Planning an Express, questions!

[soma_hero]

Hey guys, first post around here. I've been reading a ton the last few days and I'm getting ready to move forward on an Express clone. I have a few questions though..

First, the chassis in the build materials list isn't shown on Mojomusic. There is a blank chassis that is 17X6.5X2.5. Is this big enough? I was also thinking about bending my own out of an old computer case but I've heard that steel chassis are not a good choice. is that legit?

Also, where can I get fiberboard material? I guess hoffman amps is the only place i've seen it.

For power transformers, I've got two sitting here. One has a 200-0-200, 400ma secondary, with 2 x 6.3v, 3A windings. The other transformer has 333-0-333, secondary with 6 and 5V taps for heaters/rectifier tube. Can I use either of these trannys or should I just go for the Toneslut 300-260-0-260-300?

And finally, I installed a modified Post-PIMV as sold by metroamps in my MIG50 and it works alrighty. What is the advantage of a VVR or an airbrake over installing either a pre or post PIMV?

Thanks all!
Michael

[fishy]

Hey guys, first post around here. I've been reading a ton the last few days and I'm getting ready to move forward on an Express clone. I have a few questions though..

First, the chassis in the build materials list isn't shown on Mojomusic. There is a blank chassis that is 17X6.5X2.5. Is this big enough? I was also thinking about bending my own out of an old computer case but I've heard that steel chassis are not a good choice. is that legit?

Also, where can I get fiberboard material? I guess hoffman amps is the only place i've seen it.

For power transformers, I've got two sitting here. One has a 200-0-200, 400ma secondary, with 2 x 6.3v, 3A windings. The other transformer has 333-0-333, secondary with 6 and 5V taps for heaters/rectifier tube. Can I use either of these trannys or should I just go for the Toneslut 300-260-0-260-300?

And finally, I installed a modified Post-PIMV as sold by metroamps in my MIG50 and it works alrighty. What is the advantage of a VVR or an airbrake over installing either a pre or post PIMV?

Thanks all!
Michael

Your first stop should be the BOM that Ron collated. Inth at you will find what you need and where to get it.
The chassis, boards etc are all identified in there.

I'm sure people have put a TW in a smaller chassis. Just consider it is touchy on layout. I would be interested to see how much room you have for the trannies if you follow the TW layout.

Hoffmansells generic material. McMaster is the best source in my opinion. I'm sure others have their preferences. If you want it to look like the old perfboard you need to drill it yourself or get the resource from the BOM. Turretboards.com sell an Express set

People have used different transformers and been very successful, there is no reason why you cannot use something different. The Toneslut gear is proven though with numerous builds. Someone else might be better able to speak up regarding the trannies.

General feeling on a TW is that the MV's don't work so well. Dana has had success with VVR and by all accounts it retains the character of the amp better. Airbrakes are good to a point but will not have the flexibility of the VVR.
Another plus for VVR is that it is "kind" to your tubes as it lowers the power instead of dumping output through a soak like most attenuators.
If you consider VVR, can caps are generally prefered. Check one of Danas (UR12) express builds.

Hope that helps

[soma_hero]

Your first stop should be the BOM that Ron collated. Inth at you will find what you need and where to get it.
The chassis, boards etc are all identified in there.

That's what I mean. The chassis listed in that XLS sheet says its available at Mojo but it isn't listed on the mojo website.

Any thoughts on using a steel chassis? Is it as bad as I've heard for grounding issues and current flow?

Oops, I guess it was listed at ampbuilderparts.com, that's what I meant and it's not listed there either.

[gearhead]

Highly recommend chassis from Allynmey. Top notch quality.

As far as turret board blank material, McMaster gets my vote too. I made some drill templates (in photoshop) for both express and liverpool preamp/power boards. Will post if I can find them.

[soma_hero]

I remember reading a quote from Ken fischer when asked what voltage was best to run an EL34 at, and he said 325V or something close, while two other guys said like 480V.

Tubes sound different at different voltages is the take away, so if I want to run the express with a 200-0-200 secondary, and have lower B+ values can I expect much variance from the normal express tone/voice? I know KF used random transformers at times so a lot of Trainwrecks sound very unique.

Would this ultimately just result in lower volume? I ask, since Dana has had luck with the VVR circuitry to low voltages with no real tonal massacres.

