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acg

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acg last won the day on March 25 2018

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About acg

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    I can't jump puddles
  • Birthday 31/07/1973

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  1. They are great to deal with. They will also machine the heatsinks to suit...for example they knocked a few mm off a strip at the top and bottom of the last sinks I bought so they top and bottom plates of the amp case would sit flush with the top of the sinks...looks great...they charged an extra $10.
  2. 71a is a nice old DHT that can source a fair bit of current. If going battery B+ with one of these, I would use some newer generation LiPO4 batteries, the same ones used in home solar installations, because of their low internal resistance compared to any older battery chemistry of which I am aware. The 9V LIFePO4 batteries only give 8.4? volts, and are pretty expensive, but are rechargeable of course.
  3. The 10y/801a is quite a linear tube, have a look at the curves below... Plate 200V, 20mA, about -6v bias is an excellent linear region. When used as a preamp with say 2V input voltage there is plenty of space up there to choose an good operating point.
  4. Personally, I would be reluctant to let go of the LCLC B+. Surely the inductors have a suitable current rating and have enough inductance to do the job. If you do need another 30V just make the adjustment with the power transformer, allow for the hefty voltage drop across some SiC Schottky's and you are likely good to go. Another point of thought is the operating point for the valve. Because It is being used as a pre-amplifier it will have limited input voltage (circa 2V from your dac/phono probably) so does not need to be biased as heavily as it would be if using it as the power tube in a power amplifier. The first operating point I intend to try with the 801a is 200V anode voltage, 20mA plate current, -6V cathode bias. Not sure if you have already aimed at this low a B+, you may be as high as 400V, but I reckon that 200V-250V B+ range is a good spot to aim.
  5. Just as an aside, Thomas Mayer, at least a decade ago, used this sort of filament supply for 801a/10y... ^^^^ was in the psu chassis. Yes, 40,000uF of filter caps (electros of course). In the signal chassis was another LL2733. In that diagram he has connected two power transformers in serial so he can get the correct voltage for the tube. I would just have one custom wound to the correct voltage. The Coleman Reg. may or may not be a better solution than this, not sure.
  6. As @aussievintagesaid later on, low impedance is where we should be aiming for a power supply. That link I gave to the Stephie Bench information on valve regulated power supplies displays voltage lag as power supply impedance rises and gives some better ways to achieve achieve the optimum. My valve amps use unregulated power supplies, and I remember at the time wondering if I should be trying to snub the power transformer ringing, but the necessary RC circuit would also increase the impedance of the power supply so I did not even bother to try a snubber.
  7. I believe you. There are huge electros the size of my forearm in the power supplies of the bass channels of my 6 channel SET amp. Treble channels are not as tight, with less overall capacitance, and only then are the final filter caps film. Yep. If I were to ever pursue tube rectification, Stephie Bench is a great source of information. See here... https://jacmusic.com/techcorner/SBENCH-PAGES/sbench/reg1.html Low Z, tube regulated power supplies are possible but just take some effort. Yet to be tested by me, but the sand is only to provide an optimal load for the tube in which to operate, so I reckon you may actually hear more of the tube and fewer of the of the power supply issues. Either way, in some circumstances such as mine, it is pretty much the only way to drive distortion and output z down to a suitable level while achieving maximum gain. We will see when it is built.
  8. @Red MacKay, I'm told by someone in your neck of the woods with a lot of knowledge and practice in the area that winding an OPT for a 10Y/801a/VT25/VT62 is next to impossible to get right, that that family of tubes are a proper prick to wind for, and to not even bother with an OPT because I would not be happy. This is from the guy that wound a SE output transformer for me so a 6c33c could drive a 1r speaker load, so he knows how to get around problems! Choke loaded might be a little better, but really, for this family, in a lot of circumstances, the mu-followers are probably going to get the best results from the tube. Have a look here to see something similar to what I am planning, and what I mean by the advantages of the hybrid mu-follower load for this tube: http://www.bartola.co.uk/valves/2017/03/26/vt-25-dht-preamp-update/ Bandwidth is 3Hz - 800kHz at full gain of the tube. No OPT, no choke, and at 8v output is about 0.05% THD, all 2nd harmonic. Bloody excellent numbers for what it is. Regardless of all that, I think you should certainly resurrect your 10Y preamp with some expensive SS Schottky silicon diodes and personally I would retain the LCL filter over RCR. You will probably need to drop the B+ voltage a little once you go to SS rectification, but that is a simple series resistor. Go for it! Photos?
  9. I do not have much experience with valve rectifiers, but am put off them because of the generally high impedance of the supply which makes current harder to source making things sound "slow". Another problem, depending on your amplifier, may have been the output impedance of your pre-amplifier...if it is too high for your amp you may notice the issues you raised. Slow is not what I am after, and because I have in-room bass response to 18Hz and treble to 40kHz+ the pre will need lots of bandwidth to hopefully go an octave either side of those numbers. Red, how did you load the plate in your preamp? Because of the 5K input impedance of my amplifiers I am going to need low output impedance for the pre-amplifier. Also, to get the aforementioned bandwidth a nice "tight", low Z, B+ will be necessary (which is SS rectifiers and decent capacitance) and an optimal plate load to minimise distortion and maximise gain. The only topology that I think will really tick all those technical boxes is a hybrid mu-follower such as that which Ale Bartola champions. It all sounds pretty simple, but like you say adds up to a lot of space and a lot of coin. As planned, my preamp will be: input >> 6 way input selector (remote controlled) >> 801a/45/71a/YO186 DHT gain stage, CCS filaments, hybrid mu-follower >> Slage autoformer volume control (remote control) >> output The autoformer may have to be shifted to the other side of the valve, but technically it will be capable of transferring the full output voltage without a problem whilst lowering the preamp output impedance logarithmically with attenuation. Without the autoformer I expect circa 25r output Z. With the autoformer at output as soon as I attenuate from maximum gain the output z will crash below 1r...which is ideal. So fingers crossed it all works as planned. Red, perhaps a nice low impedance B+ and perhaps a different plate loading will help to alleviate your 10Y problems...not sure.
  10. I hope so. Along with the 71a's, there are also some 245/45, 801a, and YO186 DHT's to try out. That latter DHT will push 85mA, which will drive anything!
  11. Hey Rob, I'm not using Hypex anything in my system, but do need a preamp with gain and real current drive to manage my six channel SET amps with passive line level crossover filters for all six channels at the amp input. It is a tough load, circa 5K, with inductors, caps et cetera making it NOT an easy resistive load. So no passive preamp for me, only active, and something that can turn 1v from my sources into 4v+ so I can clip the amps, and given some nice "quiet" recordings I am going to need maybe up to 8x gain to make things really loud. I have a handful of SS active preamps here that vary from acceptible to not acceptible in use, but nothing really exceptional, and philosophically I would like complete hollow state from source to speaker: 2 stage valve LCR phonostage >> DHT preamp with autoformer volume control >> DHT/IDHT SET amplifiers. The DHT preamp that I intimated to earlier is really the only shot I think I have for valves to work in my particular circumstances, and the parts are pretty much all here to get started...just have to design up a chassis and get to building. But this is Art's thread, about a driver stage for a Hypex amp module. I'll stay tuned in here to see what he comes up with... Anthony Hey @Art Vandelay, are these amps for fullrange or just the bass? Or not sure?
  12. @mwhouston, I have some 71a's here to try in a preamp at some stage, and have been warned about their microphony and other suscpeptibilities. Not sure I coul live with them iving in an alfoil nest...
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