new Express build problems

[fishy]

You can also build an audio probe and use that to step through the circuit.

Something like this although when I made one I increased the voltage on cap

http://diy-fever.com/misc/audio-probe/

[Randy Magee]

Does anyone see anything or have any suggestions what may be wrong? Something tells me it might be a ground, but I can't for the life of me find it... the problem is not intermittent, but instead of the extreme volume I should have, I have a nasty sounding low volume... no clean at all and the more I turn it up the worse it gets... the signal overloads. Could it possibly be an oscillation causing this? The output transformer doesn't get hot like I would expect if there was an oscillation... I'm really confused as to where to look next...

[Gibsonman63]

Have you verified power tube bias current and run the bias adjust up and down a bit to make sure all of that works? If you have a plug in checker, it may be a good sanity check.

Verify all grounds with a meter.

Double check the wiring on the shielded cable from the input to V1. I have recently read that some of those cables have multiple layers and that some layers can be conductive.

[fishy]

Anything strange happen when you turn any pots?

Do you have a noise with V1 pulled out?
If you can identify when the noise is present, it will give you an area to focus on.

Are any pots scratchy?

Many problems get solved by walking away, regrouping and going over the same stuff again......

I would check all the ground relative to the safety ground. The buss is a good candidate for issues since it is soldered although you do have cathode voltages referenced to the buss that look ok. That does not mean all references associated with the buss are OK though. Make sure the pots are all secured properly. Make sure all jacks are secured properly.

Check the input jack and wiring up to V1. It is easy to compromise shielded cable with excessive heat.

I really think an audio probe will help out here. You can listen to the signal through the various stages. and you can inject a signal at different stages. I have used a radio signal or a tone generator to do this before and it is a valuable tool. For the price of entry, it is well worth it.

[Randy Magee]

Anything strange happen when you turn any pots?

No strange noises from any of the pots and they all seem to function properly...

Do you have a noise with V1 pulled out?
If you can identify when the noise is present, it will give you an area to focus on.

I haven't tried that as yet...

Are any pots scratchy?

No noises from any of the pots...

Many problems get solved by walking away, regrouping and going over the same stuff again......

That is where i am at this point... I have grown weary checking stuff and I'm taking a mini vacation from it hoping that at a layer time, the problem will become obvious...

I would check all the ground relative to the safety ground. The buss is a good candidate for issues since it is soldered although you do have cathode voltages referenced to the buss that look ok. That does not mean all references associated with the buss are OK though. Make sure the pots are all secured properly. Make sure all jacks are secured properly.

I'm considering changing the grounding scheme to a buss not soldered to the pots. All the grounds check out with a DMM, all the pots and jacks are tight, but I'm almost convinced the problem just about has to be a ground.

Check the input jack and wiring up to V1. It is easy to compromise shielded cable with excessive heat.

I have rewired the input jack from scratch, but no joy...

I really think an audio probe will help out here. You can listen to the signal through the various stages. and you can inject a signal at different stages. I have used a radio signal or a tone generator to do this before and it is a valuable tool. For the price of entry, it is well worth it.

I have constructed an audio probe, but haven't started tracing the signal... I'm supposed to leave town tomorrow until Sunday night and I think I'll wait until then to continue... I may need some help figuring the audio path to probe for a signal at that point. I appreciate everyone's suggestions. I know it must be something simple and there is a "duh" moment in the immediate future...

[passfan]

I had the wrong feedback resistor installed one time and this is exactly what mine did, no volume , no cleans. Lift your feedback and see what you got.

[Tillydog]

Does anyone see anything or have any suggestions what may be wrong? ...

Since you sound desperate (!) I'll chip in my 0.02p:

I can't verify from the photos that the centre tap on the PT is grounded. (it probably is, but it's bugging me )

[analog_stud]

Try flipping the OT primaries around (at the sockets). Some are sensitive to proper polarity but don't tell you? My 2 cents

[Randy Magee]

Does anyone see anything or have any suggestions what may be wrong? ...

