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02-28-2006 Post does not mapped to Knowledge Tree
AnonymousUser
Posts 19
Joined on 11-27-2004

Post #: 1
Post ID: 2139
Reply to: 2139
Grid bias power supplies
Hi Romy,
   Your amp schematic is intriguing. I have a question about the dual supply for the input tube grid bias. If my preamp uses an output coupling cap, do I need plus and minus supplies ? The interconnect will be biased at the same voltage as the grid, but that shouldn't be a problem, should it?
Floyd Andrews
02-28-2006 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,345
Joined on 05-28-2004

Post #: 2
Post ID: 2140
Reply to: 2139
Re: Grid bias power supplies

Floyd,

Regardless your preamp uses output coupling cap you would still need a positive supply. Positive supply compensates the negative DC voltage at the amp’s input. Theoretically if you do not connect or disconnect the cable between your preamp and power amps then you would not need the positive supply as it will be grind on the preamp output. However, even if your preamp has ultra low output impedance (a few own, that is very unlikely with capacitate coupling) then still you will have some auditable click via loudspeakers what you change value on your preamp. It will be due to the present of DC at the power amp input. To compensate it with a positive supply is an elegant and comfortable solution.

You might subtract a positive voltage via a deviser from B+ but since it is not regulated it will be drifting with main fluctuation. In a past Dima and I made quite a few experiments to find a correct solution. Dima design quite slick circuit that monitored the voltages drift and automatically composite it. The problem was that since it dealt with grid everything we touched affect sound very dramatically, always in negative way. The gas regulator tube in negative supply, in the way how we use it in the Milq did not affect sound negatively, but quite in contrary it affected it superbly positively. From there to put in the game the same positive supply was not complicated. Do not think about the gas regulators as the regulators but rather as a decupling devised that provide the necessary compliance of the bias PS to the grid current’s rollbacks….

Generally, having the Milq up and running and contacted it to the preamp it is possible to shut down the positive supply. Then you might need it on only if you need to connect or disconnect the cable. This increases current through the cable is also works very beneficially. But I would not advocate this mode. I would do it myself, but would not suggest if for others. I did not detect any sonic advantages to do so, still if I wish to do it than I plug the cable and put my Cat to guard the cable, making sure that no one would disconnect the cable while the amp it running.



Rgs,
Romy the Cat


"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
03-02-2006 Post does not mapped to Knowledge Tree
AnonymousUser
Posts 19
Joined on 11-27-2004

Post #: 3
Post ID: 2149
Reply to: 2140
Re: Grid bias power supplies response
Hi Romy,
Thanks for your response! I have never tried grounding the cathode in any of my tube projects. Even my amp output stages have been mostly cathode biased. But I am going to try grounded cathodes (and the +/- input stage supply) in my next amp, which is going to have a 404a input tube.
 Floyd Andrews
03-03-2006 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,345
Joined on 05-28-2004

Post #: 4
Post ID: 2152
Reply to: 2149
Do incremental steps.

 AnonymousUser wrote:
Thanks for your response! I have never tried grounding the cathode in any of my tube projects. Even my amp output stages have been mostly cathode biased. But I am going to try grounded cathodes (and the +/- input stage supply) in my next amp, which is going to have a 404a input tube. 

Floyd ,

Why do you have to have a cap in cathode and why juice should flow form ground to cathode via this hugely auditable capacitor? Even grid cap affect sound less then a cathode cap…. Grounded cathodes are the way in which I feel everything should be used. Anyhow, Floyd, I do not think that +/- input stage supply is a panacea solution and would sound correct in all cases. In my Baby Milq (headphones) with 6C19P it is not as affective as with 6E5P. You might for deigning try to use +/- bias milling the B+ and negative supply via the voltage groping resistors. If you find that this INCREMENTAL STEP will work out for you and “sound” better then cathode bias then you might try the isolated the +/- bias supplies via regulators. Please let know how it will work out for you

Rgs,
Romy


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