I know I'm a bit off-topic, but I just registered and entered into this forum (Hi everyone!) to thank you all for this thread and the forum.zaphod_phil wrote:IMO this is not such a good idea, as it fundamentally alters one of the distinctive aspects of the Marshall 18W design. First of all, as we're not doing hi-fi here, there's nothing necessarily wrong with having smallish amounts of crossover distortion. I may be mistaken, but I also reckon the crossover distortion is what helps give the 18W it's very distinct Marshall grind versus the usual kind of distortion you hear from other EL84 amps. Obviously, you don't want too much, or it will get buzzy, so it has to be a limited amount. Also, as Paul Ruby demonstrated, with good quality OTs, you don't hear a buzz. And Paul's zener/diode fix can always be used if necessary.Chappy wrote:... increasing the cathode resistor will run the tubes colder at idle, in effect shifting the output's bias more towards class B. This will introduce more crossover distortion when the amp is cranked. I confirmed this with my scope. As a sidenote I've managed to kill almost all of the crossover distortion by lowering the grid bias resistors from 470k each down to less than 200k each. I'm also doing this with resistors jumpered in parallel. The output stage still has the ability to distort due to saturation and clipping, with less of the harsher distortion due to crossover.
The other thing is that reducing the EL84 grid leak resistors (or the PI load resistors) makes the amp breakup later. Now, some folks may prefer that, in which case its fine. One of Jason's mods reduces the grid leaks from 470k to 220k, to get more clean range. However, the amp is then no longer true to the 18W design, and IMO its a pity to loose the early breakup which is one of the things that gives this amp its charm.
I should also point out that the WEM Dominator used 470k grid leaks on the EL84s along with the 220 ohm bias resistor, and everyone who's ever heard one says how great they sound!
I have been looking for a solution for my need of a bit more clean headroom, and here's where I found the hint of the grid resistors.
I am a guitar player from Spain, I have been using a self made 18 Watter, with certain modifications, for around 12 years, after trying many other amps. And well, just to try, last week, after reading this thread, I swapped the resistors that the Jason Mod proposed.
IMHO, to my ears, the only thing I got was a noticeable improvement in the definition when the amp is not hardly crancked, what helps a lot: the amp has lost the bass "farting" when playing fingerpicking style, and still breaks as hell when I over-drive it. IMO, it's a very interesting, cheap, and easy mod.
I just wanted to say I saw very minor differences, and wanted to write this here, for the record. It's an easy to do mod, and 100% worth a try.
Even it's recorded awfully, with a phone, I guess this video helps a bit to document that the amp still got guts (no pedals on in this song, just a Telecaster jumped into the amp):
https://www.facebook.com/eugenio.pachec ... 187078278/
That's by band, Los Brazos, last friday, I dig the bit softer breakup after the mod. I bet it will remain like that for long.
Thank you all for your help and sorry for the off-topic!