Ampeg sj 12 T 6L6 and EL34 Biasing

[Smitty02]

Hi guys. Does anyone out there know how to bias a Ampeg sj 12T this model will use 34"s or 6L6's I was hoping someone might be able to tell me what the correct way to bias this amp would be ?? Does it have bias as I cant see any but it does have a bias pot. AP1 ....Do I need a biasrite or can I just use a meter. It is a cathode biased amp so I read but calls for bias adjustment for tube switches and tune ups. which is what I want to do is just put some new 6l6's in but would like to know the procedure I got a request in to Loud for a schematic but if anyone could explain the process I would be grateful thanks guys

[Bruce / Mission Amps]

As far as I know they are not cathode biased, they are fixed bias.
You'll need to learn how to set the bias voltage for a correct amount of measured current through the power tubes.
Not too many of us old guys here use those biasrite gimmicky things but for a novice tech, I can see why you should have one.

Not sure if it is your model but here is a place I just found a schematic:

http://elektrotanya.com/PREVIEWS/634..._sch.pdf_1.png

EL34s will run cold in amps set up for 6L6s... 6L6s will run hot when used in amps set up for EL34s!!

When you buy your 6L6GC power tubes... ask for a matched pair and tell them you need a set that is in the middle of their tested idle current ratings.
I'd be surprised if they don't just pop right in and be close enough.

[Smitty02]

As far as I know they are not cathode biased, they are fixed bias.
You'll need to learn how to set the bias voltage for a correct amount of measured current through the power tubes.
Not too many of us old guys here use those biasrite gimmicky things but for a novice tech, I can see why you should have one.

Not sure if it is your model but here is a place I just found a schematic:

http://elektrotanya.com/PREVIEWS/634..._sch.pdf_1.png

EL34s will run cold in amps set up for 6L6s... 6L6s will run hot when used in amps set up for EL34s!!

When you buy your 6L6GC power tubes... ask for a matched pair and tell them you need a set that is in the middle of their tested idle current ratings.
I'd be surprised if they don't just pop right in and be close enough.

hey thanks Bruce I just submitted a request this morn and Ive got it already (schematic) says set bias for slight crossover notch at full output at .65-.70A,, but shows no test points I guess they want ya to do it old school by measuring the pins ? and a bias calc.? which I really need to learn to do ha ha so this might be a good time It is ver. U.schemo. Ok question if it is fixed bias then can I use a 1 ohm to ground on pin 8 ? I see 1 and 8 already go to ground. I am not real familiar with ampegs Although I have a vintage reverberocket that date by SN to 1965 but all pots code to 1964 and it is a odd duck I think, as it has 3 ax7's and 1 au7 in v2 with 7591 ouputs a R12R M version and I couldnt find no info hardly at all for that too..I did get that running new cap can, old one leaked all over I need to get the trem and reverb right but it does sound good as is, need to check the caps and resistors yet but that is one of my to do list projects. but back to this jet I see no mention of a 6L6 or a el34 anywhere for outputs.. Can you point me in the right direction? I have done some reading on it already but if you got some directions I am a good follower LOL and I live on these, forums they are great so much info and Ive learned so much already ... Thanks

[Jazz P Bass]

A one ohm resistor from pin 8 to ground is the safest way to do it.
The one limiting factor when going from 6L6 to EL34 is the bias voltage range.
EL34 tubes are typically idled hotter tha 6L6 tubes.
So you need to go less negative on the bias voltage.
So you need a range of something like -35 to -55 volts.
Please post the correct schematic if you can.

[Smitty02]

I know this sounds stupid but I cant figure out how to post it Im better with amps and guitars than computers, tried to copy paste and attach ?? Anyways the one Bruce posted is pretty much the same as i can see pins 1&8 are going to ground already so technically I could put mount baising points in the chassis ? with a 1 ohm resistor.. and use bias calc to adjust? Jazz

[Jazz P Bass]

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[Bruce / Mission Amps]

I know this sounds stupid but I cant figure out how to post it Im better with amps and guitars than computers, tried to copy paste and attach ?? Anyways the one Bruce posted is pretty much the same as i can see pins 1&8 are going to ground already so technically I could put mount baising points in the chassis ? with a 1 ohm resistor.. and use bias calc to adjust? Jazz

The basic idea is to determine the idle power in watts by measuring the idle current from cathode to plate and then multiply that number, in fractions of an amp, times the measured plate voltage.
6L6GCs, a 30 watt tube, sound really good to me and will last a fair amount of time with that wattage # being around 14 watts to 18 watts.
El34s, a 25 watt tube, sound good to me at around 12-13 watts up to as high as 18 watts too.... not sure why, but that's my ears. The EL34s will not last as long at 18 watts... as a good 6L6GC will idling at 18 watts.

