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Anyone want to help with a step by step DIY bias?
by Martallika666 on 2009-08-22 09:51:09.6000

I am familiar with electronics. Was top in my class when I was in school, however that was 5/6 years ago! I have successfully modded a valvetronix ad30vt with a fx loop and all worked out fine.

What I am asking, is anyone up for walking me through, step by step, biasing my 112. I mean literally like "take this off, put this in there, wait,... and, your done" know what I mean?

It would be awesome to do it live like on facebook chat or something. Forgive me if this seems a little far fetched and ridiculous but I am a student with very minimal cash and the local guy wants to charge £40 ($75) to bias my amp! No thanks.

Re: Anyone want to help with a step by step DIY bias?
by pikeral on 2009-08-22 12:24:38.1370

$75 is a small price to pay to not have your heart stop.... Here's a link for a biasing discussion that i found valuable, it's a long read but all the info you need is there. WARNING! you could DIE doing this yourself. A qualified tech should perform this.

Re: Anyone want to help with a step by step DIY bias?
by Martallika666 on 2009-08-22 13:10:02.5020

Thanks. I realise the safest option is to buy a bias probe rathe rthan prod around with a normal multimeter so i am getting one next week.

Re: Anyone want to help with a step by step DIY bias?
by Daniel-Ganfield on 2009-08-23 17:03:08.0340

I think we are all a bit afraid of what could happen to you and are not willing to be the one to give step by step directions on how to bias your tubes.  Having said that, I did it and had a good result.  Just take your time. Research safety tips, like keeping one hand in your pocket so that you reduce have electrical current go though your heart if you touch the wrong thing and get a shock.  Maybe work with a friend.  I spent several hours just testing the amp with my multimeter to see which parts produced what voltage and how long that voltage remained after the amp was turned off.

Re: Anyone want to help with a step by step DIY bias?
by jmgalante on 2009-08-24 08:29:01.2460

Sure - here ya go...

How to bias SV112.

Bias is the amount of Plate (+/- cathode) Current flow at 0 input signal, it is “idle current”.  It is dependent on the negative Grid (signal) voltage.  It is the basis starting point for the incoming ‘input’ signal.  Input signals from the guitar raise or lower this bias voltage more or less negative.

Some notes & info…

“Desired” Amp Bias is based on the following formula:

W / V * 0.70 = A

W is the tube's rated dissipation (in watts)
V is the plate voltage (high voltage); Pin 3 of tube vs. ground
0.70 is a desired P, percentage of the tube's dissipation capacity (watts). This percentage should be between 0.65 and 0.85.  *Line 6 recommends using 35mV for A.
A is the bias (cathode current in milliAMPS); this is adjusted to a desired level by controlling the voltage on the grid with the pots.  In the Spider valve it is measured in mVOLTs at the test points.  *Line 6 recommends using 35mV for A.

Where to get "W": Call your tube dealer or check spec sheets for rated dissipation (typically 25w or 30w). i.e “My tubes are a pair of matched *6L6GC's and have a rated dissipation of 25 watts.”  *Spec sheets show 30watts max; most recommendations for 6L6GC however base it on 25w.

Where to get "V": From your Amp. This is Plate Voltage, from the actual tube pins.   See below. 

Where to get "Percentage of tube dissipation": If you have "W", "V" and "A" then rearrange the formula to P = A / (W / V), where P is the percentage.

Where to get "A": From your Amp – you are setting this!  You measure & set the bias,

in mVolts, via the Amp test pins; these readings are the same as bias current in mAmps at the tubes (Line 6 has 1ohm resistors in place). 

**Note: You test and adjust one tube at a time but you must have both tubes installed any time the amp is on!

When getting values for "A", it is always better to make sure the amp is powered off before making any connections!  With proper socket-type probes most of these measurements though can be made 'hot, and the Tube Plate Voltage must be measured with a needle probe - with the amp already on - BE CAREFUL!   When turning the amp on, be sure to take the time to warm it up, make sure you have the speaker connected, make sure you have 2 tubes in place. 

Note: Meter leads with sockets that fit the test pins are easier and safer then alligator clips – I made some using the socket parts of insulated Panduit “Fast Ons’ stuck/secured to the ends of the stock (Fluke) meter needle probes. These were totally insulated (shrink tube) and can be slid over a test pin and will stay in place w/o your hands getting close to the circuit.


***Note:  Bias milliVOLTS are measured at the test points, set for +/-35mV.  In contrast, setting the amp at 35mA from the test points runs the tubes wayyy hot! After biasing my amp to 67%, the test points showed 37mV, and 12.56mA; which means mistakenly configuring the amp for 35mA via the test points is biasing the amp to more than TWICE the recommended setting. This brings me to another point...

