Anyone tried a tube rectifier in an Express?

[MCK]

Why mess with something that's perfect right? Don't know. Just curious I suppose. I love the sag introduced by the tube rectifier and wonder how it would act out in an Express circuit. Has anyone tried it?

If I was to go for this I would need to consider a different PT than what is most often used by builders right? I need to compensate for the V drop that the tube rectifier will induce. Any thoughts on this?

Thanks for your input.

[Colossal]

No reason you couldn't try it. You can simulate the sag of a tube rectifier by adding a similar resistance after the rectifier. No harder than that and no audible difference and you don't have to mess around with getting another PT. Start with 100-200R and dial to taste. I do see your point however about going with a slightly higher voltage secondary so that with sag you are still running the typical Express voltages.

[MCK]

Thanks. I can try that approach first and see how it feels. I could even go with a pot there and make it into an adjustable sag control. I like it!

[Jana]

You're going to need more wattage than just a plain old pot for this--something along the lines of 50 watts depending on how much current you run through it.

[Reeltarded]

Switches work best for that from some experience!

I like selectable NFB too. Great place to tune and tweak.

[M Fowler]

Or use a choke in place of the 1k25w resistor and leave it SS rectified to soften then amp.

[reswot]

I built an inspired-by amp with the basic Express circuit, a Lar-Mar PPIMV and a GZ34 rectifier.

It's one of my favorite amps.

[geetarpicker]

Many many years ago Callaham built a few amps (Exp and Pools) that were "TW inspired" but in addition used tube rectifiers. I don't think he's built amps in many years, but these were the first effort I'd seen with such. The examples of these that I played sounded decent however didn't have a very good clean to mean range. This could have been due to the OT used, or perhaps the power supply balance was somehow off... That said, Express and Liverpools have quite a bit of controlled power supply sag probably due to the use of a big power resistor instead of a choke. I would assume the current capabilites of the PT were also highly considered as part of this equation. In the end I feel an Express or Liverpool really doesn't need any more sag than it already has.

If you really crank an Express wide open and hit a power chord, you can actually hear the hiss levels go down then ramp back up slow enough that you can notice it as a swell in the noise levels. Almost like a studio compressor with a fast attack, but a slow release. Some tweed amps do this, but typically they are much mushier amps than a wreck. What's cool about the TW setup is you get the compression somewhat like a tweed amp, but without the slop.

Not to say I'm against experimentation, but darn Ken got it right with these amps in the controlled compression department. This IMHO is the magic of the accentuated clean to mean range these amps have over most other designs.

[Cliff Schecht]

That said, Express and Liverpools have quite a bit of controlled power supply sag probably due to the use of a big power resistor instead of a choke.

I think it's the 9.1k resistors after the big 1k guy that give you the awesome dynamics and controlled sag. It adds up to almost 30k of resistance in the preamp section alone which is enough to cause significant sag when you clip the preamp tubes hard (including the PI) and draw much more current than at idle. You ever wonder why we bother searching out the relatively hard to find 3W 9.1k resistors? I promise you Ken did a lot of playing around to find out that three 9.1k resistors give just the right amount of sag/compression when you hit a hard note so that the distorted sound doesn't easily overpower the clean sound. When you roll back your volume and get that nice spanky clean with the instant attack, that's because the preamp is drawing less current so the power supply can readily give you all of the power you want.


The way the gain stages are laid out (very AB165 Bassman-ish) combined with the right amount of gain on each stage so that the PI clips before anything else gives you the awesome range. I've calculated the gain to be about 30,000x or ~89.5dB total on eht Express from input to power tube grids (IIRC, it was a while back) which is a shit ton of gain, much more than is actually used in a playing situation. This is why these amps can oscillate so easily and are naturally hissy, there is a LOT of gain on tap that even if we don't use it all, the noise itself is random and therefore gets amplified even after the tonestack very heavily.

[dartanion]

Tube rectification is cool and all, but in practical terms, you can achieve the same thing with ss diodes and a big resistor. This has already been mentioned, and Glen has it right. That 1k is giving significant sag when pushed. In essence, you would want to match the 1k, so ~200 ohms for the GZ34 + 800 ohm resistor. So, you now you have replaced about $0.10 worth of diodes with an expensive tube. I would high recommend against using any new GZ34 as they are all "crap". The new stuff is still $20+, and a good NOS example is going to be "expensive". Great idea. Good thought process. Try it if you want to, it may work great, and it may not. Experimentation is half the fun.

[funkmeblue]

I built a 2 hole liverpool with a 5y3 and a choke with no sag resistor. I like it, does have a bit more sag when I hit her hard. If I were to do an express circuit with a tube rectifier I think I would look to the komet 60 for inspiration.

[KellyBass]

If I were to do an express circuit with a tube rectifier I think I would look to the komet 60 for inspiration.

My thoughts exactly.

[Normster]

I thought that's what the Komet was?