Since I know about a few ongoing attempts out there by different people to build Melquiades I would like to share with some of my observations regarding corrections, simplification and possible modifications of Melquiades amplifier.
Obviously you have read the Melquiades release page:
Please take very seriously my insistence for high sensitively speakers. It was not the paranoia or preferences of a horn guy. There are deep underlying reasons (that I live aside) why I insist to use high sensitively speakers.
Below are a set of randomly organized thoughts about Melquiades that the people who have considered devote/invest into Melquiades might find useful. All below comments will be regarding a regular full-range Milq, not about the Milq DSET.
1) Melquiades, Milq, Melq – across this site they are the same names of the Melquiades SET
2) The Milq’s Driver state tube is easily available on eBay for slightly more then one dollar. It is ridiculously wonderful as the Morons pay a lot of money for 6H30, 437A, “kinky” 6922, and many other drivers but they completely disregard the dirt-cheap 6E5P that in fact sound order of magnitude more interesting then any other indirect heated drivers.
3) There is a mistake in Milq circuit. The B+ of the driver stage has almost insufficient amount of critical inductance. It was because I drew the circuit having the Super Milq in my head where the PS was feeding 3 stages and consequently drew more current. I recently opened my full range Mils and confirmed that I have not 15H coke in there but 30H. So, ether you increase the current you draw across the PS lowering the R23 bleeder or increase the value of the L1 choke (Hammond has 30H and 40H chokes that do very nice). My presumption is that any PS with input choke should be tested in continuity, so it is easy to see what is going on….
4) It is not necessary to run input choke filtration for the Milq’s bias supplies. If you wish to do for CRC filter before the gas tubes then it is fine (use input cap of a small value thought). You will also need much lover voltage on the secondary of the bias supply transformer. With input choke the bias need to be 220V as the input choke will eat up ~10% and the gas tubes need 190V to light up. With input cap you will have an increase voltage from bridge and I would go for 140V-150V on secondly if you use CRC…
5) You might consider do not make a dedicated PS for positive bias but to take it via a resistor right from C2 capacitor. I did it and I did not see any worsening in sound.
6) Do, not screw with Milq’s bias principle for both states. The input stage should be in Milq exactly as it is and the output stage should be fixed biased. Keep the R6 of very good quality, and keep it very short and right at the grid. Bring the return to ground of C5 right to the V2’s ground.
7) The C12 capacitor is not relay necessary if your C9 capacitor physically located close to VR2 attenuator.
8) If you never used 6C33C before then print many posts at this site about this tube and put them in your restroom. Sometimes when you spend long time in there please read about the tube. Do not forget to heat the tube for a long time (40-60) minutes before REALLY stressing it.
9) Amorphous core output transformer is absolutely mandatory for this amplifier. There are a number of companies that do it. Find it, pay premiums and get the amorphous transformers.
10) In the circuit I marked the out stage operation at 225V and 180mA. However, these markings are only for 200mA Lundahl transformer. Lundahl can not gap them for more current as they have fixes core size and any increase of currant will reduce inductance of primary with consequential loosing of bass. Even at 225V and 180mA with 200mA on LL1627, loaded to 8:1 (depends of your speakers) the Milq does very fine. Still it is for my speakers. If you are not using the off the shelf Lundahl but go for a custom transformers then I would suggest to stay with 190-200V and run 250mA across the tube, with a transformer gaped at 300mA. Sure, it will greatly depending from your speakers - do not forget the Milq has no feedback and therefore it is very sensitive to load. Do not use the transformer taps under any circumstances. Anyhow, you can go as low as 170V and as high as 250V, driving from 180mA to 330ma across the tube, loading it from 5:1 to 12:1. Increase current lower plate impedance and adds transients… You will need to play with current and loading in order to please your speaker… BTW, do not forget that the bass section of your loudspeakers and the HF section of your loudspeakers might have different optimum loading demands…. but I promised don’t talk about DSET in this thread….
So, my recommendation for the folks with 300mA transformer would be for average circumstances of above 12R loudspeaker to go for 200V/250mA (on full tube). It largely would be OK for people who use normal, mid colorfulness Alnico compression drivers. With very flowery drivers you might slightly drop current and with very dull drivers or with ceramic drivers you might drive more current through the output stage drop voltage. It is always a good idea to have an extra gap in transformer that would allow playing with current. The 6C33C will slightly fluctuate current with signal, even with low PS impedance, so to have extra DC capacity in your transformer is good idea to keep away form saturation. Let your transformer maker to advise you about the balance between the transformer core size and the diameter of the turns… Let them also advise you how much excessive gap would be desirable. From one perspective with amorphous core it is hard to get bass out of transformer from another perspective the amorphous core tolerate proximity to saturation with more damage to sound then regular non-amorphous core…
11) Do not save money on the output transformer. It is perfectly OK if the output transformer will cost you as much or more even a few times then the rest of your Melquiades. I assure you that Milq will do sonically wonderful thighs. You assure yours that you will get for Milq a deserving transformer.
12) Put two 10R, 5W resistors (in each leg) between the rectification bridges and secondary of the power transformers.
13) Read my article about the gas regulator tubes: http://www.goodsoundclub.com/GetPost.aspx?PostID=2792
14) The 6C33C tube sockets are very bad, unless you go for expensive Johnson tube sockets. Still it is not too difficult to replace the cheap 6C33C tube sockets once in 2-3 years. You will extend the live of the 6C33C tube sockets if you clean the 6C33C pins of your every NEW tube. The pins are covered with some freakish candle-like material. Use fine sand paper to brash the pins up and to remove the material.
15) If you do not cool down the Milq internals then allow some space for 6C33C to breathe. Make 3”-4” around the tune as the parts-free zone and drill some ventilation holes around the 6C33C - it runs hot. Twist the filament wires to the 6C33C socket and then solder it. I have seen as in not cooled down tight enclosure the heat of 6C33C melted down the solder on the filament pins. Use the 6C33C pins a high temperature solder. BTW, do not use heavy silver solder anywhere in signal pass. The very light-silver solder (like Wonder solder) is tolerable.
16) If you have a very good quality VR2 that can handle some power then use it and get rid of the R19, R18 chain. I used the 10-turns Burns that are 2W I believe and the R19, R18 are juts to keep the attenuator cooler. Keep the VR2 as good quality as you can get and put R20 as close to the 6C33C’s grid as possible (do not burn if by the tube though)
17) There are a very few wires in Milq – it is very simple amp and most of the parts might be connected by their natural leads. Sure no silver wires must be use anywhere but almond all equally good cupper wires that you will chose to use some wires are more important in Milq then other. You will have a wire between negative pin of C9 and R18. Pay attention to that wire. It layout, the proximity to ground and etc is auditable…
18) R2, R12 and R23 should sit right in the tube socket.
19) The Milq even without loading resistor has enough impedance from the bias resistors but I still like to current-load my cables using 20K resistor as R3.
20) Use good quality tube socket for 6E5P. it is very fast tube and it has some minor tendencies for microphonics when is it getting worm (not as horrible as some other tubes through). The Connex 70032, the 9-pin tube socket does very well and it is not too expensive. Expect approximately on year from a single 6E5P with approximately the same from 6C33C. it is $12 per year – it will be all maintenances that you need to do on Milq.
It is all that I have atop of my mind for now. If something else come to me then will dump it here….Good luck,
Romy the Cat
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche