• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Replacing a power tube (used as driver for mosfet car amp)...how to bias?

replacing power tube

Hi all,
I'm new here.I've seen there are many expert here,and i absolutely need your help.
I bought a used butler tube driver td1500 on ebay,but a power tube got broke during the shipment.
I just ordered a replacement,but i read that you need to adjust the bias on tube amp.How do i do that?
I phoned many car audio installer and they don't know how.
Music instrument store want to work only on home tube amplifier
Any help?Someone with that amp?
The amplifier is that :

It is the original or the ppi version?I can only find the word phaze audio on it,no sign of butler and no sign of ppi :D
I don't know how it is biased...since you have to set it..it looks like fixed bias. In that case you should be able to find a little potentiometer with a track or wire leading to the grid of the tube in question.

What kind of tube is it? Looking at the specifications it looks like a pre or driver tube is being used and not a power tube. It is unlikely (with my limited knowledge) that the driver tube is fixed bias...leading me to think that you might stuff it into its socket and voila.
I don't know if that specifica amp has to be biased,i just read that the general rule for for replacing power tube amp is to bias them(have done some google search,but found only home amplifier topic).
The tube is that :

I found a little screw near the 2 socket(there are 2 power tube) ,between the 2 of them, that could be a potentiometer ,but why there is only one if the tube are 2?
I will post some picture when i return home.
The photo would help I think..self bias is very inefficient..so it is likely that fixed bias was chosen..

here is the datasheet

From this you can see that pin5 is grid one..ie that is where the bias needs to be set...to something -20VDC... so measuring between earth and pin 5 should give you around that voltage. The manufacturer however should have listed what the bias voltage needs to be.

NB...Your amp probably has lethal voltages inside. BE CAREFUL.

ps...since this thread concernes tubes..I'll move it to the tubes section...hopefully others with knowledge of this amp can chip in...besides in the tubes section there are more folks with the relevant knowhow..if you don't mind that is...
Hi Christian...

And i have 1 potentiometer only,does that mean i will change the bias of both?Is this possible?

I've never used fixed bias myself...but I think that is very possible, but not so smart (I think)..as you cannot set the bias seperately for the 2 channels...????

Let's hope others chip in. :)

Unluckily i didn't got any manual from the seller so i hope it is -20v,or you where referring to the manufacturer of the tube(sovtek)?

That is just a general figure I got from the datasheet.
I don't think that thing has power tubes in it, and the link that the guy provided was only to explain the bias thing, NOT meaning that there is a 5881 inside there (how would you fit it? :bigeyes: ). Probably the guy doesn't know how to distinguish a pre tube form a power one. It will help if he posts any writing that's visibile on the damaged tube... I bet that's a 12AX7 or something.

Tell us, you tube looks like this

or more like this

What's the height of the tube? A pic or the model number would help...
Oh excuse me I didn't read carefully the whole thread.

How can only 2 5881 make a 150W amp???? :bigeyes: I'd expect either class D (I don't think so) or a quad of those PER CHANNEL... so 8 tubes!!!

However it's possible that the power rates are inflated. Expect about a tenth of what is advertised. If the amp is really a SE of 5881, expect no more than 10-15W per channel. Btw, are even the output transformers inside the unit??? Can you provide a photo of the amp with the top cover removed??

It is quite important that we know the topology and the type of the amp in question before we can give any advice on tube replacing and biasing... because I think that's not a conventional topology!
Actually i just found googling that the amplifier is not a true vacuum tube amplifier.It just use tube for the input and mosfet for the output(haven't truly understood what that means :D ).
More info here :

What a pity,i didn't know that :(

BTW i got a reply from the butler technical stuff :

"The amp is self biasing as the tubes are run in class A.
No adjustments are necessary.
Butler Audio Staff"

So all i need is to replace them,but it is normal that they are soldered?
More shots coming.
In fact it is an hybrid amp. It means that vacuum tubes are used as voltage amplificator, where the heavy duty of driving the speakers is made by transistors. It is not a good sign if the tube is soldered on the PCB: a 5881 directly soldered is quite hard to remove IMHO... are you absolutely sure that the tubes are 5881? Are they very big (about 10 cm height)? I'm still sure that they're some other kind of tube... a 5881 to drive mosfets is too much... even in 2-3kW amps I think.

