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Title: B class PP modulator input & output calculations Post by: AMLOVER on March 17, 2013, 03:33:29 PM Modulator is a B class triodes in push pull.
Modulator's requirements for full power as per tube manufucturer are: V grid to grid and peak to peak = 560v af drive input = 116w I use an audio output transformer 0-16 ohm to 1700 ohm with CT for bias connected the known way. It is a ClassicTone 100w (40-18026) As per my calculations when in full driving conditions the primary will see 345 ohm per winding or 1380 ohm for the total winding, is that right? In case I wish to test the driving transformer voltage capabilities before installing it in the amp which resistor should I connect to the total primary, 1380 ohm or 690 ohm and what is the right voltage I must see in the ac voltmeter? Using 690 ohm resistor though I can get 280V rms with very small distortion and negligible phase shift, with 1390 ohm I can get 400V rms with almost the same good results but I'm not sure which resistor to use and which is the right voltage I should get in order to optimize the setting. About the output modulation transformer now, I have 3500 ohm RFload and 5100 ohm plate to plate at the push pull side. The voltage at the RF side is 5500V and at the AF side CT is 3500V. According Don's calculations I can get 3500V X 1.6 = 5600ac volts from AF side which is almost 100% positive mod assuming small losses with a 1:1 turns ratio iron but according the ohms to ohms calculation I should have a 1.2:1. Don says to fix the right voltage ratio and so I should use a 1:1 iron (5600v to 5500v). Dahl and other mod. transformers manufucturers fix the right ohms ratio and so I should use a 1:1.2 iron (5100ohmPP to 3500ohm RFload). Which is better direction? Stefano Title: Re: B class PP modulator input & output calculations Post by: Steve W8TOW on March 18, 2013, 04:19:28 PM "When the primary impedance is the higher of the two impedance, the transformer must be connected step-down" (Radio Handbook, 10th ed, 1946).
In your case, the Primary is 5100 ohms Z to Secondary 3500 ohms Z.... so step-down. gl es 73 Steve 8tow Title: Re: B class PP modulator input & output calculations Post by: AMLOVER on March 18, 2013, 07:37:57 PM So you agree by 1.2:1 step down mod. transformer BUT in this case the AF voltage will be step down, too. With step down and according Don I'll have 3500Vdc x 1.6 : 1.2 = 4660 peak audio volts which is 92.45% positive modulation BUT right ohmic ratio 5100 ohm : 1.2^2 = 3540 ohm. As more I study it so I come closer to Don's side, better to stay with 1:1 iron and 100% positive modulation peaks than to care about right ohmic ratio. About driver transformer Z calculation anybody to help? Stefano |