Laney LC15R - too much heat - should I install a fan ?

[kleuck]

OK, so, it's normal temp for this PT actually
They are tiny, i remember the PT of my VC30 being hot enough to fry an egg on it too, but it took some time playing dimed.
I remember too that firsts LC15 had reliability issues, maybe related to the PT, anyway i had a VC15 on my bench recently, and there was no big temp issue like this one, i don't know if they use the same PT though.

[Jazz P Bass]

So did we measure the current that the amp is drawing from the mains?

[chazpope]

199ma with HT fuse in - 140ma without it. It was quite a tricky measure as cheap meters go to 200ma max and the next hole for 10A is dc only. I think I killed 2 meters trying.

Anyway - 200 ma is 48W at 240V. 60 ma is 14W HT idle , heaters should be 15W so there's 19W to account for inefficiencies - isn't it too much ?

[kleuck]

No, tube amps efficiency for a PP is usually about 33%.

[Jazz P Bass]

Well, the schematic does show '50 watts maximum'.
So that seems to be good.
The high temp of the PT may be what it is.
After all you cannot grab on to a 50 watt lamp for very long
Put a fan in it if you feel inclined.

[chazpope]

well ok, still, is it normal that it draws 48 of its 'max 50W' when I am not even playing it. And is't 200 ma at idle too close to the 250ma fuse blowing limit ?

[kleuck]

Heaters + anode should use about 30 watts-33 watts.
A ferromagnetic transformer has an efficiency of about 80% max, so should give about 40 watts on the mains.
Then you have to consider that in the real world, most power tube have a heater consumption above what is given in the datasheet (do you use JJ EL84 ?) so it's actually credible.
Remember too that though PP are AB class, with EL84 the most part of the cycle is similar to A class, they are usually biased around 8-10 watts.

[J M Fahey]

Since you have an HT fuse, why don't you pull it and measure across its socket, to *know* what the idle current is, instead of all this guessing?
Just sayin'.

[Jazz P Bass]

Did we ever check the bias voltage to see where the output tubes are biased at?

[Steve Conner]

Since you have an HT fuse, why don't you pull it and measure across its socket, to *know* what the idle current is

Did we ever check the bias voltage to see where the output tubes are biased at?

It was quite a tricky measure as cheap meters go to 200ma max and the next hole for 10A is dc only. I think I killed 2 meters trying.

I think the guy may be out of meters? In this situation, I've used a switch to short across the meter and then unshort it once the power-on inrush is over.

[chazpope]

I think the guy may be out of meters? In this situation, I've used a switch to short across the meter and then unshort it once the power-on inrush is over.

Well that's what I thought I was doing (by hand) but then 0.2A is the top of the meter range so I popped 2 meters internal fuses until I got it right on the 3rd one Still I have 2 meters left, plus today I ordered a slightly posher one just to not have the dreaded 200ma limit. When I was a kid my needly old russian meters went much higher I must say ..

I have put the amp it back together now and played a gig with it last night (with external fan). I will take it apart again for these HT fuse and bias voltage measurements, and I will try measuring the heater current too.

[chazpope]

ok so current across HT fuse is 55ma, 300V, mains is 240V, bias voltage on C22 12.4V. (it says it should be 11.5V)
couldn't measure the heater current - no meter for big AC currents yet.

[chazpope]

Ok so this is my Laney LC15R with a fan! I am really pleased with result - I have played several gigs with it and it does not 'cook' as before, heat levels are barely above room temperature so I guess the dodgy pcb is not going to give me problems for a while.

I added a small pcb inside with a Delon voltage doubler that turns the heater 6.5VAC to about 13.5VDC and then I have a 1 transistor regulator to drive the 12V computer case fan.

now where do I find me a grille - looks like this model did not have one ..

[Jazz P Bass]

This may be a little too big at 15.8 x 2.4 ". (400 x 60mm)
Maybe not.
VENT GRILL for Amp Heads - $23.00 : AMPCLONES.COM, Tube guitar amplifier PCB boards, chassis, transformers and DIY kits

[Chuck H]

Probably not an issue, but... Motors use a lot of energy. Rectification and regulation use a little too. It's possible the heater wind could be stressed. How hot does the PT get? After the amps been on for awhile, can you hold your finger on it comfortably?

The only time I ever installed a fan on an otherwise fan-less amp I used a 120V fan. I ran it through a fuse straight off the mains. No worries about taxing the PT.