've been playing with, and I am extremely proud of how it turned out. The very messy prototype is surprisingly hum free, and can be driven to 1.4w clip, but can sustain a clean 1.2w continuously, using a ~300 volt supply. My calculations point to around 2w would be possible at higher voltage around 400 or so without too much change.

Now here's the goofy parts- I am using Antek toroidal power transformers as outputs, and due to the low current and power, 10VA units work great, and are very affordable. In order to keep the current balanced, I'm using the garter bias arrangement, which works way better than you think it should, and at such low current and voltage is not a big loss at all having extra cathode resistors. I drop 9.2 volts across each 1k resistor, so my tubes are running at 282 volts, sitting at around 2.6w idle dissipation per triode, 5.2w per tube, not counting the heater. It's got NFB too, just to make everyone upset

Here's a messy teaser pic of the first rig, but I'll get up a schematic soon. I've got an aluminum box coming tomorrow and will work on a layout for it too.
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Last edited by Lingwendil; 16th February 2017 at 05:11 AM.
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Old 16th February 2017, 03:48 AM   #2
AquaTarkus is offline AquaTarkus  Canada
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We demand to see the schematic :-)

Sent from my phone with Tapatalk. Please excuse any typpos.
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Old 16th February 2017, 05:55 AM   #3
artosalo is offline artosalo  Finland
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Quote:
I am using Antek toroidal power transformers as outputs...
How do these perform at bass frequencies (< 100 Hz) ?

Last edited by artosalo; 16th February 2017 at 05:57 AM.
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Old 16th February 2017, 11:57 AM   #4
Shoog is offline Shoog
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A man after my own heart. Well done.
I went SE parafeed for my flea powered amp, but always fancied building a mini PP flea amp.

Shoog
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Old 16th February 2017, 12:01 PM   #5
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Some time ago I made a 6CG7 (very close to the 6SN7 in 9 pin bottle) and I got about 1W using 250VDC and the bigger trafos that can be found in ol push pull transistorized radios (Laminations of about 30mm wide) in class AB.
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Old 16th February 2017, 10:23 PM   #6
Lingwendil is offline Lingwendil  United States
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6SN7 push pull flea amplifier project
As promised, the schematic and more details. You may notice I'm not one for exotic parts values, I like easy and simple if they work well.

The power transformer used for the psu-

Antek AS-05T240, $28

The power transformers that I'm using as output transformers-

Antek AN-0105, $10

The primaries are wired in series, and the secondaries are wired in parallel. Since impedance ratio is the square of the turns ratio, 230/5= 16928 ohms for an 8 ohm load, 33856 ohms for a 16 ohm load. Output tubes are Russian (6H9C) 6SN7. While this sounds great and works fine at 8 ohms (especially considering we are using negative feedback to keep everything in line) it would likely work with better THD on a 16 ohm load. Since I plan on building a nice pair of speakers, I think they will be running 16 ohm in the long run. They will eventually drive either a pair of Karlsonator or Cornu style designs in my bedroom, depending on which sounds better once I get around to building some...

Input/phase inverter tube is a russian (6H9C) 6SL7, although a 6N1P, 12AT7, or similar would work as well. I arbitrarily set the feedback resistor at 15k, considering that I am running a 1500 ohm cathode resistor at the input stage, although I might play with increasing the feedback resistor to increase the overall gain. I found when plugging directly into my cellphone that the gain was a little low. I might try replacing the 15k with a 39k or 33k pair in parallel with a switch to swap them in and out to deal with different sources. The capacitors I used for the output stage bypass are 35 volt rated electrolytics from a local Radio Shack that is going out of business. Use whatever brand snake oil you prefer if you must.

Power supply is pretty basic, and built using parts on hand. The smaller caps are some film caps I have in my parts bin, electrolytic capacitors should work as well. Make sure you mind the DC bias for the heaters! On the next build I wil likely use a higher voltage secondary, as the AS-05T240 is only giving me 305 volts rectified. The Antek AS-05T280 would do very well in its place, and has a 280 volt secondary with a 260 volt tap. This would give more room to play around, and let you squeeze a little more power out of the outputs with just a few component changes.
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Last edited by Lingwendil; 16th February 2017 at 10:40 PM.
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Old 16th February 2017, 10:37 PM   #7
Lingwendil is offline Lingwendil  United States
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6SN7 push pull flea amplifier project
Quote:
Originally Posted by artosalo View Post
How do these perform at bass frequencies (< 100 Hz) ?
Excellent. Some very bass heavy music has no problem on these guys when played through an 8" celestion that I have on hand. I am not set up for any more testing than DMM or ear for now though.

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Originally Posted by Shoog View Post
A man after my own heart. Well done.
I went SE parafeed for my flea powered amp, but always fancied building a mini PP flea amp.

Shoog
Your Toroid 6080 experiments are what led me to try them out in this guy! Push pull just makes more sense to me, slightly more complicated, but worth the fuss IMHO. I've played with garter bias back when I was in love with the 6AS7G (which I still am) and it seemed like a natural way to go on a low power design. Garter bias truly does work extremely well, and sounds excellent. It seemed like a natural way to go for toroidal output stages to keep everything balanced without getting into active circuitry.

I like them so much, I'm going to order a few toroids for the five channel 6AS7G amp that I'm working on for the home theater system, but thats another story. Think this guy, but with an output stage B+ of 165 volts, garter bias with 330R resistors, 100 volt across each triode, ~100mA, and around ~32 volts of bias across each resistor. Hot, heavy, and wasteful, but it sounds very nice and works very well

Last edited by Lingwendil; 16th February 2017 at 10:43 PM.
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Old 17th February 2017, 01:05 AM   #8
Lingwendil is offline Lingwendil  United States
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6SN7 push pull flea amplifier project
Well, like a goof I left my circuit a little bit unoptimised. With a change at the input stage of just a little more negative grid voltage, I should be able to increase the gain and maximum swing available a bit. The cathode resistor will be changed out to 2700 ohms, with a corresponding change of the feedback resistor out to 27k. I want to leave in the option for less feedback, so I may experiment with something like a 33~39k feedback resistor(or a couple of resistors of 56~68k switching in parallel). I could always go with an LED and a small resistor underneath for feedback, and this would increase gain even further, so that might be my next experiment if I can find an LED in my bin that is close to the needed voltage. Of course the mail came right after I left for work, so I wont get to mock up the chassis until tomorrow.
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Old 17th February 2017, 01:34 AM   #9
Shef is offline Shef
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Instead of 2 x 470 lytics with same if not better result you can use a single non-polar one between cathodes. For 2Wa 10 uF will be well sufficient, smth like 2 x Murata 4,7uF 50V WLCC would do.
I have had a similar SHAX flea power project on my breadboard singing for already 6 weeks, waiting for some parts to put it in final incarnation.
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Old 17th February 2017, 01:39 AM   #10
Lingwendil is offline Lingwendil  United States
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6SN7 push pull flea amplifier project
Quote:
Originally Posted by Shef View Post
Instead of 2 x 470 lytics with same if not better result you can use a single non-polar one between cathodes. For 2Wa 10 uF will be well sufficient, smth like 2 x Murata 4,7uF 50V WLCC would do.
I have had a similar SHAX flea power project on my breadboard singing for already 6 weeks, waiting for some parts to put it in final incarnation.
I've got a handful of 6~8uF film caps I may try in the cathode then. My 6AS7 garter bias arrangement used back-to-back 470uF caps, with a 1M resistor at the junction shunting to ground, and a nice 8uF film from cathode to cathode, and it worked very well in that application. Poor man's nonpolar cap of sorts
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