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In the Forum: Melquiades Amplifier
In the Thread: Planning my DSET
Post Subject: The fuse in theMF ChannelPosted by anthony on: 6/5/2015
 Romy the Cat wrote:
 anthony wrote:
 The fuse.  You show a fuse on this schematic (that traces back to the B+ supply) but omit it on the 6-Channel schematic that I linked to earlier in the thread.  Do you use this fuse or have you removed it?
  Yes, I use 7.5A slow blow at the very entrance of the 6-Channel schematic with the amp drown in cruse mode in mid of 4A. The slow blow is critical as the amp will start up and begin to saturate the chokes it will draw instantly over 3kW. The normal consumption is 375W if I am not mistaken. 

Do you still use the fuse shown in the MF channel schematic linked above (not the 6 Channel schematic)?

Regarding the mains fuses I will have to recalculate them for the mains voltage where I am...they should roughly halve for 240v.

 Romy the Cat wrote:

 anthony wrote:
HP Filter 50R.  In the schematic above you use a 50R resistor leading into the 6E6P but omit it on the 6 Channel schematic.  As far as I can tell it is not required for the high-pass RL filter so do you use the 50R or has it been removed?
Well, the 6E6P is very fast tube and have tendency for microphonic effect, in particularly while it I cold. It is a common practice to put a very small resistor (50-100R) right before the grid entrance to stop any stray noised to come to the grid and arouse the tube. This is call grid stopper. The ferrite bids serve the same purpose. So, the grid stoppers with fast tube are not part of semantics but rather a part of regular design hygiene and they are implied with those type of tubes. On the 6 Channel schematic I just did not have space to depict it but I still would use it in particularly if your wires from the coil are too long as they act as antenna picking up all imaginable dirt in the amp chasses. 

Well I would have missed that thanks for that.  These will be my first amplifier builds so I will probably miss those kinds of things.  Can you think of any others that I will probably miss?

 Romy the Cat wrote:
Yes, that is exactly how you describe the process. You adjust the 100R in order to kill the noise on the output. What I do is use mili-voltmeter on the terminal of S2 and try to set the voltage in there under 3mV. I think anything above 20mV is auditable, Anything under 10mV is dead silent. Another very important factor when you change the tube and try to compare them is that they all will have slightly different gain. So here is what I do. My tuner has 1kHz calibration tone, it is convenient and I do not need to bring a generator or test CD). 

1)    Drive the calibration tone and measure the AV at the speaker (I prefer to do it electrically  not acoustically)
2)    Follow your procedures to replace the tubes
3)    Drive the calibration tone with the new tube and measure the SAME AV at the speaker. If the AV not same then use the post OTP attenuator to make it the same. 

There is some “kind” into it. Some tubes you do not mind to drive louder (mostly dark tubes:  PX25, RE604, YO186, 2A3 with single plate for instance) and some you would like to run softer (type 45, 30, 2A3 etc…). You will end up with whatever you prefer eventually. Be advised that the output of Fundamentals, HF and Insertion channels will greatly impact how you would like to hear your MF.

Thanks Romy, but I don't know what you mean by AV in the text above.  Do you mean dB?

One more question.  The filament supply of the MF Channel has the hum management circuitry on the 4v you also use an artificial centre tap on the 2.5v secondary?  I just thought that it might be one of those "hygiene" things that you left off the schematic.  Likewise for the filament supplies of the other channels.



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