Bias and re-Biasing tube gear.

[cannikin]

HI guys I have a few questions about Bias and resetting Bias.

Today I heard an Orange Amp that was fitted with 6550 Tubes, it sounded amazing and beefy! :shock: After Playing it for 6 Hours in a studio, I decided I want to fit some 6550 tubes in my 1978 Orange. Of coarse, Its not easy as just popping-them-in. The tech mentioned Re-Biasing the amp so the tubes with properly work.

I have a few questions..

First; understanding Bias has been quite elusive to me. I have read a bunch of stuff on bias... after the first couple pages, the articles move toward the theoretical and everything flys over my head. Can anyone give me a better understanding of Bias?

tonight I read on a site that you may need to re-bias any gear when replacing the tubes, especially if you replace the power tubes. If you don't re-bias the gear your tubes may run excessively Hot.

Which I realized; last week I bought an entirely new set of Tubes for my Gates Sta-level, After popping the new ones in - When I turned the Sta power on - within 10 minutes my Tubes were extraordinarily HOT.

Could this be because I need to re-bias the unit??

So How do you correctly Re-bias a unit?
:grin:

Thanks

 

[NewYorkDave]

WAY too much hot air has been blown on the Internet in attempts to explain biasing, which is really a pretty straightforward thing, but a topic on which too many people seem to feel the need to try to show off how much they know (or more often than not, how little they know). Instead of adding to the noise level, I'll simply refer you to some good reading from Randy Aiken.

http://www.aikenamps.com/WhatIsBiasing.htm

http://www.aikenamps.com/Biasing.html

 

[cannikin]

thanks NYD.

Cheers :guinness: :sam:

 

[cannikin]

Just read those articles.

The vail has been lifted.. sort of..

 

[cannikin]

Ok to put it in simplistic terms.

Bias is having the proper current going through a Tube so performs and functions properly within its rating.

Am I delusional and off base??

 

[CJ]

After you read 100 opinions, you end up so confused that the ear replaces the voltmeter.

Make sure the iron is warmed up good before you drop the hood. That's all I have to say. Warm iron sounds looser. Or maybe it's because your ears are cooked by then.

It's a tradeoff between tube life and sound. And don't forget the poor power supply. If you have plates that aren't glowing red hot, and you like the sound, you are there.

 

[cannikin]

You guys sound like eastern philosophers of TUBES :grin:

I like it :razz: Just don't ask me to answer the question: what is the sound of One hand clapping?

So The Hotter the Tube, the more harmonic distortion it creates, which the human ear equates with Warm, Fat, Rich, and Deep sound. The draw Back - less life of the Tube.

the optimal life and tone of a tube is rated by its recommended Bias current?

So I replaced the Power Caps in my Sta-level, Which I assume is affecting my bias of the tubes which is making them run extremely Hot.
I could literally use the unit as a space heater.

This means my bias is to high?

Also If I wanted to throw 6550's into my Orange running them hot is fine, But at $60 - $90 per tube, I would like them not to Burn out too quickly, my pocket book .

How would I figure out for both these units what the Correct Bias is?

 

[CJ]

Ask PRR.

 

[PRR]

> Ask PRR.

The questionS asked are far too general. Guitar-amp? Sta-Level? Sta-Level vari-gain stage, fixed-gain stage, output stage?

A typical high-power guitar amp's output section, if there is no bias measurement and adjustment scheme, will work OK with any tube of the right type.

> The Hotter the Tube, the more harmonic distortion it creates

No. In non-clipped work (gitar amps are funny) the tube runs more linearly at high current (up to a point that you probably can't reach).

But it also cooks itself and everything around it. Don't run more current than you need.

In a single-ended amp, well-designed, the idle plate current times the plate-cathode voltage will be almost equal to the tube's plate dissipation rating. (If the amp would run well with less power, they would have used a smaller tube.)

In a push-pull amp, with adjustable bias, not designed for low idle heat, the optimum current will usually be between 50% and 90% of the plate dissipation rating. I defy anybody to prove they can tell a difference between 50% and 90% in sound.

70% is mentioned. It will work. It is not always optimum. In fact the theoretical optimum for the high power very high voltage amplifiers tends to be higher than the plate dissipation rating. Fortunately the optimum is very broad, and you can sit well below optimum without bad sound.

My Fisher ran over 100mA at full power but 15mA idle. This is far below optimum. They did it that way because they ran very large voltage on fairly small plates. With negative feedback, it was pretty much the same from 15mA to 100mA idle, but 100mA idle made the plates glow cherry-red, tube life would be very short. Going below 15mA, even with feedback, did give audible low-level distortion (fine at high levels).

The Sta-Level is a complicated case. I would expect most of the stages to be self-biasing, and any overheating implies something broken. The vari-gain stage can't be self-biased and could be prone to trouble. Figure out WHAT is getting hot and we can maybe talk about it.

 

[cannikin]

[quote author="PRR"]> Ask PRR.
The Sta-Level is a complicated case. Figure out WHAT is getting hot and we can maybe talk about it.[/quote]

Hi PRR

thanks,

I posted my Sta-level issues in another thread

http://www.groupdiy.com/index.php?topic=106626#106626

 

[CJ]

Here's the beef:

1) Heater current- the 6550 needs twice as much current to light up the heaters. About 1.8 amps compared to 0.9 amps for the 6L6G/C. So if you plug four 6550's into 4 6L6 slots, you cook the 60 dollar pwr trans somewhere down the line after the varnish crysyalizes..

So you need to find a place on the chassis for another transformer. 6.3/12.6 x-formers are cheap and not that big, so that won't be a problem.

2) Power transformer- the 6550 is going to draw more plate current also.
You can run a 6550 anywhere from 40 ma to 75 ma each. You might be able to get away with using your stock power transformer if you add the auxillary 6.3 volt heater transformer, as the core will not have to transfer flux for the heaters on the power tubes. I would leave the preamp tube heaters where they are.

3) Output transformer-if you had two 6L6's in a circuit and they put out 40 watts, then two 6550's in the same circuit will put out about 60 watts. And if you had four 6L6's putting out 80 watts, the 6550's will bump you up to 120 watts. I can see an output transformer that will get angry after a few hours. Maybe that's why the amp you played sounded so good. So you might get away with the stock output iron, but on a hot day, you might have a problem durring the second set.

4) Bias- you will need to modify your bias circuit to get the 6550's where you want them. I would start with 50 ma ea. and see how you like it. That might be a little cold for what you play, but it's a good middle ground to get your bias circuit resistor values in the ballpark. Once you dial in your bias, I would disconnec t the bias pot, measure from both ends to the wiper, and sub in two resistors instead.

Here is a schematic of an Orange Mk 2 bias circuit. Maybe you can lift some ideas form it. It uses EL 34's, which might be closer to the correct bias voltage for 6550's.

Here is another Orange bias circuitz:
'

Here are a couple of Orange schematics:

http://vacuumbrain.com/The_Lab/TA/Guitar_Amps/orange_1.jpg

http://vacuumbrain.com/The_Lab/TA/Guitar_Amps/orange_2.jpg