Tube DIY Asylum

RE: confused again

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This Post Has Been Edited by the Author

In Reply to: RE: confused again posted by Caucasian Blackplate on February 13, 2019 at 09:12:37

To try and get a handle on all this I decided to bracket the operating points by using several different cathode resistors. The results I got are as follows:

Cathode resistor plt volts plt current grid volts bias B+
1.2 k 367 90mA -13 13 492

1.8k 376 65mA -20 20 509

2.7k 375 46mA -24 24 515
To make sure we are all on the same sheet of music, the definitions I used for the above values are as follows:
Plate voltage - voltage difference between the cathode and the plate
Plate current - voltage across the cathode resistor divided by the value of the cathode resistor
Grid volts - voltage difference between the grid and the cathode
Bias voltage - cathode to ground voltage - grid to ground voltage
To my ear the best sounding combination was using the 1.8k resistor. This gave very close to the desired plate current of 62mA, but the grid volts are way off of the desired -51vdc.
I was feeling bummed about the grid volts until I remembered a posting that Jim McShane made a while back called "tube problems that are not tube problems". In it he says "DO NOT TRY TO SET THE VOLTAGE AT THE GRID TO A SPEC"! He says it should be whatever it needs to be to set the cathode current to spec. I am not sure if he meant even a 30 volt difference, but I hope so. I find it interesting that that is also the setting at which it sounds best to me.
I occurred to me that I should probably try another tube to see what I got with it. Unfortunately, I do not have any nos 845's, only new stock Chinese units. I tried another one that is from a different maker (at least the markings are different) and got virtually the same results using the 1.8k resistor on the cathode. I wish I had an old stock tube to compare with. It my guess at this point that perhaps the grid voltage given in Mike's numbers may have been from an old stock tube and they are quite a bit different. If any one can shed any light on this or anything else in my results, I would appreciate it.
I think tomorrow I am going to take the plunge and try substituting an 813 for the 845 and see what happens

Edits: 02/13/19 02/13/19

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