Help needed with faulty Lonestar Special

richardhomer

I'm getting some cracks coming through the speaker once it's been switched on for a few minutes, which I gather is often a symptom of a shorting output valve. The volume of the noise is unaffected by gain or master settings - so I assume it must be a problem in the power stage? It is loudest on the 5 watt setting, quieter on the 15 watt setting and at its quietist on 30 watts.

So far, the rectifier valve and fuse (which is the correct rating) have survived.

I'm guessing it's time to replace the output valves?

I realise a massive amount of rebranding goes on, so what EL84s should I be buying - and who from?

Do I need four which are matched, or will two matched pairs suffice?

Amps are really not my thing - time for the wisdom of @ICBM!

richardhomer

Bumping this as I'm keen to get on with ordering valves. Any advice would be appreciated.

meltedbuzzbox

I cant offer too much advice but I believe its a good idea to replace a pair at a time. I dont think you have to buy 4. 

I think you might need to reset the bias afterwards, unless your mesa can do that its self?

richardhomer

^ I believe Mesas are 'fixed bias' - whatever that means.

I'm sure four matching Mesa-branded EL84s will sort the problem out but money's tight at the moment. Saving a few quid is an appealing prospect but if it only results in further expense down the line, it's a false economy.

jpfamps

I had one of these in the work shop with a similar fault, and it was a cracked resistor on the PCB.

The the revalve doesn't cure it, it will need to be looked by someone.

richardhomer

jpfamps;344183" said:

I had one of these in the work shop with a similar fault, and it was a cracked resistor on the PCB.

The the revalve doesn't cure it, it will need to be looked by someone.

Thanks for this. Hopefully the valves will sort it....

ICBM

Most Mesas are fixed-bias, and not adjustable as stock. (For good reasons in some ways, if you understand Mesa's philosophy on matching the valves to the amp and not the other way round.)

But these ones are cathode biased, and have separate resistors for the inner and outer pairs, I'm fairly sure - because of the half-power mode - so you can get away with two matched pairs, although a matched quad is better.

The best valves for it in my opinion would be NOS Mullards, Brimars, GEs or Sylvanias. Personally I've preferred the US-made ones in the EL84 Mesas I've owned myself. If you don't want to spend that much and would prefer modern, easier-to-buy valves, I would go for JJs. As far as I know Mesa are still using Sovteks although they have switched to JJ preamp valves.

Karltone is excellent for JJs.

richardhomer

Thanks @ICBM.

richardhomer

Quick update. New valves are in - the amp is working perfectly again.

Thanks for the advice gentlemen.