My First Express Board

[geetarpicker]

I should mention on my two original Express amps & two clones the clean to mean range is noticeably wider (most notably better cleans) with the bright switch on. The trick is to have the volume knob up enough so the bright cap isn't adding too much to the mix, perhaps running the amp on 1/2 or a little further up. If you have fairly dark lower efficiency speakers (I prefer old G12M) it balances out just fine. Point is don't assume the bright switch isn't useful on these amps.

[Littlewyan]

I used standoff turrets for my resistor

[tony hunt]

I should mention on my two original Express amps & two clones the clean to mean range is noticeably wider (most notably better cleans) with the bright switch on. The trick is to have the volume knob up enough so the bright cap isn't adding too much to the mix, perhaps running the amp on 1/2 or a little further up. If you have fairly dark lower efficiency speakers (I prefer old G12M) it balances out just fine. Point is don't assume the bright switch isn't useful on these amps.

Excellent advice.
I, for one, plead guilty to not exploring bright switches thoroughly!

[hans-jörg]

[quote="DaveWell"]The .47uf is in parallel with the 1.5K

I put a 3 position Switch for bypass

My way to add voicing in pre-amp.
Position 1 : 25uf + 1.5K
Position 2 : 1.5K
Position 3 : .47uf + 1.5K

quote]

Oh, I see.
My fault.

Hans-Jörg

[hans-jörg]

Hi,
I built the 1st tube system (V1b) like the Komet (2k7/0,68µF) and use the place for the bright switch for a fast/gradual switch. But don´t want miss a bright option, so there is a 220p permanent at volume pot.
A push n pull pot could be an option, but I don´t like their behaviour and they tend to microphonie (mainly at this place).

Hans-Jörg

[Littlewyan]

I should add that I had my amp out of the headbox today and felt that 1K Resistor and it does get pretty dam hot when the amp is running. Too hot to touch but not so hot that its gonna melt. Going to try and find a solution to this and doesn't involve attaching it to the chassis as there isn't room in my amp.

Edit: If it helps I measured the voltage drop under load across this resistor and it was dissipating 10W. The chassis mount resistor I used is rated at 12.5W when not mounted to the chassis so should be ok but 10W is a bit close to the max rating.

[stuben]

The 1k/25w cement resistor shown in original Trainwrecks were mounted to the board without problems. I see no problem with this case resistor bolted to the board rather then chassis.

As I recall metal cased resistors are not capable of their full power rating unless they are attached to a heat sink of some kind. Does it need the full 25W? Probably not, but I would be inclined to put a U-shaped piece of aluminum under it, and put a couple of washers on each mounting bolt between the aluminum piece and the board.

I've always been puzzled as to why the 25 watt wirewound resistor here. I do not think it is necessary. I used a 5 watt wirewound cement resistor here with good results, just leave some air space between the board and resistor. Even a 3 watt metal oxide is adequate.

[Littlewyan]

Have you felt the resistor immediately after using the amp? When you play through the amp you get a large increase in screen current draw on the EL34s which causes a big voltage drop. 15watt would work ok but 25watt means you never have to worry about it. Also reading through old posts from Glen on other topics he states that on his amps even the 25watt gets a bit too hot to touch.

[stuben]

Never done that Littlewyan, that is touch the thing, but I know that this resistor does get very hot! At first I had it resting against the board and it turned the G10 epoxy under it a nice golden brown! So perhaps I'm flirting with danger here. Since leaving an air space between the board I've not had a problem, but I rarely run the amp hard.

Think I'll install a 25 watt aluminum housed unit bolted to the chassis just to be safe. Sure would not want to start a fire! I've seen Fender twins with 10 watt cement wirewounds in place of the choke, but they are usually 470 ohms and not 1K. There is a math formula for calculating the watt rating for a resistor. I think it is current squared times resistance.

[Littlewyan]

The way I work it out is by measuring the voltage drop across the resistor, divide the voltage drop by the resistor value, then multiply the current you get from that by the voltage drop and then you'll have the dissipation in watts.

Hopefully that makes sense, not the best at explaining things!

I think you should replace that resistor so it doesn't worry you or cause you problems in future. Stick a 25W Chassis Mount or Cement Resistor in there and sleep well at night! I'm building a new PS Board for my Express and going to use a 25W Cement Resistor, its so big I barely fitted it onto the board! (Mine isn't a standard Express layout).