Micro Superlead Circuit
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- เผยแพร่เมื่อ 26 ก.ย. 2024
- This is the description of the micro-Superlead amp. I basically lifted the circuit topology straight from a Marshall Superlead schematic and modified it. I did my best to scale down the power from 100 watts to 1 watt whilst retaining the original character. Instead of 4 EL34s, I put in a single 12AU7 (still push-pull) in the output stage. The main problem was that the voltage I was dealing with was a lot lower, about 270v instead of 400-450v. Therefore, I had to re-bias everything.
At some point I'll build a little box or something to contain it. Carrying it is awkward and I don't want to keep getting shocked every time I pick it up. I have a demo of this amp elsewhere on my channel: • Micro Superlead Amp Demo
Here's my build blog: thetuberoaster...
Nice amp.
I've decided im gonna do a baby plexi build, would you want to share you're diagram with me? I'm plannig on going ecc99 with a bit bigger OT. Love to hear from you!
I've only heard of the ECC99 but it looks really good on paper. JJ claims 5W plate dissipation at 400V with more gain than a 12AU7. I may have to try this one out someday. Bigger transformers in general would be ideal, it's just a matter of size and cost I guess. Just bear in mind the OT has a non-trivial influence on the tone and dynamics. The main thing I realized about the Micro-Superlead output stage is that it has too much gain and too much input sensitivity. The phase splitter will drive the 12AU7s well into clipping before the volume knob is maxed. This makes it sound super compressed and weird sounding. Part of this could have been the tiny output transformer I used. I ended up adding some 150K grid stoppers to make a potential divider with the 470K grid leak to reduce the signal level somewhat on the 12AU7s. That helped but did not remove the problem. (In fact, I think it contributed to the hum by reducing the dynamic range.) I didn't want to make the grid stopper resistor any larger because I thought it might infringe too much on the high frequencies. (Low-pass filter with grid capacitance.) The wise thing to do would be to redesign the preamp for less gain but it might sacrifice some of the "plexi" sound. If you give me an email, I can send you my schematics.
@@thetuberoaster8321 Sounds like its going to be a interesting project! jesse_rulez@live.nl is my mail. Looking forward to your reply, thanks man!
This is amazing! I just watched the video that you play through this amp and its sound. What's the calculation the get the proper cathode bias resistor values? I was thinking about adapting this to a very low voltage operation, like 24v DC. Thanks!
For really low voltages like 24V, regular cathode biasing probably won't work. Grid leak biasing is pretty much the only thing that works below a certain voltage. Tie the cathode directly to 0V and adjust the grid leak resistor value. Typically, this value is something like 1M, and the tube will pretty much bias itself normally. To set the bias closer to 0V on the grid, use a lower value resistor (100K or so). To set the bias to near cutoff (more negative), use the max value of 2.2M or 2.7M. You may find that 12AX7s are more finicky than 12AU7s since their anode current will be lower for a given voltage.
Of course, you won't be able to get exact bias values this way. The calculation for cathode biasing basically involves finding a location on a dawn loadline on the characteristic curves and finding the expected cathode current. Using Ohm's law, you find the resistance required to get the voltage drop (grid to cathode voltage) to bias the tube at that current. If you do this at low voltages, this will add to the overall bias voltage and drive the tube into cutoff, which is probably not what you want. At normal voltages however, this is almost always the process you would use.
Looks Friggin' cool, how much volts is your PT and hows the cleanish tone?
The PT is a 380V CT at 30mA. There is also a 6.3V winding at 1.2A. This circuit basically uses the whole transformer. The four tubes draw exactly 1.2A for the heaters and I'm sure the HT draws near 30mA when cranked. I have another video on my channel demoing the amp and you can hear what it sounds like at various gain settings. The cleans are honestly not the best. At least not compared to a Fender or something with hot biased 6v6s. The sound is sort of brittle at low levels and really only comes into character when pushed a little. Part of this could be the speaker not responding as well as it could since the clean sound is very quiet. There is a palpable lack of headroom with this amp. I can't get crazy wide clean-crunch dynamics like I can with my Marshall 18 watt clone. I imagine this is due to the low HT voltage of only 270V. I think the real Superlead has a HT voltage of 450-470V or something.