Also, after full wave rectification as shown in the Express schem, is the resulting output DC voltage .6366*Vp ? So if I have a 400V supply I'll only end up with 250V after rectification right?

[guitarsnguns04]

Does McMaster have a website?

[Colossal]

'guns, it's http://www.mcmaster.com/

[doctord02]

Tubes sound different at different voltages is the take away, so if I want to run the express with a 200-0-200 secondary, and have lower B+ values can I expect much variance from the normal express tone/voice? I know KF used random transformers at times so a lot of Trainwrecks sound very unique.

200-0-200 is way way way too low. You really want something at least close to 300-0-300. The Express runs EL34's at apx 400VDC - thats kind of a magic voltage to get the correct sound out of it.

Ken used different vendors to supply his transformers, but they were anything but random. Pacific, Heyboer and off the shelf Stancor output iron. Save yourself a lot of grief and buy the Toneslut set. Otherwise you are compromised on the primary key elements of the circuit - the proper voltage and current and the correct output iron.

[CaseyJones]

+100 on Allyn's chassis and ToneSlut iron.

A whole cottage industry has sprung up to supply the correct parts for these. Do yourself a favor and buy the correct parts to begin with. It's a waste of time and money not too.

The correct transformers have the correct current ratings. 270-0-270 @ 80ma won't cut it for instance.

Turretboards dba Watts Audio sells the circuit boards.

http://www.turretboards.com/

installed a modified Post-PIMV as sold by metroamps in my MIG50 and it works alrighty. What is the advantage of a VVR or an airbrake over installing either a pre or post PIMV?

No takers on this one?

If you stand in front of a cranked Express blowin' through a slant front loaded w/ nicely broken in genuine British greenbacks you'll never play it any other way.

Tubes sound different at different voltages is the take away, so if I want to run the express with a 200-0-200 secondary, and have lower B+ values can I expect much variance from the normal express tone/voice? I know KF used random transformers at times so a lot of Trainwrecks sound very unique.

Would this ultimately just result in lower volume?

You betcha they sound different with different voltages!

P.A. tubes all have their own unique character. That's counterintuitive, they're supposed to be hi-fi devices so in theory they should all be relatively transparent. In practice that's BS, they sound different. 6L6s have a different character compared to 6V6s compared to EL34s compared to EL84s etc, etc.. In hi-fi the difference is less obvious, driven to clipping like we use 'em the character becomes obvious.

Different tubes were designed by different manufacturers in different countries for different circuits, not surprisingly the supply voltages the tubes were designed for vary widely. If you look up the specs for a given tube the manufacturer generally suggested a maximum plate voltage for design center values. That means that if your tube is installed in a conservative circuit use that rating and your tube will live happily ever after... or close enough to it. Manufacturers didn't suggest outrageous specs 'cuz why would they? Build a tube torture chamber, tubes fail, pissed off customers and all that. However... some amp manufacturers tossed caution to the wind and ran things way on the hot side.

There is a range of usable voltages for each tube. Too high especially on the screens kills tubes. Too low, the amp loses drive and definition.

Having said that I generally go low. KF generally went low. Low but not too low.

250 volts for EL34s is well below the sweet spot. If you really must use that 200-0-200 my choices would be 6AQ5s, 6BQ5s or 6V6s in that order.

[gearhead]

installed a modified Post-PIMV as sold by metroamps in my MIG50 and it works alrighty. What is the advantage of a VVR or an airbrake over installing either a pre or post PIMV?

No takers on this one?

If you stand in front of a cranked Express blowin' through a slant front loaded w/ nicely broken in genuine British greenbacks you'll never play it any other way.

Like Casey said. It just seems counter-intuitive to me to build an Express and then do nothing but try to throttle it down. If you have a volume issue, build something else, because anything other than a cranked, throttle-open Express is a compromise.

On the other hand, if you're looking for an option to cut back every once in a while, then you've got a point. Distortion in an Express is kinda opposite of most amps, so a PPIMV wouldn't do it justice. As you preogressively apply more signal, Distortion starts at the power tubes, then at the PI, and then back into the preamp section. With PPIMV, you'd be missing that vital PT distortion. Imho, an Airbrake is the best, but still a comprimise. The VVR is quite intriguing and well worth a look into also.

Yeah, but what do I know? LOL.