The trim pots are sensitive! I had plenty of room leftover after biasing. If anyone is running out room then something is wrong. If you find that the numbers aren't changing (or beginning to change in the opposite direction) as you turn the trim pot, then the bias is probably too hot and things are unstable.

Spider Valve Circuit Board:

amp cb.JPG

OK, let’s get BIASing…

-         Unplug everything from amp. Remove rear panel (4 screws).

-         Lay amp on its side.

-         Remove amp from chassis (4 top screws).  Carefully lean amp assembly against amp frame so you have easy access.

-        Plug speaker load in; have BOTH tubes in whenever powered!!!

-         Locate Test Pins V3, V4 and Ground on amp CB – it looks like a computer connector. Locate pots T3 and T4 down about 1.5 inches from pins. Locate V3 & V4 tube socket solder points (See pic)

For Cathode Current Bias – @Test Pins

-         Set Multi-meter to DC mV.

-         Connect meter ground / black probe to ground pin (cntr), & connect red probe to V3 test pin.

-         Plug amp in, turn on (make sure you have speaker load), let it warm up for 30 seconds & take off standby, let it warm up more.

-         Look at ohm meter, turn T3 pot until it reads “35”mV. ______mV : (_____mA)

-         Take red wire from V3 and connect it to V4 test pin, leave black probe connected as before - to ground pin.

-         Look at ohm meter, turn T4 pot until it reads “35”mV. ______mV : (______mA)

For Measuring Plate & Grid Voltages – Tube Pins

Adjust your meter’s range to read at least 500V DC. Ground the black probe to the Ground test pin.  These measurements are taking at the tube sockets on the circuit board.  Each pin on the amp board is labeled.  Small static may be heard at probe to pin contact – this is normal.  Grid Voltage can be measured the same way at Pin 5 for curiosity’s sake.

-         Set meter to 500+DCV

-         Leave Ground probe connected to ground pin, use Red needle probe to measure at tube pins

-         Plate: Measure voltage between Ground and tube Pin 3 (450V) v3______V : v4 ______V

-         Grid: Measure voltage between Ground and tube Pin 5 (-60V) v3______V : v4______V

Here are my numbers & some more notes:

W = 25 (tube rating))
V = 450 (measured pin 3 vs. ground)
A = 0.0372 (set bias = idle cathode current)

Therefore, P = .0372 / (25 / 450) = 0.668 =~ 67%

There is idle noise at high Master volumes, but it is small and considered normal. The pre-amp tubes make an impact on this (lower gain tubes = lower noise).

*The only time I notice a real noise (hum) is when there is a significant variance in biasing levels between the 2 power tube circuits (as when adjusting bias, which is done one pot at a time).

*Note: I had correctly biased a pair of JJ 6L6GC to 35mV.  When I 1st put in the Sovtek 6L6wxt+s, the reading was near 60mV!  Bias your tubes!

Other Tubes:

V2 is the phase splitter.  It is closer to the input jack (to the left looking from the front, on the right looking from the rear) and is traced to the power tubes.

“Pre-amp” tube V1 is further from guitar input, on the right, from the front.  It has the input cable, and is also easier to get at from the back (behind power tube cage)

Re: Anyone want to help with a step by step DIY bias?
by sinlimo on 2009-08-25 14:37:12.4460

NICELY DONE jmgalante!!!!! [insert applause here]

THAT was freakin informative, should be a sticky someplace.

Re: Anyone want to help with a step by step DIY bias?
by Martallika666 on 2009-08-25 15:25:37.5950

wow! Thanks jmgalante. That was awesome. WHen i get m yprobe I'll give it a go. Cheers, M.

Great explanation
by JKorzyp on 2010-08-28 08:16:50.0060

Thank you. Very detailed.


Re: Anyone want to help with a step by step DIY bias?
by spaceatl on 2010-08-29 19:25:54.4800

Probes that insert for true current are the best...I ordered $4 in parts and moved the adjustments and test points to back of the amp.

Here's the pics...Took 30 minutes...No danger of shock during the mod...

Here's the original thread.

Re: Great explanation
by mtrash14 on 2010-08-30 16:41:15.6180

In my opinion - any amp that does not have an external bias adjustment - - -  is not user serviceable.

I say this because I don't know you nor do I know your technicial ability.

