Finally an amp not built in a Valve Junior chassis. I put a venerable Fender H.O.T chassis in a homemade head cab, made a faceplate of sorts and went with a push pull design instead of single ended. I just like push pull better, what can I say…
The redo started as a semi-clone of a Fender 5G9 Tremolux. I wanted something simple with the potential for raunchiness and came across the 5G9 on Schematic Heaven. This was my stripped down 5G9 schematic:
It sounded fine like that but I, of course, couldn’t leave well enough alone and started messing with it. One thing led to another and it’s gone through many iterations. However I thought it was about time that I tried out a paraphase phase inverter so I went with the Supro Thunderbolt. I considered the 5D3 Deluxe but thought the Supro looked more unique and maybe I would get some Jimmy Page tone!
It’s pretty close to the stock schematic and that’s unusual because I have a hard time sticking too close to schematics most of the time. It has 6V6s instead of the original’s 6L6s. I also changed the B+ dropping resistors to try and get my voltages close to what I had seen in schematics.
The Thunderbolt tone control is interesting because, as far as I can tell, it’s the Big Muff tone control except the Supro pre-dates the Big Muff by a few years. I tweaked some values based on parts I had and what I was seeing in Duncan’s tone stack calculator.
The stock Thunderbolt also has a left over triode so I did a two channel type thing, close to the arrangement in a 5D3/5E3. It has one channel that is like the stock Thunderbolt with the bypassed 3.9k cathode resistor and 100k plate resistor. The other channel is “Supro-ish”. It’s pretty sort of inspired by based the tremolo channel (sans tremolo) from the Supro S6688 with the 500pF cap across the volume pot. The new channel is brighter and has a little more gain so it’s nice to have both. The channels are internally jumped so I can blend them and get a pretty good variety of tones.
Another thing I came across building this amp is that some of the Thunderbolt schematics floating around out there, I think, are incorrect in regards to the phase inverter. This is one of the schematics I think has the phase inverter wrong. The signal goes from the tone control to the triode with the 3.9k cathode resistor. The inverting triode has a fully bypassed 2.2k cathode resistor. When I first build the amp I tried this and it did not sound good at all as the triodes were switched. After some research I came across other schematics that I believe have the correct phase inverter like the Supro S6688 and the Harmony (made by Valco like the Supros) 420. So that is the phase inverter I’m using.
For iron I’m still using the Weber W022772 power transformer and I installed an old Blues Junior OT. The OT is not ideal but, for now, it works.
It’s safe to say that it is not a 5G9 anymore.
As far as how it sounds…pretty cool! This is probably my most vintage sounding amp I have built even though it doesn’t have a tube rectifier. At lower volumes by mixing the two channels you can get a good tone for just about any guitar. My favorite setting is turning the tone to about 2 o’clock and turning the Thunderbolt channel up about all the way, the other channel off. It’s actually not as loud as you might think and it’s got GREAT power tube overdrive! Turning up the other channel really adds to the volume.
Here is the schematic.
This is it just cranked up, only the Thunderbolt channel. I’m using a Lopo Line 1×12 with a Celestion G12L-35. I added some reverb on the computer.
Sangria T-Bolt tone
This is the same setup but on the other, S6698 channel. It seems similar but brighter and less mids to me.
The front. It’s my standard cabinet “design” utilizing cheap shelf wood and floor molding.