18W 1958 / 1973 / 1974 Original Schematic

[Thomthom098]

Hello,
Has likely already been clarified on the Forum but can't find my way about this yet: what is supposed to be the original 18W amp introduced by Marshall in 1965: I find some in the download but it mentions they are not necessarily correct vs original amps. I find the following:

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Thanks, and again, sorry if this has (likely) been clarified already

[geoff 1965]

You have to take into consideration that the early Marshall amps were built in a small shop by demand at that time and components used that were available at the time “hence” the variations that can be found.
one area that can be misleading and is on all 3 schematics is the spec of the PT, the 300-0-300 secondary HT is obviously “open circuit” voltage not “loaded” to come close to the B+ voltages shown on the schematics.i’ve lost count of the amount of posts where people have used a 290-0-290 PT “including myself” and have to use current limiters or zeners to drop the B+ to get in the ballpark.

[Thomthom098]

Thanks geoff, so you are telling me there is no real "officially recognized" 18W Marshall schematic ?

[geoff 1965]

i for one would like to know which schematic Marshall recognize as the original from the 1st 18W built!
but seeing as the 1st schematic is colin's 1966 amp that was reversed engineered by the forum i would think that is the closest.

[Daviedawg]

Colin's 18 watt was reverse engineered to provide the schematic on which all the early work on this site was based. But as mentioned above not all early models were built identically due to parts supply issues, we believe.This was the case across the Marshall range at the time.
That schematic is now accepted widely as the most correct basis to use in building replicas.

Dd

[colossal]

Colin's 18 watt was reverse engineered to provide the schematic on which all the early work on this site was based. But as mentioned above not all early models were built identically due to parts supply issues, we believe.This was the case across the Marshall range at the time.
That schematic is now accepted widely as the most correct basis to use in building replicas.

Dd

In the Classic 18W section of Downloads, Zaphod Phil notes that Colin's amp may or did have some changes, so I've always viewed that amp with caution, and not necessarily indicative of the type specimen. Of note on that amp is that the coupling cap from the Normal channel to the PI is shown as 0.022uF and from the Trem channel to the PI, 0.01uF. Usually it's the other way around. On any real 1974 18W amp, I have seen 0.01uF from the Normal channel. Also the tail resistor of the PI of Colin's amp is listed as 56k, not 47k. I would think 47k would be more credible as the "true" value, because Marshall copied and co-opted other designs. 47k is typical of the Wattkins Dominator and Vox AC15/30 PIs. But, maybe as pointed out, that amp really did get a stock 56k based on parts availability on the Marshall bench that day. The 56k tail seems to have made its way into the clone world, as seen on Ampmaker's variant. On the 18W, what is interesting to me is the change of PI the grid leaks to 470k/470k. That seems to be uniquely Marshall.

[ViperDoc]

I put a 56K on my PI tail of my latest 18 watt TMB simply because the layout called for it (obviously not an original!). Would a 47K tail resistor lower the gain?

[colossal]

Would a 47K tail resistor lower the gain?

Not really. Lowering the tail from 56k to 47k will increase the voltage swing of the PI very slightly, by a whole 0.02dB

[geoff 1965]

I also use the Marshall 1974X reissue schematic for reference as they claim it’s built to the specs of the originals,that has a 56K tail resistor and a secondary HT of 275-0-275 which makes sense as the WEM’s it was derived from had that spec which also allows the hotter biasing of 100 & 125R shown on those old schematics.

[colossal]

I also use the Marshall 1974X reissue schematic for reference as they claim it’s built to the specs of the originals,that has a 56K tail resistor and a secondary HT of 275-0-275 which makes sense as the WEM’s it was derived from had that spec which also allows the hotter biasing of 100 & 125R shown on those old schematics.

The 1974X schematic shows 91R for cathode bias, so their PT secondary has got to be much lower than the typical 18watt.com spec of 300VAC unloaded/290VAC loaded.

[geoff 1965]

Yeah the reissue has a primary of 0-230 and secondary HT of 275-0-275 colossal,i’ve switched to that spec but my wall voltage is high and I would need a variac to get into the 91/100R bias range

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[colossal]

Yeah the reissue has a primary of 0-230 and secondary HT of 275-0-275 colossal, i’ve switched to that spec but my wall voltage is high and I would need a variac to get into the 91/100R bias range

Hi Geoff,

I am very interested in this 91R bias resistor business, as shown on the reissue amps. To further the mystery, some of the older tracings show 125R. Was this again a "use what's on the bench for the order" choice? Some of the fellows (neikeel) that I've seen build 18W copies using NOS parts and Radiospares transformers use 125R. I am interested to see where you end up with your new Dagnall 275-0-275 PT. With an EZ81, this would imply around 320VDC rectified and under load. This is right around where the old Voxes would operate.

[geoff 1965]

hello colossal,
my PT's i had wound by the same company that makes the valvepower transformers,the dagnalls are what marshall use.
my wall voltage is very high at 249VAC and with a 275-0-275 HT i'm getting 358VDC at first filter cap "i like the tone/headroom of the higher 350/60 B+" and i've biased the el84's at 160R giving me 11.5W dissipation.
i'm going to experiment with the lower value biasing when i eventually get a variac,god know's what dissipation those reissue marshall's are running at with 91R!

