Another nice new express after a dumb start-up mistake

[Broz]

Finally got my new Express up and running and what an amazing machine. Sounds spectacular ( although way loud), better with el34's, but good with 6v6's. Started this thing early summer wth a home-brew chassie, Moose's trannies and Dana's boards and then had to put her away for 3 months while I spent my summer in Kandahar, Afghanistan. Started up again around Thanksgiving and ran into a wall with a very strange problem. Bias at zero volume would start at about the right level, and then track with increasing volume to larger negative voltages such that the output would stay exactly the same whisper quiet across the volume range. Perfect power-scaling if you never want more than about .1 watt out of your 35 watt amp!

I tore her apart about ten times, scoped things, moved everything, to no avail, and finally set it down over the Christmas holidays to avoid throwing the whole thing in the trash. I was hesitant about bringing any or these issues up on this forum because I had settled on a non-standard grounding scheme ( more about that in a moment) and wanted to be sure I hadn't done something dumb with that before I asked any questions. Anyway, came back with a fresh look and decided it must be a power supply problem and - low and behold - the wire to the second power supply filter cap (hidden behind the draining resistors) was only crimped and not soldered. Enough of a connection to sort of work, but would poop out with any real current draw. What does eveyone say - check your solder joints and grounds, but this was the one connection in the amp that was hidden behind stuff that had to be moved to check.

Anywho, after fixing my stupid error, she came to life, and things are very, very nice. I initially used a grounding scheme that was a cross between that suggested by rhinson, Paul Ruby and the AX84 P1, but tore it out trying to fix what turned out to be my power supply connection problem. Got it up and running with something close to the stock ground stuff and it sounded really good, but with quite a lot of hiss and hum. I figured at this point I could go back and try the alternate grounding and about 90% of the noise went away. I used a separate ground buss (not connected to the backs of the pots) as suggested by rhinson, and only connected it to ground at the guitar input end like the P1. I also got some more improvement in hiss by adding shielded cable from the first gain stage to the treble pot (in addition to the normally used input jack to the grid of the first triode), and in moving the grounds from the power stage octal sockets to the appropriate part of the ground buss.

The amp is now relatively quite (for a gain monster) and sounds totally unbeliveable. Way too loud for my old ears, but I ordered one of Weber's Mass lite attenuators that makes it possible to play in the house. This is the unit that has 2 rheostats to separately attenuate low-mid and mid-highs, and works quite to my ears a lot better than the mini-mass I use on my 18watt TMB.

Thanks so much for all of the info and help available on this board. It has made it possible for me to end up with something that I think is spectacular. Next stop is building a cabinet. Thanks again.
Broz

[UR12]

Broz

Nice job man, she looks great!! Congrats on a beautiful build! Glad you got it sorted out. I would also like to say (and I hope I'm not the first or the last) Thank You for protecting out Freedom!!

[sliberty]

Glad you made it home safely, and thanks for your service. But if they call you again in the middleof an amp build, try telling them your busy working on an amp - I am SURE they will undestand

[Broz]

Dana and Sliberty,
Thanks for the kind words. I'm just a DoD science guy, not in the forces, but we were flying aerial recon and imagery missions in support of US and ISAF troops. Most satisfying work I've ever done, and we're hoping to go back this year. The folks working on the ground, outside the base fence deserve the credit. They carry 80 lb packs in 120 degree temps with the constant threat of IEDs and ambush.
Broz

[smokin' tone]

Very nice build. Glad you got it going. I'm sure you are going to enjoy it.

[Allynmey]

Very Nice looking build!