Arbitrary biasing question

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NerineNerine Frets: 1244
Afternoon.

Decided to swap the output valves out in my amp today. It's been used extensively for the last two-three years, mostly at gigs, and I had a spare set of JJs laying around, so thought I would pop them in just to freshen it up now that gigs are starting to happen again. The noise floor was also creeping up a bit, so I did some minor swaps in the preamp too. Good stuff. 

So, the question:

I have not biased the amp after swapping the valves.

I was going to see what happened and then potentially buy a Bias Master or something so I could be sure that the mA was correct. 

However, the amp is dead quiet, sounds great, I've had it up past gig volume, have looked intently at the plates etc whilst it was cranked, it's been on for about three hours and and everything looks and sounds fine. The filaments don't dim or anything like that under load. It's difficult to know whether they have that blueish glow, as it's daylight. 

If I accept that the bias may be slightly hot and the valve's lifespan will be shortened a bit, or it may be slightly cold, but it sounds as good as ever, is there anything in particular to worry about? It doesn't seem like the chassis or anything is getting too warm, the valves seem to be glowing normally, and touching the valves yields them hot - but not beyond what I would expect through touching various hot output valves in the past. The amp doesn't really seem to sound or feel different after the swap. Perhaps a little more strident and punchy, but that's difficult to determine for certain. 

In this scenario, is there anything to really worry about? I'll keep an eye on them at this weekend's gigs, to make sure they aren't red plating or anything daft, but everything seems to be pretty normal, and the amp seems to be running and sounding as I would expect. 

Cool?

(EDIT) - If anything the chassis seems a little cooler than usual, but it's currently sat outside the combo shell, so the heat from the valves is dissipating upwards rather than being directed into the top of the amp and therefore the control panel etc.  


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Comments

  • crunchmancrunchman Frets: 7798
    What amp is it?
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  • NerineNerine Frets: 1244
    crunchman said:
    What amp is it?
    Friedman Small Box.

    So, Marshall-based.

    Not cathode biased/EL84 etc. 
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  • crunchmancrunchman Frets: 7798
    In that case, you would bias it in an ideal world.

    A quick Google doesn't come up with any information about a 1 Ohm resistor in the cathode of the valves to enable easy measurement, which would make it easy with basic multimeter.  Something like the bias master might be the best option.
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  • Modulus_AmpsModulus_Amps Frets: 1892
    tFB Trader
    Turn it on in a dark room and watch the power valves, make sure they don't red plate at idle
    then play as loud as you can and make sure they don't red plate when playing or blow the HT fuse

    If all ok, you can probably get away with them until you get them biased properly. Red plating shows is the valve is running beyond the maximum power dissipation allowed, normally quite a bit beyond that point so definitely get them biased as soon as you can, they may also be running really really cold and the amp might sound better with a bit of tweaking.
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  • NerineNerine Frets: 1244
    Turn it on in a dark room and watch the power valves, make sure they don't red plate at idle
    then play as loud as you can and make sure they don't red plate when playing or blow the HT fuse

    If all ok, you can probably get away with them until you get them biased properly. Red plating shows is the valve is running beyond the maximum power dissipation allowed, normally quite a bit beyond that point so definitely get them biased as soon as you can, they may also be running really really cold and the amp might sound better with a bit of tweaking.
    Cool.

    Appreciate the response, thank you. :)
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