Matchless Chieftain

[Pdavis68]

I posted this over on diyAudio and Enzo suggested that I might get more information here. (I actually hadn't found this site before, somehow. Glad to be here. Clearly another excellent source of information.).

I was watching a demo of the Matchless Chieftain on YouTube and I really liked the sound. I can't quite describe it, but it just sounds powerful. I pulled up the schematic for it and I noticed it's unusual in a few ways.

The first is that the preamp and reverb driver are 12AX7s with both sides of the tube wired in parallel (share plate resistors, cathode resistors and bypass caps, when used). Anyone know why that is? (Enzo, in his message to me, suggested that it can average out noise and it affects the impedance, but didn't really elaborate)

Another thing that's unusual is that the tone stack is split between the first and second stage. The bass and mid tone controls follow the first stage and the treble (and volume) follow the second stage.

I'm just curious about it from a design perspective. Can anyone kind of give me an overview of the whys of the paralleled tubes and the split tone stack?

[MarkusBass]

Two triodes wired in parallel cause the noise to be lower (twice). This is a circuit copied from hifi amps. With guitar amps noise is also critical so they tried parallel triodes. This also causes that the current can be twice higher and the output impedance is twice lower (this is important with reverb driver).
Regarding the tone controls I'm not sure. Most probably they wanted to design an amp, which is not a copy of Fender - just to have different amp from other manufacturers.

Mark

[Enzo]

It was suggested elsewhere that splitting the controls into more than one place prevented them from interacting. The typical Fender stack, all the controls interact substantially. So I guess it means you can adjust the treble without having it upset your mids and bass tone.

I hadn't thought of that.

[Chuck H]

Another couple of slightly unusual design aspects (and one goofy one) would be that this design runs a pair of EL34's in class A. Other than some 'other than Matchless' cloney, boutiquey wannabe's I don't think that's been done with a guitar amp since the original AC30 (The very first one. Not the famous one with 4Xel84's). And then there's the branched power supply to reduce shared HV rail resistance. I did this with a custom recently. I had seen some Matchless schems before and took the tip. It worked out well. The only thing I would call out as goofy is the effects loop. High output voltage and higher than desirable impedance. I suspect they put it there because it was a contemporary feature at the time, but they didn't want to compromise the intended circuit. If you don't use it, it doesn't matter.

As to the separate tome controls... I can get behind what Enzo and Mark are saying. Both real possibilities. I have a design in pencil right now that does something similar and the reason "I" did it was to avoid having a stage devoted to equalization. Since stages dedicated to overdrive typically need some padding it might be possible to have your cake and eat it too. That is, if tone control losses are reduced then some clipping can still be achieved. If you cascade stages then you may be able to reduce the total number by avoiding a stage so lossy that there is no clipping. Since Matchless placed the bass earlier and the treble control later in the circuit, which would help keep the early stages generating higher harmonics and keep the tone defined, I suspect this was the goal.

[J M Fahey]

Most Guitar amps are based on Fender, including Marshall and Vox; this Matchless looks unusual because it's based on a Gibson, in fact it's basically a knockoff of one of them whose model name today I don't remember by heart.

Rather than going through 200 schematics to find it, here's one which illustrated the basic Gibson ideas:



Some typical Gibson traits:
1) a midrange killing T net .
In this Gibson it's fixed; in the model I mention it has a series pot between it and ground, to make it adjustable.

2) a Bass cut only control, in both a very small cap in series with a resistor to ground (which may be a volume pot) bypassed by a pot which controls how much it cuts.

3) this one is easy: a treble cut only cap to ground, with a series pot to control how much.

Then, how can such crude excuses for a "tone control" exist in such a good sounding amp?

Because Matchless starts on the right foot: a guitar tube amp is a *speaker driver* which sets the main sound; if properly chosen it has lots of punchy bass and biting treble even run flat, so much so that cutting the excess is fine.
FWIW Matchless always used killer speaker combinations.

The amp head by itself , plugged into a generic run of the mill speaker, sounds bland, lots of amp home makers here felt disappointed and asked me about it (there was a rash of cloners after a couple local guitar gods started using them) .
My answer was: "that's the way it sounds, plug it at least into a Marshall 4x12" and you'll see"

Here's a free sample: a classic Argentine folkloric Gaucho song, played the modern way, Strat into a Matchless combo:

Similar to early AC30 (pre Top Boost) versions: tone control was only a crude Cut one, plus it had massive fixed Bass cut , but amp was incredibly clear and jangly, and "the loudest 30W ever seen" because it drove killer Celestion Blue speakers.

