18Watt.com

Can I just substitute the EZ81 with a solid state recifier (four diodes)?
Will it change the tone of the amp ?

A lot of people will say don't do it because of the insane plate voltage. But the Marshall 20watt uses a 300-0-300 volt PT secondary with SS diodes and they work. The plate voltage on mine is around 390v. It will change the tone though. And not for the better, IMO. Otherwise this site would be called 20WATT.COM. ha!
markd

Groove Tubes makes a solid state plug in replacement for the EZ81 that I believe is just 4 diodes. I tried one and there was only a subtle difference in tone. Just a little more of an aggressive distortion edge to the amp with slightly sharper highs. Just a different flavor not better or worse.

The EZ81 has a series resistance of 230 ohms, so you could use SS rectifiers followed by a 230 ohm resistor, like Graydon does with some of his amps. As 230 ohms aren't easy to get, 220 ohms is probably close enough.

I've used a weber copper cap in place of the EZ81, and to my ears, it is almost indistinguishable from the real thing. Since my first copper cap fell apart, I saw the inside, its just some diodes with a huge resistor across it.

Someone here used a 6.0 Amp Silicon Bridge rectifier that screws right to the chassis and has solder lugs, and they report very good results. Its an all-in-one solution. Mouser # 821-SBT607

I would think that since you need to add a serie resistor to tame the higher voltages obtained with solid rectifiers, this reproduce the sag effect of the EZ81.

It does add a similar sag characteristic. The benifit is that it costs a fraction of a tube rec, doesn't need a heater connection so it draws less power, and should never need to be replaced.

Personally, I would go with a tube rec. It's one more glowing bulb with a purpose, and it adds just a little more to the coolness factor.

I'm runnign my personal 18 watt Marwatt with strictly solid state (diode) rectifiers. This is to grab a bit more head-room and reduce sag. The result is decent enough. The DC voltage from my Heybor (290VAC-0-290VAC) ended up about 370vdc. This is lower than the theoretical 410 one might expect due to the sagging and winding losses of the transformer itself and the ripple effect on the 32uF cap I use. Without the apparent impedance of the Tube Rectifier plates there is a little more strain on the transformer due to charging currents of the capacitor. However after several hours of continuous use I've found the transformer runs reliably and there are no signs of unreasonable thermal stress.
I'd be interested to here other peoples results with similar or other brand transformers.

Bob Dettorre
DST

DSTengineer wrote:This is to grab a bit more head-room and reduce sag.

That's OK if you want to do something different from the classic 18W tone and dynamics. It's all a matter of personal; preference, but for someone building their first 18W I would recommend they keep with the usual amount of sag and headroom.

Ultimately, there's no need to use an EZ81, when UF4007 diodes followed by a 220 ohm sag resistor will do exactly the same job, giving the same tone and dynamics.

zaphod_phil wrote: ↑

Sat 04/25/20 10:44 pm

Ultimately, there's no need to use an EZ81, when UF4007 diodes followed by a 220 ohm sag resistor will do exactly the same job, giving the same tone and dynamics.

I like both the sag of an 18 Watt and the somewhat stiffer response of a 2061. I wonder if using UF 4007 diodes with a sag resistor which can be switched in or out of the circuit might be the best of both worlds.

Synchro wrote: ↑

Tue 08/04/20 7:34 pm

zaphod_phil wrote: ↑

Sat 04/25/20 10:44 pm

Ultimately, there's no need to use an EZ81, when UF4007 diodes followed by a 220 ohm sag resistor will do exactly the same job, giving the same tone and dynamics.

I like both the sag of an 18 Watt and the somewhat stiffer response of a 2061. I wonder if using UF 4007 diodes with a sag resistor which can be switched in or out of the circuit might be the best of both worlds.

I probably wouldn't use a switch. But you could maybe use a 250Ω 5W potentiometer and have a tunable sag, which might be kinda cool.