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Post subject: Bias setting hot/cold v clean headroom
Posted: Mon Oct 10, 2011 7:17 am
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Hi,
sorry to keep on about bias settings but I am trying to learn more about valve amp theory, and i am really getting confused as to what effect the bias current has on actual power outputs and clean headroom before clipping.
I have also read that bias should be set using a scope to eliminate/minimise crossover distortion, although this may apply to HiFi amplifiers.
I also read there are mixed views as to the actual max safe power rating of a 6L6GC winged C, and other 6L6 valves that some claim to be able to handle 30w.
What seems illogical to me, is that if you set the bias colder, ie the max power the tube will supply before it clips is reduced, this gives more clean headroom. Is this right, or have I misunderstood something along the way. It seems to me that the higher the bias current is, the more headroom one should have, assuming the safe limit for that valve's power dissipation is not exceeded.
Very confused over here in the SouthWest of England :? :?
mel


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Post subject: Re: Bias setting hot/cold v clean headroom
Posted: Mon Oct 10, 2011 8:52 am
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The higher idle bias serves to drive the output tubes more quickly into saturation, which produces the complex even-order harmonics that give a tube amp so much of its sonic character. From a technical standpoint, this is a trade-off of the amp's architecture as it relates solely to fidelity. But as we all know, a guitar amp is not, nor has it ever been designed to be, a "hi-fi" amp. The other half of the equation relates to tube longevity -- the hotter the bias, the shorter the lifespan of the bottles. Thus, an ideal idle bias setting requires some measure of compromise to keep both the ear and the wallet happy.

HTH

Arjay

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Post subject: Re: Bias setting hot/cold v clean headroom
Posted: Mon Oct 10, 2011 9:14 am
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I see Retroverbial is quick on the draw today. Great post, I'll add mine to the mix.

In simple terms, the amp bias controls the amount of DC current through the output tubes while at idle (not playing). I don't agree with using a scope to look at crossover, it isn't representative of actual playing when using a signal generator, and impossible to see when using a guitar. Using a scope to set bias is done by practically no one, forget that. You want to look at the wattage dissipated by the Plate by measuring the Plate voltage and current, and then multiply those two numbers to figure the wattage. Ideally, the setting should be between 50% and 70% of the tubes rating. A little overlap is beneficial to make sure that there is no crossover notch, so my minimum is 55%. I like clean headroom and typically set my tubes around 55% to 60%.

The consideration for setting the bias is Plate voltage, tube Plate wattage rating, and breakup point. The Winged C should be a 30 watt tube, and as I said before, check with the tube supplier for wattage rating.

You are correct dawntreader, the lower the idle current through the output tubes, the more clean headroom, and the higher the current, the earlier the breakup (within the 50% to 70% range). The idle current is basically raised and lowered during play to produce the output signal. The higher the current through the tubes, the closer they will be to clipping, and if you start with a high current to begin with (idle) the tubes will clip sooner (earlier breakup). The colder bias doesn't change or limit the amount of power available from the tubes, it merely determines at what point on the volume scale that clipping occurs. Clipping is when max power is reached and breakup starts. Do you want breakup at 3 on the volume (early), or closer to 6 on the volume (late)?

It only seems complex at first, but break it down to simpler terms and it will become more clear.

I should add that some tubes can handle higher Plate voltages than others, even if they all are rated for 30 watts. See the tube specs if possible, or ask the supplier. The Plate voltage might be what you were referring to, as far as what the Winged C can handle, not the wattage. :idea:

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Post subject: Re: Bias setting hot/cold v clean headroom
Posted: Mon Oct 10, 2011 11:26 am
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Thanks guys, the discrepency I referred to is the power output in watts.
There seems to be disagreement with 6L6GC s as to whether they are rated @ 23 watts or 30 watts depending on which 'technical' site you look at. Although one did qualify this : NOS valves or the Original winged C s are of a higher quality than the current output from China say, and will be ok at 30w compared to 23w for the Chinese valves. Maybe thats why Fender rate my twin @ 93W and not 120W : because of the lower quality of the Chinese or Russian valves they currently use. Am I right in recalling that there was a previous version of the twin that was rated at 120W, but still using 4 x 6L6GC s?
So, the bias at idle has nothing to do with the actual power rating of the tube when driven by a signal, its an offset that could reduce clean headroom if set at the higher end of the recommended range. Assuming my 6L6GC s are rated at 30W, then I am deffinitely not running them hot :)
I have some old Harma ecc83/12ax7 preamp valves. These were manufactured in the USA and carry 5751 marks underneath the Harma label. Comparing these to theGT valves that were made in China, I can see a difference in the quality of the internal construction. Some of these USA 5751 ones are desirable because of the miltary spec?
Thanks Guys
Mel


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Post subject: Re: Bias setting hot/cold v clean headroom
Posted: Mon Oct 10, 2011 11:38 am
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dawntreader wrote:
I have also read that bias should be set using a scope to eliminate/minimise crossover distortion, although this may apply to HiFi amplifiers.


