I had added another preamp tube at one point and tried all sorts of circuits. I also tried something like a more standard 18 watt lite but it still wasn’t quite doing it for me. This one ended up under the solding iron longer than I expected and went through a bunch of incarnations before I decided to go simple.
The amp starts with the first gain stage using a 100k plate resistor and 1.5k cathode resistor bypassed with a .47uF cap. That’s a fairly small bypass cap for me but it keeps the bass in check. There is a coupling cap and the tone control. I’m not sure where this tone control came from originally but I first heard about it in the Bitmo mod kits for the Valve Junior. It’s sort of like a tweed tone control but it’s built on a resistor rather than the volume pot. I think it’s a really good sounding basic tone control and not too ice picky. The negative side is that it’s a little more lossy than the tweed tone control but it works well in this amp.
After the tone control is the volume pot and the next stage which is 1.5k/22uF on the cathode and 100k plate. Following the second stage is the master volume going to the phase inverter. I used to be not a fan of master volumes one stage from the first volume. However, after doing my GA-5E30 I’ve changed that opinion since in some configurations it is effective in going from preamp overdrive to power tube overdrive.
The phase inverter is pretty much like an 18 watt Marshall LTP which I really like for EL84s. The rest of the amp is pretty straight forward. I tried a more 18 wattish output section with .01 coupling caps and 470k grid leaks but I went to 220k grid leaks and .02 couplers after trying it for a while. The latter seems to produce a bigger sound to my ears. There’s no negative feedback as it’s in the 18 watt/Vox vein and I was looking to get the full effect of the overdriven EL84s.
I suppose the circuit bares some resemblance to the Orange Tiny Terror in that there is 2 triodes then the LTP driving EL84s and the controls are volume, tone and master…but it’s also pretty different.
The power transformer is a trusty Hammond 270DX and I have about 345vdc on the EL84 plates. It uses a MPS OT15PP output transformer with an 8k primary. After some experimenting with other output transformers this amp really brought to light for me how much more picky EL84s are about OTs compared to 6V6s. I think it’s much easier to get a good sound from 6V6s using a not so great OT.
Here is the current schematic:
Here’s a clip: