Train wreck Master Volumes?

[Justin Thomas]

Hey All,

I had a question regarding Trainwreck Types 3 & 4, and examples of which amps use them. My reasoning is because I don't really like looking at a parts diagram to figure out the schematic representation, because if the layout is different, then obviously it's confusing... And since I mainly build true PTP, I prefer a schem.

From what I can gather, Type 3 is the "Crossline" MV, used by Matchless, correct? And is Type 4 the mid-70s Fender, except Fender put it right before the PI instead of "directly after the tone stack? I'm focusing on these 2 because I'm not quite at using dual pots yet, and both of these use single pots. I am very happy with the MV in my Spitfire clone, and I'm not building uber-gain monsters, so the Fender-type is okay with me. I'm only using MVs for small-scale volume reduction, not "stack-in-a-bedroom."

So, for brevity: Type 3 = Spitfire, Type 4 = 70s Fender?

Thanks all!

Justin

[Gingertube]

I'm assuming you are discussing the Master Volumes at teh end of this document.
http://www.blueguitar.org/new/schem/...reck_pages.pdf

Types 1 and 2 are the dual gang Post Phase Inverter style (which I prefer), the type 1 has the additional DC blocking caps required by a fixed biased amp, type 2 does not have these additional caps and is therefore for cathode biased amps ONLY.

Type 3 is the Post PI Cross line master - I haven't ever tried it but should be OK (check if it has the blocking cap(s) required for fixed bias).

Type 4 is the "standard" volume control hung from the treble pot wiper and I would not call it a Master Volume at all.

The "beauty" of Post PI (Post Phase Inverter) Masters is that the Phase Inverter becomes another stage which can be overdriven - The standard Differential Amp style "Schmidt Splitter" has very pleasing overdrive. You then use the Post PI Master to set the actual output volume.

Check out the circuit for an Orange Tiny Terror to see where they have taken advantage of this in an elegant minimalist design.

IMPORTANT NOTE: See on the Tiny Terror Circuit that there is no global feedback around the power amplifier. You can get strange effects with post PI masters if the power amp has a feedback loop coz the master ends up inside the feedback loop. Post PI Masters are not a good idea when feedback is used.

Hope there is something of value in this ramble.
Cheers,
Ian

[Justin Thomas]

Thanks Ian, definitely worth considering. I'm thinking #4 is like the way Fender did the MVs in stuff like the Bassman 100, where the Master Volume is basically schematically exactly like the channel Volume... not really for getting overdrive, but probably more of a "Flavor" control. There is a difference in the sound on those amps with one cranked and the other low, and then switching the settings on the knobs. Just not what many would expect.

I've been fortunate in my three builds, where the cranked sound is very much to my liking. Especially #3, by a 50W amp with only 2 knobs may not be exactly "tolerable." I never suffer a shortage of "gain," even in my minimalist designs. I have yet to build with more than 3 knobs - I'm sure that helps!

Justin

[robrob]

The negative feedback loop is only an issue at very low master volume settings. See The Trainwreck Pages here for more detail. Here are the source documents for Type 1, 2 and 3 Master Volumes:

The Master Volume Type-1 is overly complicated and I do not recommend it. The Type-1 inserts a dual 1MA pot (1 mega ohm audio or log, dual gang pot--1 shaft turns both pots together) between the phase inverter output coupling caps (C) and the power tubes' grid leak resistors (upper set of resistors). Note in the diagram below the capacitors and grid leak resistors have been separated. The pots function as simple volume controls (variable potentiometers). The 0.1uF caps attached to the master volume pot wipers are coupling caps that keep the power tube grid bias voltage out of the master volume pots. Note the grid leak resistors are not replaced.



The Master Volume Type-2 Is the most "transparent" and my recommended MV because at max volume the amp circuit is unchanged. The Type-2 simply replaces the power tubes' grid leak resistors (upper set of resistors) with a dual 100KL pot (100 kilohm linear pot, dual gang--1 shaft turns both pots together). Note in the diagram below the grid leak resistors have been removed (see the Type-1 diagram for comparison). The pots function as variable power tube grid leak resistors. The pots' left terminals are tied together and connected to the old junction of the original grid leak resistors. That junction is either tied to ground for cathode biased amps or to the bias power supply for fixed bias amps. There may be some pot scratchyness in fixed bias amps due to the bias voltage in the pot. I recommend a 100KL dual gang pot when the original grid leak resistors are 100K. For amps with 220k grid leaks like most Fenders I recommend a dual gang 220KA pot like this. Another option is placing two 1.8M resistors in parallel with a 250KA dual gang's pots. Solder the resistors across both pot's--center terminal to output terminal. This will improve the pot's taper and drop the pot's max resistance from 250k to 220k. The "Lar Mar" MV is simply a Type-2 MV using 2.2M resistors added to a 250KA dual gang pot.



