JTM-18 EL84 Build Suggestions

[Rontone]

I've had these Elstone transformers for a while from a 2x EL84 Push Pull cathode biased amp with 3x preamp tubes and EZ81 rectifier, they test ok and in fair condition for their age, then a chassis and head cabinet that was perfect turned up recently, so they would do nicely for an 18w JTM preamp and LTP into a cathode biased EL84 power section

Mains TX is 300-0-300 100mA, 6.3V for EZ81 Rectifier, and 6.3V w/CT for heaters

Output TX is 7200 ohm : 16 ohms

Choke is ~300 ohm, I believe around 10-20H, and its big enough so I'd say definitely over 50mA





Would anybody see a problem doing this, considering the 100mA HT rating and cathode biased output stage?

I've made a .sch file schematic and a DIY Layout along with JPG and PDF for each, if anyone can see any obvious wiring or connection errors in the layout please let me know

There are values I have not settled on, such as power dropping resistor values and maybe heater elevation, cathode biasing values and pot/cap values/voltage ratings, so they may differ between schematic and layout while I refine the idea a bit

[Rontone]

Here is a Schematic JPG and a Layout PDF

[Rontone]

Some updated Schematics and Layout, a few mistakes corrected and parts added

Need to get the Presense control worked out

[tubeswell]

The B+ should drop quite a bit with cathode biasing of the EL84s (compared to fixed bias from the same HT winding) - but don't be too gentle with the cathode biasing of the EL84s - or the B+ won't drop enough. IMO you want the B+ around 300, and preferably under 320V - unless you're running old stock EL84Ms which can take higher voltages. Under these conditions, they can afford to be biased at 100% - Others may have other opinions.

[Rontone]

Slightly updated Layout File

There was an unnecessary filter cap in the middle of the board, now removed

[Rontone]

Back to this build for the new year

Just thinking about cathode resistor choice and how power tube grid leaks can interact and affect the bias [they are going to ground in this case, as the amp is cathode bias]

18W Marshalls use 470K grid leaks with 125 ohm
Vox AC10 or AC15 use 220K with 130 ohm
Watkins Mk1 Dominator has 500K with 220 ohm

They all have very similar HT and EZ81 rectifier, as will mine, but I was going to separate the EL84 cathode resistors

I have some wirewound 9W in 330 ohm, so presume they should be OK and not cool it down too much, MK1 Dominator uses a 220 ohm, which separated would be 440 ohm, and they break up really really well...

I have various odd value resistors to use up and had some 360K left over I could use for grid leaks

Attached is a more up to date schematic and layout

[Rontone]

I've just been reading about Rh-k Max values and on the EL84 it is 20K ohms maximum

So the heater elevation voltage divider is not correct on these schematics and layouts with 150K and 33K resistors, the second resistor will need lowering accordingly

The HT should be around 310V at the 2nd node [screen node]. with 150K and 20K the heaters should get about 35VDC for the elevation

[Rontone]

Its getting closer, I found all sorts of great salvage over the weekend on some old test equipment, solder tags, 4BA and 6BA nuts & bolts, some lovely McMurdo sockets, Belling Lee fuse holder, Arrow switches and loads of good wire

Some NOS stuff from joblots, secondhand transformers, chassis and plywood cabinet, only had to use a few brand new parts

[SEL49]

Scary looking transformers. 

[Rontone]

Scary looking transformers. 

  In which way? You mean the exposed power wiring on the outside, or the rough look...

They test fine voltage wise, although when it all starts flowing....  we will see

Pretty clean laminate wise, the output and choke have a little corrosion, they will be checked over by a good local tech first

[SEL49]

I mean the exposed transformer terminals. Lotta high voltage AC and DC just begging for you to poke around haphazardly, or just to hurriedly flip the chassis over while on the bench, or maybe ever reach inside the cab to change a tube.

20 years ago I built an amp with the filter caps on top of the chassis. Didn't take long to realize I needed some kind of shield. The cap board would not fit under any currently available doghouses, so I made a plexiglass shield. I felt a lot better afterwards. 

[Rontone]

Yeah its a bit of biter if you touch it the wrong way, probably why Marshall changed to the laydown w/cutout type TX in 1963  even if its not turned on, caught my finger on them turning it over and thats bad enough, the tags are sharp, I probably need a tetanus shot now

That plexiglass is a nice touch, I will look into that, Two L shaped pieces on either side could also lift the TX a bit for a venting gap underneath, I've got some paxolin sheets too

But this has reminded me I forgot to put any heat-shrink on the exposed bits, I completely missed it out...