Express volume and tonestack issues

[peckjed]

I'm going to do my best to describe my issue and I'll admit this is my first go-around and I'm def still learning.

The issue is when the volume when on 0 doesn't completely cut the signal out. The tone stack (while I believe is adding the various treble/middle/bass frequencies) is also adding a decent amount of volume/gain. Almost like a volume control more than frequencies.

I added a PPIMV which will completely cut the signal when on 0.

I followed this (http://site.triodestore.com/TWEXPRESSLAYOUT.pdf) layout and attached are a few photos of my build. Any pointers would be wonderful!

[RJ Guitars]

peckjed - compare your build to the layout that is posted here: https://tubeamparchive.com/viewtopic.php?t=5691 although I expect the Triode layout is OK.

From the info you provided I am suspect that you have missed a ground wire to one of your pots... most likely the volume control.

Your pictures are good - high resolution and if you really zoom around the tonestack and volume control pots you can get a good look at your solder joints and wiring. I want to encourage you so I'll begin by saying Good job in getting your amp together and making it work.

As you move into this hobby work hard to tidy up things. I suspect a bad solder joint maybe along that buss bar on the back of the pots. Additionally, there is a real art in tying the bussbar grounds altogether and not frying the pots. If you reflow all the solder joints and verify that all your grounds are there... then consider replacing that pot.

As an aside, take a look at this post. http://diyguitaramps.prophpbb.com/topic69.html. In this post I am slowly taking a Rocket build from start to finish with an alternate way of adding a grounding bussbar. Although this build is a Rocket, this idea would work well for n Express build. This gives you some options to consider for your next build or if you ever decide to rebuild the tonestack on your current build.

Bottom line -- Since you have a volume control issue, I suspect the problem is either a ground or the pot associated with that control. You'll need to tidy things up a bit and reflow some solders to start help you find it.

[M Fowler]

It looks like in photo that the input jack shielded wire could have some coax shielding touching the input signal post. Photos can lie.

It looks like your depending on the pots to be the ground source for the buss wire soldered to the back of each pot. Usually you run a wire from the buss wire to a grounding tab on the chassis as well.

Mark

[peckjed]

peckjed - compare your build to the layout that is posted here: https://tubeamparchive.com/viewtopic.php?t=5691 although I expect the Triode layout is OK.

From the info you provided I am suspect that you have missed a ground wire to one of your pots... most likely the volume control.

Your pictures are good - high resolution and if you really zoom around the tonestack and volume control pots you can get a good look at your solder joints and wiring. I want to encourage you so I'll begin by saying Good job in getting your amp together and making it work.

As you move into this hobby work hard to tidy up things. I suspect a bad solder joint maybe along that buss bar on the back of the pots. Additionally, there is a real art in tying the bussbar grounds altogether and not frying the pots. If you reflow all the solder joints and verify that all your grounds are there... then consider replacing that pot.

As an aside, take a look at this post. http://diyguitaramps.prophpbb.com/topic69.html. In this post I am slowly taking a Rocket build from start to finish with an alternate way of adding a grounding bussbar. Although this build is a Rocket, this idea would work well for n Express build. This gives you some options to consider for your next build or if you ever decide to rebuild the tonestack on your current build.

Bottom line -- Since you have a volume control issue, I suspect the problem is either a ground or the pot associated with that control. You'll need to tidy things up a bit and reflow some solders to start help you find it.

Thank you for your detailed response and encouragement! I was checking the volume pot with my multimeter and it was giving me strange results... so I swapped it out with a new one. Linear taper, not audio which is slightly different. When at 0 the volume still isn't completely OFF, so I would suspect a ground issue as well. I checked, and resoldered most areas and I still have this issue. I am possibly missing something. If the pot wasn't grounded, should it even function as a volume?

Does the ground bus need to be grounded to more than just one of the ground lugs the tone stack? Also, does the wire physically need to be wrapped around the ground bus or would the solder create enough of a connection to do so?

Could the PPIMV be impacting my current issues at all?

Also, thank you for the link to the Rocket documentation. That is my next build, and hopefully my gigging pedal platform amp.

[peckjed]

It looks like in photo that the input jack shielded wire could have some coax shielding touching the input signal post. Photos can lie.

It looks like your depending on the pots to be the ground source for the buss wire soldered to the back of each pot. Usually you run a wire from the buss wire to a grounding tab on the chassis as well.

Mark

Thank you! None of the layouts I've seen have the ground buss grounded to the chassis, unless I've missed something or it's implied and i didn't know. Would that possibly cause any ground loop issues anywhere else?

[RJ Guitars]

As you probably know, the controls on these amps are all about throwing signal away... that is the volume control doesn't add volume, it just sends some part of the signal to ground and likewise the tone controls don't add tone, they also send the part of the signal that you don't want to ground.

