Express preamp - first attempt

[markmalin]

Hi folks. Haven't posted in this forum for many years - hope you are all well!

I've built lots of D-clones, and D style preamps and know basically very little about Trainwreck. I thought "what if I put together a TW Express preamp stand alone, could I get overdrive out of it?" D and Marshall preamp circuits seem to ooze overdrive and creamy distortion. So I built my first TW Express preamp and, well, it has what I'd call "overdrive", but nothing like what I had expected. With the preamp volume down low, it's nice and sparkly and you can tell the quick pickresponse. But ...with the pre-volume on 10 (master down low) and I get "drive", but not what I'd call distortion ...not like you see in the youtube videos of TW Expresses!

Here's the circuit I used. The A4. Basically I built the preamp section in a stand alone box, and added a 1M Audio pot as a master for my box. Should I be expecting more gain out of this circuit? If you do the same thing with a JCM 800 style preamp you get Van Halen overdrive/distortion.

I'd appreciate any advice. I have an extra triode stage in one of the 12AX7's and can add another gain stage, but I kind of wanted to stay close to the real preamp design.

Most humbly,
Mark

kf - 1.jpeg

[xtian]

Hi, Mark. The Express circuit doesn't have a heck of a lot of gain, but it hits the power tubes hard and early, causing power tube distortion. And that, there, is the Express in a nutshell.

[markmalin]

Ah. I guess that makes sense. That must be how it gets that "turn up the guitar volume and get overdrive, turn it down and get clean" feel to it. It seems more along the lines of the gain in an AC30's preamp stage. Maybe I'll have to do something with that spare triode.

[wpaulvogel]

This was what I attempted to explain to one guy about pure power tube distortion and he kept going on and on about somebody’s website waveforms. The Express and Liverpool both hit the power tubes so hard and sound fabulous doing it but really don’t overdrive anything else until late on the volume knob and by that time the amp is so pushed that I don’t think they sound good at that point.

[Mark]

The pre-amp is the standardish Fender pre-amp with a cold clipper. The Tweed phase invertor has quite a bit of gain. Dumble did something similar with his Hotel Hog amp, but without the cold clipper. The Dumble doesn't roll off bottom end like the Express/Liverpool amps do.

I think you'd have to built the whole amp to get some joy.

BTW, the treble and bass pots are 1M ohm, the mid is 25K. The grid leak to the cold clipper needs to be higher than 56K, it should be 150K ohms. The coupling cap prior to the 56K (in the attached circuit needs to be between 0.0022uF and 0.0047uF.) All the anode resistors are 100K without bypass caps across them.

[markmalin]

Mark, thanks for your comments. I can change out the pots and the anode resistors and remove the cap, as well as change the coupling cap before the 56K - I'll have to see what I have in stock for caps.

I've been reading several threads on the board (many of which mention you ) and decided to take this thing in a whole different direction. Especially after reading what Silvergun did (with tubenit's advice) and diverged from the straight up Trainwreck circuits. To that end, I installed a 500kB pot between the 2nd and 3rd gain stages (you suggested about 150k for that 56K resistor) and that certainly brought the thing go life. I found about 190k - 200k seems to my liking, anything above that doesn't seem to change much.

So...I'd like to try to move this thing more toward a "gain monster" preamp, while retaining as much of the "spank" and "touch sensitivity" that Ken's original design offers. I understand so much of what he did is AFTER the preamp, but for my new "TRAIN-HACK" I'd like to take this preamp and get it towards something that sounds more like the overdrive of the TW on 9 or 10.

It's interesting, this thing plugged into the front end of another amp, though the overdrive is not "van halen", it really has some audible and very interesting overtones. Most of the amps I've built are Dumble style - this is all very new to me.

I'll look up the schematics you've drawn up. Again, thanks for the input...

Humbly
Mark

[jarrodthebobo]

I did the exact same thing you did here with my own little build; I got gobs of gain offa it. I did however, add an additional preamp valve in order to simulate PI gain and add a cathode follower for a better output impedance match.
It's since been converted into something very different however, since I can never stop tweaking. https://www.youtube.com/watch?v=_SasstOKS8c&t=253s

[markmalin]

Wow, that's exactly the kind of result I'm looking for. So that youtube video, did that have the extra valve? Or is that video of just the Express preamp (kind of like I'm doing) but with some tweaks? (like Mark A's suggestions?)

Mark

Actually - if you had a schematic to share that would be great. I'd like to keep it to 2 tubes due to the size, but am open...

[romberg]

I've found that a PPIVMV works reasonably well in an express. No it does not exactly sound the same. But to me it sounds close. And it still does the clean to mean thing.

Of course in this case it is the PI that is being overdriven and not the power tubes. The PI starts to clip shortly after the power tubes. So, you need to increase the volume just a bit.

Now, from time to time I've thought about building a pre-amp only express. I think the key would be to include a full phase inverter as part of the pre-amp. This would pretty much produce the same effect as the PPIVMV. Only problem is that the signal from each half of the phase inverter would need to be combined back together like they are in the power amp,.

I think this could be done with an interstage transformer with a center tapped primary. I think this would re-combine the PI signal in a way similar to what the power amp does in a full blown amp, However, I've been thus far unable to locate an off the shelf interstage transformer with a suitable primary impedance. I suppose one could get one custom made. But that might be expensive just for doing an experiment.

Mike

[jarrodthebobo]

Wow, that's exactly the kind of result I'm looking for. So that youtube video, did that have the extra valve? Or is that video of just the Express preamp (kind of like I'm doing) but with some tweaks? (like Mark A's suggestions?)

Mark

Actually - if you had a schematic to share that would be great. I'd like to keep it to 2 tubes due to the size, but am open...

Sorry for not replying till now; must have missed this.

In the video posted, the preamp only has the two tubes; 3 triodes for the wreck and 1 as a fake PI on the end. Its just a triode stage biased at 2.2k I think, but any value will work realistically; tweak to taste. All you really want with that stage is to reintroduce some distortion the missing real PI would add, along with a bit extra to make up for the lack of PA distortion.

Again; it sounded great! However, the tweaker/perfectionist in me just kept wanting more... But I've since stopped tweaking it as I'm very satisfied with how it sounds currently with the 6SN7 PA