Grid leak biasing

[makinrose]

Hello, 

I was wondering about grid-leak biased input stages we sometimes see in older designs. What are the con's and pro's (if any) of such arrangement? I have this Supro Coronado schematic and channel one tube is biased like this: https://el34world.com/charts/Schematics/..._1690t.pdf

First of all it looks as if the channel was maybe designed for a microphone originally , but apparently the amp was not marketed for it. On the other hand a rather high level of gain is called for here and this way of biasing the tube saves the cost of  one resistor and one electrolytic cathode bypass cap which otherwise would have had to be put in there. 

I thought it was built this way to introduce some grid distortion as the whole amp seems intended for distorted sound and output signals from either channel easily drive the splitter grid to conductivity. I scoped the input stage expecting to see half wave clipping but there is none. I used a 1kHz 128mV (peak to peak) sine and got a clean output wave 50 times the input signal (6,4V peak to peak). With the 6,8M grid leak the bias voltage is -650mV with the plate sitting at 77V. I had to increase the signal voltage to almost 2 volts to clip the grid. 
Looks like the triode could've been easily biased using a cathode resistor with a bypass cap and do exactly the same job. 

Was there any particular reason why the first input stage was built this way that I'm unable to see. I'm aware this is most likely another case of bad amp design, I'm just interested in hearing some background about it. 

Tomislaw

Grid-leak bias was used on lots of early amps and it can sound really good for certain kinds of playing.  If you want gritty 50's tone it's great. Particularly for players who don't use pedals for gain sounds.    

That said it has some major limitations.  Grid leak-bais was developed at time when input signals where not expected to be high so the amps with were not designed for distortion.   You'll see grid-leak bias all the time in 40's and early 50's Hi-fi amps. Most early instrument amps are simply adaptations of those same designs.    These amps were developed prior humbuckers, hot pickups and fx pedals so they were far more acceptable sounding with low output single coils.  Grid-leak biased stages will distort easily and cannot handle large input signals because so don't hookup a Fuzzface a grid-leak bias amp.  Since you cannot voice the cathode with a bypass cap the voicing is limited to the input and output caps which are typically big values reflecting their hi-fi roots.

One thing further. The Supro you reference has number of other design choices that limit the headroom of the amp. Whether you like it is all up to your ears but the mix of relatively low plate voltages, cathode bias, a see-saw inverter and light magnet speakers all contribute to the sound.