Federal Limiter - How to make center tapped output transformer obsolete

[bernbrue]

Hi,
At the moment I'm studying the federal limiter schematic and while searching for suitable transformers I remembered that Kingston in his M670 project substituted the center taps for connecting B+ by a simple two resistor voltage devider. Somewhere PRR mentioned that this would also be possible for the federal limiter output transformer. We know that in some cheap transformers the center taps can cause imbalance and instability especially in a circuit like the federal limiter.

Here are the two schematics for reference:

M670 (T2 and T4): http://groupdiy.com/index.php?topic=51478.0

Federal limiter:  http://www.waltzingbear.com/Schematics/Misc/AM-864.htm

My question: how do I calculate the resistor values? My guess is that a 10K/600 transformer wants to see 10K (2x4K7) at its primary.  Did I answer my own question?
Best
Bernd

 

[PRR]

Changing from transformer-loaded to resistor-loaded typically needs *double* the supply voltage and gives *one-tenth* the power efficiency (maybe only 1/4 here).

It's a major compromise.

 

[bernbrue]

Thanks a lot for the reply. I saw this in Kingstons M670 schematic and thought there might be other advantages besides avoiding imbalances in some cheap transformers and having more transformer options.
Bernd

 

[emrr]

imbalance is mostly a red herring.  if it's there, it's the tubes and rarely the transformer.  DC resistance difference is normal, and is not historically an imbalance factor of any merit. 

 

[bernbrue]

Hi Dough,
We're are planing a prototype pcb for a federal based limiter with modern tube substitutes and other smaller changes. We want to implement a cathode balance pot for the output tube and a plate balance pot for the two 6BA6 input tubes.  I thought that it might be a good option to provide some solder pads for a resistor loaded output section.
Best
Bernd

 

[emrr]

Since neither PM660 or Federal intend to generate large output levels, resistive can work fine, but consider you lose headroom with resistive.  Less so with lower Z (10K) output with minimal current draw.    Many of the classic single ended preamps with direct feed outputs generate +18dBm no problem, and the corresponding era resistive feed preamps struggle to deliver +8dBm cleanly.  If you can double the voltage (impractical) on the resistive type, you can get the headroom back.  If you don't need the headroom, who cares. 

 

[bernbrue]

It's exactly because of answers like yours giving me insight into a complex theory that I'll never understand in all its aspects I like this forum so much.
Best
Bernd