AX84 Variable Bias

[DeepBlue]

Hello all, I have not posted in sometime since finishing my first Two Stroke. Which I am still loving! So in the meantime I have built the SEL from AX84.com if anyone is familiar with those guys. Now I have posted this same question on that forum, but this forum seem very responsive so I thought I would give it a shot. Basically I am wondering has anyone have any experience with the Variable Bias option that is shown on the site? I have this set up at the moment and I am having issues with the voltage on my pin 8 of my output tube. I am using a KT88 tube at the moment at per the SEL schematic. Well if anyone has any knowledge about this option or the SEL amp that would be great to get a thread going on this site for interested peeps as well. Thanks again and hope all is well.

[Robin]

This sounds like a question for beelzebum or maybe Mark Fowler.
I’m gonna check it out.
Thanks for posting

[beelzebum]

What problems are you having on pin 8 exactly. to start, once everything is wired in take a reading from test point 1 to ground. try to get it around the 430R of the original schematic by adjusting the pot. if you can’t get it to that point then something is wrong. also do you still have it wired with the elevated heater voltage because that will throw off your reading. cut the wire between pin 7 and 8, and see if you are still having voltage issues.

[DeepBlue]

Thanks guys. So Beez, when you say check from point 1 to ground? What are you referring to as point 1? The first test point in the variable bias? I am just not sure what point 1 refers to? I am on my way to take a look at things now.

[beelzebum]

Yes, I was just referring TP1 in the adjustable bias circuit, really just pin-8 of V1.

[DeepBlue]

Awesome. That is what I thought. You know I am bone head. I just realized in my stupidity! I have a 4.7K resistor not a 470ohm!!! Good god. I think as soon as i put the correct value in things should be much better! I still appreciate all the help. I think talking to others is really helpful even when I just make stupid errors.

[DeepBlue]

So I have a new question/problem. I am fairly sure I am measuring everything correctly and my specs look fine with the voltages all being withing very close to spec to the SEL schematic. However when I calculate the power across the bias pot it seems to be fine, about 1.4 watts for a 3watt pot, but after a few hours of playing I notice my master volume pot becomes scratchy and begins to cut out at higher volumes. At the same time that this scratchy noise appears in the Master Volume, the bias pot appears to become stuck? I am not sure if it is overheating and melting? It seem to be fine in terms of the power it has through it? I am measuring the voltage drop across pin1 to 3 on the pot? Is this in correct? Has anyone else had this problem appear and maybe have any suggestions? I at first thought I had a scratchy volume pot, but after replacing the bias pot, since it became stuck, I noticed the scratch was gone, and sure enough when it appeared again the pot was stuck again.

[beelzebum]

I have not built this schematic, but it is missing a grid leak resistor on the KT88. KT88s have a maximum grid leak resistance of about 270k. On power tubes the grid leak resistor works differently than on small input tubes. On power tubes if your grid leak resistance is too high it will drive your grid more positive, biasing your tube hotter. This is most likely why you are burning out your cathode bias pot. If your volume pot is all the way up your grid resistance will be over 1M. It would probably help to add a grid leak resistor after the grid stopper resistor. I would suggest between 270k and 470k since it will be in parallel with your 1M volume pot. This should fix your problems.

[Robin]

Great stuff from beelzebum (as usual). Here is a link with more details:
http://www.freewebs.com/valvewizard/gridstopper.html

[DeepBlue]

Awesome ! I agree great info. I realize also now that its when my master is up that the current goes too high for the tube. So I will check out both the grid leak resistor and make sure I have my tube biased correctly. It was a bit hot I think especially at the high volumes. I will post what I find this weekend! Thanks again. I did notice some other set ups had an extra resistor in series and I wondered why…now I know.

Just a quick note, sorry for the dumb question but is this going to be on the screen grid or control grid? Pin 4 or pin5?

[beelzebum]

Just connect around a 360k from pin5 to ground. This should give you about 270k of parallel grid resistance at max volume. Or you could swap out your 1M master volume pot for a 250k pot. This will also work.

[DeepBlue]

Thanks again! So just one last question any preference if the grid leak goes before or after the grid stopper? I was going to place it before as recommended by B. Merlin. Says otherwise acts like a voltage divider and will attenuate the signal. Any thoughts on his opinion here? I was going to put either the 370k or maybe 1M 2W resistor on here right out of the volume pot to ground.

[beelzebum]

Anywhere you put it, it will throw off your volume pots taper due to the fact that your volume pot is already a voltage divider. It will also attenuate your signal no matter where you put it. That being said, the only reason you need it is because at full volume the 1M is to high of a grid resistance. Even with another 1M right off the pot itself, that will give you 500k at full volume plus the grid stoppers resistance. This might might work though. You can put it anywhere you want. The main reason for this is to keep the grid, DC referenced to ground, and stabilize the grid current. Try it where it’s easiest for you though and see how it works. You never know till you try. The problem with the schematic is it was made with an EL84 in mind. EL84s had a max grid resistance of 1M. People have tried to put larger tubes using this same schematic, but they did’nt account for the lower max grid resistance of 6L6 or KT88.

[DeepBlue]

Thanks again B. I am just not sure what is going on here. So…well to be short things are still not working. I have played with resistor values and such for a bit now, as well as added my grid leak resistor, however I am still seeing my problem. So here is basically the situation, maybe I am missing something. i will also try to post some shots of my wiring, perhaps I have something wrong. So the Va is at 359V, which looks good and my cathode is is showing around 25V. These are both with the master volume completely turned up, which is a 1M pot. Now the problem is that with this spec, and the adjust bias pot set to 0R, the current is very high, around 180mA. So when I adjust the bias pot to bring the current down to spec my Cathode exceeds the voltage the bypass capacitor is rated for of 50V and the capacitor begins to smoke. Now the problem seems to be the range for when I adjust the master volume, that I can get the bias set, to around 62mA and the Cathode voltage to be around 25V when I have the master volume turned down, but as soon as I turn it up the values are way out of range. I am not sure if I have perhaps something wired wrong, as I have tried numerous values of the resistor for the cathode. I am currently at a 100ohm. I am also running a KT66 not a KT88 but I do not think this is the source of the problem, as I tried my KT88 originally cannot find a value that works for that tube as well. As a note for the bias I have been setting it to ( 25W * 0.9 / 359V). I am using a Shuguang KT66 Treasure. Sorry for the extensive email, but I am really not sure what is going on, as I see nothing wrong with my set up at this time. I will take a picture of the set up and post ASAP.

[beelzebum]

Try to take a grid voltage reading when your volume pot is all the way up. It sounds exactly like your grid is going positive. Also just for shits and giggles, try putting a 360k or less resistor from pin5 to ground. less would be better.

[Author]

Posts