Grid leak biasing

[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.

Thank you, makinrose. 

I'm experimenting with Supros as I have a specific sound in mind and some of those old Valco designs suit my taste quite well. I'm all up for a snarly, aggresive tone with limited headroom. 

I have built copies of two Supro models the Coronado 1690T and the 6424T and have modified them in many ways ever since. On the 6424 I got rid of the second channel, converting the spare triode into a tapped gain stage. I'm under impression that both models work better with humbuckers as the input impedance is very low and eats up a lot of signal from single coils. I actually used a typical 68K/1M input on the Coronado where I also added another preamp tube. The original tremolo circuit had the B+ voltage on the intensity pot but only used one triode. I had a bit of a problem getting it to work properly and decided to replace it with the complete tremolo ciruit on two triodes from the 6424. 

The main difference between those two amps if obviously the power amp - the 6424 uses two 6973's whereas the Coronado runs on 6L6's. I went for Heyboer transformers on the 6424, but decided to use the easily available Hammonds for the Coronado. The ones offered by the Classic Tone are quite pricey and they're dedicated for other Supro models, so I didn't want to make an expensive mistake. 

Unlike the Model 24, the Coronado has two gain stages in the tremolo channel and it's quite a hot rodded piece of an amp. I had an unused triode in there too so I stacked it with the first channel stage and added a three way switch to choose A/B or both channels. This way I can have 4 gain stages voiced in different ways, but on the other hand the amp has deviated quite a bit from the original circuit. I'm actually using it for trying out different ideas and experiments with methods of voicing, having temporarily put aside the main goal of approximating the peculiar tone I had been looking for. Originally I was a bit disappointed with the amount of gain I was getting from the original circuits, although the overall tone was promising. Kevin pointed out to me that there are quite a few aspects of the Supro tone and those can be achieved with different methods, which encouraged me for a bit of experimentation on my own. 

Tomislaw