Tweed Super Bias

stringgears

Member
Messages
116
My tweed Super which is a replica is biased at -38vcd but the originals were biased at -42vcd. Anyone know what this would do to the tone?
thanks ,
SG
 

hasserl

Member
Messages
4,708
It's a pretty meaningless figure. That is the bias voltage applied to the control grid of the tube. Depending on the tube itself and the operating voltage the bias voltage may need to vary substantially. You're looking at the wrong place to determine if the bias is set properly.
 

beefour

Member
Messages
24
hasserl is exactly correct.

You need a 'scope and a sinewave generator to properly set bias. The difference in the voltage on the schematic means only that the guy who wrote the schematic also wrote those numbers.

Proper bias for your amp is based on your own desires. The considerations are:

1. Elimination of crossover distortion
2. Output tube life
3. Small differences in tone.

Those considerations are interactive. More bias shortens tube life, and can serve to enhance tone to some degree. Less bias introduces crossover distortion, and extremely low bias decreases output power and bandwidth.

The difference between the two specs you quote are not spacious, and neither is necessarily right for your particular amp. Each unit is slightly different.

Bias in general is a lot like balancing the tires on your car. Having matched output tubes is good, just like having the same make, model and size of tire helps your car hold the road better. Minute differences in bias are much like adding little tiny weights to get things 'perfect', or adding or subtracting a half pound of pressure.

I can tell when motorcycle tires are two pounds low or about a pound high. I can also tell when they are even slightly out of balance. It's much harder to tell this in a car.

And so it is with guitar amps. If this was laboratory grade stuff, we'd be a lot more concerned about exact specifications. But guitar amps aren't built with this sort of 1% or .2% or .002% accuracy. And that's why one unit sounds better/different than another. It's also why a particular set of power tubes can sound better/different than another. Again the tire analogy works here. Tires wear out. Power tubes wear out. Good tires wear better and perform better. "Sport" tires perform very well but don't last as long.

If you buy a new set of tires, you mount, balance, and adjust pressure based upon specifications. You can run them with too much air and make them last longer, but there are handling ramifications. You can under bias your amp and make the tubes last longer, but there are ramifications.

But in the real world, if you buy decent tires (tubes) and install and inflate them correctly (bias w/ a scope or at least check that idle current isn't just wacky) and you're likely fine.

Forgive the length of my explanation. I'm thinking of writing an overall post on bias, as there seem to be a lot of quesitons concerning that very subject here.

Regards,
Bill
 

Trout

Member
Messages
7,549
stringgears said:
My tweed Super which is a replica is biased at -38vcd but the originals were biased at -42vcd. Anyone know what this would do to the tone?
thanks ,
SG

The trick here is partly which circuit did they base this replica on?
For instance.

The Tweed Super Model 5D4 (Wide Panel) is cathode biased (self biased)

The Tweed Super Model 5E4-A Narrow panel uses 6V6's Bias voltage on schematic is -32V

The Tweed Super 5F4 Narrow panel uses 6L6's Bias voltage on schematic is -40V

Often replica or clone amps are not even based exactly to a given model outside the use of a name or model number. Example:There are lots of guys selling amps on ebay as "5E3 Deluxe" yet have tubes like EL84, EL34, 6L6, none of which are common to the " Deluxe".

If your in doubt of the correct voltage setting for your replica, It would be helpful to get the the correct model # and go from there assuming its built to original specs, Or contact the vendor for further circuit information/identification. Most builders are very helpful and willing to help out on these issues.

Trout
 

stringgears

Member
Messages
116
Thanks for your info.
The guy that built the amp for me does a hell of a job. He mentioned that he set the tubes that way for less noise and less crossover distortion. I"m not concerned of any harm to the tubes but was just wondering if it made much of a difference in tone. The amp doesn't seem to have a lot of that tweed tone as in "mids" that I expected. I have played and owned many original tweed amps over the years that I thought had more mids. The amp does sound great its just not what I quite expected, though I have never owned or played a tweed super before besides this replica, so actually I have never heard what an original sounds like. thanks again.
SG
 

