Thanks nerdorama. I'm wondering if I made a mistake. I was thinking about the 22k plate resistors above the 6sn7. I thought that when you had 2 resistors in series like that, that the sum was your plate load. I've been reading yesterday and today about it. I get that the cap shunts AC to ground, so I could see how the impedance would not be the sum of both Rs. But for a DC load line, wouldn't sum of both still apply there?
Also, I found this other schematic where someone changed a very similar board to suit their needs. The thing that interested me about it, was the temperature coefficient compensation on the lm334:
Definitely like that idea, so I looked up the lm334 datasheet and found the formula for zero temperature coefficient operation.
Had to dig through the resistor bin for a combination that would work, keeping in mind a ratio of R2:R1 of around 10. Total current is a combination of the flow through both these resistors, as well as the 334 bias itself (although the bias flow is already added to the equations). And also figuring out the forward voltage drop of the diodes I had, Vd, which was .65v for the 1n4007.
It's late and the writing is messy, but cool, now this thing can have the current source without the temperature wander. I set it to stock bias, 4.2ma each half. I believe your boards have it already don't they nerdorama? I know that both retailers offer it as a tiny pcb that mounts off board, surely with similar values...
Thanks all,
Loren
Also, I found this other schematic where someone changed a very similar board to suit their needs. The thing that interested me about it, was the temperature coefficient compensation on the lm334:
Definitely like that idea, so I looked up the lm334 datasheet and found the formula for zero temperature coefficient operation.
Had to dig through the resistor bin for a combination that would work, keeping in mind a ratio of R2:R1 of around 10. Total current is a combination of the flow through both these resistors, as well as the 334 bias itself (although the bias flow is already added to the equations). And also figuring out the forward voltage drop of the diodes I had, Vd, which was .65v for the 1n4007.
It's late and the writing is messy, but cool, now this thing can have the current source without the temperature wander. I set it to stock bias, 4.2ma each half. I believe your boards have it already don't they nerdorama? I know that both retailers offer it as a tiny pcb that mounts off board, surely with similar values...
Thanks all,
Loren
It's funny but I don't think my board has the temp comp even though Pavel sells both. I've already bought the resistors so I can add it but I may just buy his little board to keep things more neat. Not using or working on the Mk2's right now as I've got other amp things on my mind.
I believe you are correct about the DC load for the input tube since no other load taps off at the capacitor.
I believe you are correct about the DC load for the input tube since no other load taps off at the capacitor.
It's funny I thought about getting the boards for that exact reason, just didn't want to pay for what I knew I already had. I did look for a micro sized circuit board like that, or a larger one with perforations where I could break off a little 4x4 hole chunk of the board, but could not find any. I have board I could cut, but that would negate the whole "looking cleaner". I'll probably just mount the lm334 topside, and the extra r and diode bottom side. Besides at some point in the future I might change it anyway. Having the negative supply voltage handy...I could do better.
Ha! Thanks for posting that. Yeah it definitely looks like the same thing. Some different values, and some measurements that are the same, haha. I bet there have been a few slightly different values subbed in here and there depending on what was at hand. The circuit is so simple it definitely has some flexibility, and I'm sure that's the point to some degree for ease of use.
Loren
Ha! Thanks for posting that. Yeah it definitely looks like the same thing. Some different values, and some measurements that are the same, haha. I bet there have been a few slightly different values subbed in here and there depending on what was at hand. The circuit is so simple it definitely has some flexibility, and I'm sure that's the point to some degree for ease of use.
Loren
Another couple of things that I have done is to increase filtering on the bias supply. I could see the ripple on the output tube grids. Just not enough voltage to go too far with this. Possibly a doubler would help. The other is increasing the negative feedback. Can't remember what I measured but it was low and the bass seemed a bit loose. I tack soldered a resistor across the feedback resistor and it seemed an improvement. A 6SL7 first stage would allow for more feedback without killing the input sensitivity.
