New Half Power Express

[katopan]

Lower cost would have to be less iron. As the others have said same circuit and same OT impedance at a lower B+ gives you (almost) the same tone and response at less output power. Yeah the tone at 18W is slightly different than at 35W, but when you can't wind it up to full for comparison like you can with VVR installed, you won't notice the difference. Down at much lower powers tone and response falls apart but only scaling to half power it's apples and not much different other apples.

I did a lot of experimenting with a low powered Express circuit using different output valves as well as building a full size one. My final conclusion now is that a 20W (B+ of about 300V or so) Express with smaller transformers but everything else the same including using EL34s would be perfect.

[M Fowler]

I bought some smaller transformers to build a 25-30w version using EL34's but never built the thing.

I will have to dig those out to see what combination I came up with, you guys have me curious.

First off if Ken F. was going for a 30w why use 300mA behemoth transformers cut that down to 150mA 290-300v and keep OT 30w rated 6k6.

Mark

[Littlewyan]

I think the Express would definitely be better at lower power. Its just too much for most venues in the UK, not sure about the USA. I know 18W isn't a huge decrease in volume (roughly 3dB) but its still easier to manage.

The only issue I can think of with a lower B+ is less headroom. I guess you can adjust for this though by for instance, increasing the NFB (use 47K instead of 100K resistor).

[RockinRocket]

Isnt there more to it other than reducing plate and screen voltage to get it to sound "right"? Wouldn't a drive compensation pot or some sort of fixed value resistor be needed like how London power does it?

Now I wonder wouldn't just using a different output tubes, say 6v6 be a better option. The 6k6 for El34 is a higher imperdance than the typical 3.5 for el34s. Using the stock Express transformers of 6k6 for 6v6s would be under the normal of 8k and make the power amp stay cleaner like a deluxe reverb. Wouldn't finding the right transformers like a 10 or 11k 18 watt transformer be more correct keeping a lower watt amp on pair with the 3l34 version?

[Colossal]

My guess would be two EL34s running at 9W each in parallel single ended or Class A push pull.

[Littlewyan]

The trouble with using 6V6s is they just sound so different to EL34s. Although I guess you can try and tweak the rest of the amp to compensate. Anyway in this case JM has used EL34s and apparently still got the same tone but at a lower wattage.

Wouldn't a drive compensation pot or some sort of fixed value resistor be needed like how London power does it?

If the voltage was only lowered a bit then I don't think there would be too much tweaking required. As I said earlier it might just be a case of changing the NFB resistor to 47K.

I've not looked into Class A Push Pull yet. I think there is an article on Aiken's site about it. Will have to do a bit of reading.

[RJ Guitars]

This is interesting from a lot of angles... JM was sorta put down at one point because... well he wasn't Ken and he was making Trainwreck amps. This kinda made him just another guy cloning TW amps - except he had access to a ton more authentic info than the average cloner which is a substantial advantage.

When I look at his work it looks pretty decent to me and talking with the guy I find him altogether likeable - decent and humble. Now with this little trick I'll give him some engineering and creativity credibility as well. Kudos to JM for taking TW into some turf that always has gigging guitar players asking for help. Trainwreck tone that doesn't leave your ears bleeding - did he succeed? Has anyone heard one?

There are some tricks you can do with an EL34 to knock down power level. I like Colossal's thought about dual single ended technology although I am more inclined to think of some sort of throttling technology.

[Littlewyan]

I vaguely remember there were a few clips posted of a Trainwreck which were recorded on an iPhone. I can't remember which amp it was though. Rocket comes to mind.