Volume control issue / Trainwreck Express

[Guitarjo]

Hey!

Finished my express build and absolutely love it!

But there’s an issue with the volume control. It’s a CTS 450T with 10% taper. But it actually behaves like a switch on one point. Until maybe 10 o’Clock the volume increases like expected, if you turn it a little (and it’s really impossible to find something in between) further up there’s a jump from a little bit gainy to full overdrive/distortion. Is this a normal behavior in an express? I’m not able to find anything wrong in this amp and it sounds fantastic without hum and anything. It’s really quiet.

I unsoldered the pot and measured how the volume pot increases its resistance by turning it and everything seems fine and within tolerances including the taper. Which is actually lower than 10%! If I remember correctly it’s 75-80K have way up.

Anyone else have had this problem and solved it?

PS: My middle pot shows the same behavior on almost the same spot. The thing they have in common is the ground connection.

Greetings

[romberg]

On an express, the very first stage to overdrive is the power tubes. It is all gain all the time pushing those tubes until they "can't take no more captain!". When the power tubes start to overdrive, all the other stages in the amp are still dead clean. You can look at this with a scope and see it. So, the volume (and overdrive) comes on pretty quick. I'm guessing that Ken liked overdriven power tubes and made the express go straight to that .

As a result, you don't really control this amp with the volume control located on the amp. You just set it to a value of "maximum crazyness" you can deal with. For most folks this is around 12-2 oclock (depending on the guitars pickups). Then you control the amount of overdrive using the volume on your guitar. An express is usually always loud because it is usually running so much gain that the power tubes are almost always near the point of overdrive. But you can control the amount of distortion pretty well using the pot on the guitar.

Don't be afraid to turn you guitar's volume WAY down. Many folks (I was one) just keep their guitars always on 10. This does not work real well for an express. See this video made by one of the members here:

[youtube]https://www.youtube.com/watch?v=Uc3Xi6aAG80[/youtube]

[Guitarjo]

Thank you for your reply!

Actually my Express behaves exactly like described in Glen‘s video and it sounds fantastic.

But a volume pot behaving like a switch at one point doesn’t seem right to me. It’s like pushing a distortion channel switch on a „normal“ amplifier. The guitar volume doesn’t influence this behavior. But it cleans up really well with guitar volume in this area.

I totally agree with you. The express is the only amp I’ve ever played I like it with guitar volume level set to for example 1.

Best Wishes from Germany

[romberg]

Hmm... If it does come on that abruptly then that does not sound normal. Is there also a massive change in volume when the overdrive kicks in? On both of my expresses there is a small range (maybe between 0 and say 8/9 oclock where you can hear the volume ramping up to full (like in Glen's video). But it sounds like you may be going from low volume to FULL ON all at one spot on the pot?

You've already done one of the things I'd do and pull the pot and check to see that it operates in a continuous fashion with a DMM. Having had a similar problem with a pot on one of my expresses I'd try a couple of things:

1) Check the treble pot as well. Mine had a treble pot with an intermittent wiper. For the longest time I thought that it was my volume pot. All your signal goes through the treble pot. And if it is wonky like mine was that can do bad things.

2) Your volume pot may also be intermittent (resistance varied alot due to bad wiper). You might try swaping the volume and or treble pots.

3) Finally (and probably a good idea to do first) check for suspect solder joints related to the volume and treble pots. If either the pots themselves or a solder joint is loose then things can go crazy when an expresss is running especially if it is sitting ontop of a cabinet as things rattle around. It is not unheard of for even folks who consider themselves good solderers to miss a joint every now and then.

[xtian]

Typical logarithmic pots are made from several discrete segments of resistive carbon track, where each segment has a different composition or resistance, whereas a linear pot is just a single composition. I think you're hearing the transition as a sudden step. Definitely try a different pot and see if it's better.

https://en.wikipedia.org/wiki/Potentiom ... entiometer

[pdf64]

I seem to remember some of the Bourns ranges being recommended for their smooth taper.

It would be nice to be able to eliminate ultrasonic oscillation as the root cause here though.

[Guitarjo]

Thank you all!

I ordered two new potentiometers now (for treble and volume) just for the case both are faulty components or one of the ordered ones is faulty as well. Actually my supplier informed me CTS had some trouble with the tolerances of the 1Meg potentiometers. That’s why he didn’t sell them for a while. He didn’t tell me what tolerance of the potentiometers in specific ... so maybe it’s really about the discrete segments.

And yes, there is a big jump in volume as well ...

Regarding ultrasonic oscillation I’ll quickly add a 68K resistor going to the grid of V1 and maybe V2, too. This grid stopper(s) should eliminate this as a root cause. But in V2 one half of the volume control acts as a grid stopper so the issues are mixed up here. We’ll see!

Thank you for your ideas!

I’ll come back to tell you what worked or not!

Best wishes!

[Guitarjo]

Hey again!

Changed the volume control for a new one. Everything works now!

Thank you all!

[Colossal]

Hey again!

Changed the volume control for a new one. Everything works now!

Thank you all!

That's good news. What pot did you end up with?

[Guitarjo]

I ended up with a new CTS 450T - 1Meg (the ones with solid brass shaft from mojotone). I really like the taper on that ones. I selected one with 7% taper, the other one I ordered has 8% taper.

Maybe it was not a faulty potentiometer from the beginning. Possible I cooked it when I tried to solder on the back of them. I used a concentrated hot air stream (420°C) to heat it up to solder on them because one soldering iron was not enough to get this done and I don’t have a second one ...

Best wishes!

[Colossal]

Yes, advisable not to solder to the back of pots. That is bad grounding technique and it's easy to damage them. Glad you got it sorted out. Great amps...

[M Fowler]

If do solder to a pot I turn the pot to full rotation so that if the trace got wrecked it would be full resistance not minimum. Maybe that doesn't work but I do it that way.

[Guitarjo]

Thanks Mark, I’ll do it like that in future, too.

Why is this considered to be bad grounding technique? Contrary to the overall grounding in an original Express (which I have chosen as well) I thought of a (copper) bus grounding following the signal path being a good solution. It‘s possible to use local stars on the bus as well if somebody wants to ... just curious ...

[M Fowler]

well all the Express amps I've built are noisey so I later changed my grounding to what we consider better scheme.

With the buss bar soldered to the back of the pots, each pot is bolted to the chassis and thus creating ground loops.
We know the current will take the shortest path to ground but still we set our selves up to creating ground loops with the Trainwreck buss bar method.

I connect the input jack, all preamp grounds and preamp filter cap grounds near the input jack.
I connect the Phase Inverter filter cap ground and main filter caps, both filament CT and HT CT together near the PT.

We really should use Cliff jacks to isolate input jack and speaker jacks, especially in a high gain circuit like the Trainwreck Express and Liverpool.

Mark

[Guitarjo]

Thank you for enlighten me! Everything you said couldn’t have been explained better and makes perfectly sense! Applying this logic to the original grounding scheme there are many grounding loops inside an express. There is a ground loop as soon as you decide to install a third grounding spot. Hopefully you ended up with a quiet amp with your two spot grounding!

Since my amp is really quiet I’m not gonna change it. Unbelievable how different these Expresses turn out using almost the same or similar components.

Thank you!