So install your 1/2 w to 1 w, 1 ohm @ 1% resistor from lug 8 to ground and measure with a DVMM on the lower scale.
You'll be looking for a milivolts DC reading which will correlate directly to the same number in miliamps...
SO if you read 40mv, you can call it 40ma.
Then measure from lug 3 to 8 to see what the actual plate voltage is...
if you read 450vdc, then you just write out the 40ma as .040 amps X 450vdc = 18 watts.
Now the bad news is that you are measuring the plate current and the screen current with that method.
That's not a big deal with a beam forming tube like a 6L6 which draws only a couple miliamps of screen current, but the screen current in an El34 can be higher... so you'll have an error factor there that will have you slightly over biasing (deeper negative grid voltage, hence cooler plate current)...the EL34.
I do not use the cathode current method myself but if I do, and we are measuring the idle current of EL34s, I will add a few ma to my allowable max current level to allow for the additional screen current.
Hope that was not too confusing... and
also:
BE REALLY CAREFUL... those high voltage DC supplies can really put you in a bad mood when you get poked hard and wake up in a puddle of your own pee.

[Smitty02]

sj 12 t

ampeg-j12t-jet-ri.pdf Good stuff I learned today thanks Jazz & Bruce

[Bruce / Mission Amps]

[ATTACH]20763[/ATTACH] Good stuff I learned today thanks Jazz & Bruce

Me too... the schematic you supplied shows the power tubes are neither EL34s or 6L6s but El84s (6BQ5) and they are cathode biased, not fixed bias!
Wow...

[Enzo]

That schematic is for a J12, not the SJ12.

Here is an SJ12.

[Smitty02]

Yes I seen that Im sorry I was searching for the sj 12T. In my search I had downloaded this one and a couple others . I was happy that I was able to follow jazz's posting instructions that I picked the wrong one lol it was late and i was tired. and noticed after I did it it was the wrong one but I learned how to post but not remove one lol I went to bed guys sorry But it looks like Enzo beat me to the punch ( thanks Enzo ) My wife is a computer geek, I am limited in all the little tricks and pathways of these wonderful tools known as laptops When I downloaded the schemo from email I renamed it, hey I was close, an I need new glasses lol seriously.. thanks guys I did spent some time looking at the sj 12t and find in odd that they have a bias pot, but no bias points, that I could find anyways.. most of the newer stuff has points if they have a pot. Im sorry guys Thanks

[Jazz P Bass]

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[Smitty02]

Bias Point

Im back sorry wife and family have had me busy, but Im back into the amp again. I spent some time looking over the schematic, really looking it over with it magnified. I did find a bias point in the schematic at R21 it says -38v typical I measured here and got -42. My question.... is this a true bias point ? I also tried the 1 ohm resistor from pin 8 to ground, now pin 8 and 1 are to ground already, I got no reading at all using the 200mv scale on my dmm with a 1 ohm 2 watt 1% resistor. I ended up going to Webers bias calculator and plugging in my numbers... class AB 6L6GC 406 volts plate voltage (pin 3 and 8 ), It comes up with 51.7 very far off the -38 and I tried running same numbers for a EL34 and got 43.1 which is about where it was set when I measured it at the bias point at R21 when I started this biasing quest. So amp runs ok and sounds the way it was running with 6L6 gc's,but a slight buzz/rattle, which i think is the left power tube starting to rattle. I am getting ready to put new JJ's in it and want to know if I am on the right track... Thank you guys for all your help so far, for your patients I know you probably are reading and rolling your eyes at me but I look at alot of posts and try to learn as much as I can from you guys I truly love this forum and have used it so many times i cant count..

[g1]

First, your 1 ohm resistor must REPLACE the connection from pin 1&8 to ground. If you leave the original connection, you will not get a reading with your DMM.
As far as the R21 test point, it is not showing bias, it is showing grid voltage. It is a negative voltage that will affect the bias, but will not give you a very accurate reading of the actual bias (idle) current. The numbers from the Weber calculator are milliamps. You will measure millivolts across the 1 ohm resistor. The 1 ohm resistor is used because it is easier to measure millivolts across it than to measure the milliamps. To measure current requires breaking into the circuit so it's easier to use the resistor. Because the value chosen is 1 ohm, the millivolt reading comes out equal to the current in milliamps (I = E/R, or current equals voltage divided by resistance).
It is just coincidence that your calculation came out to be 43.1 while your grid voltage was -42 volts. 43 milliamps and minus 42 volts are completely different things.
Hope that makes some sense

[Smitty02]

Yes it makes since, thank you. I thought I would have to lift the connection but before I go cutting a trace I was hoping to get some input Now I am torn between installing probe points or not since it will be on the bottom of the board. I think I will have to, it will make it a lot easier to swap between tubes. I dont get why they didnt put a bias point on this amp?? Also are the bias calculators really reliable in your opinion? Bruce explained to me the process and i understand what he means but it seems like for a noob like me and alot of others trying to learn these electrical laws the calculators are much easier but I still want to learn the old school way. Laugh if you may I will learn it I know to alot of you guys this stuff is like 101. Ive built amps pedals and fix alot of stuff but i am always trying to learn....I will get there ......... Thanks G-one and Jazz and Bruce and Enzo ......