phsyconoodler

Member
Messages
4,301
In my opinion it is not necessary to bias with a scope.It is however,necessary to know what the tubes are biased at.That means knowing the plate voltage,the current draw through both the cathode and the screens and knowing how to alter the bias if necessary.But,the reason for bias is not just to save tubes.What do we all like about tube amps? The TONE.Right? So why bias your tubes so they will outlive you?Bias them so they sound the best and are within reasonable limits.
When the rest of the amp is set up with components that are 20% + or -,why fret over exact bias? As long as the tubes are not going to explode and burn out in a month or two,set the bias so the tone is good.They may surprise you and like to be biased colder.Hot is not always the best tone.And this matched tube scenario is way over done too.If they are within 10ma of each other they will work fine.Some guys purposely use dual bias to get them mis-matched.The tonal advantage is interesting.Case in point:I had an old peavey Ultra with 2-6L6's and one tube was on it's way out.The tone was heavenly!The bias was about 15-20ma off.When I finally put in a matched set of tubes,the magic was gone.They did not sound anywhere near as good.They were the same brand of tubes.If you are an audiophile and minimum noise and the clearest,cleanest tone is the only acceptable result,by all means use a scope and agonize over your bias.If you like great tone out of your guitar amp and are not worried about periodically changing tubes,don't waste too much sleep over bias.
 

beefour

Member
Messages
24
Old Tele man said:
re: "More bias shortens tube life, and can serve to enhance tone to some degree. Less bias introduces crossover distortion, and extremely low bias decreases output power and bandwidth."

...did you possibly mean "More current..." instead of "More bias...shortens tube life,..."?

SUMMARY: More current = more tone today, but less life in the future!


Thanks for asking for the clarification. I'm quickly learning the dangers of brevity vs. clarity. Good thing I've got cats keeping an eye on my terminology. :Spank

As you suspected, 'bias' in my statement refers to the bias CURRENT flowing through the output tube, not the negative VOLTAGE applied to the grid to establish that idle current. It's a worthwile distinction.

The original question: "My tweed Super which is a replica is biased at -38vcd but the originals were biased at -42vcd. Anyone know what this would do to the tone?"

As the Tele man says, Less tone at -42, and longer output tube life. And more tone and less tube life at -38. The difference between the two may not be spacious, or may be quite different depending on the particular tubes in the particular circuit.

The difference in tone is easily discovered by playing the amp with the bias set at both voltages in question, and deciding for yourself. But those numbers really are meaningless by themselves. A meter and a bias probe will tell you what the idle current(s) are. Many folks here will be happy to offer opinions on what those figures ought to be.

Regards,
Bill
 

Blue Strat

Member
Messages
30,686
stringgears said:
Thanks for your info.
The guy that built the amp for me does a hell of a job. He mentioned that he set the tubes that way for less noise and less crossover distortion. I"m not concerned of any harm to the tubes but was just wondering if it made much of a difference in tone. The amp doesn't seem to have a lot of that tweed tone as in "mids" that I expected. I have played and owned many original tweed amps over the years that I thought had more mids. The amp does sound great its just not what I quite expected, though I have never owned or played a tweed super before besides this replica, so actually I have never heard what an original sounds like. thanks again.
SG
What you really need to ask is at "what bias current" and "what plate voltage" the bias is set at. Forget about crossover distortion and scopes.

Comparing grid voltage is only relevant for one particular pair of tubes. One pair might bias perfectly at one number and not the other, or somewhere in between. Another pair of the same brand/type of tubes would bias completely differently.

There are few absolutes without getting into more detail.
 

phsyconoodler

Member
Messages
4,301
Well said Mike.
Every tube responds differently unless they are tested like Boogie tests theirs.And even then many boogie owners complain about their tubes not sounding the same even though they were 'Matched" at boogie land.
There are no absolutes when it comes to tubes.
Here is one for the scope guys out there:how often do you get your scopes calibrated?Are they accurate?
I bias for tone and measure the current draw to make sure I don't fry up some nice tubes.As an example;JJ tubes seem to draw more current than other tubes of the same variety.they require re-biasing to get them to live and sound their best.
-38v or -42v means nothing if the tube current is not consistent.It's just an idication that the bias supply is working.It may be at -38v the tubes in the amp are drawing 10ma more current than the ones you took out.
 

Blue Strat

Member
Messages
30,686
Old Tele man said:
re: "Another pair of the same brand/type of tubes would bias completely differently."

...reminds me of the joke: two Chinese twins are being introduced to two African twins...and one turns to the other and asks "...if THEY'RE twins too, then why don't we ALL look alike?" (a parody on "matching" tubes)

Heehee! That's good!:D
 




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