Express Builder? - Newbie Post

[amplifiednation]

Yes you are correct. You want the first jack to short. Sorry to add any confusion. I havent used shorting jacks on the speakers of my builds...probably should.

[lions den]

Very good,
thanks,
sean

[lions den]

Any thoughts on the effects of the grid resistor on V2? I can quickly take it out and see/hear what happens, but any hints on what to expect? That is the only deviation i've seen from most stock express layouts.
thanks,
sean

[lions den]

Is there any downside to replacing the shorting input jack on an Express with a normal non-shorting one? I've been trying to troubleshoot a nasty noise problem on my Express. It sounds like a grounding issue and I suspect the input jack as sometimes when I unplug the guitar everything goes quiet as expected, and sometimes it doesn't. The top of the cabinet is very microphonic and tapping on the top can generate a loud building of noise.

I thought I would swap out the input jack with a normal one as I never turn the amp on without being plugged in first, just to see if that's where the problem is.

I've already swapped tubes multiple times as well.

thanks,
sean

[Reeltarded]

Be weally quwiet, and tuwrn de amp down a widdle bit.

Using a long thing that can tap a tube (like a cut off arm with a stiff finger on it, or a narwhale tusk) tap V1 and V2. The one that is most verbose is likely as microphonic as one can be without wailing itself into occilidamnation. The tube may be so microphonic that you can't tell the difference from the one you are tapping and the offending tube even with the amp on 1.

Does your amp have any shielding tricks on the front end?

[lions den]

I'll give it another go and do some tapping to see if I can find an offending tube. Not sure what kind of tusk i have lying around, maybe some Havalina

I can play for 5-10 minutes with everything working great, then I stop and the noise level of the amp in terms of hiss rises. It's not a hum definitely a hiss. I can sometimes unplug the guitar and it goes away, other times if I tap on top of the cabinet a few times it either gets louder or will all of a sudden go away with a tap.

Not sure how to describe any grounding tricks other than I know it has the shorting input jack, and ground of the input jack is connected to the ground buss at the tonestack. The cable to V1 is grounded on the input jack side but not on the V1 side. There are pics at the opening post of this thread.

Thanks for your help.
Sean

[geetarpicker]

Sounds like a preamp tube issue to me.

Just for the record I've had noise issues on my original Express and 2 clones where the input jack would get loose enough to not be grounded. However on an original the jack only makes it's ground connection via it's mounting, and coupled with a wood panel it can sometimes get loose and you can't tighten it really snug for fear of cracking the wood. I've also pulled the jacks on my amps and cleaned the aluminum with alcohol every few years to clean up the connections. Still, on your amp you have a dedicated ground so that gets around that issue. I did notice some non standard grounding on your main filter caps. On an original all the filter caps get grounded to the terminal in the middle of the map (some ganged up by different ground busses) but yours has a terminal near the front panel which is not like an original. Also on my original both the output jacks are of the shorting 3 terminal type, though they are simply wired without the extra terminal hooked up so the shorting effect was not implemented. Probably doesn't hurt to have it wired up though.

[lions den]

Glen,
Thanks for the feedback. I didn't notice the extra grounding point towards the front panel that was used on mine. I need to take another look and see what is going to the main grounding point near the positive side of the filter caps, if anything.

Anyone have a resistor on the grid of V2 pin 2? I guess I could take it off and see what happens, not sure if the previous owner had it installed to fix a particular problem or not.
Thanks,
Sean

[selloutrr]

Do you know which state the amp came from? I can probably tell you who built it based on the logo.

[lions den]

No not sure where the amp was built. I understand my friend originally had the parts shipped to him in Arizona then decided to have it put together instead. I'm not even 100% sure that he got the cabinet from the same person who built the amp.

Thanks
Sean

[selloutrr]

Then it is anyone's guess.

[lions den]

Today is a great day!! I've been battling issues with chasing down a tube or grounding problem. The amp would periodically have a very loud hiss problem and recently also would go to no output.

Had the amp out of the cab today and was determined to find the issue. At one point I could tap on the chasis between V2 and V3 and at will have the amp go to no output. I was fearing the B+4 wire under the preamplifier board was not making a good connection. After ruling that out I could also reproduce the issue by picking up the chasis.

Finally found a cold solder joint going from the V1 cathode resistor/cap (R5 C7) over to the ground buss. On my amp the cathode resistor for V2 R8 is also joined together with R5/C7 using a single connection to the ground buss. Resolder and now everything is back to normal

Now I can go back to some of those nice NOS preamplifier tubes I put into the "suspect" bucket.

Sean