I am doing some experimentation on winding my own OPTs using microwave oven iron. I previously posted a thread on that topic in general, but this is more of a specific question. I would like to attempt to use the 6AS7/6080 in SE as a headphone amp. Not OTL. I've thrown a design together that is running at 120v/plate @ 75mA into a 125:32 ohm OPT. I gather that the 6AS7 has a low Z-OUT, but how low is too low to wind the OPT, y'know? I'm feeding it with a basic 6DJ8 gain stage with 235v B+ and a 10k plate resistor. ~8mA. This is in no way a final design. It's just how everything kind-of fell into place.
It sounds good to me (maybe I'm just not picky...dunno) but I'm curious what a "good" operating point for that 6AS7 would be. I'm getting better with curves, but it's still a little hazy. I saw another post here on DIYAudio where someone was running one way up at 375v B+. ( https://www.diyaudio.com/community/threads/6as7g-se-output-stage-operating-point.383207 ) I don't mean to second guess those that are more knowledgeable than I, but is that not crazy high for this tube? I don't need a huge amount of extra gain. There is already more than enough where I'm at. I just want it to be all that it can be in my scenario.
Most any B+/Z-OUT is achievable within reason. I've gotten better at winding custom transformers. Just kinda winging it with the interleaving pattern, but I've brought down ringing and distortion. Separating each and every layer with paper tape and keeping the turns absolutely neat/straight has been the key for me. I've honestly been very pleased with the level of performance that I've gotten out of what is basically scrap iron. Performance is nearly identical to similar commercial OPTs that I have on hand at this point. I am fortunate enough to have a large cache of old microwave iron to pull from. Might as well make use of it, right? I've also wound several power transformers. Have to interleave the laminations for that, but that's not a big deal. They work great.
It sounds good to me (maybe I'm just not picky...dunno) but I'm curious what a "good" operating point for that 6AS7 would be. I'm getting better with curves, but it's still a little hazy. I saw another post here on DIYAudio where someone was running one way up at 375v B+. ( https://www.diyaudio.com/community/threads/6as7g-se-output-stage-operating-point.383207 ) I don't mean to second guess those that are more knowledgeable than I, but is that not crazy high for this tube? I don't need a huge amount of extra gain. There is already more than enough where I'm at. I just want it to be all that it can be in my scenario.
Most any B+/Z-OUT is achievable within reason. I've gotten better at winding custom transformers. Just kinda winging it with the interleaving pattern, but I've brought down ringing and distortion. Separating each and every layer with paper tape and keeping the turns absolutely neat/straight has been the key for me. I've honestly been very pleased with the level of performance that I've gotten out of what is basically scrap iron. Performance is nearly identical to similar commercial OPTs that I have on hand at this point. I am fortunate enough to have a large cache of old microwave iron to pull from. Might as well make use of it, right? I've also wound several power transformers. Have to interleave the laminations for that, but that's not a big deal. They work great.
You'll want the OPT to be about 600R:32R...
Is your design like this? Post a schematic.
![URL]](/community/proxy.php?image=http%3A%2F%2F%5BURL%5Dhttps%3A%2F%2Fwww.diyaudio.com%2Fcommunity%2Fattachments%2Flilliput_web-jpg.33944%2F%5B%2FURL%5D%2F&hash=28077ed8294755e0dcc61cd99bee1fa1)
https://www.diyaudio.com/community/attachments/lilliput_web-jpg.33944/
EDIT: Since the forum doesn't want to link it's own damned image, I'll upload a second copy.
Is your design like this? Post a schematic.
EDIT: Since the forum doesn't want to link it's own damned image, I'll upload a second copy.
Thanks for the reply. It's similar to that one. I don't intend for this to be anything approaching a final design. I just threw it together to get an idea of how the OPT would work, and posted here before I started getting down to refining anything. Below is a schematic of what I have so far though. I've used a 2500:32 and the 125:32 OPTs with it so far. I'll likely get started working on a 600:32 shortly. It's oddly relaxing to wind these things. I didn't include the power supply, but it's just a bridge rectifier + doubler so I can give the 6DJ8 a bit more voltage. All solid state on that end.
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I'd make the 6DJ8 plate resistor 22k, and the cathode resistor 330R as to not beat the crap out of it. Likewise, 250R for the 6AS7 is too low IMHO.
