third stage in a tw

[Andy Le Blanc]

I'm curious as to how the third stage in a train wreck is percieved as a design element....although new to the forum I have been a tube and guitar amp freak/ builder for a couple decades and have hand built numerous rigs....this stage in a TW looks for all the world like a low gain buffer....with a lot of degenrative feedback.....fed by a simple RC net work......I am very familear with marshall and highwatt and fender......is it the simple fact that TW captures the simplest and or optimal design for the player? I live in an area that has not afforded me the opportunity to try one. I'll most likely convert an old project amp to a TW configuration to check it out....the schematics and layouts in the forum look like just another tweaked fender....fill me in..... please......

[UR12]

I'm curious as to how the third stage in a train wreck is percieved as a design element....although new to the forum I have been a tube and guitar amp freak/ builder for a couple decades and have hand built numerous rigs....this stage in a TW looks for all the world like a low gain buffer....with a lot of degenrative feedback.....fed by a simple RC net work......I am very familear with marshall and highwatt and fender......is it the simple fact that TW captures the simplest and or optimal design for the player? I live in an area that has not afforded me the opportunity to try one. I'll most likely convert an old project amp to a TW configuration to check it out....the schematics and layouts in the forum look like just another tweaked fender....fill me in..... please......

Ken Fisher created a publication a long time ago called The Trainwreck Pages. In it he explains various Mods he does to Marshall and Fender amps. You can see first hand where a lot of the circuits that ended up in Kens amps came from. We get a lot of questions regarding Protection Diodes, Master volumes. Third Gain Stage, What is a MOV? All of the questions are answered by Ken himself in The Trainwreck pages. The 3rd Gain Stage is explained on pages 15 and 16.

Here is a link https://tubeamparchive.com/viewtopic.php?t=16

They are at the bottom of the page

[Andy Le Blanc]

Thank you... I hadn't chanced upon that yet....its a great resource....its nice to see a culmination of one man work....I was grateful the see the explanation of bias.... I've been thru the O-scope bit my self....it gives you some thing to look at.... its a good approach to voicing too....its easy to get side tracked tryin to voice multiple high gain stages only to end up with cheese..... I did find a question in his explination of class A....has any one done the math to find what the conduction angle of typical TW is?

[roberto]

Does anyone ever tried a different 3rd/cold stage?
Something like a soldano 100k-39k or an engl 220k-2k2?

[dave g]

I just added a 2nd "cold" stage to my 18 watt Lite. 100k plate, 6.8k cathode, no bypass, .002 coupler, output taken from the middle of a 470k/470k divider. Sounds pretty sweet!

[LeftyStrat]

I've played with using the 10k/no bypass cap stage in other amps. I tried it in the last gain stage in a JTM45. It tends to increase the perceived sustain of the amp. However, you seem to loose some Marshall character, I think the standard 820 value there lends some of the Marshall crunch to the sound.

Here are some scope shots of a wreck:

http://home.polstra.com/amps/wreck1/scope/

Look at figure 4. You'll notice that when the third stage waveform starts to flatten, it changes the duty cycle of the hard clipped output of the amp. Altering the duty cycle will change the harmonic content. A square wave only contains odd harmonics, but as the duty cycle is changed it will introduce various even harmonics into the signal.

I think this stage, because of the degenerative feedback acts like a compressor to give more sustain, and then as it clips, gives animation to the harmonic content of the output waveform.

The high pass filter before it makes the effect happen at higher frequencies first. Reminds me a bit of how the Aphex Aural Exciter works. Tap off some high frequency part of the signal, distort/animate it, and then combine it back with the original signal.

This is all conjecture/opinion on my part, so take it with a grain of salt.