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In the Forum: Melquiades Amplifier
In the Thread: Planning my DSET
Post Subject: So far so good...Posted by Romy the Cat on: 1/3/2019

 anthony wrote:
Thanks for your thoughts Romy.  Last night I roughly wired in some random crossovers….
I wrote a loooong replay to you after that post but my 2.5 yo son turned off my computer while I was posting it. Hate to rewrite it….Your chasses design look spectacular by the way. In many year what your will dive back to that amps you will not believe you did it.
 
 anthony wrote:
So, six channels are now operational in the first DSET.  By up I mean the anode voltages on all tubes is about right and the currents are in the ballpark of where they need to be.  I am yet to check the DC Offset so hopefully that goes ok.
Congratulations. Do not be too anal about the anode voltages. For both stages anything above 175 will go, do care about plate current, particularly on the driver stage and at DHT stage.

 anthony wrote:
The Channel B is still an issue of sorts.  I have reduced R5 from 12k to 5k to get down to 200v at the anode of the 6e5p.  For the life of me I can't figure out why because all components in the bias string are the correct value and the cap is polarity correct.  Channels A & D also operate from the same arrangement for the positive bias and they hit their targets first time, so to my thinking it must be an issue in the negative bias string, however that string is correctly installed.  So, I have a decision to make.  In Channel B, assuming that I use a similar first order  RRC Low Pass filter at the start, do I reduce the 30k resistor by 7k, or do I reduce the 12.1k resistor that sits closer to the grid by 7k?  I am leaning toward changing the crossover resistor, and am not even sure either will make an audible difference.  
I am not sure what you have in there, I would keep the R5 as is. You should have -150V at point #06 that should burn in the 1K and 420K resistor, arriving at driver tube grid at ~-4V. You need to short the chela B input and to measure the -4V at grid. At that time of you have 200V on plate the tube will be completely operational. Then you apply positive bias on the left side of the filter (resistor 30K). The voltage on the left side will be different and you need to turn you positive supply trimmer to set it to 0mv. The p[point is that positive supply need ONLY to get rid of current flow over the filters, the amp should work fine of you short the inputs.  Make sure that you .043uF cap to ground is not shorted. From what you describe it might be the case.

 anthony wrote:
One thing has become very clear to me while sorting out this first DSET...the crossovers in Channels A-D affect the operation of the bias, so when changing filters I may have to re-tune the bias for that channel. 
Yes and no. Your bias is your 12.1K resistor, pretend it as a virtual buttery. Or you can pretend it a firewall for you bias voltage. Now, when you calculate a filter you do it considering the load impedance, in my case it is 12.1K and I intentionally too keep it the same in all channels. There are too many variables to equate if you begin to change the bias rector. The key point is that if you change filter resistors AND bias resistors then the amount of voltage you have available in positive bios supply will not be enough for you to set 0V at input. So, you would need to assure R11 resistor to make sure that you VR1 will be able to set 0V in the middle of own operation. To me it is too many changes and I prefer to keep my bias resistor at 12.1K and if I want to change the crossover point then I just adjust the series resistor and the cap to ground. There are two things you need to recognize. First, in the channel B the first restore 30K and shunt 12.1K are not a filter but voltage divider that mitigate the volume of the channel output. The filter is a combination of 30K resistor and cap to the ground. Second, is that when you change you filter make sure that in your circulation you use ONLY Bessel curve, it is important.
 
Generally speaking I would be interested to know how you like to change the crossovers. I feel that sine you have the same configuration as I do I know that result you should be getting and there is a HUGE amount of time invested into this thing to do experimentation, listening, testing, measuring etc and there is not a lot known to me room for improvements. Trust me it has  nothing to do with ego  and I will LOVE to learn if you fine any better way to organize what has been organized, unless you use other compression driver…

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