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understanding tube bias/retubing


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Hi guys, I'm just starting to read about the basics of amp bias cuz I have a buncha tube heads and I'd love to be able to work on them myself. I just ordered a Weber Bias Rite and should have it next week.

 

From what I've read it boils down to multiplying the plate voltage of the amp by the cathode current (mA)...and getting them to equal to ideal wattage of the tube. For example, I'll be working with EL34's...which are supposed to max out at 25 watts. Let's say I wanna retube my Bogner Shiva. I've read on the Bogner forum that the plate voltage is around 500. If my math is right I'd have to bias the Shiva at 50mA to get one tube up to 25 watts. This where I get confused...the Shiva puts out 80 watts. Soooo...to do that, each tube must reach 40 watts. That means I'd have to set the bias at 80mA (.08 x 500 volts = 400 watts). From what I've read, a bias of 80mA sounds really high and I don't think it's even possible to set that high in my amp. On the forum, the guys are saying that the newest Shiva's are biasing under 20mA. If that's the case, each tube would only be putting out 10 watts (.02 a 500 volts = 10 watts). That'd be a 20 watt amp! Where did the other 60 watts go? What am I missing here?

 

Also...how do I know which power rating of tubes to buy? I wanna make sure they're in the right range for my bias pot to cover them...without having to install a resistor to change the range. Ok, thanks for reading this long ass post and thanks in advance for bearing with my ignorance on the topic.

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Take the bias rite. Get mA reading. Take ohm meter. Measure plate voltage on power tube (connect positive end to pin #3, neg end to ground). (Some bias-Rites have the plate voltage switch - use that if you have that version) Multiply pV x mA. That will give you IPD of tube.

 

EXAMPLE:

 

mA = 32

pV = 495

 

Multiply 495 X .032 = 15.84 watts of plate dissapation.

 

EL-34 MAX plate dissapation is 25 watts. Being that this is a fixed bias amp, you want to run this tube at 70% of plate dissapation = 17.5 watts MAX.

 

So, 15.84 watts of IPD per tube is acceptable. You want to bias to the tube pulling the most current.

 

Last time I biased a Shiva, it sounded real good at around 14 watts of plate dissipation. Use your ears and use the math and you will be OK.

 

- WOT

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Postscript: I usually bias amps as cold as possible while still trying to retain the best tone. Hotter does not always mean better. New production tubes - well, from what I have seen they aren't as stout as the old ones - so take the "70% of max plate dissipation" with a grain of salt.

 

- WOT

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So, 15.84 watts of IPD per tube is acceptable. You want to bias to the tube pulling the most current.


Last time I biased a Shiva, it sounded real good at around 14 watts of plate dissipation. Use your ears and use the math and you will be OK.


- WOT

 

Thanks! So at 15 watts per tube...the amp is putting out 30 watts?

How does Bogner come up with saying the Shiva is an 80 watt head?

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Thanks! So at 15 watts per tube...the amp is putting out 30 watts?

How does Bogner come up with saying the Shiva is an 80 watt head?

 

I m guessing that they just state the max possible for the given valves used (which is virtually never achieved), not the effective amount the amp puts out at any given bias.

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take the 5150 for example...they rate it as a 120watt amp. a dual recto, with the same power tune compliment, is rated at 100watts. both of them are biased cold as {censored} from the factory, and in reality might put out 75 watts at max volume, which you really don't want to do anyway.

 

i run KT88's in my amp, and they are capable of 70 watts each, but that's at 700VDC plate voltage. does that mean i would rate my amp at 280 watts? nope. i run them at 25-30 watts each, so my amp is truly a 120 watt amp.

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  • 12 years later...
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On 12/7/2007 at 12:13 PM, djpatb said:

Thanks guys. Just wanted to make sure I wasn't missing something.

What you are missing is that plate dissipation and output power are separate things. When you bias a tube, you are setting its idle point in terms of current flow. There is an upper limit to how high you can set it, before the heat you generate fries the tube. That heat dissipation is measured in watts. This wattage rating has nothing to do with the actual output power of the amp. Just because a tube is only rated to 25W dissipation does not mean it can only produce 25W of output. 

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