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Home » Melquiades Amplifier » Planning my DSET (187 posts, 10 pages)

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09-09-2019

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anthony
Posts 338
Joined on 08-18-2014

Post #:	181
Post ID:	25589
Reply to:	25588

More to do

Romy the Cat wrote:
Anthony,

First, your measurements again the “appropriate load resistor” has very little practical meaning. The drivers is a dynamic system that has impedance not resistance and the impedance varies with frequency. So, your measurement against 1R might be accurate for 30.5Hz but might not be accurate at 24.7Hz. So, the proper number at which the clipping might take place would be observable at the real load, or the real driver(s) you use.

I agree completely, of course. I made these measurements with the purpose of checking the input voltage levels that I should aim for with the pre-amp project. A secondary motive was to make sure the Bass OPT was doing something like the job it is supposed. To be honest, driving sinewaves at circa 5V into the DSET is LOUD, so I have ordered and now received some appropriate load resistors for all channels so that I can be more thorough with re-testing without making my ears ring.

If I look at the impedance/frequency/phase plot of the Bass Cannons (below), 18Hz is pretty close to the most difficult load that amplifier channel will see. Most of the passband is in resonance so *should* be easier to drive, however I do plan to re-do the measurements using the Cannons as the load.

DATS First Cannon (Left).jpg

Romy the Cat wrote:

Then there is a definition of clipping. The clipping is situation when a channel has no power to drive the driver but power is current and voltage. You should not be concerned about juts clipping but you need to assure a SYMMETRIC clipping against you real driver across the whole range of the channel. You need to connect a scope to the channel output while driving your driver and drive a single frequency from a generator and increase the input voltage. Observe the shape of the sinusoid and as the input voltage rise you will see that the top or bottom of the sinusoid will bet distorted, or clipped, or flatted down. The top of the sinusoid is voltage insufficiency and the bottom of sinusoid is current insufficiency (or vise versa, I do not remember already). So, you objective should be to get the absolutely symmetric clipping point when the current and voltage clip at the same time. Then it means that you get out of you amp the max power again your given load.

My procedure was pretty much as you described: put a sinewave into the DSET, watched that sine on the oscilloscope (software), change the voltage level to find when the top or bottom or both of the sine starts to flatten. Not all channels showed symmetrical clipping and I cannot accurately remember how the Bass Channel clipped, but I think it was the bottom of the sine first having problems and will certainly take more notice when I re-do the measurements now that I have more load resistors to quieten the room.

Romy the Cat wrote:

Second important aspect is that if you deal with an amplifiers then you need to understand when clipping comes from. If might come from many locations. The PS in driver stage, the coupling (in case transformers use), or the output stage PS, or the OPT and so on… The Milq was designed in a way that the any power restrictions are coming from output transformers, the way how SET should be designed and the DSET topology would take care of that limitation. So, the primary focus of your in the given topology should be the only the Channel A output stage. If looks like you beef up enough inductance in you OPT of the Channel A, which is good, not you need to make sure that you can pump power to the output tube. Here is there is another limitation. The 6C33C is indirectly heated tube and they type of tube as the enter to the operation what grid voltage approaches to bias voltage (class A2 or the mode of grid currents) then this type of tubes do not do so well and they distort rather hard. The direct heated tubes if you feed them with enough current they can push through a little bit but the indirect heated cannot. You it should be very important to you that in your case your Channel A as the input voltage goes up the Channel stags in Class A1 and the input voltage in the grid of 6C33C does not rise to the rise voltage. In fact, knowing how the Milq is designed this would be the ONLY measurement that I would care as it ejectively demonstrate the efficiency of you LF speakers projected to the acoustic size of you listening room. I had at my site a post where I described the measurements I took and posted my measurements in my room. Here it is:

http://www.goodsoundclub.com/Forums/ShowPost.aspx?PageIndex=1&postID=6057#6057

Thanks Romy. This is the stuff of which I am uncertain. I'll work through it...I think it is important for me to understand this aspect of the design, and also to evaluate the Bass OPT.

