Do you re-bias your amp after changing your power tubes?

[zhivago]

Hello everyone,

my search for mad tonez in the Marshall vain finally ended about two weeks ago when I scored a Friedman Smallbox Wildwood edition. The amplifier is everything I ever wanted and more when it comes to going from Plexi sounds to JCM800 and beyond.

This is my first ever EL34 amp, and after a couple of weeks of rocking out on it on a nightly basis the power tubes went bad.

I quickly ordered a fresh pair of matched EH EL34s and dropped them in yesterday. The amp was back to normal and kicking ass, as per usual. I ended up playing for an hour or so, and afterwards it occurred to me...should I have the amp re-biased?

Every amp I have ever owned before did not need biasing, but from what I read online these amps need it. Do I really need to re-bias every time I change power tubes? In every thread I came across there really was no explanation that made sense...it felt a bit like internet folklore.

So I ask you, OSG folks...if you have an amp like mine (not one with fixed bias - like my Tweed Deluxe, or Matchless Lightning, for example), do you re-bias every time? Sounds like a bit of a pain in the rear?

I thought we'd have a poll, so the lurkers can join in the fun!

[øøøøøøø]

If you have a way of knowing that the new tubes draw the same plate current as the old ones, you don’t need to.

I remember Mesa used to write plate current draw of matched pairs on the box.

without that info, you should.

[zhivago]

Many thanks for chiming in, Brad.

I don't know what the old ones were drawing at all, so I think a trip to the tech is in order.

Thank god I have the head version of the Small Box 50!

[Dr Tony Balls]

(not one with fixed bias - like my Tweed Deluxe, or Matchless Lightning, for example)

This is going to get you in trouble using the wrong terminology. The two basic types are:

Fixed Bias - As in a fixed amount of negative voltage is supplied to the grids, biasing the tubes to correct operating currents. The amount of that voltage can be adjustable, via a pot, but it is always set to a fixed level. This is the standard in most Marshalls, most Hiwatts, most larger Fender amps like a Bassman, Showman, Twin, etc., and power tubes running this way are more efficient, generating more power. A pair of 6L6s in this arrangement can generate 40 watts or more.

Cathode Bias - Is a method of self biasing or automatic biasing and is sometimes called those terms. The bias in these amps does not need adjustment with new tubes. This is the standard on a Tweed Deluxe, Vox AC15, AC30, and is less efficient generating a bit less power but more compression. A pair of 6L6s in this arrangement will generate about 30 watts, max.

Anyway, on a fixed bias amp, yes I would always at least check the bias when changing tubes. I might adjust it, but if it's withing acceptable range I might leave it be.

Also, matching tubes is important but not all there is to biasing. You want tubes to be both matched AND biased correctly. There are tolerances to each but both are important.

[JSett]

I always check them. One of the most handy things I bought was a BiasRite / Bias Master which means pretty much anyone with half a braincell can swap power tubes and get it roughly where it needs to be. They're overpriced for what they are BUT it pays for itself very quickly...

This is the one I have

[zhivago]

Thanks for all your help guys

I guess a trip to the tech is warranted for now...I'll look into that BiasRite device though for the future...getting the amp to the tech and back every time sounds like a bit of a pain in the rear, so I should figure out how to do it myself, I guess!

[JSett]

Thanks for all your help guys

I guess a trip to the tech is warranted for now...I'll look into that BiasRite device though for the future...getting the amp to the tech and back every time sounds like a bit of a pain in the rear, so I should figure out how to do it myself, I guess!

Yeah, it's super easy to use. It can all be done with a multimeter but that's way more involved and means you're likely to have to get involved with the high voltage madness inside. These you literally plug into the amp with the new valves, check what -mA number your amps wants to see and then adjust until it's right. It's a 5min job once you've done it a couple of times.

But, for now, a quick tech visit won't hurt for sure.

[Dr Tony Balls]

I put bias test jacks and adjustment pots on the back panel of every amp I build to make this sort of thing a breeze. Friedman really ought to be doing it too, imo.

[JSett]

I put bias test jacks and adjustment pots on the back panel of every amp I build to make this sort of thing a breeze. Friedman really ought to be doing it too, imo.

I saw those on yours and did think it was such a sensible idea.

[zhivago]

Yes, it is a bummer that my Friedman amp doesn't have an external pot for it. It's a Wildwood edition Small Box 50, so not cheap at all!

I'll definitely look into doing it in the future myself!

[øøøøøøø]

There's a good reason for not making bias easily user-adjustable.

Some users might think it's another way to "tweak tone" and then complain when their amp burns up output tubes.

There's a less-good (but totally-understandable) reason for not including test point jacks on the rear of an amp that's designed to emulate a vintage design: guitarists are way too superstitious and are irrationally-suspicious of anything that deviates in any way from 'vintage correct" (see: volute, Gibson).

There's a high likelihood that a manufacturer of vintage-style Marshall-clone heads would sell fewer amps by adding such a useful feature.

Yannis, I'd really recommend getting a Bias-Rite or similar. Then you can check bias without even opening the amp... it might be pretty close to in range already, and that'd save you a trip to the tech. And if it does need adjustment for the new tubes, it's just about being comfortable adjusting an internal trimpot

[zhivago]

Yannis, I'd really recommend getting a Bias-Rite or similar. Then you can check bias without even opening the amp... it might be pretty close to in range already, and that'd save you a trip to the tech. And if it does need adjustment for the new tubes, it's just about being comfortable adjusting an internal trimpot

Good advice there, Brad...I contacted a tech earlier and I am waiting to hear back...I will check youtube to see if there are any videos showing the process using that gadget. It sounds pretty straightforward, but I do get nervous whenever I open an amp!

[s_mcsleazy]

i usually try to get ones in the ballpark of the ones i removed.

[hexes]

Fixed Bias - Absolutely.
Cathode Biased - I will at least measure. Takes 15 seconds.

every amp I build has adjustable bias and bias test points. I’ve moved to individual bias adjustment per tube with exception of bias-vary tremolo amps, then I use a bias pot and a balance pot.

[andy_tchp]

There's a good reason for not making bias easily user-adjustable.

Some users might think it's another way to "tweak tone" and then complain when their amp burns up output tubes.

Accurate. I remember reading people 'biasing a little bit hotter to get more out of these' way back in the usenet days...