Matchless combined both design ideas into their own.

[Mick Bailey]

These amps can be pretty hard on the output tubes - I've had them approaching 110% at idle. They do sound good, though.

[J M Fahey]

Yes, they run so hot that the plastic front panel bends and distorts .
And Electrolytic caps live a short life, I replace a few regularly,even 105C rated ones don't live long.

Yet I try to install low speed fans to at least move some air around and Musicians in general don't want that.

[Chuck H]

Gerald Weber, speaking about the VOX AC100 says something like 'Back in my day these amps were on fire. As in "Hey! My amp is on fire! Someone get me an extinguisher quickly!'

Always bad to run the tubes in class A hanging in a combo. Unless you're an arsonist. Yes, they are loud. AC30's are loud too. The right combination of how the amp is tuned, the cabinet and speaker peaks (and they are efficient speakers too) gives them a lot of cut in the mix. The funny thing is that listeners and real players don't complain about a lack of bottom end. Which is what you really need watts for. They're amps for musicians that know where a guitar is supposed to be in a mix. Bedroom wankers are often disappointed when they clone them. Making the cloned amp problem worse is that homebrew projects rarely include the cursory Celestion Blue speaker/s.

[Dave H]

Then, how can such crude excuses for a "tone control" exist in such a good sounding amp?

Because Matchless starts on the right foot: a guitar tube amp is a *speaker driver* which sets the main sound; if properly chosen it has lots of punchy bass and biting treble even run flat, so much so that cutting the excess is fine.

Also, unlike that Gibson Matchless and Vox don't have power amp negative feedback which boosts the bass and treble when driving a speaker load so they only need to cut the excess (as you said). Add output stage feedback and it flattens the frequency response and fancy tone controls become necessary.

The pre-Top Boost AC30 was the first Vox I played, It's pure magic and did I say it was LOUD! It was too much for the small pubs I played so I used a 2 x EL84 version of it with one 12" speaker which was perfect.

[cyclone]

I have one of the first 40 Superchief 120's. It is fixed bias class A supposedly, 540 volts on plates of 4x el34's. Stock emission was 54 ma a tube with silicon rectifier, slightly less than that with it's "dual rectifier" twin coke bottle 5u4's Run ridiculously, stupidly hot. Great sounding amp.

I concur on the separate tone controls being less interactive. Plus, stripping the bass out first is a good idea. The bridged t mid control is very nice and you can move it around in a circuit as its standalone. Lately I've been using the slightly more sophisticated "new for 1971" vampower bridged twin t middle which has a tighter slope, tune it to around 770 hz. Thats top secret. Since the NSA is turned off, I figure it's safe to let that one out

The matchless amps are players amps. Described as bright, that's not true. They are brutal and unforgiving like your mother's criticism. You cannot suck and hide behind marshall bugzapper distortion. Everything that happens on those strings can be heard at the back of the hall.

[g1]

fixed bias class A supposedly, 540 volts on plates of 4x el34's. Stock emission was 54 ma a tube

Whoa! 29W each, is that what you still go with?

[Jazz P Bass]

Class AA?

To add: it is a cathode biased output section.

[Chuck H]

So, at least the cathode voltage can be subtracted from the gross plate voltage. So, 510ish volts plate to cathode? Now less than 29W total per tube and the screens will be dissipating some of that. I'd guess that on the bench the actual bias is a proper class A. Still, what I'd like to know is what current production EL34's can be run like that without a light show?! I honestly don't think I'd use an amp that I had to feed a quad of NOS EL34's once or twice a year.

[Jazz P Bass]

True Class A is a tricky one.

Load line, tube dissipation, actually monitoring plate current to see that it never shuts off.

Generally, on a true Class A setup you will see a much lower plate voltage & a much smaller cathode resistor.

Resulting in a much lower output wattage.

[Chuck H]

True Class A is a tricky one.

Load line, tube dissipation, actually monitoring plate current to see that it never shuts off.

Generally, on a true Class A setup you will see a much lower plate voltage & a much smaller cathode resistor.

Resulting in a much lower output wattage.

I only meant "proper class A" in the gross sense that the tubes are idling at about 100% dissipation and not actually over that amount. And yes, that's not all there is too it and you would typically see Class A done with a lower voltage and a higher current. But look at all the later era Champs that pull it off with the 6V6 running at 420V or more! The caveat of the lower plate voltage is the reason I brought new production tubes into question.