Now I read the post made by agent.5: at this point I'm quite sure that the tube isn't a 5881 but a lighter and smaller one.
Thanks agent.5,at least now i know i have the original butler tube driver pre '99,since i have different tube.
Now i know why the butler technician were so fast to answer(i don't think they are so kind with the ppi version).

Here are the photos:

You can see the board layout and the tube model(it's about 9 cm tall ).
The screw near the TIP31c is what i thought was a potentiometer.
Yeah, it's a real 5881!!! excuse me for not believing in you.

Ugh... that tube is pretty bad damaged... and very cheap mounting tecnique. I suggest that you desolder the tube and before replacing it install a proper tube socket. Probably the tube is cathode biased and won't need bias regulation. But maybe a better brand over the cheap Sovtek...
Don't know about getting a socket..if you do you would have to get one with a clamp that would hold the tube in the socket...otherwize its better to just solder it in again. One would also need to drill quite a lot to get the socket in there in the first place.:xeye:

I would clean the solder resin though..it can absorb moisture (hygroscopic) and create a short.
okay, the tubes you have are 6L6's. You can replace them with any kind of 6L6, it doesn't have to be a SOVTEK 5881. I would still use that same tube, unless you want to change tehm as a pair, since it's one tube per channel, and you might not want to have two "different sounding" tubes in it. This isn't something that's going to blow up the amp, however.

What I would do would be to use a desoldering doodad (your preference), and then remove the old tube. You probabally have to cut off a few pins on the new replacement. Be absolutely sure you check the orientation of the new tube. That little plastic thingie in the middle of the pins has a notch on the side to designate the break between pin 1 and pin 8. it must be in the same orientation as on the "old" tube. putting the tube in at an incorrect orientation is like putting in a diode backwards, or an IC the wrong way around, and it has a good change of letting that magical blue smoke out. This was a problem on tubed equipment with octal tubes since sometimes you broke that little plastic locater key, and you had a tube that could be inserted 8 different ways.....gotta watch those ones :eek: I would just resolder it back in and be done with the job. You can add a socket, but you would need to support the tube somehow so that it doesn't wobble around (a removeable glue at the base, spring clips like in Fender amps with upside down mount chassis, or some high-temp foam stuff). Soldering solves the mechanical issues, but makes tube replacement a chore. I still think it's kind of a ghetto solution compared to a socket and retainer.

The reason many of us were sceptical at first is that we figured that if this were a typical hydrib amplifier, there would be a dual triode tube or two, usually a small 9 pin 12AX7/12AU7, or the larger 6SN7 (like a grown up 12AU7). If the amp were completely tubes in the output stage, you would need, per channel, one tube (SE) to get ~10 watts out, 2 tubes (push-pull) to get 50, two tubes (parallel SE, not that common, since you might as well go push-pull) to get 20, 4 tubes for 100, and 6 tubes for 150, and tube amps do not have that property where the lower the impedance is, the higher the power is. Tube output stages also require a fairly large output transformer. The buttler design is pretty unique in that it uses a largeish power tube to directly (?) drive the output MOSFETs
Well i have already ordered the tube,i can't change model now.
It is true that i don't have to touch the glass when i replace them?Because i saw the guys of the music shop that was touching the tube without caring at all.
Actually the tube was attached with a glue on 2 point,on the top of the valve(you can see that on the photo) and on the bottom of the pcb.I don't know why the glue did loose,since the package was very strong and well done,the courier must have really throw that from the airplan still flying!
Too bad to pay less import taxes i insured it for almost anything.


Yes,it should have been made between '97 and '99 if i understood well,because then in '99 there has been the ppi model.