The Making and the Breaking of an Express

[ODwan]

The Making and the Breaking of an Express
!!! This is gonna be rather long!!!
For all the info I got from this forum I wanted to give something back, so I try to give a build-report of my Trainwreck Express Clone. I’m from Germany so forgive me if my english is dotted with errors.
I wanted to use version A0 as a starter and see where I would end up after tweaking. The deviations I made from the start are a two-position bright switch (as in A1a) and the B+1 decoupling elcap is an 100µF/450V type with a 220k resistor across it.
I used a Hammond aluminium chassis and had to use a Hammond 1645 OT because Heyboer trannies are not so readily available here in germany. If I ever get to the USA I will try to get me the real deal and see what difference it makes.
As PT I used a toroidal of german manufacture with 290-0-290@0,4A
The board is layed out after Wrecked2K4’s preamp-layout and the PS is layed out after the wonderfull 3D-Layout in the files section of this forum.
Caps are Mallory 150’s and Silver Mica, resistors are 2W metalfilm types.Pots are from Alpha. Only solid-core teflon coated wire is used. Tubes are EH12AX7’s and a pair of EH EL34.
After I had everything assembled, soldered and wired I triple-checked EVERYTHING and found no error! That had to be a good omen because it had never happened before in any of my builds!!! I recommend everyone to at least triple-check your circuits… I so far one smoked PT, two roasted tubes and a “ride the lightning” experience I would rather not have again.
Time to fire up:
Sqeeeeaaaaaaaaalllllllllll…………enter the “How did you reach stability?”-thread:
In trying to tame this amp I followed the considerations made in this thread. I put a shielded wire on the grids of the first two stages, put a 47pF Mica Cap across the PI plates and upped the 2nd stage grid resistor to 2k2. I left out the 1st stage grid resistor because I feel it reduces an amps ability to react to changes of the guitar volume. I did it all step-by-step and with each the stability was increased a little. Swapping tubes did nothing and moving the wires did almost nothing, too. However, I found out that the OT is emitting a strong electromagnetic field which couples back into the first stage. Now I know why KF put the OT right next to the PT and away from the preamp tubes. Solution was found in a bit of sheet aluminium which shields the preamp tubes from the OT. My Express clone is stable now and I can get around to building a cabinet for it.
I named her ARIANA to honor my girlfriend who had to miss me much too much while I built this amp.

[toddyjoe]

And now for the most important question: how does she sound?

[ODwan]

Pretty good! And loud, my neighbours are getting angry!
She responds very well to pick attack and guitar volume. I tried a lttle powertube swapping just yesterday.The EL34 I used are a little agressive for my taste but with JJ6V6 she's much creamier (think womantone). I also tried a pair of chinese KT66 made for the german tube-vendor "Tube-Town", which got some very good reviews from hifi-folks. Very, very good tubes! Just the perfekt mix of bite and smoothness. I always liked the tone of those old JTM-45s with KT66 tubes and it was just there, only more open and responsive not to mention much more gain. I can recommend everyone to try their Express clone with KT66s.

Timo

[Allynmey]

Hi ODwan, congrats on your build. Which layout did you use? Maybe you ought to post it here. How about some pictures? Thanks,
Allynmey

[Guest]

the B+1 decoupling elcap is an 100µF/450V type with a 220k resistor across it.

What is that resistor supposed to do?
If you have two caps in series to up the voltage rating you need a 220k (or similar) resistor across each to provide equal charging.

I see no purpose for that resistor on a single cap.

[markd]

the B+1 decoupling elcap is an 100µF/450V type with a 220k resistor across it.

What is that resistor supposed to do?
If you have two caps in series to up the voltage rating you need a 220k (or similar) resistor across each to provide equal charging.

I see no purpose for that resistor on a single cap.

To drain the charge from the caps when the amp is turned off.
markd