Un-ajustable fixed bias?

[boobtoob]

Hello techies::BluesBros

Just getting to know tube amps and their intrals...looked up my DC-5 on schematicsheaven.com and noticed that there is NO cathode resistor. There is a connection at the grid from the PT. Looks like -49v. Would this be considered an unadjustable fixed bias amp? No bias knobs anywhere in sight...

Wouldn't cathode bias be a better way to bias tubes? The cathode bias kinda automatically adjusts with the current flow of the tube, right? The unadjustable fixed bias would stay steady even when a cheap, low current tube is installed.

any input is appreciated

[brad347]

you usually get more power with fixed bias.

And yes, there are unadjustable fixed bias amps. Most brownface Fenders fall into this category. :)

[John Phillips]

Yes, the DC-5 (like all Mesa's fixed-bias amps) is un-adjustable... or not without changing a resistor, anyway.

So what? Just use good tubes and it will be fine. That doesn't mean you have to only use Mesa's own branded ones, either... just good ones that are close to the correct spec for a 6L6. I once tested about forty or fifty used old-production tubes in mine, and not one was outside the correct bias range. Mesa know what they're doing, and although this approach isn't fashionable or popular among people who like to tinker with amps, it does work and there are good reasons why. I have never had to rebias any Mesa amp I've ever worked on when fitting good tubes, BTW.

If you don't want to use Mesa tubes, NOS or used old-production, just get a reputable tube vendor to give you a set that will work in Mesa's bias range. If they don't, the problem is with the tubes, not the amp.

Non-adjustable fixed bias has one advantage over cathode-bias... you can change the bias point by selecting different tubes. With cathode bias, you're pretty much stuck... even using different tubes won't change the bias point much, and many cathode-bias amps run far too hot for a lot of modern-production tubes to really take, no matter if they're 'hot' or 'cold' tubes - and there's nothing you can do about it without modifying the amp.

Yes, I like cathode-bias too (it's particularly handy if you want to use really different types of power tubes, since it's possible to arrange it so that more than one will work - eg 6L6s and 6V6s) but it has as many disadvantages as advantages.

You may ask what the disadvantage of adjustable fixed bias is, since it would seem to be the best solution... it's quite simple: people who don't know what they're doing can fiddle with it. Mesa believe that this is likely to cause more problems than the variances in tubes do on a non-adjustable amp. From experience as a tech, I agree...

[Lespaulsignature 74]

Mr. Phillips.....What is Mesa's bias range for the DC5...I need to replace the 6L6's and I don't want to order from Mesa. I usually buy my tubes online from The Tube Store.

[JJman]

I have amps that are adjustable-fixed, adjustable-cathode, and non adjustable cathode. Adjustability and fixed-vs.-cathode are separate traits as you already appear to understand.

Non-adjustable fixed is strange to me. I wouldn’t want to be limited to using tubes that need to be pre-sanctioned to bias correctly in an amp. Or have to change a resistor each time. I think some of those setups are marketing ploys to get people to buy the “proper” tubes from guess who? -The amp manufacturer.

[Ronsonic]

Non-adjustable fixed is strange to me. I wouldn’t want to be limited to using tubes that need to be pre-sanctioned to bias correctly in an amp. Or have to change a resistor each time. I think some of those setups are marketing ploys to get people to buy the “proper” tubes from guess who? -The amp manufacturer.

Like John says, this is a non-problem.

It's fun to fuss with bias points and all that but there's a big, wide sweet spot that isn't hard to hit with a properly functioning tube in a well-designed amp if that's a design goal. For Mesa it is. I've tubed dozens of Mesas and always check the bias and not found any problems with an off-the-shelf matched set.

The other marketing ploy is "hey kids, lets all neurose about our bias settings and make sure each tube is within a 0.1mA and it's all adjusted to some number from my book or according to this magic formula."

[skhan007]

Can I ask you guys this question: I have a Mesa Stiletto Ace that runs EL34's and I would love to swap in a vintage NOS set to see if I can get rid of some of 'harshness' that the current set up has (I'm also swapping in some NOS 12ax7's too)...what do I need to know when I'm looking to purchase a matched pair of EL34's for the Mesa? Is there a particular bias range, like some of you have mentioned here, or can I feel safe knowing that a good matched set will simply be good to install? This is a great thread, so the advice is much appreciated!!

[Mr. SD-1]

Simply use Mesa tubes or Groove Tubes in a 5 or 6 rating. I had a stock DC-5 running Groove Tube KT-66HPs. Perfect bias and awesome tone.

[RedMan]

Both fixed and cathode bias amps can have thier bias adjusted by changing a resistor.

[skhan007]

Simply use Mesa tubes or Groove Tubes in a 5 or 6 rating. I had a stock DC-5 running Groove Tube KT-66HPs. Perfect bias and awesome tone.