Arjay and shimmilou covered things pretty well, so I'll just respond to this part of your question.

Don't waste your time. Buy something like this

http://www.amp-head.com/index.php?cPath=21

(pick which one matches the power tubes you have) and a decent digital voltmeter and you have everything you need to physically set your bias (assuming you have a amp with adjustable bias).

If you want a more technical explanation about bias than Arjay and shimmilou provided, a good bit of info can be found here:

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

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

http://www.aikenamps.com/Why70percent.html

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Post subject: Re: Bias setting hot/cold v clean headroom
Posted: Mon Oct 10, 2011 11:45 am
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dawntreader wrote:
Thanks guys, the discrepency I referred to is the power output in watts.
There seems to be disagreement with 6L6GC s as to whether they are rated @ 23 watts or 30 watts depending on which 'technical' site you look at. Although one did qualify this : NOS valves or the Original winged C s are of a higher quality than the current output from China say, and will be ok at 30w compared to 23w for the Chinese valves. Maybe thats why Fender rate my twin @ 93W and not 120W : because of the lower quality of the Chinese or Russian valves they currently use. Am I right in recalling that there was a previous version of the twin that was rated at 120W, but still using 4 x 6L6GC s?
So, the bias at idle has nothing to do with the actual power rating of the tube when driven by a signal, its an offset that could reduce clean headroom if set at the higher end of the recommended range. Assuming my 6L6GC s are rated at 30W, then I am deffinitely not running them hot :)
I have some old Harma ecc83/12ax7 preamp valves. These were manufactured in the USA and carry 5751 marks underneath the Harma label. Comparing these to theGT valves that were made in China, I can see a difference in the quality of the internal construction. Some of these USA 5751 ones are desirable because of the miltary spec?
Thanks Guys
Mel


True 6L6GCs are 30 watt tubes. Older 6L6, 6L6G, 6L6GB, etc I believe are 23 watts, as are most 5881s. Some companies have their own nomenclature. Lacking a spec, I consider nomenclature other than 6L6GC as questionable and treat as 23 watts.

5751s have a lower gain factor than 12AX7s but are otherwise fully compatible. I favor JAN Philips 5751s.

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Post subject: Re: Bias setting hot/cold v clean headroom
Posted: Mon Oct 10, 2011 12:02 pm
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The site for thetubestore.com lists the Winged C as 30 watts, that's what I would go with. I am unaware of 23 watt versions of the Winged C. There is a question about the previously used 6L6 in Fender amps, which was the Sovtek 5881 WXT, as to whether or not it is 23, 25 or 30 watts. But, no question about the Winged C as they have never been stock tubes in Fender amps before.

http://thetubestore.com/6l6templeton.html

Spec sheet for Winged C 6L6:
http://evatco.com.au/svet1_files/sv6l6gc.zip

The current production Chinese, Russian, and other modern tubes are as good, or better in quality as they have ever been. I don't see how you can judge the "quality" of tubes by looking at them. All tube manufacturers have had trouble with various tube types over the years, but not because they lowered the quality standards. They are all made differently, different materials and will look different. At this point in your learning, I don't think that you are aware of how tubes are made or about the materials used, enough to determine quality based on appearance. Not a knock on you, just that you can't judge tubes by looks.

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Post subject: Re: Bias setting hot/cold v clean headroom
Posted: Tue Oct 11, 2011 12:38 am
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Quote:
At this point in your learning, I don't think that you are aware of how tubes are made or about the materials used, enough to determine quality based on appearance. Not a knock on you, just that you can't judge tubes by looks.

I know you cant judge a book by its cover, so to speak, but this particular valve has the plates obviously installed on the slant, compared to the others I have. having said that, I am a microwave radio tech. I used to install complete systems from the dishes up the towers, the feeders from the dishes to the indoor equipment, and then align and commission. Its a black art as anyone involved in radio interferrence issues will tell you and on several occasions dish alignment has looked off but as far as the radio waves were concerned it was perfect :)

Quote:
But, no question about the Winged C as they have never been stock tubes in Fender amps before.


