Fixed Bias Question

[darkshadow54321]

If an amp has fixed bias, does it matter what brand of tubes you use, as long as it's the right type?

eg: EL84's... EH versus JJ/Tesla

[midnite_man]

Re: Fixed Bias Question

I would make sure the tubes you install are matched sets; that the values they read on a tube meter are the same. That'll get you close to a proper bias.

[Gearjoneser]

Re: Fixed Bias Question

In a fixed bias, many people believe that it's best to go with the original brand of tubes that the amp came with. My attitude is that the best tubes are the ones that should be put in there, not the originals. If the bias runs a little hotter, it may be a better tone, so that's what matters. You're pretty safe just going with Tesla JJ's. Most amps I've heard have sounded better with those, anyway.

[chcjunior]

Re: Fixed Bias Question

I was told that Sovtek tubes come closest to a consistent standard off the line so far as ratings go. At least, that's the word from Soldano's Tech Dept.

[Pandemonium]

Re: Fixed Bias Question

Stick with matched sets near the middle of the range and you should be fine. If you want to go with JJs, Bob at Eurotubes can sell you the best tubes for your particular amp and his prices a great. As an alternative, you could go with a rated tube, like something from Groove Tubes, then once you found what you liked it would be easy to keep that same sound. Me, I just use a matched pair of NOS or used OS in my fixed bias Bandmaster and never give it a second thought. Never had a problem. My favorite tubes in that amp are Tung Sol 5881s though, which are what came in it back in '59, but that's just a personal preference, it sounds great with 6L6GCs too, and lots of people would prefer the extra headroom.

[STRATDELUXER97]

Re: Fixed Bias Question

Fixed bias is a term used and the term cathode biased...The cathode biased amp literally allows a guy to place tubes in and out of an amp without having to adjust anything...

AN old Marshall and Fender amp and many many other amps are "fixed bias" but have an adjustable bias pot within the amp to actually match the current going into your output tubes...A Mesa Boogie amp is also "fixed bias" but they don't give you the bias pot so you can't tweak for other tubes and are therefore locked in to using Mesa's tubes...As long as you stick with the same output tubes,you don't have to bias your tubes,BUT: Anytime you switch over to a different brand tube,you should have the bias at least checked...A tube biased to hot is going to wear out faster and not give you the correct headroom your amp requires....A cold biased tube is going to rob your amp of power and make it seem weak and sterile sounding...Having the amp biased to the actual tubes you're going to use is the only safe way to know your tubes and your amp are going to work together "correctly"...I use 1 ohm cathode resistors across each of my output tubes and set my bias pot for x amount of current through each output tube set in millivolts...There's more to this than just sticking said tubes in your amp and rocking though!

John

[That90'sGuy]

Re: Fixed Bias Question

I managed to luck out. I put in Svetlana Winged C EL34s in place of some old EH ones and it sounds absolutely phenomenal. No weakness of tone, but it doesn't sound like it's overdriving to quickly either. I wish the Triamp had some sort of adjustment knob or something, but it's fixed which means that for me, I'm gonna keep replacing it with the same brand tubes as I have now and hope everything runs smoothly.

[BluesNRock87]

Re: Fixed Bias Question

i wouldnt be afraid

my amp runs 4 12ax7's, and 4 el84's, and is fixed bias, absolutely no pot or adjustment.

ive tested out a few new tubes with it, some nos, some new, and each and every one of them sounded pretty good, some were amazing. i am gonna get some new tubes, hopefully this weekend, im going with a ge for v1, a rtt for v2 (40 and 30 bucks) eh for 3 and 4, and jj's for the power amp, a matched quad.

get matched, o yea

[STRATDELUXER97]

Re: Fixed Bias Question

i wouldnt be afraid

my amp runs 4 12ax7's, and 4 el84's, and is fixed bias, absolutely no pot or adjustment.

ive tested out a few new tubes with it, some nos, some new, and each and every one of them sounded pretty good, some were amazing. i am gonna get some new tubes, hopefully this weekend, im going with a ge for v1, a rtt for v2 (40 and 30 bucks) eh for 3 and 4, and jj's for the power amp, a matched quad.

get matched, o yea

Cool,consider yourselves lucky!

John

[Kent S.]

Re: Fixed Bias Question

I was told that Sovtek tubes come closest to a consistent standard off the line so far as ratings go. At least, that's the word from Soldano's Tech Dept.

Which Sovtecs though,i've heard Lew speak before about there being a changeover in the manufacturing sites or something? Also, self-bias is the term used for cathode biasing, as the amp self-biases ... fixed bias is the term used for grid biasing, i.e. ... adjustable bias, meaning you *fix *the bias where you want it when you adjust it. However, I suppose an amp could be adjusted from the cathode via a pot, instead of an RC network.
Preamp tubes are self-biased, cathode biased, you plug different tubes in and get slighlty different results. Generally, I'd agree with (match output tubes always) middle range rated tubes. The real trick would be to look up the specs on the NOS tubes that were OEM to that amp, then find a matched set that has those same characteristics ... or like pandemonium said call eurotubes, and have them sort it out, if you knew the current draw, you could invest in a GT bias probe, and book ... pretty good device, that's basically what STRATDELUXER97 has built into his amp with the use of the 1ohm 1% resistors, from those he takes his readings ... the bias probe is based on the same idea, except the tube plugs into it, and it plugs into the amp, the lead from it go to your meter. One complaint that is heard that I agree with is that self-biased does run a bit hotter (heat wise) and wears the tubes a bit more, which is why some of the cheaper tubes don't like it much, for the NOS, OS, JAN ,or some of the newer high quality tubes, or burned in, tested tubes (GT's,Boogies,etc,) it isn't that big of a deal. Cheapo Chinese tubes ... err ... no comment. One more thing to consider is that those older amps, were made in a different time, the power (speaking for the US) back in the '50's and '60's was actually lower in voltage then what you get now in many locales ... 110 was that at one time, then it went to 115, then to 117, then to 120 ... heck this place measures 126 to 130 (and that's true RMS, not peak) ... if you don't think that changes the way the tubes act think again, it's not critical, but for getting your amp to sound as close to the way it did in it's hey day ... good thing to remember.