Since you sound desperate (!) I'll chip in my 0.02p:

I can't verify from the photos that the centre tap on the PT is grounded. (it probably is, but it's bugging me )

It most definitely is grounded!

[marcoloco961]

I am looking at your power supply wiring. Mostly what looks like 3 wires coming off of one side of standby, and I see no power supply to the first two caps with the other wire running to the standby switch. Double check your wiring there.

I might just be looking a the pic wrong or not see the proper routing in the pic.
Does the standby switch even work at this point?

[Randy Magee]

I am looking at your power supply wiring. Mostly what looks like 3 wires coming off of one side of standby, and I see no power supply to the first two caps with the other wire running to the standby switch. Double check your wiring there.

I might just be looking a the pic wrong or not see the proper routing in the pic.
Does the standby switch even work at this point?

The standby switch works fine... the caps are properly wired... the voltages would be totally screwed if they weren't...

I will be out of town until Sunday night, so I will revisit this issue then. Perhaps a little time away from it will give me some clarity.

[Branson]

I had a similar problem with my Weber Java build. It did turn out to be an oscillation. I wound up shielding every grid wire (pre and power) and placing rather large (4k7 to 33k) resistors on every grid. I don't know if this is your problem, as you seem to have a great deal more room in the chassis than I did, but it worked for me. Good luck!

[marcoloco961]

I am looking at your power supply wiring. Mostly what looks like 3 wires coming off of one side of standby, and I see no power supply to the first two caps with the other wire running to the standby switch. Double check your wiring there.

I might just be looking a the pic wrong or not see the proper routing in the pic.
Does the standby switch even work at this point?

The standby switch works fine... the caps are properly wired... the voltages would be totally screwed if they weren't...

I will be out of town until Sunday night, so I will revisit this issue then. Perhaps a little time away from it will give me some clarity.

Yeah, it was hard to see in the pics. It looked like there was three connections to the bottom lug of the standby.

You really should consider the suggestions of the audio probe or the multimeter used to trace the A/C signal through the circuit one step at a time. This will probably be the easiest way to sort it out at this point. You seem to have tried most of the other troubleshooting techniques with no love. You can narrow down where things go bad, then concentrate on that part of the circuit. Start right at the hot side of the input jack, then move to the grid of the first stage, then the plates of the first stage, to the far side of the first coupling cap, then the entrance of the TS, etc. Work your way through and see where the trouble starts. I like the audio probe myself. I built one then never used it for the longest time. When I finally tried it out, I was kicking myself in the ass for not using it sooner. Built mine in an altoids box.......................

[geetarpicker]

On my original Express at one point early in it's life I sometimes had some slight oscilation issues. The fix at the time was to put a small piece of foam under the edge of the power supply board to keep the OT primary wires down against the chassis. These days however the amp has settled down and the foam is gone but the wires stay put. That said, in your amp it appears your primary wires are quite a bit longer than on my original, and route back towards the rear panel then up towards the board and OT. Also you have your high voltage runs terminating under the power supply board. I'd try rerouting your B+ leads like on an original which are elevated over the board and tie to the tops of the terminals at the group of power resistors. Then I'd reroute your OT primarys to be shorter, with a slight loop or two and running more direct. On my original the OT primarys run under the PS board about have the way up, and the OT secondaries come out of the transformer towards the rear of the chassis. Perhaps you have some instabilities due to non original type lead dress in these areas mentioned. Like I said, my original was tempermental specifically in the lead dress of the OT.

[dorrisant]

+ 1 on the audio probe, Fishy

Maybe check your coupling caps... I had an input cap on an express preamp that gave me fits... Jumped around it and, there it was... Beautiful sound!

In all reallity, it took longer to build an audio probe than to diagnose this actual problem... But man is it worth it! I use it from time to time and it has pinpointed the problem everytime. Very valuable test equipment.

Tony