I'd probably just run it like this guy did... 2k5 load. You don't need 3 watts of power in headphones, right? 😛
I'd probably just run it like this guy did... 2k5 load. You don't need 3 watts of power in headphones, right? 😛
I split the difference and wound two transformers to 2k : 38R. The first is on microwave iron. I named it "Portnoy." (Names are easier to keep track of.) I then wound a much smaller version and called it "Sportnoy." They seem to perform identically on the scope and to my ear. They sound great, have punchy bass without being muddy and crisp highs. A square wave still looks like a square wave well into ultrasonic range with little drop in voltage, not that I'd need that, obviously. I'm going to wind another Sportnoy and call it a day. What I was looking for wasn't higher power, necessarily. I just want to be sure I'm operating these tubes in an area that they work well at. I.E. low distortion, etc. This produces more than enough gain, but that's fine with me. Planning on putting a gain limit switch on the chassis so that I could have the ability to get some extra stank if it's ever needed, but not have it get in the way when I don't.
Would increasing the 6DJ8 cathode resistors from 150R not pull them down into a less linear area? That 22k plate load resistor pulls it well away from the operating limit from what I understand. Been plugging everything into the "Universal Loadline Calculator." If it'd be a problem to keep it that low then so be it. Just trying to better understand what I'm doing and what the consequences of my decisions are.
Been rolling in some cheap Soviet 6N23P's as well, and I have to say that I actually like them better than 6DJ8's. I wanted a balanced design. (big tube in center and two smaller tubes to its sides, for example) So that left me with unused triodes. I just put them in parallel. Never done that before and am not sure if it's "correct" per se, but it does sound good.
Edit(s) - I can only see typos after I submit the post. It's the law... There are probably more. Don't mind the rat's next of wires in the photos. I want to keep leads as long as possible for future use, but it definitely doesn't look pretty on the bench.
Would increasing the 6DJ8 cathode resistors from 150R not pull them down into a less linear area? That 22k plate load resistor pulls it well away from the operating limit from what I understand. Been plugging everything into the "Universal Loadline Calculator." If it'd be a problem to keep it that low then so be it. Just trying to better understand what I'm doing and what the consequences of my decisions are.
Been rolling in some cheap Soviet 6N23P's as well, and I have to say that I actually like them better than 6DJ8's. I wanted a balanced design. (big tube in center and two smaller tubes to its sides, for example) So that left me with unused triodes. I just put them in parallel. Never done that before and am not sure if it's "correct" per se, but it does sound good.
Edit(s) - I can only see typos after I submit the post. It's the law... There are probably more. Don't mind the rat's next of wires in the photos. I want to keep leads as long as possible for future use, but it definitely doesn't look pretty on the bench.
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Beware of grid current runaway with that schematic, 1MEG grid leak seems too high to me.
According to the GE 6AS7 data sheet dated 6-55, 1.0MΩ grid leak is permissible when running in cathode bias.
I may try a few different values on those grid leak resistors. I saw a 1M used on another schematic, so I just used that. I'm not 100% sure how a grid leak value is correctly chosen to be honest. I'm going to have to do some more reading.
That is correct, however this may cause trouble with used tubes that have higher grid current, and not every 6080/6AS7 conforms to this spec.
Its the same problem with 6C33 tubes, those are also series regulator triodes made for stabilized power supplies. And people run them hard with high value of grid resistors, or even fixed bias which leads to unreliable amplifiers.
I'd stick to 330K maximum for cathode bias, this will load the preceding driver stage harder but it should be more reliable.
You could try a TL783 current source to bias the tubes.
My experience with the 6AS7G is that the more negative you put the grid with regards to the cathode the more likely you are to have grid current drift. 100volts, 100mA, ~30 volts bias a 1M grid resistor works fin. Go up to higher bias like 50mA, 200 volts across the tube and the much more negative grid voltage can act a little inconsistent. I've had to drop down to 100~220k grid resistors at higher voltage to get them to play nice.
Considering these tubes were made for lower voltage drops at higher currents this makes a lot of sense. 100 volts, 100mA is my favorite operating point for them- only takes ~30-33 volts or so bias making them easy to drive and a 1M grid resistor works great there.
Considering these tubes were made for lower voltage drops at higher currents this makes a lot of sense. 100 volts, 100mA is my favorite operating point for them- only takes ~30-33 volts or so bias making them easy to drive and a 1M grid resistor works great there.