Inside these SV tube amps lives over 450 volts of electricity and if you were to touch the wrong thing, slip and do an "oops", or simply disregard proper procedure, you could die.  You would not pass GO nor will you "ever" collect $200.00. 

I'm not kidding here.  Biasing, or opening up the amp in general, is very serious business and this is not for the dumb or clumsy because you wouldn't be back to complain.  You would probably be dead.  That is something I will not be part of.

I want you to be safe and happy and for you to live long and play music, like you wanted too in first place.  Just take your amp to an amp tech and let him work on it.  He knows how to do it safely and you will be free to play and be happy.  You are a musician, not an amp tech.

If you are wanting to be an amp tech then you are asking questions on the wrong forum.  You need to find an amp tech forum and an amp tech school.  Because if you don't you could get killed in there - and I will not be part of that.

Good luck


Re: Great explanation
by JKorzyp on 2010-08-30 17:08:41.4860

The question was answered. I like the mod to bring the test points and pots out the back. The pots can be tricky and the good linear ones are available from a good electrical supply house. Pay the bucks! As far as the better sound? I agree/ Harmonics, EMI and associated noise can be an issue with the cheap crap they stick in these amps like some of the garbage I get in my oversea's guitars. That mod I will do. AND just for the guy's who like to scare the hell out of people with this black magic. You could also die if you have your hand in a sink and short out the ac recepticle that doesn't have a GFI. I agree with with the warnings. For those who are not electrically capable-Stay out! The individual who doesn't know me, I get your warning but you don't know me. I have been doing ac drive start ups, board repair and trouble shooting and working with power electronics for over 25 years. The drives I play in have over 750VDC on their bus systems along with gate driver boards that will kill you. I have my hands in 460VAC all the time and am very knowledgeable of arc flash and other hazzards. I work in high voltage (13K) area's where all you have to be is within 6 feet and you will be a ground arc. I have been around alittle with this stuff. It doesn't scare me but I stay aware. I would suggest to anyone who even knows a little that guy's do this all the time. If you have no electrical knowledge-you shouldn't be asking how to do it. WE ALL GET IT! Some of us actually know electricity but do not know the particulars of this amp and don't want to spend $100.00 every 6 months and wait a week or better until my "guitar" amp guru gets to it. I just had a guitar "god" set up my LP because I was too busy and he screwed it up. I agree, let them fry themselves. Relax already. I take full personal responsability for my own actions in my life-What a fresh statement huh! Any young kids listening? I am also smart enough to know when I am over my head. I also look at these things from many directions and have many electricall talented people to confide in. I usually have a thought out game plan before I just "go for it".

Soooo- Thank you everybody for your concern & help. Many great offerings and solutions! Yes, I also appreciate the guy's who don't want me to kill myself-Me neither! GREAT mod-Thanks for the items. I will check the PIV ratings and current ratings on the components to make sure they have enough capability to give me a lifetime of trouble free service.Special thanks for the "Bleed the Power" solution-Needed that question answered and it makes sense.



Re: Great explanation
by spaceatl on 2010-08-30 17:45:28.8230

Everytime we have a thread like this, Trash sticks his "It can and will kill you, no second chances..." response...

I am very glad he does that, even us guys that have worked around this stuff for a long time it is good to have a reminder here and there...I might be putting words in his mouth and HE WILL correct me if I am wrong on this, but I have often thought that trash isn't directing these posts only at the person he is responding to...Sure, at the moment it's like that. Trash puts a lot of thought into what he does...I am pretty sure another part of his motivation is all the folks that read these threads and never post anything. I know he has caused more than a few people to think twice about these sorts of endeavors...And that's just about all that is needed...

Anyway, on your pots comment. I just got an SV MK2 and I am going to be doing the bias mod again...However, I think I am going to go with a 2W 10 turn wirewound...stupidly expensive I know...But that is as about as precise as I have been able to find.

What I really want to find it multi-turn that my dad told me about from Apollo days...NASA has lock-set multi-turn wirewounds with G ratings are are pretty incredible...The G ratings become important in a combo because of the vibration...Vibration really doesn't get much worse than a rocket as far as I can tell...Still looking around...

I found these 4 watt ones...They say military grade...I think the stock pots are 1/2 watt so a big 4W should not drift much under vibration...

very curious what you are thinking about using for any pot?

Re: Great explanation
by mtrash14 on 2010-08-30 18:18:17.5580

Thanks Space for the backup.  It sounded like I was needing one here.

