Help with biasing procedure

Bamadawg

Member
Sorry for the newbish question. I have used tube gear over the years but have a lil Stromberg-Carlson EL84/7189A power amp coming that allows bias voltage and bias balance tweaking. Amps I have used were self-biasing I guess because I never had to adjust biasing. The amp was refreshed by Bruce Brown @ Vintage Tube Audio and he provided instructions to set the voltage and balance. I have the SAMS sheet that also provides adjustment instructions but they are a little different than Bruce provided. I have to admit that I am rather dyslexic when it comes to reading, but once I see something done... I never forget it. I just want to make sure I use my Fluke in the proper way to adjust the voltage and balance.

I think the way Bruce is having voltage adjusted is for a reduction in voltage due to modern lines voltage vs what was used in 1960 when the amp was designed. He has a target of -16v thru -22v with a balance of zero. The original instructions said to target the voltage to +.3v which I semi understand in the A/B amp typology to minimize distortion (I know there is another term, DUH moment). Bruce added new 10k pots which I am guessing are the pots that will determine bias voltage. His instructions are a bit different than the original. I will post the way he explained it. I fully understand the part about clipping alligators on the stand-up pins located behind the power tubes and adjusting the balance controls to achieve zero as the balance reading, or as close to it as possible. The other part to set voltage is a little unclear to me. I am not clear as to the 'pin probe wire' he references and if I should use a alligator lead to go from the + probe to that pin before I put the probe into the gray jacks of each channel to adjust for the -16v thru -22v.

Jus want to wrap my head around it before it gets here. A pic the seller had posted here before he had it rebuilt is included.

Many thanks in advance...
Nate

Instructions:

Here is the procedure for setting the bias and bias balance:
1- Turn meter on to the 20 volt DC range (left of off position)
2- Clip the black lead to the to the chassis (gnd)
3- Clip the red alligator clip to the pin probe wire
4- Insert the red lead in either of the gray pin jacks on top of the amp and
turn the pot closest to it to get -16 to -22 volts (see note above)
5- Repeat with the other channel
6- Now disconnect the red and black leads from the chassis and pin and
connect the alligator clips to the small ring terminals behind the
output tubes. (Be careful not to ground them and be careful of the
tubes they get hot)
7- Now adjust the bias balance control for a reading as close to 0 as you
can get. (Plus or minus up to 0.5 to 1 volt is OK)
8- Repeat with other channel
9- Disconnect meter, turn off, turn up the gain and enjoy!
 

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IMHO, I think I would not worry about it and hope or know that it all has been set before shipping. It's possible that it could get knocked out of alignment during shipping but in my experience, that really hasn't been a problem.

That Bruce guy pretty much explains it but I can see having someone else there with you. Anybody from AK in your neck of the woods?
 
I understand everything except the 'pin probe wire'. If I need to put a pigtail alligator to one of the pins behind the output tubes down to my Fluke probe then insert it into the gray bias voltage measuring port... I can handle everything. I will look at the amp when it gets here and see where those pins attach to under the chassis. Original instructions say to attach a connector across the 'Tube A' points to set balance then move the - lead to chassis ground and set voltage, which would leave a connection on one of the pins behind the power tubes. I am working it out in my mind that the way I have it figured out is correct.
 
If all else fails... I will jus give Bruce a call when I receive the amp and let him walk me through it ;)
 
If all else fails... I will jus give Bruce a call when I receive the amp and let him walk me through it ;)
Before that, just think the process through.
The term "pin probe wire" is a poor choice of words and confusing, and probably just means the probe tip for the meter. To adjust the voltages of a "fixed bias" amp is the same principle for all push pull amps. Essentially, you want to balance the voltage between the two (or more) power tubes and then adjust the bias voltage. Some amps just have a bias voltage adjustment for all 4 tubes, but this doesn't seem to be your amp. Anyway, if you have set the balance then adjusting the voltage is next. But, this will cause the voltage to change a little as the new setting will tend to do. So, you need to check your voltages again and redo the procedure until all your values have stabilised. Most fixed bias amps instruction manual suggest after changing power tubes that the bias settings be checked and adjusted during the first power up after about 5 minutes, then 30 minutes , then 2 hours. After the amp is on for 2 hours all the temperature and voltage readings should be stable and the final adjustments should be good for at least 1000 hours of use on the power tubes. More conservative audio guys would check the bias reading after 10 hours then 100 then 1000 hours of use. Be aware that if you check the bias the next day after first turning on the amp, the readings will be much different, most likely higher than when you set it the day before. This is because you last set the bias when the amp was warmed up after 2 hours of use. So, if you want consistent results you need to check the bias after the amp had same amount of time to warm up. Also, most DMM had accuracy out to 0.01volts or lower and trying to set the bias to that accuracy is an exercise in futility since bias voltage will not be stable to that number .
Hope that helps your situation.
PS. i have an audio buddy who sold his Fisher SA 300 amp because he was tired of adjusting the bias voltage every day. It seems that after setting the voltages the last time he had the amp on, he would check the bias and they were off from the day before. So he readjusted them. Next day he had to do it again. Finally he got tired of doing that and sold the amp! If he had asked me if that was normal i would have told him what was going on.
 
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Thank you sir... yes, all information is helpful. I been watching some wonder vids on the youtube from a guitar amp builder, very informative.