The Master Volume Type-3 adds a 1MA pot that simply mixes the two mirror image audio streams from the phase inverter together. The two streams will cancel each other out. This master volume is very easy to try by simply alligator clipping in a 1MA pot. I installed this master volume in my 5F6A clone and I'm very happy with its performance.



If you are anal you can completely eliminate the Type-3 from the amp circuit by adding a switch to disconnect the circuit. Use a 1MA pot with a push-pull DPDT switch and wire one leg of the master volume through the switch so when the master volume knob is down the circuit is completely disconnected. Pull the knob up to activate the master volume. To do this you would run the wire from the pot wiper terminal to the upper left DPDT switch terminal, then run a wire from the middle left DPDT switch terminal to the circuit board's right grid leak resistor. The wire from the #1 (left) pot terminal would be wired as normal to the left grid leak resistor (see layout below).

[Justin Thomas]

I'm not anal so much as lazy... only one-third of my builds have even had a Master Volume! In my ideal world, I'm not too loud - your room is too small!

I was still wondering, the Type 4 can pretty much go anywhere, right? I don't see much difference between the Volume on a Bassman 100 and the Master Volume on said amp, schematically... Some old PAs had a volume in front of the first tube stage... I guess in theory I could use one ahead of every stage except the power tubes... Hmmm. An amp with nothing but Volume knobs...

Justin

[robrob]

Yep, just replace each stage's 1m grid leak with a 1MA volume pot. Run them at max and the circuit is unchanged but you have the option to control exactly how much signal hits every grid.

[g1]

Anybody have schematics for the 4 types?
Layouts may help when you're working on it, but don't do much for discussion or educational purposes.
That's not directed at you robrob, I realize you were just posting the trainwreck stuff on your site. That being said, I would encourage you to always include a schematic with any layouts on your site.

[Justin Thomas]

I think the Orange TT is a type 1, Type 3 is a Spitfire, and I think Type 4 is the Fender Bassman 100 style... Type 2 seems to be for cathode biased amps only, but what the heck, a couple extra caps to block DC wouldn't affect a cathode biased amp, anyway, right?

My laptop is toast, so I can't gather these various schema (oh, hey - THAT'S the plural and not "schems"!) into one thread. And I do prefer to have a whole schem to look at, instead of just pulling out the "relevant" portions...

Justin

[pdf64]

Regarding the tone stack to master vol relationship, the type 4 seems more akin to the modded Marshall approach, eg which got incorporated into the JCM800 2203 etc http://bmamps.com/Schematics/marshal..._100w_2203.pdf rather than the SF Fender approach.
The latter doesn't tend to facilitate much pre-amp overdrive, due to the attenuation of the tone stack after the first stage.
Two stages in cascade aren't enough.

The Marshall JCM 800 4210 looks to use a type 1 http://bmamps.com/Schematics/marshal...Combo_4210.gif

I wonder why Ken Fisher specified a linear taper for the type 2 but audio for the others? It doesn't make any sense to me for different tapers to be used.

[Justin Thomas]

Would not facilitating preamp overdrive, and the "only" two gain stages be why the advertised "any desired degree of distortion" just doesn't work in the Fenders? I mean, they distort, but it's only grunge distortion, great for punk, that's about it...

Thanks for the tip on the JCM800, now I know what to check out!

Justin

[pdf64]

Well, the Fender thing does provide any degree of distortion, in that the signal can go from clean to distorted; but not with enough gain for a good degree of overdrive, eg sufficient to get any useful sustain!

[Justin Thomas]

Sounds they are arguing a technicality, but nobody bought the argument anyway, cuz it still wasn't what the people wanted!

Looking at the 2203, schematically, it looks just like the Fenders - the difference being the actual location in the rest of the circuit. You could stick a Type 4 anywhere where there is only one signal to be attenuated, and it'd "work." Not that it would meet any expectations, but that's okay... I like wacky and am willing to try it. I agree that it's not a "Master Volume" in the normal way we understand it...

Theoretically, could I not stick a type 4 before the first gain stage (right after the input), after it, and another after a second gain stage? Assuming only two gain stages you would be able to stick 3 in? Anywhere before the PI... I ask not expecting a result, but that a Type 4 is stupid-simple and easy to implement almost anywhere...

Justin

[robrob]

Type 2 seems to be for cathode biased amps only, but what the heck, a couple extra caps to block DC wouldn't affect a cathode biased amp, anyway, right?

The Type-2 will work with fixed bias amps but you may get some pot scratchiness when you adjust the pot. You can't add coupling caps to the Type-2 because the DC bias must travel through the circuit to the grids.

[Justin Thomas]

So I'll just stick a type 1 in the cathode biased amp. Big enough cap makes no dif, right?

Justin

[robrob]

A Type-2 is definitely the best solution for a cathode biased amp and it's simpler to implement (you have to separate the grid leak resistors and coupling caps on the circuit board with a Type-1).

If you do go with a Type-1 then yes, a larger cap won't hurt anything.