@@thetuberoaster8321 The Tube Roaster Very nice, I have been thinking about such a build for a while. Probably with an ecc99 for output tube to increase the output for a bit. Do you think a rather big OT will improve the headroom? The original super leads had rather huge Iron ofcourse.typically for a 12au7 or ecc99 a hammond 125A would be sufficient but you think a 125B or C would do the trick? I do want to be able to cover the inbetween plexi sounds, as you said the clean crunch dynamic sound. Also most PT's I can get over here seem to have rather low HT voltage. 300-325 ish.
@@pietlut2245 I forgot about the ECC99. It has a low enough Ra that you could use "normal" output transformers with it. The issue with the one I used is that it is only rated for one watt, but at full power the amp makes like 1.2W. I had to use that one because it had a primary impedance that was high enough for the 12AU7. The HT voltage is more of a question of what can the output tube(s) take. The 12AU7 is only rated for 300V if I remember correctly. You could also use the 12BH7. It would be beefier than a 12AU7. I think if you used an ECC99 with a Hammond 125A transformer with a primary impedance of 4.5K to 5K, and put the HT voltage around 350V to 400V, you could get a sound much better than the amp I made. The headroom would be there for sure. It would also be more powerful; making maybe 1.5W to 2W of power.
Very cool. I'm looking at building some type of micro in my next few builds. I'm thinking SLO 1 or maybe a 2204..
Hi! Great project! I want to refactor my JCM800 EL84 SE amp into PP based on ECC82 or ECC99. Please tell me how you recalculated preamp cathode resistors for lower anode voltages and which anode voltages were your reference? Thanks!
It's been a long time, but I remember getting the datasheet with the characteristic curves for the 12AX7 and plotting loadlines for the anode load to find the cathode resistor. I wanted to keep the gain relatively the same so I kept the 100k anode load resistor. I wanted the anode voltage to be just above half of the available HT voltage. Ohms law gave the required anode current in order to make that happen. I settled on a desired grid voltage of something like -1.5V. Tying the grid to ground and putting a resistor of 1.2K between the cathode and ground yielded ~1.6V. The second stage is biased colder for some reason at something like -3V near cutoff. I do not remember the reason for this nor the reason for why the two input stages were so different. I do remember trying to emulate the real superlead circuit and keep the bias voltage and anode current ratios as similar as possible. The bypass caps are different between channels because one is the bright channel with less bass and the other is the normal channel. I think I did recalculate those values as well using the standard -3dB RC filter equation. The stage after the volume control is biased the same as the normal channel input stage. (The bright channel is the one with the bright cap on the volume control). I do not remember changing anything in the cathode follower driving the tone stack. I may have left it the same. The tone stack is literally cut and paste from the original. I'm pretty sure the phase splitter long-tailed pair stage was also original. The only thing that I changed was the feedback resistor in the presence knob circuit since the output stage had a different amount of gain. I did this experimentally and settled on 47K but I might have been able to go lower. The output stage I had to change the grid leak resistors to something lower to drop the gain a little and added big grid stopper resistors to hopefully knock down RF and prevent oscillation. I'm pretty sure the 12AU7s in the output make way more voltage gain than El-34s do. This causes the amp to compress the sound a bit too much at full volume. The sweet spot is about 2/3 of the way up. You could probably decrease the gain somewhere and make it more manageable. I could not tell if this excessive compression was due to the power transformer sagging, the output transformer saturating, or the gain issue.
hello sir ! thank you for the quality of your video! excuse me for my english i'm french. Can you please share the layout with us! Thank you very much
There's a link in the description for my build blog where I have a schematic and explanation of the circuit.
@@thetuberoaster8321 hello sir! yes I was able to read your blog this afternoon! Thank you very much for the quality of your explanations! I think I will get started on this project too! thank you sir and have a good day