Thats exactly right.  There seems to be several people that want "how to bias my amp" instructions and again - I want no part of the responsibility that writing such instructions might mean.  I would have no control over who might read or try it (open forum) and what the outcome might be in each of those cases.  lol

And if you JK0 have worked with such high voltages as you say for all those years then you should completely understand where I'm coming from here.  Knowing the danges as you should from your experiences at work do you think biasing instructions should be posted on an open forum such as this where everyone (anyone who might read this thread for that matter) might try it himself for the first time - thinking its an easy and normal practice?  I think not.

I'm not a "stick in the mud", nor am I crying wolf.  I'm trying to keep these guys (anyone that might ever read this) from danger.  Thats all.

In my opinion - if anyone wants to learn how to bias a tube amp then you're come to the wrong forum.  If you don't believe me then ask L6 yourself for their biasing instructions.  Its their forum.

Again IMO any amp which does not have external biasing adjustments is not user servicable.  Take it to the shop.   ----- what else can I actually say?

Good luck


Re: Great explanation
by spaceatl on 2010-08-31 12:12:02.8720

I do know...I see the warnings a sufficient...They should cause pause and second thought...It's all you can do...After that point Darwin takes over...

So, now that is out of the way...what sort of wirewound would you suggest Trash? Seriously I am interested in your take on that...After all, you found the awesome mustard caps...

Re: Great explanation
by mtrash14 on 2010-08-31 17:22:36.4010

Those 4 watt military pots sure look good but boy are they high dollar.   Since a practical 'good enough' pot is only about $4 each, those might be a tad overkill.   But at the same time I would say they would be about the best you could find and something to be very proud of not to mention - probably rock solid.  But I'm having trouble believing these amps will ever have a truly rock solid bias regardless of the pots we might use.  lol  As you know there are many variables here.

Short extention leads.  Good connections that are removable when needing to remove the board from the chassis.  No noise added to the amp meaning keep those extension wires away from the signal chain as far as you can, and keep them as short as possible.  And of course test then test it all again.  Adding noise is not exceptable as you know.  Moving those bias pots to the rear chassis does present its own set of problems and each must be tested as necessary.

But to the point now.  Just how important is a close bias on these amps anyway.  How many of use can actually hear a difference in 35.2mV and 36.1mV?  I say none of use can.  I can hear the difference in my amp from 35mV and 45mV, but that is completely out of this better pot question because its not going to drift that much anyway regardless which pot I use. lol

If you can hear a few points difference in tone then IMO its completely worth whatever it costs to fix it, but if you can't hear it then what does it matter.  On these amps with all their clean headroom, with all my modding time and testing time and playing time I have on my amp now, I can not hear even a 5mV drift in the bias (say from 35 to 40mV) and pin-point it as such and be 100% sure thats what I'm hearing in the first place.  Setting there cranking up that 5mV and playing the amp all the while then YES I can hear it, but if the amp were to somehow drift 5mV without me knowing it - I completely doubt I could tell tomorrow that something sounded wrong.  Even if it does sound somewhat different I'd probably just blame it on hearing fatigue or having too much to drink.   Hearing a bias drift is debatable IMO, because in my experience it would haft to be quite a bit for me to pinpoint it as the problem without knowing it was off in the first place.

Like the OP above said, he needs to set his bias every 6 months so he will have the best tone.  Well I say this.  I've cranked the bias many times on a stock SV amp and unless you're willing to really raise the bias level by a good amount you're not going to hear much of a different nor gain much of a difference.  I bet you could not hear the difference between 35mV and 38mV without knowing firsthand the bias was indeed different.

I know you all don't want to belive this but I'm afraid it is pretty true.  I know because I've tried all of that before.  Yea changing the bias does change the tone.

These amps are actually the digital DSP board.  The tube section is only a clean tube amp.  The SV is what it is because it was designed that way.  To change it does take a lot, I know - because I've tried to.

I'm tired now.  Later guys

Good luck


Re: Great explanation
by spaceatl on 2010-08-31 17:59:03.4320

Great post Trash... Thanks man...

Ah...The .25% precision value is not really all that important as that only the deviation measure measurement between any two pots...But hi-precision does usually mean tighter turning action in my experience with wirewounds...

The spec I am actually after is g-rating...Not many pots will have it specified because most are not tested for it...This is a deviation or precision measurement on the pot exposed to some pretty serious vibration...This can get upwards of 10-20%...or even oscillate between full min and max resistance...

But you are right...this exercise is likely nothing more than an "audio placebo" for my piece of mind...You have seen the garden hose that I use for speaker cable, yes? ...Well, there ya go...

Re: Great explanation
by mtrash14 on 2010-09-01 16:27:13.0960

HaHa.  Yea I've seen that garden hose.  (((((( you talk about overkill)))))   (only kidding man).  That is the nicest speaker cable I've ever seen!!!!