07-26-2020

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anthony
Posts 338
Joined on 08-18-2014

Post #:	182
Post ID:	25895
Reply to:	25589

I had it wrong all along...

Work has been so crazy busy all last year and through these strange times right now that I have not been having any time to think about the audio project let alone indulge in advancing it. One DSET/Macondo has been playing mono and I have really been enjoying it. Then a few weeks ago we took a family holiday to the beach (no C19 community transmission in my state of Australia...restrictions are reasonably loose) and I resolved to try to take a weekend for myself here or there.

The first weekend was spent cleaning and reorganising the room. It is amazing how much dust gets through things and how long those Bass Cannons take to wipe down properly. That was a load off my mind. It felt so good to do something towards the audio.

Then this weekend, even though I should be working, I decided that the time was right to have another go at biasing the second DSET properly. That week by the ocean had given me time to think about the problem I was having with the amp, and although I had already spent probably more than a week (or two) over the past 18 months trying to get things to work properly, my electronics beginner status seemed unchallenged. By having the problem leave my head for six months, then revisiting it, where the problem lay became clearer and over the next couple of weeks I snuck moments to think about the method I would use to verify and ammend it.

It worked straight away. Plus it has led to me finding a problem with the DSET to which I have been listening since March last year. The first stage of Channel B has been running hot...270V at the anode where it should have been 200V. Not sure how I could have missed that...but I did. The fix for that issue is more challenging.

Romy, perhaps you are able to remember some of the lessons from when you were developing these amps all those years ago. Basically, the problem is that the first stage of Channel B is biased at -7.15V where the remaining channels are in the vicininty of -4.2V. I can change either the 12.1kR resistor to a smaller value (about 2.9kR) to bring B back into the fold, or I can double the size of the 402k resistor (between the aforementioned 12.1kR and 10uF cap). Both will work according to my calculations. I think you are going to suggest to change the 402kR to circa 800kR rather than 12kR to 2.9kR, but the former is much easier for me to do than the latter.

Happy days though! I am going to let the amp soak in downstairs on the test bench this week and assuming no problems will grab a couple of blokes for the lift upstairs into position in a week or so.

07-26-2020

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Romy the Cat

Boston, MA
Posts 10,183
Joined on 05-28-2004

Post #:	183
Post ID:	25896
Reply to:	25895

Play It Again, Sam

Anthony, I would recommend you go other direction. I very much assure that with over 7V on fist stage bias the channel is dead, it should be around 4V. Let see how to fix it. I would certainly do not change neither 12.1kR resistor nor 402k resistor. Both of them form loading impedance for the filter and it you change it then you need to move the value of the cap to ground (it forms subtracting low-pass filter at 500Hz). It is not a big deal to do and it might be that you want to move the crossover pose to one or another direction. It is your room and your sound, you can do whatever you want. Still, remembering how much time and efforts I spent to find the configuration that I published I would recommend you to try it once as is and then move to whatever configuration works in your room and for your ears. So, instead to changing the values let think why you have wrong voltages if the PS voltages are as they should be. I am sure that you tried to change the tube but I think it is not the case. I think you out in there a wrong resistor accidently. It is easy to confuse them, and particularly is you use my Dales who mark the values by a position of a dot. If instead of 402k resistor you put in there 40.2k resistor then you will end up with what you have. If you do not want to take the resistors out and to measure them you can measure the resistance by measuring the voltage drop before and after resistor. I am sure that wrong resistor in bias cha is the problem. Even if you do have access to the resistor and can read the proper value on it then take it out and measure, it might be faulty.

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche

07-26-2020

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anthony
Posts 338
Joined on 08-18-2014

Post #:	184
Post ID:	25897
Reply to:	25896

R4 is the Culprit

Thanks for your help Romy. Have a look at this (barely legible) sketch so we have a common reference:

Channel B Bias V2.jpg

This is Channel B First Stage bias circuit. All is identical to your published schematic apart from R9 which I had to increase to get the other stages to bias up around -4V. In red are my voltage measurements at those points with the current calculated according to the nominal resistor value: they seem to correlate. Note that R4 has a voltage drop across it and contributes -2.9V to the bias for the 6E5P. This is different behaviour to all of the other channels where the crossover filters do not impact the bias for the first stage tube. Channel B filter is unique in that it has a shunt resistor R4.

