Biasing

[Dave1234]

Friend has a Jet City JCA20 head and wants me to check Biasing. It has 2 x EL84's and I have never biased and amp before as mine are all cathode biased. I am not sure when the bias points are or what to set it to. A suggestion I have read says 18-20 mA. Does anyone have any ideas?

 

[Wally]

Caution....live circuit voltages..... google Aiken amps go to the tech section and read about biasing. kUnless you have a biasing meter rig, you are going to have to use the transformer shunt method. Make sure you understand what is going on before you get off into it.
IN short, you are going to be reading the current draw directly with your multimeter's current section...amperes. Black/Negative probe on the power tube's plate adn the red/positive probe on the center tap of the OT....B+ connection.
The amp will go silent when you make this contact, and the meter will reveal the current draw in ma. IF you are not comfortable with this procedure, don't do it.
FWiw, this method is valuable for cathode biased amps as well. Just because an amp is cathode biased doesn't mean that a person might not want t o know what the current draw is in that circuit. Each set of tubes you put into a cathode biased amp actually rebiases the output section....so knowing what is going on there might help one maintain continuity of the sonic by installing tubes with the same operational parameters. So....if one has tow sets of tubes for a cathode biased amp and those two sets of tubes have very different operational parameters, the amp will yield two very different sonics when one goes form one set of tubes to another. Same thing for a single-ended amp. (;^)

I have one of those amps sitting here right now. When I got it sorted out...bad connections causing all sorts of crackling....I went to bias it. ONe tube is drawing twoce as much current as is the other tube!! Before I started biasing it, one tube was at 93% of max plate dissipation while the other one was at 47%. After biasing, the 'hot' tube is at '67% and the 'cold' tube is at 33.5%. No, the owner does NOT want to replace the tubes. (8^O I have already let him know that I am not responsible for whatever happens. In other words, he has been warned that the amp sounds terrible and is operating in an undesirable situation, imho.
IF I had one of these amps, every tube would be thrown away. The preamp tubes....Chinese 12AX7's... are of no use in a guitar amplifier, ime.

 

[Dave1234]

Thanks Wally. I am used to using a DMM, I use one all the time at work, so no problem there. I will check out the Suggested site, for methodology. Thanks again.

 

[Wally]

Sure thing, Dave. I forgot that you have a lot of electronic experience.....way more than I do, I am sure.

 

[Dave1234]

Sure thing, Dave. I forgot that you have a lot of electronic experience.....way more than I do, I am sure.

No worries Wally. I have donated some matched tubes to the lad, but he has also ordered some NOS ones of his own. He has also put in a 5751 JAN GE in V1. I have suggested changing out the other two 12ax7's as well.

 

[Dave1234]

So to re-iterate:

"N short, you are going to be reading the current draw directly with your multimeter's current section...amperes. Black/Negative probe on the power tube's plate adn the red/positive probe on the center tap of the OT....B+ connection. "

I measure, in milliamperes, from pin 7 on the EL84 to the B+ on the OT. According to a handy table from Weber, I am looking for roughly 18mA, which should give me approx 70%......I then adjust the pot to get me as close as this as I can, if it is not already there. I am reading that the standard config is quite cold, so I am expecting it to be lower than this. Both tubes should be about the same, if not, then I will need to strike a happy medium.

Is this correct?

 

[robrob]

Keep in mind when either probe touches its intended measuring point the other probe will have B+ voltage on it (full B+ voltage runs right through the meter). Be mindful how you release your probes because they are both hot until both are clear of high voltage.

Dave, use my bias calculator web page to verify what your chart is telling you. All you need is the plate voltage and plate current to plug into the calculator: http://robrobinette.com/Tube_Bias_Calculator.htm

 

[Dave1234]

Keep in mind when either probe touches its intended measuring point the other probe will have B+ voltage on it (full B+ voltage runs right through the meter). Be mindful how you release your probes because they are both hot until both are clear of high voltage. Dave, use my bias calculator web page to verify what your chart is telling you. All you need is the plate voltage and plate current to plug into the calculator: http://robrobinette.com/Tube_Bias_Calculator.htm

Many thanks Robrob. I do have a Fluke and an excellent Fluke set of adapters, including insulated needle point probes and insulated clips. I should be ok. The calculator is brilliant!

 

[tubeswell]

Anywhere between 60% and 85% ought to work okay for Class AB1. 70% is just somewhere in the middle. Also relevant is the reflected load and the plate voltage. If the reflected load is lower than optimal for centre-biased operation, then its better (for tube longevity) to be biased colder. But if its too cold it can sound horrible so...

