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In the Forum: Melquiades Amplifier
In the Thread: Bias help
Post Subject: God, it was so long time ago!!!!Posted by Romy the Cat on: 4/3/2018
 G wrote:
Should the grid bias voltage change when I adjust VR1? In my case it does – if I keep increasing dc on inputs I can get to -1.5v on the grid and around 60v on B+.
   
Nope, it should not, well it is and it is not. Are you sure that you put the VR1 resistor in positive side of bias? Let me to explain. What I typically do to experiment is shorting the amp input and disconnect positive bias. The resistor R5 in this case acts as a bias resistor separating voltage between negative supply that goes the tube grid and ground. Let pretend that you have 4V on the driver grid while input is shorted. Now the certainly is balanced perfectly. If you un-short the amp input then bias will move but adding the positive supply on the left side of R5 resistor and setting 0V in there you introduce a virtual ground for the left side R5 and the bias in case you have  0V  at input should be identical to the  bias when the inputs are shorted. Under normal circumstances you would never adjust 0V at input, I did it one and over year is stays 0V. Even change of the driver tubes do not effect it. The only time you would need to do some adjustment would be what you change gas tubes as all of them stabilize slightly different voltage. Generally this topology might be recognized as “virtual battery”, where R5 resistors acts as voltage emitter because it has 0V on left and minus 4V on right.  Anyhow, if your bias voltage move too MUCH when you adjust VR1then there are two possibility:  
 
1)    You put the VR1 accidently on negative side, between R10 and C5, or somewhere in that negative supply.
2)    Your pream has very high output impedance.
   
Bothe on the case above are not good but they also are not bad. If you adjust 0V at input then you do not care about anything and you filter out all external factors.
 G wrote:
The DC on the inputs also drifts ~+-50mV. 

It should not be this way. If you say that it drifts ~+-5mV I would say that it is too high but let it to drift. The 50mV is too much. Switch the gas tubes, I think one of them are faulty. In case you use a direct coupled preamp make sure that it does not send any DC drift to its output.
 G wrote:
Is there any difference which side legs you connect the VR1? I connected mine so I get 0 to 20k. If I was to connect it the other way round then I will get 0-5k-0.

Sorry, I do not know what VR1 you use and I do not know the specific of it’s connection. You can use in there any good multi-turn in order to set 0V at input. If you have not enough turn to do so then slightly adjust R9 or R10. The better would be is to change the gas tubes, put positive to negative, or to get a different set all together.  They stabilize anywhere from 144V to 156V and a half volts of stabilization differences would reflect good 10K in VR1 resistor. Do not hold me responsible for exact numbers I juts pull it out of my ass now. Anyhow, flipping the tubes between the positive and negative bias supple always works for me.
 G wrote:
Where should I take the measurement for the V1 current draw? Do I connect my multimeter in series between R11 leg and tube anode?

You did not mean R11, it is juts next to Point “B”, you rather meant R2, which is point “M”. Yes, you can put a multimeter before R2 and to measure current.  It should be around 25-35mA, depending of the tube. A better solution in my view is to put anywhere in series with R2 a good quality 1R resistor and then to measure drop of voltage before and after resistor. You will be using multi-voltmeter in this case, you can add and remove it in real time, or live it permanently if you want and the voltage you will be getting will be your current in mA. You can even measure the voltage drop over R2 resistor but then you need to calculate the current by Ohm’s law.
 G wrote:
The amp is very quiet when I test it with my old ipod however if I connect it to my preamp or dac it’s noisy so I need to double check my grounding probably.

Here is where you need to figure it out yourself. I do not think that anybody would help you debug grounding loops remotely.
 G wrote:
I tested the amp with midrange and upper bass and it sounded very promising. It’s very fast and harmonically rich.

I know. Please contact my wife and suggest her that I am not an idiot. :-)
 G wrote:
In your correction thread you recommend: 12) Put two 10R, 5W resistors (in each leg) between the rectification bridges and secondary of the power transformers. Is this necessary with my bridges ? I haven't added them yet. 

Nope it is not necessary. I do remember that I did those experiments but I do not remember details or results now.

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