Revalving a Mesa Amp - Colour question!!!

welshboyo

OK, a few weeks off gigs so its time to give the flock a little TLC!!

1st on the surgery bed is my Mesa Stiletto..its probably due a revalve (Powers and Rectumfrier valve)

So...all of this Mesa nonsense/malarkey about using their branded valves with the right colours etc etc - is it complete bollocks or do I need to keep within the rules as its fixed bias ? - never really understood it!!

Amp currently has a pair of "Yellow" EL34 STR 447's in it, I have a brand new pair of premium matched JJ's here which have never been used - as they are "matched" does that mean I'm on to a winner here and I can just chuck them in?

The JJ's are labelled as PC 90.6/TC 11.7 (HotRox Premium)

Cheers in Advance me dears

jacksparrow

I think the idea behind the colour's is so you don't have to re bias if using the same colour.

If you put the jj in you will need to Re bias your amp for the jj. tubes. If you do not have a bias rite i would recommend getting one, and learn how to safely bias your amp, saves a lot of money and hassle in the long run.

welshboyo

jacksparrow said:

I think the idea behind the colour's is so you don't have to re bias if using the same colour.

If you put the jj in you will need to Re bias your amp for the jj. tubes. If you do not have a bias rite i would recommend getting one, and learn how to safely bias your amp, saves a lot of money and hassle in the long run.

but its fixed bias? i.e it doesn't have a trimpot...

I have a Bias-Rite, and have maintained non-fixed amps in the past.

jacksparrow

Hi i am not that familiar with the stiletto, fixed bias is usually still adjustable but you say there is no bias pot. maybe get a bias pot fitted or just be easier to stick to mesa tubes.

Good luck i am sure someone like ICBM  can advise you.

CarpeDiem

My understanding is that the colour coding on the Mesa valves determines their break-up characteristics. They are similar to having numbers on valves. I think they test that their (power) valves are in the right range so as not to require biasing.

RiftAmps

welshboyo said:

jacksparrow said:

I think the idea behind the colour's is so you don't have to re bias if using the same colour.

If you put the jj in you will need to Re bias your amp for the jj. tubes. If you do not have a bias rite i would recommend getting one, and learn how to safely bias your amp, saves a lot of money and hassle in the long run.

but its fixed bias? i.e it doesn't have a trimpot...

I have a Bias-Rite, and have maintained non-fixed amps in the past.

Three main biasing types:

- Cathode Bias (self-biasing)

- Non-Adjustable Fixed Bias

- Adjustable Fixed Bias

The Stiletto has non-adjustable fixed bias, so there's no pot to turn. Like CB, you can just plug and play. Your amp's bias voltage is fixed at a certain point, one that is relatively safe to allow plugging in any matched set of valves and it'll run them quite happily. This is not true of all amplifiers, so take each one on a case-by-case basis.

welshboyo said:

So...all of this Mesa nonsense/malarkey about using their branded valves with the right colours etc etc

Wish I'd come up with that one, I'd be a rich man!


A quick test if you're unsure...

Plug in the JJs and turn the amplifier on, watching them as it warms up. If one or both look like this one on the right then you can't use them. If you see this, turn the amp off straight away.

If both look like the left, then play test and if it sounds like it should then you're good to go.

ICBM

What RiftAmps said - although any redplating could be a whole lot less obvious than that and still be a problem! That looks like a valve failure rather than a biasing issue. If there's any red glow at all on the outside of the plate, even if you need to look at it in the dark to see it, it's running too hot.

For what it's worth I don't remember ever having a problem with any properly-tested valves in any Mesa amp - the bias is set quite conservatively so it isn't very critical - you're actually more likely to have a problem with it being too *cold* than too hot. Cold bias sounds like a raspy distortion on clean sounds at low to medium volume.

And don't bother changing the rectifier, they either work or don't work - they don't 'wear out' as such. Although a spare is useful to have if you're buying valves...

ecc83

Agree about the thermionic rectifier IC and on the subject of a spare? Might be an idea to get a Solid State one? I dare say there is some change in the sound but it can live in the gig bag and will never get broken but save your A and the gig? Get a hot replacement when you can.  Do NOT forget some spare fuses! Those nasty, unreliable valve things tend to eat them.

Just had a thought? Digital mains power meters are now pretty cheap. I am wondering if you could put one in the amp's mains feed and read the standing power draw then replace the OP valves and check again? No idea what the resolution is but worth a do?

Dave.

olafgarten

ICBM said:

What RiftAmps said - although any redplating could be a whole lot less obvious than that and still be a problem! That looks like a valve failure rather than a biasing issue. If there's any red glow at all on the outside of the plate, even if you need to look at it in the dark to see it, it's running too hot.

For what it's worth I don't remember ever having a problem with any properly-tested valves in any Mesa amp - the bias is set quite conservatively so it isn't very critical - you're actually more likely to have a problem with it being too *cold* than too hot. Cold bias sounds like a raspy distortion on clean sounds at low to medium volume.

And don't bother changing the rectifier, they either work or don't work - they don't 'wear out' as such. Although a spare is useful to have if you're buying valves...

A spare is useful to have but with the Stiletto you can also use the Diode rectifier as a backup. 

welshboyo

Cheers All, will try these JJ's and chuck the bias probes on to see what it's pushing out before and after replacing.

I have a spare Rectifier as the amp came with a complete set of used spares so I have enough to get me through a gig.

ICBM

olafgarten said:

A spare is useful to have but with the Stiletto you can also use the Diode rectifier as a backup. 

Yes, but I'm pretty sure you have to remove the rectifier valve, if it's shorted - the most common failure mode - the solid-state rectifier bypasses the valve rather than the valve being disconnected from the circuit. The early 2-channel Dual Rectifiers were the other way round though, they did disconnect the valves fully. The later 3-channel ones don't.

welshboyo

Thanks to All - done the swap and all good - no redplating, amp is definitely back to its former glory.