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Discussion Starter #1
I've started on a Marshall 18 watt TMB build. First amp I've ever built with EL84 power tubes. A new adventure with an endless supply of learning opportunities. I've never played one of these, but I've heard they are tone monsters and that's all the encouragement I need.
Chassis & face plates from ZH Music Parts, board from Antique Electronics, transformers & misc parts from Next Gen. Pretty sure it's going to be a single 12" combo. But it could end up being 2 x 10's. You never know......

Chassis with tube sockets and iron mounted, ready to put the turret board in place
18watt-2.jpg
 

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Discussion Starter #5
Gentlemen, a few questions if I may......... :D

18watt-3.jpg


I've never seen diodes in line before the rectifier tube before. Is the EZ81 not a full rectifier tube like say a GZ34 or 5Y3? I've looked at as many EZ81 power supply schematics as I could google, and none of them had diodes in that position. What's up with that?

The .01uf capacitor across the HV terminals of the rectifier tube? 3KV? Any idea what the purpose of that is?

The 5 watt 1.5K resistor between the sections of the filter cap. I've got 3 watt, 1.5K resistors. Would that do? Hammond says that transformer is only good for 14o mA, where is the rest of the current going to come from?

All new to me, I just want to have some sort of idea what the heck is going on and why.

*#*(
 

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The diodes are protection if the rectifier shorts. I think a few of the modern Fender amps might have them. Should the tube short, you still rectify the AC rather than allowing full AC on the caps. And no stoppage in play. ;) Having the tube in series means you still get the 'sag' effect from tube rectification. The little cap there is for noise suppression as far as I know. (diode switching noise)

140mA is plenty to create wattage at that kind of voltage. 'I squared R' gives almost 30W if it were all going through there. However, it's not going to get all that. You would have to calculate with the voltage drop in the circuit. I have a feeling 3W will manage, but 5 is probably more reliable long term.

edit: 64 Vibroverb Custom is one of the new fenders that has the diodes at the tube rectifier:
http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-64-Vibroverb-Custom-Schematic.pdf
 

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Discussion Starter #11
Here's the progress, or lack there of it. Most of the wiring around the power transformer is in place now. Those 2 diodes before the rectifier tube are in place, and the 5 watt resistor between the sections of the filter can is in place. Grounds in too.
It's kinda nice how it all fits and how Marshall kept all the AC in the same little area rather than spread it halfway down the chassis like Fender did.
18watt-6.jpg

Only thing I'm not enjoying is the tiny 9 pin tube sockets. I think if I ever build another 18W, I'm going build it with a GZ34 (or solid state rec.) and 6V6 power tubes. That's how much I dislike 9 pin sockets.
 

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Discussion Starter #19
I've built over half a dozen variations- heads, combos, high gain, EF86, octals, etc. It's a very simple platform, sounds good and is excellent for modding. Have at 'er Lincoln.
Funny you mention EF86. I have a drawing on my desk right now for an amp called a "Creme Brulee". Real simple little 2xEL84 amp with an EF86 preamp tube and a Vox style tone selector switch.
 

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I bought one of the Trinity Amps version of this build and as a tool maker with no electronics training, I was able to finish the amp with no problems. The documentation is over the top and the pictures make it as easy as can be.

 
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