Well, I have Mesa's in there now. I would like to try NOS/Vintage tubes to see if they are less 'harsh' sounding than the new production tubes that came with the amp.

I guess I should find out 2 things: 1) what is the bias range for the Mesa Stiletto Ace and 2) what NOS/Vintage el34's can I get in that range as drop-in replacements.

[Ronsonic]

Almost any of the tube-mongers who burn-in and match tubes themselves (the guys you'd want to go to for NOS) can pick you a set if you tell 'em what you're putting them in.

[AR-305]

Like John says, this is a non-problem.

It's fun to fuss with bias points and all that but there's a big, wide sweet spot that isn't hard to hit with a properly functioning tube in a well-designed amp if that's a design goal. For Mesa it is. I've tubed dozens of Mesas and always check the bias and not found any problems with an off-the-shelf matched set.

The other marketing ploy is "hey kids, lets all neurose about our bias settings and make sure each tube is within a 0.1mA and it's all adjusted to some number from my book or according to this magic formula."

+1 guys. Seems like all the tube dealers hawk some of these forums and create a whole bunch of propaganda designed to sell "their tubes". Many of us remember when it was not all that important that the output tubes be perfectly matched, that is untill a certain guy found the ultimate marketing ploy for his new brand of tubes. We've all played many older amps with "unmatched" and "incorrectly biased" output tubes that sound like a million bucks.

[Mr. SD-1]

Well, I have Mesa's in there now. I would like to try NOS/Vintage tubes to see if they are less 'harsh' sounding than the new production tubes that came with the amp.

I guess I should find out 2 things: 1) what is the bias range for the Mesa Stiletto Ace and 2) what NOS/Vintage el34's can I get in that range as drop-in replacements.

I don't really think NOS bottles will tame that harshness. Regardless, these are some great tubes that should bias perfectly in your amp:
http://mesa.stores.yahoo.net/elsiemst.html

[skhan007]

I don't really think NOS bottles will tame that harshness. Regardless, these are some great tubes that should bias perfectly in your amp:
http://mesa.stores.yahoo.net/elsiemst.html

Thanks for this link!

So, I guess that these Mesa-branded Siemens tubes are drop-in replacements for Mesa EL34 amps? No re-biasing/visit to the amp tech required? If so, this might be the ticket.

[rockon1]

+1 guys. Seems like all the tube dealers hawk some of these forums and create a whole bunch of propaganda designed to sell "their tubes". Many of us remember when it was not all that important that the output tubes be perfectly matched, that is untill a certain guy found the ultimate marketing ploy for his new brand of tubes. We've all played many older amps with "unmatched" and "incorrectly biased" output tubes that sound like a million bucks.

I thought exactly like this up till the point I bought a bias probe and learned more about biasing. 25 years of slapping tubes in amps,yes, with no problems. That said Im sure I had my share over overly biased tubes that wore out prematurely. At least with a bit of knowledge I can buy tubes from one of the "tube hawking" tube dealers that are tested for lower current draw so I dont end up with an amp idling over 70% needlessly. Its a lot easier than modding the amp IMO and I feel better knowing the amp is truely within proper operating range. Besides its amazing the wide range of bias Ive seen within the same make and model of tubes these days. I imagine thier was more uniformity back in the day. Bob

[AR-305]

It is certain that the tubes of the 50's and 60's were built and spec'd to much higher standards than the stuff we see today. I've just seen so much Mesa bashing go on because of their decision to make their amps with factory set-non adjustable-"cold bias". I learned to bias amps with an ocilloscope and my ears and as you know many of the older style amps, particularly Fenders, had a set bias voltage but with a balance pot to compensate for "unmatched" output tubes. I was simply agreeing with John Phillips and others that: those who have zero knowledge of setting amp bias cause more problems than slight variances in tube paremeters cause for a non-adjustable bias amp.

[Ronsonic]

I think Randall Smith earned some of that ill will honestly. His white paper on adjustable bias and why he doesn't use it basically calls any tech who charges money to bias an amp a fraud. Now maybe that's how he did it when he was teching and it's the old story of he who accuses all convicts one. In any case, a lot of techs have heard from clients or potential clients about what they've read on the internet about what a scam bias adjustments are. Coming from Randall Smith it appears credible.

[AR-305]

There's a really really big difference in my approach to bias as concerning say a good blues/jazz player with a classic tele and a great old Blackface and a fellow with EMG 81's and a Mesa Triple Rec. Now one of these guys (you pick) really loves his amp to have that touch sensitive yummy sweet toothy breakup. The other wants to drive a pair of 4x12's at 110db and uses so much preamp gain that he has to have a noise gate so the pickups don't screetch between notes. Which one of these two guys will be most appreciative of a tech/musician who knows the finer points of biasing his amp. Oh and one of the amps has a switch on the back so the user can switch between 6L6's or EL34's.