Noisy Express

[peesinstew]

I just finished a new Express. It's from ceriatone but I assembled the board myself. Even with no guitar plugged in, I'm having alot of noise issues when I turn up all the controls: hum, hiss, microphonics, squeal, you name it.
Chopsticking didn't reveal any obvious sources and as far as I can tell, my solder joints are good so I started trying other things.

First, I put the chassis on a sheet of aluminum foil, this definitely helped but didn't completely solve any of my problems. I also tried moving the chassis
I tried removing each preamp tube to isolate the issue. Most of the noise goes away when V1 is pulled. A little more noise goes away when V2 or the PI are removed but not much. There is still an audible hum even when the PI is removed but it is very quiet and I can live with it.
When I swapped which 12ax7 was in V1, it made a noticeable difference in the type of noise I was getting but none of them solved the issue.
After realizing that the problem is most likely before the second gain stage, I tried a couple more things.
I removed the bright switch and parts and I used a shielded wire from volume to grid2. Neither of these helped much.
I redid my preamp grounding scheme, putting a switchcraft jack in and attaching a ground bus to that. All my preamp grounds go to that in the same spots as they were before except the bus is not soldered to the backs of the pots and the preamp filter caps go to this bus now too. That didn't really make any noticeable difference either.

Then I put a 12ay7 in V1. This helped alot! No more squeal, very little hum or hiss. I still get alot of gain with everything maxed out. I'm just wondering if I'm missing out on something now without the 12ax7. Is there anything else that could help these issues and let me put an 'ax7 back in V1: dampening foam, an input grid resistor, anything else?

[BBQLS1]

I'm still new to building, but my first advice is try a bunch of different tubes! Tubes can be very finicky!

[geetarpicker]

The 12AY7 will really reduce the gain in these amps, and you can even reduce the gain further by an AY in V2. If the sound is still ok with you then that will work to basically reduce the gain of everything. However it is a very different amp at that point. That said these amps are very picky about tubes, so try everything you got and more.

I just completed two clones of my original '89 Express. I did the grounding exactly like the original. Input jack grounded only by mounting and shielded cable grounded just on the jack side and no other shielded cable used in the amp,pot buss bar with board grounds run straight over to it, PS caps grounded with 3 separate ground leads running to tab at center of chassis, output tubes grounded to tabs right on sockets, AC cable and bias grounds to tab near back corner. My clones ended up with the same hum and hiss levels of my original no more no less. None of these amps are truly quiet, but considering the gain they have they are probably quieter than most amps with similar gain. If I set my wrecks with all the knobs on about half, the gain is similar to a totally dimed plexi Marshall but with less noise than the dimed Marshall.

For what it's worth I measured the hum and noise levels with a meter. Not really scientific but it might be of interest to test it. With an 8 ohm speaker attached, the volume off and other knobs on 1/2 I got about 7mv of level at the speaker terminals, barely audible. Then with the volume raised to about half the noise went up to about 50mv. The pots tapers might make the "1/2" volume reading quite different on another amp, but that is what I got with mine.

[harryk]

When I made my first and only Express clone, I did my groundings according my other tube amps. This means that power amp and power supply are grounded to one node near PT and preamp to one node very near the input jack. It is as noisy as my other tube amps which are very silent despite they have reasonable gain. In the beginning I did all my amps with one star ground but now I believe what Paul Ruby says about gounding to make an amp silent. Regards Harry

[peesinstew]

yeah, I read a post by Paul, and that's why I changed around my preamp grounding. I've just been too lazy to get around to rewiring the rest of the grounds yet. I used to have the grounds how geetarpicker describes, except for the input jack which was cliff style at first so that was grounded to the pot bus bar.

Also, yeah I know that the 12ay7 is lowering the gain, and I consider it cheating to fix my noise problem this way. I would love to be able to use a 12ax7 and realize the full potential of the circuit, but so far, it just drives itself out of control. I just went home for lunch and put the bright switch back in. With no guitar plugged in, the knobs maxed, and the bright switch on the 100p setting, it starts oscillating. I could press on certain parts of the chassis and make the oscillations stop so I guess this means I should try some damping material. I also took the amp off of the aluminum foil for a while and it started squealing again. I'll go get a more permanent plate to mount it on later but until then I'll be keeping it on the foil.

When I get the chance, I'll try going through all the other ax7s i have and see if I have any more that are quieter, but if anyone has any other suggestions on generally keeping things quiet, I'd like to try some of those too.

[gearhead]

I haven't finished my Express build yet, but take heart.

https://tubeamparchive.com/viewtopic.ph ... e&start=30

LeftyStrat (from this thread}: "This got me to thinking. I remember the quote from Ken that some amps that he couldn't tweak to sound good enough, he would disassemble and not sell. I use to think that meant he couldn't get it under control. Now I think it might be the opposite, that some amps may have come out too stable.

