Can Someone Please Explain.....?

[The Ballzz]

Why do most, if not all, 18 watt clones, kits, etc. NOT include a presence control in their designs, even when laid out with a standard TMB Marshall style tone stack? Is it that the vendors figure that anyone who wants a lower wattage amp wouldn't know what to do with a presence control? Or is there some technical reason? I realize that a properly functioning presence circuit is somewhat difficult to nail down, but it seems so crucial to the true Marshall experience that I simply don't get it. I mean, isn't the real goal of an 18 watt TMB amp to be able to get that same (or at least similar) sound as a 50 or 100 watt 1987/1959 and 2204/2204 amp in a lower watt package that won't get players thrown out of venues? Is it an issue of lower watt power tubes, such as EL84 & 6V6 not reacting well to presence circuits? Hell, even Friedman's "Pink Taco" (yummmm) lacks a presence feature! Curiosity is getting this cat!
Please Inform The Uninformed,
Gene

[rock_mumbles]

A "standard" 18 watt Marshall is a cathode biased amp with no NFB (negative feedback) ... without NFB an amp is "wilder"

A 45/50 or 100 watt Marshall is a fixed bias amp with NFB. NFB cleans up and controls the output section of the amp ... in doing so NFB "flattens" the tone, a presence control lets some highs go through the amp "unmolested" by the NFB circuit ... a resonance control does the same thing for the low end ...

[matt h]

(deleted)

[M Fowler]

The 18w TMB or 2061 and others are intended to have that breakup.

For something different I built an 18w JCM800 which of course has presence and I really like that amp.

[The Ballzz]

The Marshall 18watter platform does not include a global NFB loop. Most, if not all variants, utilize both PI inputs. This makes it impossible to add a global NFB loop as the "big" marshalls do.

A "presence" circuit is not part of the preamp. It is part of the power amp. Traditional implentation requires use of a gNFB loop, which is fed into the lower-PI input.

So while it's possible to build a scaled-down plexi-ish power amp that happens to use EL84's and happens to put out 18W... a "Marshall 18Watter" power amp, sorta by definition, can't (or shouldn't) include a presence circuit.

From my understanding, the TMB variants aren't trying to nail the scaled-down plexi, so much as bring plexi-preamp-flavor to the 18Watter power amp.

Looking at the November/October projects, those are essentially 18-20W projects that are scaled down plexis.

Comparing an 18W TMB to a November would illustrate the differences mentioned above.

Matt,
If I'm reading correctly, can I assume that if one were to reconfigure the PI to more closely resemble/mimic the topography of the "bigger boy" amps, all would work properly? Looking at a plethora of different versions of schematics, I see that on a 1959/1987 style amp, the 2 "channels" combine/meet before the input of the cathode follower, whereas on an 18 watter, the 2 channels remain separate until the input(s) to the phase inverter. Obviously, if the PI were reconfigured, you could not utilize the 2 channels of the 18 watt design in the same manner. I can now see why an NFB/presence circuit may not work well on the 18 watt PI configuration. Although I'm not quite getting how the "phase inverter" in either of these schematics:

https://taweber.powweb.com/store/6m18vibh_schem.jpg

https://taweber.powweb.com/store/6m18tmbh_schem.jpg

is actually "inverting" anything, as it appears to simply be a "driver" for a signal that has already had it's phase "inverted. On the "TMB" it seems to have been inverted at V3 for the TMB channel whereas on the VIB, I'm not really clear on where the phase inversion of the 2 channels takes place. On this schematic, it is more obvious where the phase inversion occurs:

https://taweber.powweb.com/store/6m45p_schem.jpg

Thanx 2 All 4 The Great Comments/Tips,
Gene

[matt h]

(deleted)

[Richie]

I might be able to shed some light on the subject. Back on the early days of the yahoo 18 watt site we had a rule that we could only talk about 18 watt amps. And Any offshoots of the 18 watter, had to keep something of the original amp.
The TMB came about because someone said they didn't use the trem. So I replaced that channel with a treble middle bass control, sort of a Marshall type plexi channel. I just called it a TMB for the controls, and it kind of stuck. And people liked it. There were also other TMB type channels and things added too. But that is where all the Lite, and Lite II,Marwatts, and others. I build TMB's a little different than the early first one posted.
Since that time, many have tweaked and posted different layouts of the TMB.

If you look at some of the later 36 watters, it has a feedback switch which is a little bit different.Not only did the switch kick in a feedback circuit, it was also like a preset presence,which you could change values on the switch to set it at a different level.

One thing though, the TMB channel had more gain and the feedback helped tame that channel. BUT if it was used on the Normal channel, it sucked the life out of it..lol, so you were able to set the feedback in or out for what ever channel you wanted to use.

Also there were are many ways to tweak or add a presence or resonance, or whatever. looking back it would have been easy to just have the presence control on a switch,so it could be used or not. Just remember, the stock 18 channel never used feedback,and one less gain stage compared to the TMB channel.

So hopefully you can see where the ideas came from. You can do about anything you want with that design.

[The Ballzz]

Yessir Matt,
I'm beginning to see, said the blind man! I do understand how the JTM45 type is not really 2 channels, but simply two different inputs to the same preamp with a separate input triode & volume control for each, which is why I put the word "channels" in quotation marks. Upon some re-reading and study, I now see how on the 18 watters, the PI is still used as a LTPI, but simply each channel feeding the different grids and how it all actually equals out to keep the 2 channels in phase in the way they drive the power tubes. I'm definitely beginning to smell what you're steppin' in! I also see how the plate resistors of the 18 are both 100K to keep both channels even in the way they drive the power section through the LTPI, as opposed to the 100K/82K commonly used to "even out" the way the two sides of the output of the LTPI in the JTM45 type of layout with only initially driving one grid from the preamp. I can additionally understand how the two different arrangements might substantially create/affect/impact some of the differences in overall response/overdrive characteristics of each respective design. Having never actually played through a stock type 18 or 20 watt Marshall, I have no real understanding of the claimed "magic" of the jumpered use of the two separate channels, but am beginning to suspect that the difference between this setup , as opposed to the JTM45-ish (2 input/2 triode/2 volume/single preamp) design may be more dramatic than I ever realized. Hmmmm........ It seems that when using only one channel, the output of the PI to the power tubes is a bit unbalanced and that when using ONLY the other channel, it is also unbalanced, but in an opposite manner. It appears that when jumpering the two channels and blending them you get to control the amount of the imbalance. Again, Hmmm.....
Thanx Again 4 Helping Inform The Uninformed,
Gene

[The Ballzz]

Richie,
I registered at 18watt.com a year or so back, but my membership seems to be in some sort of "limbo" that does not allow me much access or allow any of my attempted posts to be seen. I have been quite eager to become a part of that community but have not been able to make any kind of contact with moderators and/or the "powers that be" over there. I would love to be able to read all of the supposed fantastic info there and pick the brains of the many knowledgeable folks, such as yourself. If I try to log in, I'm told I'm not registered, but If I try to re-register, I'm told that I've already registered and logged in. All very confusing and any tips or guidance would be appreciated.
Thanks,
Gene

[Richie]

Gene, yeah it is a pain to get in there sometimes even when you are a member. I think some mods here, are also mods there too. So maybe they can help you get in.