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In Reply to: RE: What tubes to use with this Interstage Transformers? posted by Tre' on August 23, 2016 at 21:08:27
Well, it is the driver transformer used to drive large Eimac tubes.
As far as IT is concerned, it's to load the driver tube properly while providing the signal coupling to the grid of the output?
Since it's not the output stage, how's the Miller C effecting the driver tube? The driver is properly biased and loaded at the optimal load which is the primary of the IT around 10K-15K which I think is what the 801A wants.
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.Thou shall not stand where I type for I carry a bottle of Certified Audiophile Air and a Pure Silver Whip.
Follow Ups:
The reactance of the Miller capacitance of the output tube will be reflected back to the driver tube through the IT.If the Miller capacitance of the output tube is 100pf (just picking a number here), it has a reactance at 100kHz of 15.9k ohms
If the IT has a impedance ratio of 1:16, that 15.9k ohms of reactance will be reflected back to the driver tube as 975 ohms of reactance, just as if the Miller capacitance value has been increased 16 times.
Once you know what output tube you are going to use, then you can calculate the requirements for the driver tube (output impedance and current delivery capability) that will allow you to use those ITs successfully.
As a side note; transformers don't have impedances per se. Different transformers are meant to work into different ranges of impedances and that has to do the the inductances and capacitances present in the transformer winding.
Transformers only have winding ratios.......but if we take a transformer meant to work in a circuit with a 15k impedance and try to to operate it in a circuit with only 1k of impedance, as an example, there will not be enough inductance to "support" a 1k reflected impedance. Going the other way causes problems as well.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/24/16 08/24/16 08/24/16
So, the CMiller and the r at the Output tube is dependent upon the Frequency and it will make the frequency roll off somewhat problematic. It's also true the impedance from the output tube got reflected to the plate of the driver tube but as long as it's not very high, I don't see how it poses a problem.
In the case of the 801A, the data sheet suggest the 425V B+ at 18mA plate current will require about 10K ohm load resistance so if I load it with 10K-15K Primary of the IT, I think it should work fine. Would you otherwise thinking it will not work?
I just need to make sure the Secondary of the IT is connected to the overall Impedance of 160K or more if possible. Thus, the quest for the suitable output tube begins.
1:4 IT isn't a problem to drive the output tubes. I just need to find the right tubes. Tango has NC14 with 1+1:1+1 and you can wire it as (0.5:2) which is 1:4. NC14 have lower Primary load impedance though.
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.Thou shall not stand where I type for I carry a bottle of Certified Audiophile Air and a Pure Silver Whip.
"I just need to make sure the Secondary of the IT is connected to the overall Impedance of 160K or more if possible. Thus, the quest for the suitable output tube begins."Using a 1:16 IT you will need an output tube with a very low Miller capacitance to meet that goal......like a pentode....I was assuming that because you are using a direct heated driver tube that you would be looking for a direct heated output tube.
Direct heated output tubes have very high Miller capacitances.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/28/16
Thank you Tre! I appreciate your valuable input and I am now looking closely at the values for Grid to Plate C of the output tubes in my quest.
You are right that I am only looking to drive DHT output with DHT driver tube.
Thanks,
James
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.Thou shall not stand where I type for I carry a bottle of Certified Audiophile Air and a Pure Silver Whip.
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