Reeling in an unstable amp is probably a lot easier than trying to "untame" a polite one. "

IMHO agree; you can tame a thoroughbread, but can't excite a draft horse.

[peesinstew]

Haha!

I like thinking about it that way. Honestly, I thought I could get some cool overtones out of the amp before when it was squealing. Not necessarily the tone I had in my head, but I was still proud of what I did. The trouble is that no one else in the room (or the neighborhood) thought it sounded good. Oh well, I'm sure it will be better overall once I actually get her under control.

[LeftyStrat]

If you can post some pics of your layout, that would help. You may have luck with a grid resistor on the input (right at the tube). A value of 33k has been seen on some of Ken's amps.

I'd also probably say to worry about getting the amp to a reasonable state with the controls all at fifty percent. Once you can get to this point, then you can worry about diming the presence control and figuring out how to make the bright switch stop acting like a theremin.

Look at the Francesca pics and try to make your layout match. Play with the leads going to the tone controls, especially the treble control. Some people have found it helps to have the middle and bass leads running parallel, with the treble lead more up in the air. You can reduce some treble content by doing the opposite and running it closer to the chassis.

And play with tubes, mostly in V1. Different brands, NOS if you can get your hands on some.

[mdroberts1243]

In your original post you don't mention whether your voltages are all in order.

When I built my Ceriatone express there was a mix-up in the colours of the primary taps for 100V and 120V... my voltages were much higher than they should have been and I didn't check them closely, assuming they would be 'correct by design.' With 6v6 the high voltages weren't a problem, but with EL34, etc. you also got 'blocking' distortion happening.

The higher voltages made it harder to get the amp under control squeels and hiss, etc... One area in particular that seemed sensitive was the bright switch and cap... very tiny changes there were really noticeable. I followed the Francesca pics religiously and once the primary wires were swapped the voltages came right in line with Geeterpicker's numbers and everything was much more stable.

FWIW...

[LOUDthud]

One thing people tend to forget or not be aware of is that heater voltage affects a tube's gain. When you look at the plate curves on a Tektronix 570 curve tracer, the first thing you do is adjust the heater voltage. I made a little box to adapt a Tek 576 curve tracer to look at matching between halves of a 9A based dual triode (12AX7 et al). It matters whether you use the series or parallel heater connection! The changes arent huge, but could make a difference in a marginally stable amp.

[moj067]

I know it's hard to tell you, but this is the fun part of the building process.
Best thing to do is go slowly through the circuit and find out what exactly is going on. Every amp is different blah, blah, blah.
Post pics, it does help.

[peesinstew]

Hey everyone,
Thanks for all the suggestions so far. I'm going out of town for thanksgiving all this week so I won't be able to try any more changes until I get back, but I really appreciate all the help. I did bring her with when I played out tonight and I had a chance to run her wide open today with the 12ay7 and she does sound sweet, but I can tell from how it feels that I really need the ax7 in there to get the dynamics (and occasional over-the-top gain) that I'm looking for.

"When I built my Ceriatone express there was a mix-up in the colours of the primary taps for 100V and 120V..."

I had that idea too, and I measured some voltages. Can't remember what they were off the top of my head, but I think B+ was in the right range and heaters were i think about 6.2 so I'm pretty sure I have the correct primary lead.

Well, when I get back, I'll probably try rerouting some of my wires to get closer to Francesca specs. It's something I think I want to do anyways, so I'll go for that and then if I'm still having issues, I'll post some pics.

Thanks again, I'll see you guys later.

[peesinstew]

ok,
I'm back working on this finally.

I moved some grounds a little more, reflowed some solder joints, put some foam tape on the chassis, and replaced the 1n4007's in the rectifier and bias path with HER108's. It seems like it has helped some, but still nothing has completely solved my noise issues. I put a 12ax7 back in since I'm determined (at least at the moment) to get this running quietly. It still squeals with nothing plugged in if I turn up high enough.

While playing around with the amp, I noticed that I get some sound with the volume at zero. I thought this was a ground issue, but after reading some posts on ax84 and ppwatt, I now think that some of my wires are emitting signals that are getting picked up further down the chain. If I pull V2 and turn the volume up, the PI will pick up some signal from V1.

Is there a good way to prevent this (shielding, grid stoppers, etc?)? I figure that if signal is being transferred forward, it's probably also going backwards and might be giving me positive feedback and causing my squeal and noise. Does that make sense?

[peesinstew]

I'm posting some pics in case it helps.

It might not be clear, but the bus bar is not currently attached to the pots. It's floating next to them and connected to ground through the input jack only.

[Lonely Raven]

Question for you:

Did you check the value of each resistor and pot before you placed them? In my Ceriatone kit, I found 4 or 5 resistors that were more then 50% off spec. I E-mailed Nik to give him a heads up about it and suggested he spot check his stock to see if maybe he got a bad batch. I got worried when my 1Meg read something like 551k. I even double checked all my findings on a second meter (both calibrated Fluke meters) just to be sure.