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[DaveWell]

I do my best, tell me what you think :)

Pros and cons ...

Thanks rooster for the Mallory's
Dale resistor 1/2w
Sprague Premium
Polystyeren film 500pF Cap
A ceramic .0022pF parallel the .0022 Mallory
Silver mica 47pF
Metal Film 9.1K
Uf4007 diode
SC gold 10uF 160v bias cap
Piher 25k trim pot
1K 25w with heat sink
Silver plated teflon wire

I use 3% silver 97% Tin

Dave

[Bob S]

Nice looking boards Dave.
Good luck with the build. Great start.

[RJ Guitars]

Nice work Dave - you'll thank yourself later for taking some time and doing a neat job. Looks like you have the vintage coupling caps as well... should sound quite nice!

[rooster]

Beautiful!

[RJ Guitars]

Pay attention to that metal jacket power resistor, they don't meet their 25 watt rating if they don't heat sink to something. The Express can work them pretty hard and if you notice it getting hot you'll need some metal under it or have to swap it for one of those big wirewound cement resistors.

[DaveWell]

Doing the maths, say you have approx 10mA being drawn by the Pre Amp Valves and 60mA being drawn by the screen grids when playing full blast (Usually 50mA I believe) then.....

.07A * 1000Ohms = 70V
70V * .07A = 4.9W

Without a heatsink that resistor is rated at 9Watts so I don't see an issue with using that resistor.

[Vince]

"I use 3% silver 97% lead "

Ekhm... 97% LEAD? Are you sure?

[Littlewyan]

Whilst pushing a signal through my Express I measured the voltage drop across that 1K resistor and it is 100V.

100V / 1000 = 0.1A

0.1A X 100V = 10W

I used a resistor like that to begin with but it got way too hot to touch. I now use the standard 25W Cement Resistor and even that is a bit too hot to touch after playing for a bit. I would highly recommend either attaching some metal to that resistor or replacing it with a higher wattage resistor.

Apart from that it looks dead smart mate. I bet it'll be a nice quiet amp with those metal film resistors.

[DaveWell]

Vince,
My mistake... 3% silver 97 Tin...not lead..:)

Littlewyan,

Thank's for the right calcul :)
Ok, I will find a way to attaching some metal.

And I Will look for a bigger resistor if metal attachment don't do the trick.

Thank's

[Littlewyan]

Thinking about it you'll be better off using a bigger resistor. Could you fit the 25W Cement resistor on that board?

[Vince]

What about 25W resistor this type:
http://www.amazon.com/Amico-Mounted-Alu ... B0087ZD0L6
I like this resistors and frequently use in my projects.

[Littlewyan]

Thats exactly the same as what hes using.

[Vince]

My fault
But this thing not bolted to chassis has only 10W

[DaveWell]

So, the resistor (25K 1W) I choose may cause heat problem.

Littlewyan propose to replace it with a traditional Cement resistor , but , after taking the measurement, a cement resistor is a bit too big and didn't look the way I want for my board.

So I go with the second option; Adding some metal to the resistor, and this is what i did.

1- I take off a heat sink from an old PC graphic card
2- Cut the piece i need and sand it just the right size
3- Glue the the heat sink to the board with high heat car epoxy glue
4- Clean both surface with Acetone and add some of PC thermal paste
5- Bolt it back together on the board.

Voila !

This will help to dissipate the resistor heat and keep temperature lower.

Please comment !

[brewdude]

Forgive me if you've already discussed it, but why not simply mount the resistor to the chassis?