500k vol pot in trainwreck express

[bagudan]

Hi

I built this, and it sounds great.

http://users.zoominternet.net/~jdhall/amps/TwreckKelly Revised.JPG

But the volume pot is extremely sensitive. It is loud immediately, no chance of dialing in low volume at all.

Was thinking about swapping the 1meg log vol pot for a 500k or even 250, but I am nervous to do it, since I am unsure if it will affect the tonestack or other part of the amp, in a bad way?
Does any of you guys, have input on that?

Thanks
-Kasper

 

[Gnobuddy]

...the volume pot is extremely sensitive...

Hi Kasper, starting with obvious and simple things just to be sure, are you sure it's a log pot? If it's a linear pot, it will have the problem you describe. A log pot should have the letter "A" on it next to the pot value.

If it's already a log pot, I don't think lowering the value to 500k will help. What you want is a different audio "taper", but these days log pots are increasingly thin on the ground, and you are unlikely to find such a thing. The "taper" tells you how slowly the pot "gets going" when you start to turn it up from zero; the log pots available today are usually 10% taper, i.e. the resistance between wiper and ground is 10% of the full end-to-end resistance, when the pot is turned half-way.

Okay then, let's say you confirm the 1M pot is indeed a log pot. All isn't lost, though - I suggest the simplest thing to try is adding a fixed resistor to the pot, a "taper resistor" which softens the pot response around zero. You solder this resistor between the volume pot wiper and ground lugs.

The smaller the added taper resistor, the more it softens the response of the pot - but a small resistor will also, as you suggested, load down the tone-stack when the volume control is turned up to maximum (but only when turned up, so it may turn out not to be an issue - experiment to find out.)

Since you have a 1M pot, I suggest starting out with a 1M taper resistor. This will soften the volume response just a tiny bit, but also load down the tone-stack only a little bit when you get to full volume (the 1M pot and 1M taper resistor will be in parallel at full volume, and present a load of 500k to the tone stack.)

If the 1M taper resistor doesn't soften the volume curve enough, try roughly halving it (470k), and if that isn't soft enough, halve it again (270k). I've never had to go lower than that myself, but again, feel free to experiment - certainly that's what Ken Fischer himself would have done!

This is quick and easy to try, particularly if you have a small junk-box stock of resistors handy. Good luck, and let me know if it works for you!


-Gnobuddy

 

[bagudan]

Thanks for the reply. Sounds like good advice, and I certainly have a junk-box of resistors

The pot is a log pot, and that is what Kevin fischer used, but I think he might have had a box of old "magic mojo" nos pots, with a different behaviour.

I will try it out

 

[Gnobuddy]

I certainly have a junk-box of resistors

There's more magic mojo in a junk-box than anywhere else on the planet!

If a taper resistor doesn't solve your problem, I have other tricks up my sleeve.

You can wire a stereo log pot (two pots on one shaft) so that one of them feeds the other - cascaded attenuation, the second pot further dividing down the signal from the first pot.

This gives you an extremely soft and progressive effective taper; where a single log pot gives you one-tenth of the signal (-20 dB) at half-rotation, a stereo log pot wired like this will give you one hundredth of the signal (-40 dB) at half rotation! You'll still get the full signal at full rotation, but the response will be very slow as you start from zero.

There are downsides, of course: you have to buy a new pot, you need a 1M stereo log pot (which might be hard to find) because the combined load drops to 500k at full volume, and there is a chance that the volume curve might end up too soft, even for a gainy Trainwreck.

But if the taper resistor doesn't do it, this is your next more extreme option.


-Gnobuddy

 

[Mark Tillotson]

The volume control and tone controls are interacting in that design. It might be better to move one of them after the next stage so they are no longer affecting each other.

 

[Gnobuddy]

The volume control and tone controls are interacting in that design.

It's the classic Fender layout, exactly as you'll find it in, say, an AB165 Fender Bassman. By using a large-value 1M pot, the hope is that it won't load the tone-stack enough to mess with its frequency response.

As I understand it, Ken Fischer basically stuck an extra gain stage into an early Fender guitar amp topology, at a time when that was still a novel idea.

It might be better to move one of them after the next stage so they are no longer affecting each other.

But if you do, the second gain stage will be heavily overdriven and distorting heavily all the time - you have no more ability to dial in clean tones, or slight distortion, or anything other than all-out mayhem.

In contemporary guitar amp parlance, that "volume" control is really a gain control. There is no real volume control in the amp (what we call a master volume nowadays), so if you want distortion, you also get ear-shattering loudness with it. Very retro, very macho, very good for getting fired from that bar gig you finally landed, very good for giving yourself tinnitus and having your ears ring loudly for the rest of your life.

A good solution to the tone-stack loading problem would be to insert a MOSFET source-follower between tone-stack and volume control - a wee little TO-92 LND150 N-channel depletion MOSFET would do the job beautifully. Distortion from a MOSFET source-follower is far below audibility, so the MOSFET would be a completely transparent addition to the amp - you wouldn't hear the MOSFET at all, but it would allow the tone control to work properly, and allow you to use a lower-value volume control, or a taper resistor, without affecting the tone control response.

But most Ken Fischer / Trainwreck fans would consider talk of MOSFETS in a tube amp - in THE holy-grail tube amp - as sacrilege, and would want to tar and feather me and ride me out of town on a rail for even daring to suggest it.


-Gnobuddy

 

[JMFahey]

The volume control and tone controls are interacting in that design. It might be better to move one of them after the next stage so they are no longer affecting each other.

Hi Mark. Fine in a Hi Fi amplifier, but this is a Guitar one.
Usually controks ARE interactive and that´s fine.

Besides, given thye ery high gain involved, moving it to a later stage means the first two blocks will clip like crazy and in an uncontrolled way.

This is a "Holy Grail" type of amplifier and is best left as is.

Here "defects" are "features" instead.