New Express, new noise

[Smokebreak]

I fired up my second Express build tonight. My first one was all donor iron with only 6V6 capability. This one was Edcors from RJ that are fantastic! RJ was a pleasure to deal with and very helpful. I'll definitely get my next set from him.
Good golly this thing is so aggressive and loud. I hadn't planned on putting in a MV on this one but I had to just to troubleshoot.
I got rid of some preliminary squeals at higher levels with some preamp tube swapping. Now the only trouble is when I get close to diming everything, I get a low frequency feedback rumble, and sometimes a high pitched whistle, but not a theremin-like squeal. This happens with guitar plugged in or not. I've never experienced this.
I had to shut down the troubleshooting for the night, and will get to more swapping tomorrow, but I was just curious what this was, or if anyone has experienced this. I put the amp in a shielded cab and moved it a few feet from the speaker cab, but that didn't help.

Thanks, J

[geetarpicker]

Can you post a gut shot?

[Smokebreak]

I hacked in the MV last night, probably should use shielded cable to the grids. I wasn't sure the best way to run those wires, either.
Bias feed is waaay long as I took it from where the 220Ks once were, instead of the bias board.
This is a Callaham chassis so I kinda had to work with where the holes were punched. My concern is the section around the OT secondary and EL34 sockets, it's very congested, and I got a bit confused as to how to route things there.

While I'm at it, I'm getting too much 60Hz hum from single coils, too. (edit: it's quite directional, and I can get it pretty quiet if I angle the guitar in 1 spot)
I'm not a fan of soldering to the backs of pots, but the ground scheme is pretty much the same as the layout, with the except of the location of the pre buss, and it's chassis grounding point by input. I haven't got to mess with it today, but that presence ground looks troubling, as does it's NFB run, not to mention who knows what else

[img2448]http://www.whiteghostshivers.com/images ... ak/gut.jpg[/img]
[img2448]http://www.whiteghostshivers.com/images ... ak/pre.jpg[/img]
[img2448]http://www.whiteghostshivers.com/images ... eak/ts.jpg[/img]
[img2448]http://www.whiteghostshivers.com/images ... k/el34.jpg[/img]
[img2448]http://www.whiteghostshivers.com/images ... /extop.jpg[/img]

[geetarpicker]

This amp is going to be noiy with single coils, but that is the guitar's fault. Aiming the guitar to null out the hum is going to be just like running other high gain amps. Still single coils won't be any noisier than any other amp that has similar gain, and the amp itself might even be a tad quieter in the signal to noise dept. The issue is with these amps you are running high gain ALL the time, as compared to a channel switcher where most folks click off the gain when they settle down. Same goes for master volume amps, most are used to the amp backed way down but with the wreck it's like the master is fully dimed. A power attenuator might help get the volume down AND also control your apparent noise issue.

Power conditioners won't typically help much, but rid your house of light dimmers and florescant lights for starters. I often use my strats with a wreck and make use of the 2nd and 4th positions alot with the hum cancelling effects of a reverse would middle pickup with reversed magnets. These day I have a Suhr hum cancelling back plate on my strat and that really helps.

One other factor with noise and gain is the pots that Ken used were typically 10% taper, maybe even 5% on some amps. I'm not talking 10% tolerance I'm talking 10% (or 5%) taper. What I mean is the analog tapers would net only 10% of the total resistance at 1/2 rotation. Heck the pots on my '85 wreck come on even more slowly than that, and the amp is still almost totally clean on 1/2. Heck on 1/4 it's barely audible it comes on so slowly and tame, but then 3/4 to full up still has a lot of change. SO, if your volume, treble, and bass pots have quicker analog taper slopes you may have totally different knob positions than say on a real wreck. Even though all knobs on 1/2 may be a great starting point on a real wreck, you may have to back down your 3 controls that have analog tapers if they come on too quickly. I've seen some wrecks where you might have to back some controls down to 1/4-1/3 to make them comprable to a real wreck. Since the treble pot especially controls the gain alot this can make a big difference in taming the amp down a bit. You might measure your volume pot and see what kind of taper you have.

Ok, on your amp it looks really nicely built and soldered up. nice work! However I suspect your grounding scheme in the control pot area is the root cause of your stabilty issues. On my wrecks (2 originals, and my 2 clones) all have the buss bar wire soldered to the back of the pots. Others have good luck running say PEC pots where you can't solder to them but then they run small terminals to still get a buss bar very close to where Ken had it. On my wrecks IF any of the pots get loose it can cause a squeal issue same goes for the input jack, and that's even with a buss bar obviously there too on the back of the pots. The short ground paths apparently must be very important, and the redundant nature of the buss bar and the pots physical connection to the front panel for grouding is crucial.

