Hi nautibuoy, I am Italian I know very well Ares Audio, that is a commercial version of Mellow OTL, for who doesn't mind to spend lots of money well is a very good PCB and component but the cost is quite hight, you can have the same quality sound spending very small money....interested on sub-miniature tube, question why use sub-miniature tube? thanks
Hi @kissabout2002. I have the Ares Audio power supply and soft start PCBs and have dealt with them previously and they do seem to be a good company and the parts are high quality. I suspect the V4 all-in-one PCB is practical though very expensive and I'm not in the market for one.
The only 'issue' with the Ares PCBs I have is regarding the soft start board, I am not convinced about having a relay in the speaker path so I may explore alternative speaker protection arrangements - I think XRK's mosfet based speaker protection modules are able to operate at 150V DC on the speaker line.
As for 'why use sub-miiature tubes', because I have them already and it will be an interesting, and not too expensive, experiment.
The only 'issue' with the Ares PCBs I have is regarding the soft start board, I am not convinced about having a relay in the speaker path so I may explore alternative speaker protection arrangements - I think XRK's mosfet based speaker protection modules are able to operate at 150V DC on the speaker line.
As for 'why use sub-miiature tubes', because I have them already and it will be an interesting, and not too expensive, experiment.
Hi nautibuoy,
I completely agree about having relay in the speaker path is not a good idea and please if you find a reliable speaker protection just drop me a message, for now I just add a 1.5A slow fuse on the speakers path plus of course the protection in the original Tim schematic.
My personal problem is with PCB's and tubes, is 50 years that I am dealing with electronics of any type and I had so many problems with PCB when there is hight tension and hight current, I see Ares are really doing a good job and new technologies of course allowed to get hight quality product but still I will never use PCB for my personal project when tubes are involved, unless small pre-amp, I rather prefer the old style point to point circuit, I know I am an old man
All the Best
Acky
You're welcome and apologies if I implied a replacement of the 6C33C tubes and raised false expectations.
I've built several OTL amps and agree with your commet about their sound quality.
BTW, those other OTLs include Transcendent's SE-OTL and 300B SE-OTL and a screatch built 13E1 SE-OTL. I also currently have a 6C33C SE-OTL project on the back burner too, but that is very low power and only suited to higher impedance speakers (I have 15ohm Lowther horns).
Hi Acky.Hi nautibuoy,
I completely agree about having relay in the speaker path is not a good idea and please if you find a reliable speaker protection just drop me a message, for now I just add a 1.5A slow fuse on the speakers path plus of course the protection in the original Tim schematic.
My personal problem is with PCB's and tubes, is 50 years that I am dealing with electronics of any type and I had so many problems with PCB when there is hight tension and hight current, I see Ares are really doing a good job and new technologies of course allowed to get hight quality product but still I will never use PCB for my personal project when tubes are involved, unless small pre-amp, I rather prefer the old style point to point circuit, I know I am an old man
All the Best
Acky
I'm checking out X's speaker protection solution, which appears as though it might be suitable as the 'protection' Mosfets he uses are rated at 150V, what do you think?
https://www.diyaudio.com/community/threads/rtr-ssr-speaker-protection-gb5.392399/
As for PCBs, I generally prefer hard wiring for high voltage/current situations, like the 6C33C output arrangement. The PCBs I have started working on for a sub-miniature Mellow project will only be for the phase splitter/driver stages where the voltage/current is more manageable. I will use the Ares power supply PCB, though not for the 6C33C filament hookup, as it is heavy duty and designed for twice as many 6C33Cs as I would use.
BTW, I technically become a state pensioner later this month, so I'm no spring chicken myself.
BTW, Neuochrome offer a similar protection module but it's only rated to 90V
https://neurochrome.com/products/guardian-86
https://neurochrome.com/products/guardian-86
Hi again @kissabout2002. I saw the respnse from XRK regarding the speaker protection modules. I think his response is incorrect in assuming 300Vpp based on the power rails, as you say, the output of the amp is nowhere close to that. My assumption is that we are talking about protecting against a 6C33C failing with a short circuit, which would present plus or minus 150VDC at the output, which the XRK module would accomodate. I'm assuming Tim Mellow's original speaker protection arrangements would be left in place too, so an element of 'belt and braces', and I assume that would ground the power rail and blow the fuse in that rail. Does that coincide with your own thinking?
