Building again - authentic 18W this time!

[shakti]

I just got a very minty pair of January 1966 T1220s in a trade, i.e. 8 ohm 20W G12M greenbacks. These are about the earliest ceramic G12Ms you will find with a green magnet cover. The cones are exactly the same as in the alnico G12s I've had in there, as there is no doping on them. Starting sometime in the spring of '66 the edges began to have doping applied to them, which changes the character of the speakers and gives them (the doped ones) a slightly darker sound. The undoped speakers, by comparison, are a little sparklier and have some terrific touch-sensititivity to them.

I just put these G12Ms in the 18w, and I must say I think they are going to stay there. The alnicos are terrific and arguably have a better clean sound and a little more chime and sparkle, but the 18w is hardly about cleans anyway...the G12Ms just mate very, very nicely with this amp and tones down some of sizzly edginess of the EL84 breakup. Ever so slightly darker, more "compact" sounding and more classic Marshall feel IMHO, but since these cones are undoped it still retains some of the sparkle.

[Fabio65]

Shakti, very nice job indeed! Congrats on your killer amp!

I' m in a build of a TMB version of this amp and have a few doubts in respect of the power transformer. I've bought a P18RS from Mercury Magnetics which is supposed to be a perfect clone of the original Radiospares power transformer.

The iron has 316 VAC HV secondary taps whilst the rest of modern market trannies has 290, giving a plate current of 340VDC which is considered ideal for a pair of EL84 ... Mine will probably run the power tubes around 370

Mercury Magnetics told that these are the original specification of the vintage RS PT they've copied and that they prefer this configuration which lead to a clearer and more articulate sounding amp. They also said that the original schematic is plus/minus 20% on ALL voltages and that there is no consistency at all on Marshalls...

Can I ask you which is the voltage of your power tranny b+ and if basis yr experience I can run EL84 at those plate current level without frying a pair of tubes a week?

Thank you very much indeed!

Fabio
NW Italy

[neikeel]

That fits very well with my experience, as I went the other way!

I have been running my 18w 2x12 with a pair of '71 ( 3 cone) T1220s. They did sound very good but I had a pair of undoped early Alnico B024 speakers which are not really suitable for more than 1-12watts each one is a pulsonic cone the other a RIC.
Definitely crisper and less dark than the T1220 ceramics. I tend to run my 18w at 2-3 on the dial for reherasals and smallest venues so perfect for me.
The T1220s will go either in a Marshall closed back 2x12 for JTM45 duties or my Vox closed back extension cab for my 'Vox stack'

[shakti]

Aaah, the headaches of the Celestionally enabled...

[shakti]

After burn-in and some tube tweaks as well as the mentioned speaker swap, I have to say...I finally get the hype. This little thing is a touch-sensitive tone monster! The final ingredient was a pair of Mullard EL84s out of my AC15. They were actually a touch dark sounding in the AC15 where I prefer more of a crystal chime, but they sound just perfect in the 18W. Normal channel preamp tube is an I63 Mullard, but trem channel is a Brimar which has a slightly brighter character than the Mullard.

This amp *breathes*!

[shakti]

Also; sorry I missed the question of B+. I must admit I don't remember, but it was on the high side for sure. The GDS I used (Heyboer made) was actually 300-0-300 nominally IIRC, and with my wall voltage it was a touch higher than that still. I don't know how it would sound with a slightly lower B+, but typically I'd suspect it to be a touch creamier and warmer. Not really an issue with this amp, I think.

[RockinRocket]

Any chance you blueprinted this chassis? I am not sure all the clone chassis are very accurate.

[shakti]

Afraid not...the major difference though is the depth which is less than all the available chassis out there, and the lack of a flange on the lower edge. The available chassis out there is deeper than the originals, and has a flange on the rear/lower edge, presumably because...
1) the deeper chassis will also fit in a standard smallbox head cab when punched for tubes on top, and
2) the extra flange reinforces the chassis, and also allows for head cab mounting

[RockinRocket]

Hmm that's what I was thinking.

When you say deeper chassis you mean distance from faceplate to tube sockets?
And the depth is still 2 1/2" (or so).

If I had something to fold the chassis.. making one of these wouldn't be all that hard to drill.

[shakti]

I mean the distance from front panel to rear edge/tube sockets.

I agree, if you have a jig to bend aluminum sheets, and punches for the holes, it shouldn't be too difficult. I know Mike Bacino of Bacino amps has blueprinted the original chassis (at least AFAIK).

[beaulieu]

I' have an original board and OT to do one of these. I have a RS PT pulled from a Teletronics oscilascope. Wonder if it is good enough or where can I find a PT? BTW NICE build

[neikeel]

B+ once rectified needs to be 290-300v.
You need two 6.3v pairs of heater taps (one with centre tap (for the pre and power valves)

[RockinRocket]

B+ once rectified needs to be 290-300v.
You need two 6.3v pairs of heater taps (one with centre tap (for the pre and power valves)

GDSs transformers put out 330-340 (I think) at modern wall voltage. Its supposedly the first reversed engineered 18w transformers.

Is your quote at 60s wall volts?

[neikeel]

Mine is what my original RS PTs produce (HT 300-0-300 ac) at UK wall voltages (235vac) using the 240v tap option on the PT, this gives B+ before first filter of 320vdc or so

I think that I mixed up what I meant HT is 290-0-290 which through the EZ81 should give 320vdc or so on the output plates which is about right for modern EL84s