If you recall Channel A has foregone the 12kR resistor (R5) and uses a larger 1M R7. If I did this, I could remove R4 entirely, swap out C1 for 0.01uF and retain the same low pass filter frequency. It is essentially the same as Channel A but wil a different LPF. See below:

Channel B Bias Solution.jpg

R9 can then be adjusted to get the ideal bias voltage, and all three channels on this bias circuit (A/B/C) will adjust by the same amounts.

07-27-2020

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Romy the Cat

Boston, MA
Posts 10,183
Joined on 05-28-2004

Post #:	185
Post ID:	25898
Reply to:	25897

Some explanations

I see, let me to explain how the this work all together the you will be able to make your own judgement. Let look at the functional purpose of the emails in there.

The R1 resistor is a loading resistor. This is understandable. It makes sense to keep it high as we have 5 parallel channels to run that drops the impedance to 1/5.

The R2 is the resistor over which the positive bias is suppled, balancing the input to 0V. It is a good idea to keep is as high as possible as it acts as a decouple between the tube and the adjustable resistor. This DC voltage goes directly were the signal flows what so it should “conditioned” as much as possible. The higher R2 resistor the less the AC signal “sees” the elements behind the resistor. BTW, you have in there missing another resistor and a very small capacitor to the ground over which noises get shunted. This cap very impactful to sound, so you could experiment with it.

The R3 and R4 is are a regular voltage divider set to align the output of your mid-bass channel with the rest of the channel. It your room it might need a different adjustment depending on how you room response to the given position of your mid-bass horns.

C1 is a low pass filter again the impedance of everything you have on the right.

R7 is the same as R2 but for negative supply. There are a lot of missing there as well but you might did it internally to make the drawing dimplier. Do experiment with that C2, that cap, in particularly on negative supply for some reasons VERY MUCH affect sound. I ended up use an electrolytic cap, which is kind of ridicules but sounded the best to me at that time.

R6 is a grid resistor. It is very fast tube with a tendency to rind and to have microphonics, so the grid resistor helps little bit with that. You can put sone ferrite into play that would do the same. When you just turn the amp and it is stone cold you can clearly hear a very unpleasant sound with a lot of ringing. That is from 6E5P grid. BTW, the length of that ugly “glossy” sound high be a good is a good indication for you when to change the 6E5P.

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche

07-28-2020

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anthony
Posts 338
Joined on 08-18-2014

Post #:	186
Post ID:	25899
Reply to:	25898

Fixed

Thanks Romy,

It is now fixed. I have left Channel B the same apart from adding a 390k positive bias resistor on the positive signal line (between C1 and R5) connected to the 14k resistor that also connects to Channel C and F. A 402k would have been used had it been on hand.

The solution came about by thinking about simple things that Romy could have transcribed when preparing the schematic, and it worked right away. The beauty of this solution is that changing the attenuation for Channel B (R4) will not alter the bias voltage by much and also there is much less drop across that resistor from the negative signal line.

Now, all the channels can be set to about -4.3V bias so it is working.

Regards,

Anthony

07-30-2020

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anthony
Posts 338
Joined on 08-18-2014

Post #:	187
Post ID:	25900
Reply to:	25899

Two up

Impatience got the better of me, and I called two mates who prompty showed up yesterday afternoon and we hefted the second DSET into place on the horn stack. A couple of hours of connecting and checking things and it was all ready to fire up for my first sterophonic sound in 18 months.

What a delight! Even with a dying 45 in the right channel and a sweet YO186 in the left! Especially with the lovely Helix Two/Schroder CB/MC Anna in action.

The finish line is getting close now. Have t find some time to finish the lathe work for the front half of the upperbass horns and then it is general tuning and room treatment.

Anthony

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