Hi nautibuoy,
yes me too I think XRK made the wrong assumption, well is not only a catastrophic event on the 6C33C's tubes, by the way if a short circuit will take place in one of those tubes one or both 3.15A fuses on the -/+ 150 rail will blow immediately even before intervention of speaker protection, what I am thinking is also different, while testing my amp I came across a defective 6N2P that I am using instead the ECC83, result I had persistent 1.2/1.5 dc volt across the speaker even when I was trying to correct with RV2, at that stage the amp was connected to a dummy load, I even connect a 8ohm low quality speaker with 1.2 volt but the speaker still was working for 5/6 minutes with no problem then I disconnect it, so my concern is also this, will the speaker protector engage even with just let's say 1 vdc or less across the speakers or just in case of serious problem?
yes me too I think XRK made the wrong assumption, well is not only a catastrophic event on the 6C33C's tubes, by the way if a short circuit will take place in one of those tubes one or both 3.15A fuses on the -/+ 150 rail will blow immediately even before intervention of speaker protection, what I am thinking is also different, while testing my amp I came across a defective 6N2P that I am using instead the ECC83, result I had persistent 1.2/1.5 dc volt across the speaker even when I was trying to correct with RV2, at that stage the amp was connected to a dummy load, I even connect a 8ohm low quality speaker with 1.2 volt but the speaker still was working for 5/6 minutes with no problem then I disconnect it, so my concern is also this, will the speaker protector engage even with just let's say 1 vdc or less across the speakers or just in case of serious problem?
Hm. First a correction of misinformation:
We designed an OTL based on the 6C33 in 1992. It was originally designated the MA-1.5 and was later renamed as the Novacron. We featured it on our first website back when Netscape 1.0 was the only game in town, about April 1992 IIRC.
Some things to consider in this design:
1) the use of a control in the cathode circuit of the voltage amplifier will be found to be problematic. We found it better to have a good quality Constant Current Source between the cathodes and B- instead! The control really isn't needed since the voltage amplifier employs coupling capacitors. The cathodes should be directly tied to each other. This will increase gain and reduce distortion.
2) The bias means used will alter the distortion of the driver section depending on the bias setting. Again a CCS is recommended for this circuit. Bias really should be handled on the grid side using a divider network. This is possible since coupling capacitors are used.
3) the circuit really needs more grid stop resistors. 600 Ohms will be adequate at the output tubes but should be rated at least 3 Watts to prevent failure if a tube develops defects. Otherwise oscillation is a real possibility!
4) the input coupling cap is not needed. The tip connection of the input connector (apparently a phone jack) can be tied to the other 6J10B input tube, directly to the 33K resistor. Again the 100nF cap can be omitted. In this way the amplifier can accept a balanced input. If single-ended is desired, the second input need merely be grounded. A stereo volume control can be placed at the input; each deck wired in identical fashion.
5) rather than compromise the CMRR range and overall performance of the input, a separate power supply for B+ would really help. The additional B+ would help linearize the gain stage.
6) if you look at the driver section, you'll see that one plate supply is +140V while the other is essentially 0Volts, being at loudspeaker potential. This means there will be a bit of a current imbalance between the two pentodes since they share a common cathode connection. It might be worth consideration to experiment with separate screen circuits just to see if better drive can be accomplished. Pentodes being what they are, this difference may not amount to much, but it might be worth a look. Just a thought.
Are these ceramic? Where did you find them?I think good quality tube sockets are important with 6C33Cs and I am fortunate to have four of these available for my project; very substantial sockets for the tube pins.
Thanks for the history lesson, though I suspect it was an innocent error, most people would have had no visibility of products on websites that were using Netscape 30yrs ago.
Are your comments targeted at the Mellow in general or the sub-miniature schematic specifically - I ask because when I worked through the sub-min schematic it is logically the same as Tim Mellow's original, it's just the tubes that are different? It is quite a while since I looked though so perhaps I'll have another review later.
The sub-min schematics aren't well set out though so it takes a bit of concentration. I found this sketch of the Mellow schematic to be useful;
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Yes, they're ceramic. Much better quality than the typical Septal tube sockets. I bought them from a guy on Swap Meet, a few years ago now.