Hot / Cold Bias Considerations

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Hot / Cold Bias Considerations

Post by Molly »

Got the new main board in my TSL100. The bias is apparently supposed to be 90 but most YouTube videos I've seen set it at 75. Still seems hot to me at that so I've gone colder until I can get some new power tubes.

This is one of the videos I've watched which seems to suggest that it's not as critical as I'd thought and that you can get away with a colder bias without it being detrimental to tone.



What say you chaps?

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Re: Hot / Cold Bias Considerations

Post by Bg »

well apart from the obvious... will it matter when its sold in a couple of weeks?

I've played around with bias, but not by much. I've tried to get a median on the manufacturers specs, and left the rest to boosts etc. No point killing tubes when you don't need too.
So, is that low alcohol or no alcohol at all? mmmm, no alcohol, do you want to try it? Noooooooooo.

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Re: Hot / Cold Bias Considerations

Post by Molly »

Bg wrote: Sun Jul 05, 2020 6:10 pm No point killing tubes when you don't need too.
Especially when you see the fucking price of power tubes these days. :-(

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Re: Hot / Cold Bias Considerations

Post by Bg »

Molly wrote: Sun Jul 05, 2020 6:33 pm
Bg wrote: Sun Jul 05, 2020 6:10 pm No point killing tubes when you don't need too.
Especially when you see the fucking price of power tubes these days. :-(
I shit my pants when I retubed an amp with 4 kt88's, thankfully eurotubes shipping price in those days was good....
So, is that low alcohol or no alcohol at all? mmmm, no alcohol, do you want to try it? Noooooooooo.

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Re: Hot / Cold Bias Considerations

Post by Optical »

Yeah 90 is much too high. 70ma is about right and yes you can run them much colder than that without hearing much, if any, difference if you're not smashing the volume.

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Re: Hot / Cold Bias Considerations

Post by Kris »

Bg wrote: Sun Jul 05, 2020 6:48 pm

I shit my pants when I retubed an amp with 4 kt88's, thankfully eurotubes shipping price in those days was good....
Defo one of the things thats swayed me away from an AD200/mesa 400+ when theyve come up for consideration!

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Re: Hot / Cold Bias Considerations

Post by sizzlingbadger »

I haven't watched the video but I'll respond to the answers already given in this thread.

Bias is set by calculating power dissipation, you can't just say a cathode current of 70mA is correct - this means nothing on it own.

Class A (single ended), Class A/B bias settings are also different.

Most bias recommendations for Class A/B (Push Pull) like the Marshall above are to set the tubes to ~70% of their rated max power dissipation. 60% would likely be fine too, towards the colder side and will help the tubes last longer.

You need the cathode current and anode (plate) voltage to calculate the power dissipation. Here is good site to check stuff out...

http://www.tedweber.com/webervst/tubes1/calcbias.htm

BTW, a correctly biased set of KT88's will last for fucking years in most guitar/bass amps.
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Re: Hot / Cold Bias Considerations

Post by godgrinder »

Hotter bias helps if you mostly play quiet. For the volume I play at I prefer a colder bias at about 55%. At 70% they seem to break up a bit early for me.
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Re: Hot / Cold Bias Considerations

Post by sizzlingbadger »

Molly wrote: Sun Jul 05, 2020 6:00 pm This is one of the videos I've watched which seems to suggest that it's not as critical as I'd thought and that you can get away with a colder bias without it being detrimental to tone.


What say you chaps?
Unless your Mast Vol is cranked right up and you are getting power tube distortion (likely to be at considerably loud volume levels) you will not hear much difference. Most modern MV amps use the pre-amp to create the overdrive.
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Re: Hot / Cold Bias Considerations

Post by Molly »

Cheers guys. I'll check that link thanks, Nik.

Pleased with myself for having got this far without killing myself or the amp. :-)

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Re: Hot / Cold Bias Considerations

Post by RectifiedAmps »

Just to add a bit more info... Usually when we discuss setting the bias to a particular current or %-of-maximum wattage, we're talking about the plate/anode only. But the current passing through the cathode contains both the anode and screen current, although the anode current will be most of it and screen current is usually only a few mA.

So let's say you set the bias to a cathode current of 35mA per tube (like with a bias probe or voltage across a cathode resistor), then the actual anode current might be just 30mA. A Marshall usually has an anode voltage around 475V... so 475V x 0.03A = 14W (57% of max), which is colder that what you think you're setting it to (475 x 0.035 = 17W or 67%).

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Re: Hot / Cold Bias Considerations

Post by Bg »

sizzlingbadger wrote: Mon Jul 06, 2020 2:30 pm BTW, a correctly biased set of KT88's will last for fucking years in most guitar/bass amps.
Thats great, I'm sure the new owner is still very happy 15 years later.
So, is that low alcohol or no alcohol at all? mmmm, no alcohol, do you want to try it? Noooooooooo.

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Re: Hot / Cold Bias Considerations

Post by Bg »

I'm still pissed off about the price though.... lol
So, is that low alcohol or no alcohol at all? mmmm, no alcohol, do you want to try it? Noooooooooo.

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Re: Hot / Cold Bias Considerations

Post by Molly »

Sounds fucking great with the high-output Dragon 1s in my PRS. Just sayin'.

[youtube]https://www.youtube.com/watch?v=v2WEiaxJpTM&t=320s[/youtube]

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