I think my snag is coming from the way Mr. Bruce set the amp up, but I believe I understand why he did it. He added the gray test jacks... they were not part of the original design it seems. I do not have the amp, so I can not say 100% what and where he is measuring to set the -16V thru -22v. I do however believe it is as I thought, an adjustment of negative volts to accommodate the modern lines main voltage so you can safely run EL84/6BQ5's in the amp instead of the rarer 7189/7189A's.

The original instructions on the SAMS list is very clear, you clip gators on each test point pins behind the tubes and let it warm up 5 mins, adjust the Bias Balance pot to achieve 0. Then you leave a + clip on a test point pin and move the - clip to chassis ground then adjust the Bias Voltage pot to achieve +.3V.

So he has added the gray test jacks to set the -16v - -22v and I should just clip the - lead to chassis ground and insert the + probe into each channels gray jack and adjust the Bias Volt pot to read -16v - -22v.

I have a nice Fluke meter, but like you stated... you are not gonna get 100% on target. Saw that in the vids I have watched where they take volt readings to adjust bias.

Regards...
Nate
 
After some more 'learnin' I have come to understand that the amp is being negative grid biased using the 60VAC tap of the power transformer through a negative bias circuit which provides a range of -15VAC thru -60VAC through the potentiometer. This circuit provides the means to apply negative energy inside the tube to reduce current flow (plate voltage). I think I have that 'learned' correctly ;)

Nate
 
Furthermore... the original schematic used a 5K pot and I believe Bruce swapped in a 10K pot which would change the negative VAC values, but I ain't doin no math this AM to figure out the -VAC ranges... not enough coffee consumed ;)
 
Sounds like he's having you set the grid voltage rather than adjusting the bias to get to a specific current. Honestly setting grid voltage level is not nearly as useful since it won't tell you what the tubes are doing. The usual way of doing it is reading across a 10 ohm resistor on the cathode, and you set the bias pots to get however many volts is appropriate for the circuit.
 
Sounds like he's having you set the grid voltage rather than adjusting the bias to get to a specific current. Honestly setting grid voltage level is not nearly as useful since it won't tell you what the tubes are doing. The usual way of doing it is reading across a 10 ohm resistor on the cathode, and you set the bias pots to get however many volts is appropriate for the circuit.

This is the original schematic... kinda crappy, but you can see they used a negative grid bias circuit to feed up to the Bias Balance pot.
 

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right, thats all pretty typical but usually the specific voltage the bias ends up set at is not that important. Usually its more important to know how much current the output tubes are conducting, especially now with tubes being a little bit more varied than they were in the 60s. Some tubes need the grid more negative than others in order to conduct a reasonable amount of current. If this does actually have a large value cathode resistor somewhere (don't know this amp, it may) then checking across that might give 16 volts, though honestly I'd seriously doubt it for a fixed bias amp. EL84 amps typically run somewhere around 12 volts of bias,
 
Yes sir... the video I watched about this type circuit stressed the utmost importance of using matched tubes for it to be effective and accurate. I am thinking it important to measure the plate current and calculate plate dissipation to know what range you should shoot for to set correct bias with the pot for matched tubes.
 
Got it all figured out. Bruce jus inserted the nice test probe jacks back around the voltage pots so that you did not have to use the pin probes as both test points. Set up the amp while with some NOS Mullard EL84's and played some FLAC files through it about an hour and re-biased it all again. Sweet sounding lil amp. Thanks for all the help. Oh... the original specs were -15V that fed up to the bias balance circuit, so we are jus measuring the negative bias since we are not using VTVM to set bias voltage and balance.

Cheers...
Nate
 
Just a little update. Since I got the SC up and running I decided to invest in the nice lil Eurotubes Pro One 9-pin bias meter. Best thing I ever did. Can adjust bias on-the-fly since the Pro One shows actual plate voltage as you adjust the bias. Very easy to do once you have tried it a few times. Found with my NOS Mullard EL84s that I was getting about 430v on the plates when biased for 70% @ 12w - ouchy! Put in the Sovtek EL84s since they take a bit more voltage. Have a set of the NOS Russia 6P14P-ER (long-life) tubes in route from The Motherland.

Got to thinking... I have a NOS Mazda 5Y3GB in my tube stash. Plugged it in and reinstalled the Mullards. Set the bias to 22ma and the plate voltage on both sides running dead on 380v... much better! I don't get quite as much gain as I was getting with the higher current at lower bias, but at least I know I am driving them easier with the Mazda rectifier. Lil amp is dead quiet and rocks right along without a hiccup. Got some Orange Drops coming to replace the original coupling caps... only a .033uf cap.

Anywho... back to the music ;)

Cheers...
Nate
 
5Y3 has more voltage drop, but less current capacity as well. Its basically half what the 5AR4 is good for.

Even 380v is awful high for an EL84 amp. I'd expect closer to 300. 380v puts you in 7189 territory.
 
Yes sir... the amp is def a 7189A circuit but I was following advice I had read from Jim McShane about higher plate voltage on the EL84 not being as harmful (as long as it does not exhibit any arching or red-plating) as long as it is biased to operate in its intended dissipation range (12W x 70%). He explains that it is not the current that causes the tube to wear more, but not changing bias to compensate for the increased voltage and causing the tube to operate over its intended use, say pushing it to 14W-16W. I do have the NOS Russian EL84's en route and will sub them out after they arrive. I am also going to construct one of the voltage limiter outlet boxes so that I can operate the amp at its designed 117V specs.

From my understanding the NOS Mullard and NOS Telefunken EL84s are among the toughest EL84s ever made. Only reason I would even consider applying this amount of voltage to them.
 
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