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

[chazpope]

Hi I got this Laney LC15R recently - plays beautifully - but ....

even when left on standby without any playing in about 15 mins it gets too hot for my liking - you can't hold your hand onto the power transformer - most of the heat seems to be generated there and that can't be good for the dodgy pcb this amp has.

I tried it without power valves - cold. So I guess the transformer hasn't got a short. Tried it at a loud jam with external fan and it did not get hot at all.

I have not measured standby HT current to check biasing but sound is pretty clean. Biasing pot was to about 2 o'clock, I wound it back to 12h - that only seemed to increase the time it takes to get really hot. It must have changed the gain which I would not like long term.

So, if the deal is that it just has to run this hot - can I put a fan on it and how ?

I am thinking perhaps a 12V case fan could be run off the 6V heater circuit with a 2 diode 2 cap voltage doubler - how big caps should I use ?

[Jazz P Bass]

I would want to know just how much current this puppy is pulling from the mains.
The schematic states 50 watts max.
Whatever that means (at idle, at max output: it doesn't indicate)
I would advise checking it out .
There may be an issue already with the amp & a fan may mask it.

[Chuck H]

+1

If the amp worked fine and stayed at an acceptible temp in use, but gets too hot sitting at idle, that's probably a bias condition. It's not senible to install a fan with the available information before checking the bias.

[Enzo]

AMp gets hot? Or power transformer gets hot? Transformers often run real hot. Don't just twiddle the bias control, MEASURE the tube idle current.

[kleuck]

Laney often use tiny transformers, they get hot very quickly (i had a VC30) but that's not necessarily a problem.
But the bias may be in the VC15, which is fixed-bias (VC30 is cathode-biased)

[Chuck H]

AMp gets hot? Or power transformer gets hot? Transformers often run real hot. Don't just twiddle the bias control, MEASURE the tube idle current.

+1

[Chuck H]

Laney often use tiny transformers, they get hot very quickly (i had a VC30) but that's not necessarily a problem.
But the bias may be in the VC15, which is fixed-bias (VC30 is cathode-biased)

Cathode biased or fixed bias, it's still possible to be biased too hot. A fixed value resistor in a production amp, especially one that is biased class A, may not be a good value for tube brands other than what an amp was designed for. IMHE EL84's do tend to be more consistent than other tube types. And I've never seen a case where EL84's and cathode bias is involved where a rebias was needed for a tube change. But there are cases where this has happened. Some old Vox and Fender amps that use cathode bias have been known to need a little more cathode resistance to bias up correctly. Admittedly these are extreme cases due to changes in mains supply voltage and tubes that are decades removed from the original design. But I'm just noting that the level of bias is exclusive of the method of bias.

[kleuck]

I don't want to say that cathode bias always give a perfect current, and the VC30 like the AC30 is biased very very hot, can be and is often an issue, but there's not a lot difference in current between a quad of tube of another, just some brand/model that can stand this high dissipation, and others that can't.
It's a whole other story with fixed-bias, deviation can be huge from a pair of tubes to another, and not a good idea to play with the bias pot without actually measuring the current.

[Chuck H]

Duely noted. Cathode bias is a little rubbery with tubes of different current needs. That's why it's also called "self bias". Fixed bias is not the least bit flexible. So +1 on that point.

[chazpope]

Power traf gets so hot at standby that you can't put your finger on it and count to 3.

Valve anode current at P1 is 20 ma, measured it in series and in parallel over the 116 ohm primary coil.

Voltage at HT fuse is 315V, I guess up to 26 ma is ok bias ?

[kleuck]

Up to 30 mA (would be about 80%), but there's something badly wrong indeed if you can't touch the PT, and not bias-related it seems, can be a problem with the heaters, as the PT keeps cool without tubes, did you try with another pair of EL84 ?
Should not be an oscillation issue -HF, so not audible- as the temp rises even in Stby.

[chazpope]

er no I haven't got another pair short of pulling the ones from my blues junior, I guess i should buy some..

I guess measuring the mains wattage will show if there is oscillation, not sure how best to measure it though and what it should be normally, heaters and all. It seems to not get so hot when chassis is out of the box, I am still not sure if there's anything wrong with it but still I hate how hot it gets

I tried double rectifying the heater voltage and got 16v that dropped to 12.8 with a 150ma fan on. I guess that adds .3A on the heater coil. So I think I probably have a fan solution should I go that way

[Chuck H]

Power traf gets so hot at standby that you can't put your finger on it and count to 3.

Maybe I'm impatient, but I can't find a standby switch in the schematic. To know that the amp is still drawing current in standby is helpful. But only if we know about the standby circuit.

Can someone tell me where the standby switch is?!?

[kleuck]

Good point, as far as i know, there's a standby on the Vc15, but none on the LC15.

[chazpope]

no there isn't one, so by standby I meant when the amp is not being played - all HT is on of course.

I tried pulling the HT fuse out and then it does not get hot, put it in and it starts cooking