I've made a start :) 50w #102

[martin manning]

Put the main power supply board in and hook up everything in the power section and the relay switching circuits. The only things that will be unconnected on the PS board will be the leads that go to the preamp. At that point you can test all of the power supplies and the relays.

Then, with a renewed sense of accomplishment and enthusiasm you can make it through the tedium of the preamp board. Make sure you have all of the leads that pass under the preamp board in place before mounting it. Then just go across the tube sockets pin by pin, left to right, and similarly for the front panel.

[norburybrook]

Bit more progress..... bit by bit...


The relay boards are going to be tricky as they've no eyelets so I need to solder them underneath.

I feel like I'm painting myself into a corner at times


I'm in great admiration for the tidy builds I've seen on this site. I've tried my best to be tidy but it's proving very difficult.

Marcus

[norburybrook]

another couple of hours this evening.



Marcus

[norburybrook]

Ok......... I've finished all the wiring....


Tomorrow I'm going to turn it on. I'm very nervous to be honest

If anyone can see anything glaringly obvious now's the time to tell me.


So, If I'm turning on tomorrow, the light bulb limiter. Now here in the UK I cannot for the life of me find any 100w incandescent light bulbs. I've a feeling they're now history due to ecological/energy laws.


Any ideas of what to do instead?

TIA


Marcus

[martin manning]

100W would be bigger than you want for the first power-up. Can you find any specialty bulbs like for sconces or chandeliers in 40W or 60W?

[norburybrook]

I'll have a look and see what I can find tomorrow , I'm sure there must be some old fashioned bulbs still left in circulation.


IN the UK now all new builds have to have a percentage of LED low wattage light bulbs, I can't remember the percentage but they're really pushing the energy saving. It's a good thing IMHO, I have LED's through my house and also in my studio ,they're great...4watts per light instead of 50w . and no transformers.



Thanks for the help so far Martin , I've a feeling the difficult part is about to start!

Marcus


P.s. Is that your dog? I should make an avatar with my Lurcher 'Willow'

[martin manning]

Have a look. Over here the bulbs I'm talking about are called bent-tip, and come in both standard and miniature screw base sizes up to 60W. I guess maybe it's time to start looking for an alternative to the light bulb limiter, but it'll be hard to beat the simplicity and low cost.

Yes that's my Flat-Coated Retriever, a proper English gamekeeper's dog. There are more than a few dog avatars here.

[Charlie Wilson]

Marcus,
What I do before powering up the amp is check the high voltage supply wiring starting from the power transformer B+ all the way to your FET supply. No sound is a irritating but smoke coming out of your transformers is expensive. I double check the wiring and stare at every single solder joint with a magnifying glass. Also double or triple check your electrolytic capacitor wiring and orientation. The current limiter is very important but I have to confess, I did a Hail Mary power up on mine. Don't do that.
CW

[norburybrook]

Marcus,
What I do before powering up the amp is check the high voltage supply wiring starting from the power transformer B+ all the way to your FET supply. No sound is a irritating but smoke coming out of your transformers is expensive. I double check the wiring and stare at every single solder joint with a magnifying glass. Also double or triple check your electrolytic capacitor wiring and orientation. The current limiter is very important but I have to confess, I did a Hail Mary power up on mine. Don't do that.
CW

Thanks Charlie, I've no FET board on mine

@Martin, does it matter if it's a halogen or LED as long as the wattage is say 40w or above?

can you test a light bulb with a multimeter? as I've found an old bulb here but don't know it's wattage.


Marcus

[martin manning]

A halogen would work if it is powered directly from the mains with no transformer in between (just a different species of incandescent), but not an LED. If you can measure current when it's on you will have what you need to calculate wattage.

I just checked a 60W bulb at ~24 ohms cold and 0.5A current (when on) with my meter. The DC resistance of the filament will be a clue, but note the cold resistance is maybe 1/10 of the working hot value: 120V/0.5A = 240R hot, and 120V*0.5A = 60W (as advertised). This bulb will limit current to 0.5A = 500mA. You are going to see 4x the resistance and half the current at 240V, but you still want a ~60W bulb.

Another quick check you could make is measure DC resistance to ground from each power supply node. You should get tens or hundreds of thousands of ohms, and if that's the case you certainly don't have a short.

[norburybrook]

OK so I found a 60w bulb and made my current limiter.


Switched on the mains and I have a dull glow from the bulb.

I've stopped here as I presume the light should not have any glow at all.




Edit; If the light only glows dimly then it's not a complete short, what should I be looking for?



Marcus

[martin manning]

A good indication for the limiter is:

Main power on- a moderately bright pulse followed by a dull glow

Standby switched to "play"- another moderate pulse followed by a slightly brighter dull glow.

The first bright pulse is the main reservoir caps charging, and the initial surge of filament current. The second pulse when the standby is switched is the remainder of the power supply caps charging up and the increased steady current draw.

Sounds like you are doing fine. I'd take the limiter out and power up at the full mains voltage, measure the power supply nodes and and set the bias voltage at minimum (most negative). Then put the tubes in and set the bias on the power tubes.

[norburybrook]

thanks

So with just the mains on I've got AC heater voltages across all valves ( around 3.00v AC)

I've also got 5v DC from the regulator to the relays and to the footswitch.

Time to throw the standby.


marcus

[norburybrook]

Ok, valves in, everything glowing nicely.

I've got -47VDc on pin 5

450v on the plates which is spot on.


Lowest I can get the bias is 39 mA, so I presume I need to change the Bias pot resistor and or/ the resistor on the rectifier board.
I've got at the moment:

3k3 and a 27k. what values do I need to get into the range I need?


Marcus

[martin manning]

Make the 3k3 a little smaller (2k7) or the 27k a little bigger (33k) to get more negative bias voltage. But, 450 x 0.039 is 17.6W, or 59% of the 30W max, which is pretty cool... Ok for testing. Got sound?