[colossal]

hello colossal,
my PT's i had wound by the same company that makes the valvepower transformers,the dagnalls are what marshall use.
my wall voltage is very high at 249VAC and with a 275-0-275 HT i'm getting 358VDC at first filter cap "i like the tone/headroom of the higher 350/60 B+" and i've biased the el84's at 160R giving me 11.5W dissipation.
i'm going to experiment with the lower value biasing when i eventually get a variac,god know's what dissipation those reissue marshall's are running at with 91R!

Geoff,

Your mains voltage is US equivalent of 124.5VAC, so, while high, seems typical of modern voltages in places I've lived. The mains runs 123VAC where I am now. With a Mercury 18W power transformer with 301-0-301VAC (unloaded), I end up with 350VDC rectified with a Mullard EZ81, 340V on the plates (a 10V drop across the output transformer), 320VDC on the screens, and 12V on the cathodes with 150R bias resistor. The screen dropping resistor is 2k2, again, different from the reissue schematic of 1k5.

I agree. I tend to prefer the slightly higher, or at least what I'd call "normal" voltages I am seeing with the Lite IIb format. There is a nice ping/clank and clarity. I should add that my preamp and PI B+ run 309-311VDC. When the amp is variac'd down to 320V on the plates, it warms up a little more but at the expense of a little of that ping and clarity. But some of this is the preamp getting muddy with lower plate voltage as well. Using a variac is not a truly fair comparison though, because filament emission is reduced, so that lowers output.

Well, I am still hardpressed to see how those reissue 1974s are not running very hot that 91R bias resistor. I think this can make EL84s sound a bit harsh with a harder, more skidding sound to the distortion, rather than being smooth with that seamless transition in and out of overdrive.

[geoff 1965]

yeah i see the importance of a variac for repairs or accurate input but i like to keep things simple,hence the drop to 275 HT,not to replicate the original specs etc just gives me the B+ i want without adding zeners/current limiters to drop the voltage.
another point of interest you mentioned earlier is the filtering, both the reissue and colin's original have 32/16uf can cap at 1st stage,as yet i've only seen one and that was a TAD 32/16 500V.

[colossal]

yeah i see the importance of a variac for repairs or accurate input but i like to keep things simple,hence the drop to 275 HT,not to replicate the original specs etc just gives me the B+ i want without adding zeners/current limiters to drop the voltage.
another point of interest you mentioned earlier is the filtering, both the reissue and colin's original have 32/16uf can cap at 1st stage,as yet i've only seen one and that was a TAD 32/16 500V.

I guess this discussion begs the question, what really were the original voltage specs? The Lite IIb, to me, has always been a simplification as well as a refinement of the original 18W spec; that is, reduction to a single (Normal) channel, removal of the tremolo, but also improvements for increased reliability and longevity. The improvements include individual 1k screen resistors, giving the equivalent of 500R shared, instead of just the one shared 100R in the original, and the increased bias resistor to 150R. I have seen references to 1k5 to 2k2 for the screen dropping resistor. Operating the plates at higher voltages is reasonable, so long as the screen voltages are moderated and the Lite IIb spec does this. I have experimented with 100R screen resistors but of course this will change both the plate voltage and screen voltages. I would probably only use 100R only if the plate supply were 20-30V lower.

Yes, not having to mess with zeners or other measures is important to me as well. So far, I have been right on the mark with my builds using Mercury transformers. They offer two so-called 18W specs, supposedly built off Radiospares specifications. One is 316VAC unloaded, and the other 301VAC. I use the latter. Under load and even with 123VAC mains, I am getting the aforementioned voltages, so the PT is sagging to 289-290VAC or so.

[geoff 1965]

same here! i use a 2K2 dropper and 1K for the screens, annoyingly the guy who posted the pics of the reissue a while back did'nt post any voltages to give us a clue!
i mentioned the filtering because i read in the forums archives Curtiss saying that over filtering can detract the touch sensitivity,can you see using 32/16/16uf being much difference to 32/32/16uf?

[colossal]

same here! i use a 2K2 dropper and 1K for the screens, annoyingly the guy who posted the pics of the reissue a while back did'nt post any voltages to give us a clue!

I think that was Lenny/Pleximaster. He builds under his Clubman label. That amp looked to be an almost verbatim 1974 clone with a few values changed here and there. He used 100R for bias and a 1,000uF (Radiospares) bypass cap.

i mentioned the filtering because i read in the forums archives Curtiss saying that over filtering can detract the touch sensitivity,can you see using 32/16/16uf being much difference to 32/32/16uf?

I believe that's true. I personally don't care for stiff amps. Some of the early 100W Plexis, were quite underfiltered and suffer from ghost noting (1959 circuit), but for the 1974, I haven't found any lack of touch sensitivity with 32uF plates and 32uF screens. Still, I plan to try 16uF at some point to compare. I did build a 2x6V6 Plexi once from the MHuss schematic. The caps were 47-(5k/choke)-47-22-22-22. For fun I paralleled in a second 47uF on the plate supply and it killed the amp; way too stiff. The last 100W 1959 I built was to the Friedman verified '68 Van Halen spec with 50-choke-32-100-16-16 and that amp was great.

[geoff 1965]

no it was a genuine marshall reissue 1974X, i'll have to surf through the old posts to find it.

[colossal]

no it was a genuine marshall reissue 1974X, i'll have to surf through the old posts to find it.

Ah, ok. I don't remember seeing that. I thought you were talking about Pleximaster's NOS parts build.