This amp came about after some discussion at Wattkins.com about the new 1 watt Marshall amps. In the Wattkins Tone Per Buck section much discussion ensued (thanks to everyone who chimed in) and it was agreed that a 1 watt amp could be built at a fraction of the cost that produces as good or better tones.
The first attempts were using a triode, push pull output section and guesses at what sort of topology the Marshall amps were using. I tried a 6SN7 and a ECC99 (actually a 6N6P) in a push pull arrangement, a design basically like my JCM5E3 but with a dual triode for the output. Though the 6N6P was a big improvement over the 6SN7 I still wasn’t sold on triodes for the output tubes..
After getting a little discouraged by the triode output tubes I had been using I decided to look into some low power pentodes. I like cheap Russian tubes and I happened to come across the 6F4P mentioned on AX84 I believe. This tube is the Russian ECL84 equivalent and has a pentode and triode in the same bottle. They were really cheap so I got a few, I think I paid 5 bucks for 6 of them.
To keep the wattage down I went single ended and changed from a Marshall topology to a Trainwreck. With the extra triode in the 6F4P this amp just needed a 6N2P to do 3 gainstages and the power tube. I also think a Trainwreck pre seems to do well in a single ended format.
As soon as I fired it up I knew I was on the right track. The pentode in the 6F4P is very sensitive so I had to add a voltage divider before it. The tone stack is pretty much taked from the Liverpool but I increased the treble cap from 50pF to 100uF to get a little more mids. The tone stack is flexible enough to go from Voxy/Blackface scooped tones to more Marshall like tone by cranking up the mids.
Another, slightly unusual thing, is the negative feedback setup. I tried to “fix” two things with the NFB. The first is a little harshness on the high end. The other is the amount of low end available with a 1 watt amp. So I added a little NFB to fix the sorta harsh high end but added a .0047 capacitor so the NFB would not affect the low end. This gives a perceived low boost while smoothing the highs. I really like how it turned out.
For iron I’m using a Weber WSE15 for the OT. It’s way overkill but it’s the only single ended OT I had. The PT is an Edcor XPWR024-120.
At this point I still haven’t heard any of the 1 watt Marshalls other than YouTube clips. I can say I’m really happy how this one came out. It could also be built for about one tenth the cost of the Marshalls. A 1 watt Trainwreck style amp is fun to play and not worry about disturbing everyone in the neighborhood.
Here is the schematic.
And here is my layout.
Here are a couple clips:
The speaker in this one is a 2×12 with a Celestion 70th Anniversary and WGS Veteran 30. Tone controls are at noon and volume is around 2 o’clock.
The speaker here is a Jensen MOD 12-35 in a small 1×12 cabinet. Tone controls are at noon and volume is around 2 o’clock.
Here’s one more clip.