Looking for info on vintage Celestion 12" 25M speakers. (Marshall Master Lead)

[armstrom]

I'm going to be doing some work on an amp for a friend but I wanted to get some info up-front. The amp is an old Marshall SuperLead combo (Solid state) http://www.schematicheaven.com/marshallamps/jmp_masterlead__30w_2199.pdf

The speakers are 12" celestion greenbacks with no markings aside from the date code (at least that's what I'm told, I haven't had a chance to look at them personally).  The schematic seems to show two 4Ohm speakers in parallel however 2Ohms seems to be pretty low for an old SS amp like this. If the impedance isn't marked on the speaker is there an easy way to determine what it is? I know I can measure the DC resistance but is there a range the resistance will fall in to based on the impedance? For example, if I get 10Ohms for the DC resistance is that pretty much guaranteed to be an 8Ohm speaker or is it possible for it to be a 4Ohm?

The amp is dated from the late 60s while the speakers are from the early 70's according to the date code.
-Matt

[teemuk]

The DC resistance is typically few ohms below the nominal impedance rating. For example, 6-7 ohms for an 8-ohm speaker, about 3 ohms for a 4-ohm speaker, and about 13-15 ohms for a 16-ohm speaker.

A real-life example: Celestion G12M greenback specifies a DC resistance of 6.7 ohms for the 8-ohm version and 13.1 ohms for the 16–ohm version.

[armstrom]

Ok... so I should be able to find out.. Do you now if there were any 4 ohm greenbacks made back in the early 70s? So far I've only found references to 8 ohm and 16 ohm..

How about the amp? It seems to be one of Marshall's earliest SS amps but has almost NO collector value (my friend bought it for the speakers, not so much the amp). He's considering having me replace all the electronics with a hybrid setup (Tube preamp and LM3886 power amp). Would this be tragic for an old SS amp in fine working order?
-Matt

[armstrom]

Also, does anyone know what the "Presence" control does on these amps? Unfortunately the schematic doesn't label what each pot in the drawing corresponds to  on the control panel, but I've narrowed down the "Presence" knob to VR6 down in the power amp section of the schematic. I can't seem to figure out what that pot is trying to do to the signal though. does it simply change the gain of the power amp section?

For reference VR1 is the bright gain control, VR2 is the normal gain control, VR 3-5 obviously comprise the tone stack and VR7 is the master volume control.

One more question... What about the "Bright" channel makes it bright? Is it just a different filter in the feedback loop?

[teemuk]

Maybe, maybe not... Many people converted those things to tube amps or simply threw them away. There aren't plenty of them around in original condition anymore and if you think about the fact that they actually are important pieces of Marshall's history (after all, JMP was their first solid-state line) it wouldn't be surprising if after few years these amps suddenly became a highly sought after collectors item. But, seriously, who knows? Predicting that kind of stuff is quite difficult. I guess it's just safe to keep in mind that plenty of vintage equipment that today could be worth a lot of money was thrown away/gutted/butchered with modifications etc. just because people taught it was worthless old junk. I can definitely understand if those early SS Marshall's begin to gain more value when some Marshall fanatics realize that they can be collected too – at least for the sake of historical importance. Because when it comes to collecting stuff, it's a thing that necessarily doesn't have anything to do with how those collectible amps sound.

From musical side, though, I got to admit that they never were that good of a product. When Unicord introduced the JMP line in 1973 its solid-state designs were already old-fashioned and years behind the products of some other companies. For example, in the few year period after introduction of JMP line those amps already had to compete with Acoustic Control, Sunn's transistor amps, Randall's Commander series, Kustom and Kasino amps, Roland JC-120 and GA-series plus H||H solid-states. Not to mention the gigantic wave of hybrid amps and all the expensive tube amps for the pros.  I haven't actually heard about anyone who would say that these amps sound great or phenomenal. Typical comment is that they have a decent clean tone – which is not exactly what one looks for in a Marshall.

But I got to admit I like the look of them. Those odd chassis and cabinet designs look way better than the boring, stereotypical Marshall head or combo design.

[teemuk]

The presence control:
Think of a non-inverting opamp gain stage and how you define its gain with the ratio of the resistors in the feedback loop. Now, the power amplifier circuit in this amp doesn't differ much from the said opamp circuit. Crudely, the gain is defined by the ratio of R31 divided by R36 (plus one). Capacitance of C21 is so high that it basically has no effect to AC signals on the frequency range we are interested in.
However: We must now acknowledge the presence circuit in parallel with R36 as well. Capacitance of C22 is fairly low so the capacitor basically couples only moderately high frequencies. VR6, on the other hand, controls the series resistance of this parallel path. (For simplicity's sake, let's just ignore the R35 that is some initial series resistance).
When resistance of VR6 is low the impedance of the branch at high frequencies is basically very, very low. Now, consider what this does for the ratio of resistances/impedances that define the gain of the opamp stage: The gain for high frequencies increases tremendously. Think about it: At high frequencies, the circuit mostly shunts the feedback signal to ground, which basically is equal to removing feedback, which in turn increases the gain.
Now, when VR6 is dialled to provide a higher resistance the impedance of the parallel branch becomes less significant in relation to resistance of R36. Now it doesn't affect the resistance/impedance ratio that defines the gain almost at all. You can also acknowledge that less high frequency signal is shunted to ground, so more is fed back, so the gain at higher frequencies must be lower. So, now the amount of gain is quite identical throughout the bandwidth.

This is essentially the very same high-frequency control (that someone just decided to re-title "presence") that you find from many tube amplifiers as well. It works just the same way in them too.

The bright channel: Compare the coupling cap C5 to C7. The coupling cap on the bright channel has a lot smaller capacitance than the coupling cap in normal channel so it will cut bass tremendously. Then there's the "treble bleeder" cap C9 in parallel with R16 that makes the voltage divider formed by R9, R16, R19 and R20 less effective at higher frequencies.

[armstrom]

Thanks so much for the information! I think I understand now

[thekon]

i have a lead 100 head that's a model 2098 i recently picked up pretty cheap.  it sounds like a bassman to me sort of....
decent clean and if you turn the channel volume all the way up and the master down it actually sounds like a marshall.....
but then i'm weird!  no it's not the greatest amp i've heard but it's decent considering what it is.

[Jack1962]

The orginal speaks for the amp are now longer made by Celestion all of them are 8 and 16 ohm now. IMHO , retro fit the amp , the thing when it was new was poor at best.

                                     Rock On



P.S. the 2ohm output was a common thing for older amp (trying to get more from less) most solid state amps can handle those low impedances.

[armstrom]

Thanks for the info on the speakers. I'm not sure I want to take on this project just yet. The thing is that my friend wants to retrofit a circuit that has the same controls as the original, that means two preamp channels (bright and normal) with a gain control for each. Also, a power amp section with a "presence" control. I figured around $200 in parts to do this all with a tube pre and an LM3886 power amp. I'm not willing to risk $200 of his money or mine on a project that very well could fail I need to get a few simple tube amp builds under my belt before taking on something like this.
-Matt