I've recently built two budget orientated single-ended amplifiers, one using the WWII-era 1625, and the other using the 6CB5A connected as triode. Both were built to a price point - Edcor output transformers and parts sourced from Mouser. The sonic results for these unusual designs were quite gratifying, considering the sub-$700 price range.
Sonically, I actually preferred the sound of the 1625 amplifier which was used as a pentode with screen regulation. It had a pleasant forward sound compared to your average triode, and also seemed to develop much more power than expected, along with greater speaker control with the use of (yikes!) negative feedback. This project made me rethink an older amplifier I built using the EL156 output tube connected in Ultralinear. I began to wonder how a true pentode assault on the "state of the art" would sound.
With that in mind, I came up with the following concepts to guide me through the design process:
- Ability to use octal socket output tubes: 6550, EL34, 6L6GC, KT66, etc
- Fixed Bias - allowing the maximum range of bias
- Plug-in Plate cap: for the weird tubes
- Pentode operation - possibly with a triode switch
- Tube rectification using damper diodes
- Screen regulation using gas tube shunt regulation