You might also go over your soldering as even though it's very clean it also doesn't use much solder so eyeball them closely to see if they are all perfect. One of my clones had a cold solder joint on one of the pot terminals and would squeal because of it, and the soldering looked perfect but was indeed cracked.

I notice you have a much different placement of your OT. I'm not sure if that is going to be a problem or not, both for stability and or hum pickup from the PT. Still, just making a note there...

Hope that helps!

[Blackburn]

Nice build, Smoke! Might want to chopstick it a little bit and see if anything acts up. And I'd look over the solder joints, as Glen mentioned. What tubes are you planning on running?

[Smokebreak]

Thanks guys. Well y'all were right on the money. Check out the junction of the tone caps on the board. I completely missed that. I didn't have my pots cranked all the way down either. Unfortunately, I'm still having the same low rumble issue, and sometimes whistling.
I haven't got to the ground scheme yet but tha may be next.

I did discover that even with all the front panel controls all the way down, the low growl happens when I turn the MV up. If I get the MV up to the point right before instability, I can crank everything else up with no issues.. Hmmm...

Last night I had my trusty mismatched Shug tester EL34s , today I put in some JJ EL34(I was hoping this would cure), and I'm putting in whatever 12ax7s that don't howl or make weird noises. Right now I'm running a new Sovtek in V1 that stabilized my last express, and I think an old GE and sylvania . I'm thinking about getting some tungsram.

[JazzGuitarGimp]

Hey Smoke,

I am wondering if the MV / low growl issue could by caused by the OT primary wires needing to be swapped? This would cause the global negative FB to become positive FB, thus causing oscillations. This could only happen if you are employing the Lar Mar style MV.

Cheers,
Lou

[M Fowler]

Hey Smoke,

I am wondering if the MV / low growl issue could by caused by the OT primary wires needing to be swapped? This would cause the global negative FB to become positive FB, thus causing oscillations. This could only happen if you are employing the Lar Mar style MV.

Cheers,
Lou

I agree Lou. Next will be the presence circuit if noise continues.

[Blackburn]

It happens. I think you should try some Tungsrams. I'll be repurposing an amp that I didn't care for in a mispunched chassis that happens to be absolutely perfect for a 6V6 Express. I want to try some Tungs eventually in there. I think I'll be changing the circuit around a bit, but that remains to be seen. And if you're interested in a pair of NOS GT EL34Ms, I just might be able to point you in the right direction.

[geetarpicker]

I was assuming you had already tried swapping the OT primary wires. It could be the easy fix right there so try that first!

[Smokebreak]

Hey! Swapping the OT leads did the trick. You guys need to school me on this, because this is the second time in 24hrs that I've switched them, back to where they were at the very first power up.
So on first powerup(no Larmar yet) it squealed instantly. I swapped, and problem solved. Then I put in the MV and had the most recent issue with the low howl. I figured swapping them back would bring back on the instant squeal, but that obviously solved my problem. How does that work?

These should have been my clues:
Presence control didn't have much effect, and at certain levels it seemed to be working backwards.
When I disconnected NFB problem went away.
Pulling PI stopped growl.
I tried about 10 tubes in V3, including AT7, and the only one that stopped the growl was an old organ pull ax7 that had very low gain. Amp sounded like crap with it but that one had me scratching my head.

As it stands I can dime everything and it just starts to break into a high whistle when gain is almost all the way up. I'm hoping the right V1 will do the trick here. Thanks a bunch for the help guys, I'm thrilled. I've only got shield on input, and no stoppers. The cleans are indeed beautiful and seem accessible, and it's a beast on the other end of the spectrum. All is well...for now..haha.

Yea Blackburn, I may be interested. You planning a trip up to Austin?
I'm not familiar with those GTs, you like em?

[Blackburn]

Yea Blackburn, I may be interested. You planning a trip up to Austin?
I'm not familiar with those GTs, you like em?

Not in the foreseeable future, but my girlfriend is there now, ironically. I haven't played with them myself, but they were the ones Ken loaded Express amps with if he wasn't going with NOS at the time. The EL34Ms are OS now. I just know a guy that (I think) still has some. I'll PM you a link.

Good to see you got the amp fixed up.

[JazzGuitarGimp]

My guess is, when you installed the Lar Mar, you inadvertantly reversed the outputs of the PI going into the output tube grids. It's pretty easy to overlook. Glad you got it going. It sounds like a nuce amp!

[Smokebreak]

Well that's a good guess cause that's exactly what happened

[leadfootdriver]

'All my noise problems were cured when I got my flying leads from the board down to the chassis.

Cool build tho.