how do I set my Bias?

[mattbreaker]

Hello all, My name is Matt and I am new to this forum, I've recently bought a cheap Bugera amp for session gigs while my other gear is away being repaired.

I've used Valve amps for years (Got a cornford RK100 for my band www.myspace.com/breakerband) But this query is a new one for me.

this Bugera (6262-2x12) is equipt with 4 6L6's and 5 12AX7 drivers, the amp sounds a bit naff as it is so I have ordered some EL34's (matched quad) to soften the sound out a bit.

I have never touched the bias control on an amp before as I've never re-valved them myself, can someone advise me as to how to do it??

I work along side a load of electronics engineers by day so tools and test equipment is to hand, but never touched amps before so direction is appreciated!

Thanks guys,

Matt.

www.myspace.com/breakerband

[stokes]

Do you have a bias meter or a multimeter to measure the current draw?

[Tom Phillips]

Matt,
You will find a lot of good information to get you started at:
http://www.geofex.com/tubeampfaq/taffram.htm
Scroll down the topics at the left side of the page to find the bias topic.
Tom

[mattbreaker]

Alright folks, thanks for all your help I shall certainly save that link that Tom sent me so thanks for that.

I got my collegue to sort it for me, he had to take the whole thing apart to get a reading for the idle current, good old germans glueing everything down to the PCB! Gotta love that!

No doubt I will be on here again when I break one of my amps in the future!

Cheers Chaps.



www.myspace.com/breakerband

[Enzo]

Hi Matt, welcome to the forum.

If this is the same Bugera 6262 you posted about three days ago, it is not necessary to start two threads on it.
Bias adjusting on all Bugera 6262s is the same, and it is the same as for most any tube amp.

[buddhapaugh]

Bugera Biasing Of 333XL

Hi Enzo,

I am sure you don't remember me but I have been to your shop in Lansing to have some repairs done. I was kind of excited to find you on the Net--(yeah, you are a bit of a cult hero amongst musicians in the Mid-Michigan area!!).

Anyway, I have a new Bugera tube amp and saw that you were answering someone else's question about biasing that amp in this thread. I was wondering if you could tell me about biasing the Bugera and if there is a set range for measuring the Negative Voltage on the bias test point on the Bugera amps. My intitial measurement from the factory for EL34's was between -40 and -41.

Could you tell me if that is optimal for EL34's and what the optimal reading would be for 6l6's??

Any other helpful/pertinent info would be greatly appreciated.

Thanks again,
Dave from Michigan

(I'd stop by your shop and have you check it out but I am almost 200 miles away now since I moved up north to the Gaylord area!!)

Thanks!

[Enzo]

If you set that to around -42v for EL34 and around -55v for 6L6 you will be OK. That is what PV recommends of the very similar 5150 series. It will be a cool setting, but that still works.

A real bias setting is not done by bias voltage though, it is done by plate current to set plate idle power dissipation. For that you will need a "bias probe" from any number of commercial sources, or you would go inside and take live readings inside the amp. SInce the voltages in there can KILL YOU, I don't recommend that method for the inexperienced.

Do a search here for bias probe and look through the most recent threads.

And thanks for the kind words.

[buddhapaugh]

Hi Enzo,

That's great! Thanks for the response!

I have done quite lot of reading and was getting afraid to do anything because there are so many "scare tactics" used that suggest you can fry your amp by doing the wrong thing. I guess I thought it was as easy as adjusting the external trim pot and flipping a switch. A Carvin V3 I owned had that. The Bugera has it. The Peavey JSX has it. Guess it all makes me wonder why those are there if they don't serve to properly bias an amp.

Of course I would like to have my amps running smooth as they can.

So, if I may ask, are there trim pots near each Power Tube Socket that you adjust so as to individually set plate current for each tube... or is that something you have to solder in... or does it depend on the amp??

Maybe I should just come by the shop, eh??

Cheers,
Dave

[Enzo]

I doubt very much the Bugera has individual bias controls, that would be very rare in a commercial amp. One installs a matched set of tubes and then adjusts for the agregate current. Two controls is less rare, many Marshalls have that.

SOmething like the PV JSX or XXX has a very limited range bias adjuster - set it by ear. It is not likely to get so far off as to cause proglems. I don't know offhand how much range the Bugera control has.

People make out bias to be a lot more critical than it really is. If the tubes don't start red-plating, then the bias is not too hot. If the amp is not all pinched off and weak, then the bias is not overpowering. SOmewhere in between those it will be happy.

[pdf64]

Have you checked that the amplifier is specified to run with EL34s? They will draw another 2 amps total heater current, compared to 6L6GCs.
Re biassing, red plating doesn't tend to be noticeable until well over max dissipation, so just because they aren't red plating doesn't mean that they aren't very under biassed. ie if you check the bias by turning the pot until they red plate, then back it off a bit, they might be red plating at 150% of max dissipation, and you might wind it back to 120 or 100% and think it was fine. Peter.

[Enzo]

When they have an EL34/6L6 switch on the rear panel...