What tubes to use with this Interstage Transformers?

[megasat16]

I have a couple of interstage driver transformers and I want to hear opinions on the best possible driver and output tube combinations for the SET amps.

The IT is single Plate to Grid type with 10K Primary and 330K Secondary (nominal) with 20mA of permissible DC and 135V Rms Signal Max for the Primary Rating. FR is 20-20KHz with only +0.5dB and -0.5dB deviation from 1KHz during an actual sweep with Function Generator and a Scope.

Step Ratio is 1:4.

I am thinking to use 801A type as a driver with 18mA Plate Current to drive the Grid of Large (unknown / undecided) Transmitter tubes!

Suggestions Welcome!

 

[pieter t]

Hi James,
When the IT is 1:4, the impedance ratio is 10k:160k.
In case of 801A as driver, the source impedance for the output tube will be some 80k (5k x 4²), and the current will be 4-5 mA (18mA : 4).
These conditions are not very good to drive a transmitter output tube, especially when the output tube is driven into A2 (grid current).

 

[megasat16]

Hi Pieter,

Thanks for the info. I am not expecting to run A2 out of any power tube. But I need to find a suitable tube with high enough Grid Resistance not to load the 801A driver tube from the ideal A1 operating point.

It's strange that the measured Vo/Vi is 4 but the transformer is labeled that 10K ohm input Z and 330K ohm output Z. According to the label, I should get step ratio like 1:5.75 but actual measurement is different which is about 1:4.

I think I need to measure the actual Primary Z before designing any useful circuit.

 

[euro21]

"measured Vo/Vi is 4 "
Are you use series 5k resistor (as 801 plate resistance at 18ma) between generator and primary and proper (160k ? load on secondary?

 

[jerluwoo]

Remember primary z is the primary inductance in parallel with the reflected impedance and that sum is in series with primary dc resistance. If the primary inductance is on the low side it may require a higher impedance load on the secondary to arrive at the desired primary z.

 

[megasat16]

"measured Vo/Vi is 4 "
Are you use series 5k resistor (as 801 plate resistance at 18ma) between generator and primary and proper (160k ? load on secondary?

Hi,

It's measured with function generator and a 10x probe on a scope with the secondary unloaded condition. The source impedance of the function generator is 50ohms.

Remember primary z is the primary inductance in parallel with the reflected impedance and that sum is in series with primary dc resistance. If the primary inductance is on the low side it may require a higher impedance load on the secondary to arrive at the desired primary z.

Yes, I remember. I need to use an output tube with 0.2Mohm or above grid resistance for the primary Z to stay high. I think adding the high value parallel resistor on the secondary side will help to stabilize the grid impedance value too.