Indeed it would. But this is the circuit I used last year when I designed the KT88 amp, and the 6sl7 while offering that ability, was also a little microphonic, and I thought a little thinner. I'll see where I'm at when I get it going. I decreased that cathode resistor a little from 680 to 650, so right there I have a tiny bit more feedback, especially since your first half of the input tail is 1k. Also my phase splitter is running a little hotter. Mine, which I kept at the same current draw just temp compensated, started at 8ohm on the lm334, as compared to your 10ohm.
Definitely agree about the bias supply. I have some plate chokes I could use here to flatten the output. If not, I should still have more voltage to play with just because of the 110 vs 123 nowadays, on the primary. Were your transformers original mkiii? Power and output? What would really be optimal here is a small regulator board. There are some tiny ones when the demand is less than 100v and only a few ma.
Here's the boards last night. About half way done with them, and some closeups of the ground trace I cut and the temp compensation components installed. Red shrink tube is the diode (so that it's leads didn't touch the pcb)
Loren
Definitely agree about the bias supply. I have some plate chokes I could use here to flatten the output. If not, I should still have more voltage to play with just because of the 110 vs 123 nowadays, on the primary. Were your transformers original mkiii? Power and output? What would really be optimal here is a small regulator board. There are some tiny ones when the demand is less than 100v and only a few ma.
Here's the boards last night. About half way done with them, and some closeups of the ground trace I cut and the temp compensation components installed. Red shrink tube is the diode (so that it's leads didn't touch the pcb)
Loren
I have a pair of ratty Mk3's that I may get to this winter. After the Mk2's I'm not sure if I want to do the same to the 3's or mix things up a bit. I'll probably have to have the chassis' sandblasted and powder coated, or at least sandblasted. Might need some bondo if I choose to paint. The big decision will be the choice of driver board. I might even do a point to point with an aluminum plate. I recently did two pair of Heath W5's for friends using a scheme by Dave Gillespie at AudioKarma. They sound so good I might see if that could work. The plan uses cathode feedback from the OPT secondary so I would have to do some checks to see if the secondary is well balanced from the 4 ohm tap to each end.
Sorry to ramble on your thread. Now back to regularly scheduled programming. I will enjoy your comments when you have them playing.
Sorry to ramble on your thread. Now back to regularly scheduled programming. I will enjoy your comments when you have them playing.
Not an issue nerdorama, I enjoy this on the thread. Not that either of our board is terrible as is, but in searching it didn't seem another thread was needed just to say "I followed the directions and this is what happened". I like the sound of that driver board, and would assume I've seen it somewhere, but feel free to post it. These are ALL valid ideas for "what YOU can do with that old MKii or iii...or W5M, or"...there are so many smaller monoblock 6l6 family designs from the 50's-late 60's that fit the bill for a similar rebuild.
Last night I was installing the plate loads for the 6sn7 ltp when I realized, with how the pcb is set up, it's as easy as cut two traces and plug a VR right into existing through holes to create a plate balance (ac balance). Should get some more pc mount variables in the mail soon, and then boards will be complete.
Might install transformers today. I thought about going the sandblasting and powder coating direction as well. Maybe next time, if I end up in favor of the sound. 5$ and a few hours was more desirable for this build. Looks good enough, and far better than it did.
Loren
Last night I was installing the plate loads for the 6sn7 ltp when I realized, with how the pcb is set up, it's as easy as cut two traces and plug a VR right into existing through holes to create a plate balance (ac balance). Should get some more pc mount variables in the mail soon, and then boards will be complete.
Might install transformers today. I thought about going the sandblasting and powder coating direction as well. Maybe next time, if I end up in favor of the sound. 5$ and a few hours was more desirable for this build. Looks good enough, and far better than it did.
Loren
Loren
don't forget grommets where the transformer leads passas the chassies.