To be a perfectionist you would first fully understand what the bias resistor amount would be by using a temporary variable pot (like the ones you posted I guess) then remove that and solder in a hard resistor of the proper value.  In that location I'd say 1/2 watt is plenty sufficient.   That way for as long as those power tubes were in there the bias could only be spot on, and nothing else.  That might not work if you sometimes want more punch or less of it or sometimes want to turn down the bias so the mids come up for metal, etc, etc.  If you knew the resistance value without a doubt that you liked then a hard resistor is the way to go.  That costs about $0.10 cents each.  I can't see any pot being 100% rock solid in this suitation.  The size of the pot and the point that the resistance could change with heat should be enough to prove that.  (I guess)

Having a variable bias gives you (a) the ability to change the bias amount as you want/need to try and get your tone.  IMO a lower bias makes the tone brittle and more midrange (in my amp) and a higher bias gives the amp more punch and seems to bring up the bass value somewhat.  Since L6 seems to recommend 35mV (which is actually a kinda cold bias IMO and by the numbers on my amp) they are either needing a more metal midrange brittle tone for their digital DSP's output or they want to make my tubes last longer for me, with IMO both of these is true.  A hotter than normal bias (more than 70%) is generally known to make the tubes cook off sooner but maybe giving a more blues/classic tone while they last.  That I might contest in some respects (in my SV of course) but I do agree with that somewhat.

Having a variable bias also gives you (b) the ability to use tubes that are not matched to how your amp was previously setup for.  As you know some amps have solid resistors in the bias circuit which makes them require either a new matched set of tubes which are matched to each other and also matched to your old set so the bias amount will be within the specs for that amp's expected tone and output as the designer intended it to sound.  Drop in a mismatched set of power tubes that gives a different bias amount thats off by enough and the amps tone will change, which might not be expected nor respected in the end.  In either case these amps have solid value hard resistors instead of a variable pot resistor for a reason, and that reason can IMO be either for tone or to make you buy a certain brand/kind/type of tube.  Knowing what the retube is going to sound like is an advantage to a great degree.  Getting those expected results may be something else nowadays.  lol

Variables are nice if you don't know what kind of tubes you are going to install and don't know if they are going to be exactly matched to each other because you can then adjust the bias for each tube individually and try and balance the amps tone.  The variable (may) cause a drifting concern but in most cases that isn't all that important.  Making tubes usable is probably the biggest concern here.  I'm not saying thats why my SV has 2 variable bias resistors inside it.

(again - only IMO) having variable bias resistors is small time.  Having to check and reset the bias all the time is again small time.  Hard resistors, matched sets, matched tubes, regardless of the cost, or simply changing out the hard resistor values to make the tubes usable, while matching up the hard resistors to get the tone you want - - - - is big time.  Is it hard?  Not really.  Is it necessary?  Not really for a small timer,  but IMO absolutely for a big timer if he's expecting the amp to be there when he expects it to. lol

If the tubes are really a matched set then you don't need 2 bias variable bias resistors, you only need 1 and a "Y" connection between them.  The one variable would be suficient to bias the tubes properly.  If the new tubes were matched exactly with the old set and the old bias was indeed set properly then the new tubes could be safely installed with the expected results.  In that case a variable wouldn't be needed, only 1 hard resistor of the proper value.  The results would be a pretty solid bias setting.

I like a sorta hot bias in my SV but a more normal bias setting in my other amps I have.  I know its only because the way my SV was created and how it was intended to sound my its makers.  I'm afraid in this case is my bad, and as time is proving is my bad again.  I'm supposed to like 35mV.



Re: Great explanation
by i81b4u81 on 2010-11-23 06:45:45.1200

I was going to ask in a new thread but this one has pretty much the same elements I'm looking into. I have an HD 100 and will be using the Sovtek 6L6 XWT+ (they have a real good review) and tung sol reissue preamp. I believe the HD 100 is set at 35mV but on the clean channel it's rather flat (maybe lacks dimension/dynamic). So upgrading the tubes is a start but does increasing the setting to make a significant change in the tone dynamic as seems to be claimed. And is it correct that 1 trim pot/2 tubes for the HD 100 and that a 1ohm resistor is inline already( I suppose it will be evident when I open the beast up).



Re: Great explanation
by LoonyBin-Fizzbin on 2010-11-23 07:57:08.1830

Anyone care to "guess" who that was?

The information above may not be current, and you should direct questions to the current forum or review the manual.