 

[JR88]

Can go about few quick/easy ways, can do OPT Shunt/current method, or like me without any alligator clips and current DMM and both hands/arms in the amp/live... CT to plate, voltage drop through OPT then calculate it out/get close etc

Telling you right now if you are not comfortable doing it, Please Don't...you will Die.

 

[Dave1234]

Anywhere between 60% and 85% ought to work okay for Class AB1. 70% is just somewhere in the middle. Also relevant is the reflected load and the plate voltage. If the reflected load is lower than optimal for centre-biased operation, then its better (for tube longevity) to be biased colder. But if its too cold it can sound horrible so...

Thanks Tubeswell. I have read this also. I have visited a few sites, but will always defer to here, just because I trust you guys more! I am waiting for the friend to get his new tubes and then we should be good to go.

 

[JR88]

18-20ma should be pretty good get em both close.

 

[Dave1234]

Can go about few quick/easy ways, can do OPT Shunt/current method, or like me without any alligator clips and current DMM and both hands/arms in the amp/live... CT to pin 3, voltage drop through OPT then calculate it out/get close etc Telling you right now if you are not comfortable doing it, Don't...you will Die.

I am fine and comfortable with this JR88, I just haven't done it before. I have built and carried out troubleshooting on two amps, so I am well conversant with the safety aspects and correct procedures. Thanks for the reminder though!

 

[JR88]

I am fine and comfortable with this JR88, I just haven't done it before. I have built and carried out troubleshooting on two amps, so I am well conversant with the safety aspects and correct procedures. Thanks for the reminder though!

Good idea to Practice with everything off, cold/caps discharged etc, till you get the hang of it/feel comfortable etc. I don't like doing it though,I know its dangerous as ****.

You really are sitting yourself up for hand to across the chest/heart back out hand...That will Kill you its no joke.

I already damn near died/could of, checking voltages near bias pot, pulled away with left hand got bit too close to bridge/HV yeah I got ****ing Sucked in dude hand/arm grounding out...right arm was to my side thank god ran up my to shoulder/neck finally let go slumped to floor/almost past out wasn't right for like an hour dude. singed a quarter sized spot on my arm etc.

I don't want anyone to get lit up dude it ****ing sucks/hurts dude and can kill you.

 

[Dave1234]

Good idea to Practice with everything off, caps discharged etc, till you get the hang of it/feel comfortable etc. I don't like doing it though,I know its dangerous as ****. You really are sitting yourself up for hand to across the chest/heart back out hand...That will Kill you its no joke. I already damn near died/could of checking voltages near Bias Pot, pulled away with left hand got bit too close to bridge/HV yeah I got ****ing Sucked in dude hand/arm grounding out...right arm was to my side thank god ran up my to shoulder/neck finally let go slumped to floor/almost past out wasn't right for like an hour dude.

Wow! Sounds like you were very lucky. I will be extremely careful. I will probably clip both probes into place before powering up and remove once the caps are discharged, that way, I only need one hand to use a flat blade (insulated) to adjust the trim pot.

 

[JR88]

Wow! Sounds like you were very lucky. I will be extremely careful. I will probably clip both probes into place before powering up and remove once the caps are discharged, that way, I only need one hand to use a flat blade (insulated) to adjust the trim pot.

For sure insulated screwdriver for Bias Pot.

 

[JR88]

I think it made me about half crazy too. kinda did briefly hit neck/left side of head area for a good 10-15 seconds...that's been a few years ago....ever since, yeah just feel pretty unstable emotionally/personality wise. I hallucinate See/hear **** all the time dude,messed up memory, telling you it kinda ****ed me up pretty much.

 

[Dave1234]

I have had a few 240Vac jolts in my time and a nasty moment with 3 phase. That was when I was much younger......I have a very healthy respect for high voltages these days. I do get to play (work) on railways sometimes and they run at 12,000 Vac overhead here. No going back from a belt off one of those!

 

[markbt]

For reference, I biased my JCA22H head at 21-22 mA with 371 V on the plates when I put a pair of TAD EL84s in recently. Sounds good to me.

The caps also bleed off pretty quickly in this amp, so it's no great inconvenience to wait before making or removing connections with your meter.

 

[JR88]

For reference, I biased my JCA22H head at 21-22 mA with 371 V on the plates when I put a pair of TAD EL84s in recently. Sounds good to me.

The caps also bleed off pretty quickly in this amp, so it's no great inconvenience to wait before making or removing connections with your meter.

yeah you can go higher, really seen mostly the JJ EL84s can take good bit of abuse, Russian/Chinese will die if you go crazy with voltages. Other than that RCA/GE 6BQ5s.

Was never really into the EL84/6BQ5 thing though, way more prefer 6v6s