So my thought is that if you've got signal getting to the power amp section when it should all be going to ground. That makes me suspect that the path to ground in the volume pot has some resistance in it. In the Express amp, the signal path comes from the tonestack and goes through the volume pot on it's way to the 2nd gain stage. Looking at that schematic, when you dial the volume pot to zero resistance there should not be any signal that gets to the the 2nd gain stage and on to the power amp. Everything should go to ground... except some high frequency that bypasses the volume pot via your bright caps.

So put your meter on the center post of your volume pot and measure the resistance to ground as you cycle that pot. Then measure the resistance again with the pot at zero while you cycle the bright switch from each position and also at the center off position. I'll be curious what you read?

I'll leave it to Mark to advise on the ground loop question...

[M Fowler]

Yeah your right the Express layouts do not show the buss wire grounded to chassis I thought at one time it did, like the Rocket layout shows?

To me grounding the buss wire near the cap stack would cause a ground loop so I don't do that but do ground it near the input jack. However, I do not solder my buss wire to the back of the pots it just floats grounded on one end only.

Regardless I would just try an alligator clip wire from buss wire to chassis to see if it solves the issue or not. Otherwise make sure you used internal star washers on each pot to keep them tight to chassis as that is your only path to ground.

Mark

[peckjed]

As you probably know, the controls on these amps are all about throwing signal away... that is the volume control doesn't add volume, it just sends some part of the signal to ground and likewise the tone controls don't add tone, they also send the part of the signal that you don't want to ground.

So my thought is that if you've got signal getting to the power amp section when it should all be going to ground. That makes me suspect that the path to ground in the volume pot has some resistance in it. In the Express amp, the signal path comes from the tonestack and goes through the volume pot on it's way to the 2nd gain stage. Looking at that schematic, when you dial the volume pot to zero resistance there should not be any signal that gets to the the 2nd gain stage and on to the power amp. Everything should go to ground... except some high frequency that bypasses the volume pot via your bright caps.

So put your meter on the center post of your volume pot and measure the resistance to ground as you cycle that pot. Then measure the resistance again with the pot at zero while you cycle the bright switch from each position and also at the center off position. I'll be curious what you read?

I'll leave it to Mark to advise on the ground loop question...

Getting a read off of the volume pot wasn't exactly working and I'm not sure why. Possibly a shitty radio shack pot. Is it possible if one of the other pots in the tone stack was burnt out from my inexperience it would impact the volume pot functionality? Or could the PPIMV cause an issue?

[Littlewyan]

How much signal are you getting through when the volume is on 0? As its normally to get a tiny bit through. Should barely be audible though.

Also what do you mean by not exactly working? When the volume is on 0 and you've put one multimeter lead on the centre tab of the volume pot (wiper) and the other lead on ground (somewhere on the chassis), do you read any resistance? If you do then put the lead onto the buss bar instead of the chassis to see if you still read resistance.

[peckjed]

How much signal are you getting through when the volume is on 0? As its normally to get a tiny bit through. Should barely be audible though.

Its not horrendously loud, but it is certainly audible. Especially if the MV is all the way up.

[Littlewyan]

Is it louder than talking?

Sorry I edited my last post with another question.

Also there is a blob of solder on the chassis near the yellow NFB wire near the presence control. Better scrape it off before it gets loose and causes hell inside the amp.

[peckjed]

Is it louder than talking?

Sorry I edited my last post with another question.

Talking volume sounds about right, maybe slightly quieter. Definitely no louder than that. I'll take a look with my multimeter regarding your suggestions when I get home this evening. I've read that some bleed it OK, and I'd obviously never play at that volume, but Im semi skeptical about something not being quite right.

[Littlewyan]

It sounds pretty normal then. A lot of old amps bleed like that to an extent.

A few other things to look at are:

1. There is a blob of solder on the chassis next to the yellow NFB wire near the presence pot. Clear that up before it causes issues.

2. Put the audio pot back as the volume pot. You really need an audio pot for the volume control in amps, especially in an Express. They distort really quick and a linear pot will give you little control over that. They are beasts

3. Shorten the wire from the 500pF treble cap to the treble pot. This one is REALLY sensitive. If you look at other amps that people have built you'll probably see the preferred route for this wire is to run along the back of the pots and then up to the solder tab.

4. Keep the wire from the volume pot wiper to the valve socket as short as possible but make sure it can lay on the chassis as much as possible between the two connections. Again this is a really sensitive wire.

To help with the layout I recommend looking in the Trainwreck Files section and downloading photos of original Trainwreck amps. Look at how the wires are routed in them.

[RJ Guitars]

... could the PPIMV cause an issue?

I spose it's theoretically possible but a lot of folks have used them and you don't hear about it if it's a common issue. If it's easy to jumper around it then might be worth a check. I'll agree with Mark - use a jumper to ground the pot to your chassis and see if that changes things.

[M Fowler]

Looking at your pots again in the photos there is a lot of solder on them so it is possible you cooked those pots and they are no good.

When I solder to the pot backs I turn the pot to full rotation and use heat very fast, doesn't take much of a puddle to hold that small buss wire.

A better way is to use those buss wire pot terminals rather then solder directly to the pots.

Mark