Tube Help Please

[G the wildman]

The amp is glorious. I have always been underwhelmed with it. The guy I bought it off had only had it 5months from new. He clearly did not like it.

It must have come out of the factory like it.

Anyway it is truly incredible now.

 

[Ronquest]

New amp with a quad of el84s. Noticed it had a little hum and after a bit of inspection, failed heater. Part of the game.

 

[G the wildman]

Hi Aficionados,

I need more help, this time with my Mesa Boogie 5:25. This amp is great, loud, portable and versatile. But it can be quite hissy.

A new set of tubes is £200 or more if I want Mesa originals. So I may try rolling.

But engineering and electronics is not a skill of mine. I wanted to know if hiss comes from pre or final tubes.

It does not hiss until cranked.

Or could it be something else.

G

 

[PelliX]

Typically hiss doesn't actually come from valves, though you may find that some preamp valves behave "better" in a given amplifier. Often, hiss is a byproduct of the circuit around the valve, and the earlier the hiss is located in the circuit, the more amplification stages it will go through and the more apparently it will become. It's not impossible, but far less likely it's coming from the output stage.

The Boogie is a pretty high gain amplifier, so a bit of hiss is not uncommon, it's hard to express in words, though. If you don't need ALL the gain it has to offer, you could *try* lower gain valves in the first couple of preamp positions, though it might change the character of the amp a bit - so if you happen to have access to a one or two ECC82's, that *might* be worth trying. Likely the real fix would be replacing some resistors, but working on Mesa's isn't a lot of fun mostly, even for the skilled, so you're probably looking at taking it somewhere.

As for the Mesa original valves - they're obviously not compulsory, so feel free to try out others - certainly in the preamp.

 

[Pete Farrington]

I wanted to know if hiss comes from pre or final tubes.

It does not hiss until cranked.

It's pretty safe to state that your hiss is from the preamp stages then.

A new set of tubes

Replacing every valve in the amp all at once is never necessary. It's only ever done for expediency, eg where the time taken to identify any bad valves would cost more than the cost of unnecessarily replacing good valves.

To identify any bad valves, valve amp users should have a known good spare or 2 of each valve type in their amp. And preferably a spare set of output valves, that preferably all pass idle anode or cathode current, which is reasonably close to the valve set that the amp is set up (biased) for.

When problems emerge, swap a good spare valve into each socket in turn. If the spare doesn’t help with the issue, refit the previous valve and move along to the next socket.

When initially checking new, unknown or suspect output or rectifier valves in an amp, it’s beneficial to power the amp via a light bulb limiter. As otherwise, if they short, they may stress and damage the amp.

Mesa Boogie 5:25. This amp is great, loud, portable and versatile. But it can be quite hissy ... It does not hiss until cranked.

That amp will tend to be especially hissy at high settings, due to high value resistors used in the design of the input stages.
If you like the amp, you just have to accept a fair degree of hiss.

https://el34world.com/charts/Schematics/files/Mesa_boogie/Boogie_express_525_550.pdf