Modulator Driver Transformer Ratios

[W9ZSL]

How does one determine the proper turns ratio primary to secondary for a driver transformer?  I'm thinking of a p/p pair of 6V6's running about 12 watts max to p/p 811A's.  The driver transformer is 15 watts with primary to 1/2 secondary ratios of 2.6:1, 3:1, 3.2:1, 3.4:1, 4:1, 4.5:1, and 5:1; to class B grids.

[w4bfs]

the answer to this will take a bit of work .... please remember that there are several constraints to be observed for low distortion operation

1. what is the minimum driving impedance of a 811 while the grid is driven peak positive ?  start with what you know - 10W drive and peak drve voltage from RCA TT5 which as I recall is 100V (hey - I'm getting old, I'm not sure) so using ohms law R = E/I = 100/.1 = 1k Ohm    (I = P/E = 10/100)

2. what is the desired driver tube load impedance ?  for 6V6 at 300V B+ this is 10k Ohn (normal tetrode push-pull connection)

3. what is the driver transformer turns ratio, total primary to 1/2 secondary ?  square root of ratio of impedances i.e.  SQ RT of  10,000/1000 = 3.33

4. can the driver make the required voltage swing ? this is determined by the turns ratio from 1/2 of the primary to 1/2 of the secondary i.e. 3.33/2 = 1.67   this is because with center tapped primary and secondary,  half of each are used at the peak of the waveform  so if this reasoning is correct then there is sufficient swing to fully excite the secondary (167Vp primary to 100Vp secondary, each half).  distortion may be lowered by using a higher ratio
 ... also remember a resistor from grid-to-grid of the 811's of say 10Kohm to further smooth things

I would start with the 4:1 and try the others

n.b. 27 mar 15 - went back and checked 811a data sheet ...yup .... I didn't remember grid drive peak V correctly it is 160 Vp ... so recalc:

1a. I = P/E  = 10/160 = 62.5 mA and so R = E/I = 160/.0625 = 2.56k Ohm

2a. same

3a. turns ratio 2:1

4a. 160V primary peak swing ok

hope this helps

[W9ZSL]

Yeah, I've been digging through all my handbooks and info I can find on the subject all day.  Here is my other choice for a driver substituting a Stancor A-4762 for the Merit transformer shown.  I have all the parts.  Of course, the simplest route would be simply using a Kenyon T261 line to class B grids iron and be done with it.  It's rated at 7 watts and can be fed from my studio limiter amp.