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I was searching for an answer why squire wave is so screwed and I was advised by the site’s visitors that it is due to HF restriction of my channel. My HF response does look too low to me – the 23kHz; with all my ultra low capacitance magnetic I was expected way higher.

Something is not right. So, I decided to look at the stages independently.

First I looked at the output stage. I shunted the grid of 2A3 with 250K to ground and put 2uF cap between grid and generator.  Unfortunately my generator can’t swing more than 10V and I was not able to drive the tube into full power. With 10V on grid the 2A3 just started to roll off at 275kHz, but picked a lot of distortions. The clean and not distorted sine wave I got somewhere up to 140kHz, that is meaningless as it was not at full power. So, I drove squire wave there the thing at 10kHz. This time it was tutorial different picture. Obviously the output stage was fast enough and was not the problem of my initial squire wave.

Then it might be the transformer. I still do not know why that transformer in 2:1 configuration gives 30% of voltage lost. Who know, might be something else wring with it? I got rid the grid resistor and coupling cap and put the coupling transformer in, driving transformer’s primary from generator

It looks like the transformer did restrict HF up to the point as I had in the post above. So, the driver stage was one that left. I put 15K anode resistor on the driver stage, drove it from 400V and took output via a coupling cap. Here I had my 23K restriction and screwed squire wave. Hm, it looks like my line-level filter plays a bad game with me introducing too high impedance on the driver’s grid that along with tube miller capacitance of the driver tube shape low-path filter. The original Milq bias had 8.2K grid resistor, now the idea evolved to 32.1K for HF channels.

http://www.goodsoundclub.com/PDF/6-Chennal_Melquiades_DSET_Amplifier.pdf

Well, I need to think about it.  For MF I need the 20K resistor to write the high-pass filter. I can get rid the input filter all together, keeping just 10K-12K impedance on grid (that would give me around 50kHz response) and then I can make the same high-pass filter with coupling cap.  I am a bit afraid this solution as I had problem with sound in this configuration before, when the inner-stage caps acting as filter did not sound right for HF

http://www.goodsoundclub.com/GetPost.aspx?PostID=1234

also, I do not like the idea of coupling cap as with my 33 time gain in driver stage it will deliver too high voltage to the 2A3 grid and I would not wont to be involved into the coupling cap recharging and A2 operations. I might drop voltage on the driver’s grid that will again introduce too high impedance on the grid…

Frankly here is where the 6E5P gain works against me. To use the high gain, current capable driver did make sense for two-stages around 6C33C that has 60-80V on grid but with those flimsy DHT with 40V on grid I would prefer to have 15-20 time gain in driver, something like a half of 6SN7 or alike.

I kind of do not know where to go from here and I did not even start to listen the thing. I am trying to interpret the result I was getting up to now. The original Melquiades idea was perfect but then the value of the grid resistor begins to grow to 10K and then 12.1K and then to 30K that included the high-pass filter. Unknowledgeably to me the HF response of the channel was lowering from above half- hundred KHz to 23Khz. Parallel to it I was loading the MF stage harder and harder from 1.2K on 6C33C (that is VERY idea for that tube) to something that is equivalent  to 450R – a very heavy load.

I might understand why I went this way – I use Vitavox S2 drive that is very fast, very contrasty and that might go into some harshness if it not handled properly. Everyone who use S2 have problem with it but I do not. My S2 is very smooth and superbly eloquent. I was under presumption that I accomplished it by loading the stage that drive S2 but now it look like I also used less fast amp to drive the S2. It might be even beneficial but I am a bit distracted by the fact that when I switched my Sun Audio 2A3 prototype amp from cathode-biased 6SN7 to grid-biased 6E5P I did observed some transformation in transients reproduction. The cathode-biased 6SN7 has no HF limit but the grid-biased 6E5P with 12.1K resistor is effectively a low-pass filter at ~35K. Add to it the roll off of the transformer and we have second order low-pass. Well, I need to reconcile all of it and figure out how much HF I would need in MF channel.  The whole objective of the experiment of conversion the Milq to DHT MF was to inject live more into MF. My presumption was that my single stage tube has no power and no gain to be loaded idle-enough to have good transients. However, it might be ALSO the problem is not with the single stage itself but with the fact the MY CURRENT IMPLEMENTATION of single stage has too aggressive low-pass making the amp not able to throw good transient characteristic.

Hm, this opens a wide thinking about the other opportunities…

Unquestionably in the original Milq the cathode-bias or fix bias did not sound even close to grid-bias. In context of full range the grid-bias literally destroyed all other options… with THIS tube. Would it be because my driver tube itself is very fast (275MHz) and my grid-bias calmed the tube down a bit?

Well, there are many thoughts in my head at this point and I need to think what to do with my MF channel next. I even thought to DC coupe the driver and DHT, why not? I have 400V supple to drive the DHT. If I burn the 150V on DHT cathode and drive the driver at 200V on plate then I can direct-couple them. It will be no drift for PS as my whole amps is driven from AC-stabilized regenerator (good for PP2000). All DC instability might be handles and I never seen my 6E5P to die catastrophically, so nothing will burn out the output stage. It will not however address the problem with the driver stage’s low-pass… To use slower tube with less gain? To go cathode bias on 6E5P but then I need to target impedance to write my line level filter? To go with a cap filtration instead of RL filtration? To go full-range super fast amp and use speaker level filter? I do not like any of the options so far.

That all is very interesting and unpleasant so far. I did not think that the DHT projects would open so many skeletons in the Melquiades closet.

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