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In the Forum: Melquiades Amplifier
In the Thread: Planning my DSET
Post Subject: Six Channels are UPPosted by anthony on: 1/2/2019
Hi Romy,

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.

Initially the gas regulators were only drawing about 9mA each, which is a bit low, so I paralleled the 5k 12w resistors leading in to the tubes and have ended up with 15mA for both 0A2's.  This has actually helped the slow startup problem and the negative tube now starts about 30s earlier at about 65 seconds after the bias supply is energised.

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.  

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.  I think that I will figure out a way to install some test points to make everything easier.  They can stay under the covers and out of sight when the amp is closed up.  It may also be useful to be able to easily check the anode voltage of the first stage tubes when they get changed out in the future...I have some nice digital voltmeters here that could be run off the service supply or even a battery...I'll look into it. 


Cheers,

Anthony

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