This was mentioned briefly in another thread, but just to confirm that the original power valves fitted by Fender in my twin (oct 2010 build) are GT over labelled winged C s.

I will follow the links above and try and educate myself some more.
Thanks guys,
mel


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Post subject: Re: Bias setting hot/cold v clean headroom
Posted: Tue Oct 11, 2011 1:36 am
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Also, you'd be surprised how much input/gain stage/PI can add to headroom. The one tube that really gives you headroom is the 1970-80's Tungsrams ECC83. Some 5751 add a bit of clean headroom, too.

Just a thought...


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Post subject: Re: Bias setting hot/cold v clean headroom
Posted: Tue Oct 11, 2011 4:50 am
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Quote:
Also, you'd be surprised how much input/gain stage/PI can add to headroom. The one tube that really gives you headroom is the 1970-80's Tungsrams ECC83. Some 5751 add a bit of clean headroom, too.


You know, I am starting to get a feeling that what I am hearing is not the power section starting to crunch but maybe V1 is clipping when I dig in hard. I have tried the 5751s that came out of a Laney amp that needs a major overhaul, but have gone back to the GT original 12ax7-r as it seems more transparent. I have already changed the PI to one from Watford Valves here in England that is specifically chosen to be used in the PI position. can't recall the exact type but its a Harma STR with very close balance between A and B. That has sweetened the tone abit.
I am also wondering if its my pups, they are set at about 3.5mm height, but can they get muddy when playing heavy rhthym? I am toying with the idea of another strat build using Bare Knuckle pups
http://www.bareknucklepickups.co.uk/main/
These guys are les than 2 hrs away from me so I can visit quite easily.
If I play more gently the tone is so rich and clean, even at higher vol levels.
I am in a Barndance / Ceilidh band and at times really need to dig in hard to provide a strong rhythm for the dancers. Dynamics are important.
Cheers,
Mel


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Post subject: Re: Bias setting hot/cold v clean headroom
Posted: Tue Oct 11, 2011 5:04 am
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Can to drop the pups a bit? Lower them, esp on the heavy string side. See what happens? If you have a 12AY7 (or 6201), try it in the input section. It may give more clean headroom until crunch time. Kinda favorite trick of some folks.


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Post subject: Re: Bias setting hot/cold v clean headroom
Posted: Tue Oct 11, 2011 7:29 am
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I am using 11 -54s as well which I assume will give a hotter o/p all round as there is more mass than 10 - 46s .
I have just been playing with the power setting on 1/4 : 23W. I tend to forget I have this option. Havent tried it since the new winged C s and rebiasing. I like this tone, very responsive to playing dynamics and a little less compression? It just seems to get louder the harder I play. I will have to try this at saturdays gig :)
Quote:
If you have a 12AY7 (or 6201), try it in the input section. It may give more clean headroom until crunch time. Kinda favorite trick of some folks.

Thanks for the tip, I havent got a 12AY7, but I may try lowering the pups a bit more.
Cheers
mel


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Post subject: Re: Bias setting hot/cold v clean headroom
Posted: Tue Oct 11, 2011 8:31 am
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You might try rolling back on the guitar's volume a bit and compensating with an increase on the amp's volume control. That's a mighty hefty set of strings you're using -- I'm not surprised that the amp is clipping so early in the power curve.

HTH

Arjay

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Post subject: Re: Bias setting hot/cold v clean headroom
Posted: Tue Oct 11, 2011 9:46 am
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Arjay,
I generally play with the guitar vol varing between 7 and 8, with middle pup tone on 8 and neck pup tone on 10. I have already experimented with guitar vol settings down to 5, but 7 - 8 seems to be the sweet spot for this guitar.
As I suggested earlier, with this guage of string maybe its the pup that is getting muddy.
back to the original topic :) , I think I have found the best bias setting for me, and I need to try the low power setting out at the next gig as it deffinately sounds good at home.
Now that I have retired from the day job I have too much time on my hands, hence all this fiddling with the amp :) but I have managed to insulate my roof space today ready for the winter so I am still doing meaningful work :D
Cheers
mel


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Post subject: Re: Bias setting hot/cold v clean headroom
Posted: Tue Oct 11, 2011 10:00 am
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Take note of any sonic changes (for better or worse) the next time you play out with the revised idle-bias setting. If it hasn't resolved the issue at least it might provide additional clues on what to try next. BMW2002ti may be onto something with his suggestion of changing the V1 to a lower-gain tube. I think the 12AY7's "mu" factor may be too low but a 12AT7 might provide a happy medium.

Best of luck, let us know how you make out.

Arjay

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