Input shielding: Trainwreck Express

[Synaptic Amps]

In the following schematic of a Trainwreck Express the shielding on the input lead is connected to the plate. The grid-stopper is notably absent. Perhaps some sort of capacitively coupled negative feedback? I'm curious if anyone has any insight or comment on this input arrangement.

[killer blues]

no express I've ever seen had this arrangement. The shield lead should be connected to the ground lug of the jack. Also, some of those preamp values are incorrect.

[Synaptic Amps]

I thought twice at the time I posted whether or not I should label this schematic as a trainwreck or just present it as a generic amp, I should have followed my gut. I don't want to envoke yet another debate about what constitutes this amp or that. I'm explicitly looking for insight on the input circuitry shown above from anyone that has familiarity with it.

[Enzo]

Why it might matter is if no Trainwreck has that arrangement then perhaps the thing was misdrawn and that would mean there was no reason for it at all, since it was not the way the amp was wired.

Where did the drawing come from? Is it a TRainwreck drawing? Or is it home drawn from something? I am not familiar enough with the Trainwreck line to do more than ask. But circuits are circuits. Only rationale I can make up is the capacitance acts on the circuit.. Myself, if I wanted something like that, I'd just mount a small cap at the socket between the pins and use plain old grounded shield for the run top the jack.

I've seen small caps plate to grid - local NFB - but I have never seen THAT.

[Sweetfinger]

I've seen that on Marshalls.

[Alex/Tubewonder]

That piece of shielded wire has a capacitance of up to perhaps 15pF. Now think what 15pF connected directly between grid and plate does to frequency response. Hint: Miller effect.
It's not good practice though. Ground the shield and use a small trim cap between grid and plate instead if you want to achieve same thing.

[killer blues]

I thought twice at the time I posted whether or not I should label this schematic as a trainwreck or just present it as a generic amp, I should have followed my gut. I don't want to envoke yet another debate about what constitutes this amp or that. I'm explicitly looking for insight on the input circuitry shown above from anyone that has familiarity with it.

there is no debate. I've seen the insides of a few express amps, and some of those values are not accurate. As for the connection to the plate, there are some rockets that had this arrangement, probably to curb high frequency response, as the rocket is a very bright amp.

[Synaptic Amps]

I've only had the opportunity to view photographs of TWs. Of the few good internal shots I've seen the input sheilding has been grounded at the input jack consistent with the norm. However, the absence of an input stage grid-stopper is clearly shown.

The schematic above was one of a collection of 13 compiled by Steve Ahola most of which were individual builder interpretations. The example shown was noted as being "the most accurate" based on a drawing from an individual that had "seen" the inside of an Express. It was the only 1 of the 13 that showed this input sheild wiring. As such I would certainly not be one to defend the accuracy of the schematic, especially regarding component values. However, this input wiring would be a strange element for a person to introduce out of the blue without some justification. When amplifiers are built one-at-a-time with the objective of maximizing the tone from each individual build I believe it fair to expect wide ranging variability between pieces.

Enzo, I consider it noteworthy that you've NEVER seen this style of input wiring. You've no doubt seen more than your fair share of amplifiers. Regardless, I appreciate your suggested alternative of a plate-grid NFB cap (Alex, thank you as well). I will experiment with this. Could a cap so positioned be considered an alternative to a grid resistor insofar as it accomplishes the same goal of limiting high frequency response?

Killer Blues and Sweetfinger, I'd like to hear more about these Marshall and Ampeg input wirings. In the cases where you've seen it were there grid-stoppers or no? Do you consider these examples you've seen to be stock circuits or modifications? If you had any schematics showing this style of input I'd love to see them.

[killer blues]

SA,

The lack of a grid stopper increases the touch sensitivity, and the control of the amp's dynamics with the guitar volume knobs. This amp goes from clean to scream with a roll of your pinky. The preamp values are as follows:

V1a plate-100K>cathode 1.5K/25uF>tone stack (.022uF,.022uF,500pF)>vol. control>V1B plate 100K>cathode 1.5K/25uF>decoupling cap(.0022)>150Kto ground (high pass filter) V2b plate 100K> cathode 10K(no cap)

switch is single pole on-on-on with a 500pf on the bottom and a 100pf on the top. Mid pot is wired as a varistor, and the treble and bass pots are wired like a BF fender with no mid control. The pf caps are silver mica, and the .022's are polyester film/foil.

[Trace]

I've seen that on Marshalls.

I've seem this on several older Marshalls but the shield was not connected to the plate but rather the cathode.


For what it's worth
Trace

[Enzo]

Hey SA, I've only seen what comes my way. I have never seen a Bogner for example either. If someone has done it, I'd want to argue with the designer until he convinced me otherwise.

[Bruce / Mission Amps]

In days gone by, I use to talk to Kenny every once in a while.
He told me he had tried every trick in the book when it came to keeping his amps from oscillating. I asked about this co-axial B+ shielding arrangement seen in some old Marshall amps and he said it does offer a low zed path for highs to "ground" but was not something he felt comfortable with so I'm betting if someone has, or had, a real T-Wreck with the shield connected to a plate, or the B+ rail in some way, it was a fluke or one out the very few that are out there.

[MattT]

FWIW, Amp Garage has plenty of pictures of real 'wrecks and accurate schematics. I recently built an Express type amp and used the resources there a lot.

FWIW, I never saw any that had the shield connected to the plate on the input tube. But the lack of a grid stopper is correct. Many, many people at the Amp Garage add a grid stopper there though. It's not so important that you use the exact same values as Francesca (or whatever 'Wreck you choose to clone...they weren't all exactly the same anyway)...but rather that the amp sounds good and works reliably. Most of (OK...every single one) the clones I've heard are too harsh, bright, and aggressive (including mine). Even the KF Memorial Amp was harsh, bright, and aggressive.

[Synaptic Amps]

He told me he had tried every trick in the book when it came to keeping his amps from oscillating.

What are some of the other tricks that might be worthwhile trying for the general purposes of controlling oscillations and smoothing out harshness?

[MattT]

Synaptic,
One thing I have heard from some people that have cloned them is that the bottom cover for the chassis is important. Some clones are unstable without it but stable with it.