Thank you Wntrmute2 🙏 I really do too. Still debating whether I'm gonna screw pieces of hardwoods to the sides (with pieces screwed to them, in a square, for the front), or just put the bottoms back on. I did decide no components past the bottom, because I want it to be capable of stock form factor in case that's important to someone in the future.
I'll have more pics soon
Loren
I'll have more pics soon
Loren
Could you tell us what the motivation that let you swap out the original circuitry?
With the new circuitry, how much gain do you configured? With 3.9K feedback resistor, the gain would be 40x, With 6.8K feedback resistor, the gain would be 69x. Either one is too hot for me. If you use a 2K feedback resistor instead, the nfb might be too high and cause instability. It is just too much trouble for me. I would stay with the original circuitry.
With the new circuitry, how much gain do you configured? With 3.9K feedback resistor, the gain would be 40x, With 6.8K feedback resistor, the gain would be 69x. Either one is too hot for me. If you use a 2K feedback resistor instead, the nfb might be too high and cause instability. It is just too much trouble for me. I would stay with the original circuitry.
Hi Jxdking. I got these amps with the original circuitry already out of them. They had some horribly messy home made pcb driver boards in them. So that's why I had to replace original circuitry-It already wasn't there. As for why I chose the tubes4hifi driver boards, they make my life easier in designing a new amp around KT88 outputs.
As for the gain, I'm not really sure what you mean by "either one is too hot for me". Gain requirements are specific of each amplifier, and I don't see how a gain of 40, or 69, could be inherently too hot or cold for one's taste? But please, I'd love to hear why you think so in this case. I don't have the exact gain decided yet. But I know there's more than I need, which means I can tune the feedback circuitry, of a circuit I already know works well in this application, using KT-88 like I am 🙏
-Loren
As for the gain, I'm not really sure what you mean by "either one is too hot for me". Gain requirements are specific of each amplifier, and I don't see how a gain of 40, or 69, could be inherently too hot or cold for one's taste? But please, I'd love to hear why you think so in this case. I don't have the exact gain decided yet. But I know there's more than I need, which means I can tune the feedback circuitry, of a circuit I already know works well in this application, using KT-88 like I am 🙏
-Loren
Loren,
I noticed in one of the schematics you posted above that the FB connection at the input stage is labelled 8 ohm tap. Is that how you have yours connected? I used the 16 ohm tap as in the original design.
I noticed in one of the schematics you posted above that the FB connection at the input stage is labelled 8 ohm tap. Is that how you have yours connected? I used the 16 ohm tap as in the original design.
Oh I see it, that was the ieLogical schematic. No, I was just comparing it to this circuit. All 3, that one, yours, and the Tubes4HiFi boards I have are very similar, same tubes and similar operating points, and I was just showing how the ieLogical had implemented temperature compensation for the lm334. Mine is basically the tubes4hifi board with some slight value changes, a little change in the bias circuitry, and the temperature compensation for the lm334.
The tubes4HiFi board is definitely set up to take feedback from the 16ohm tap, as was the original Dynaco. The 16ohm tap is where I'll be starting, with the stock values for their feedback circuit (6.8k and 680pf), and the 680 ohm resistor from 16-0 on the OPT just as was stock from Dynaco. I'm thinking stock Dynaco was what Jxdking was referring to. Because with el-34 output then yes I could see 40x gain on a 1.5v input would definitely be too hot. But it sounds about right for KT-88.
Loren
The tubes4HiFi board is definitely set up to take feedback from the 16ohm tap, as was the original Dynaco. The 16ohm tap is where I'll be starting, with the stock values for their feedback circuit (6.8k and 680pf), and the 680 ohm resistor from 16-0 on the OPT just as was stock from Dynaco. I'm thinking stock Dynaco was what Jxdking was referring to. Because with el-34 output then yes I could see 40x gain on a 1.5v input would definitely be too hot. But it sounds about right for KT-88.
Loren
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