HELP! Learning to Bias my JCM 900 by myself

[samth3mancgp]

Hey everybody. I bought a JCM 900 SL-X about a month ago. I love the way it sounds, but I am pretty sure that it still has the stock tubes in it, and would sound much better with some new ones. Right now it has all Marhsall ECC83 preamp tubes and some Sovtek 5881s in it. I thought about switching to EL34s because that is a pretty popular change on them, but I also read of many people that prefer them with 5881s, or 6L6GC so I thought I would make my life easy by just getting a matched quad of JJ 6L6GC power tubes and a mix match of different types of preamp tubes to do my first full tube change.

I know that the preamp tubes are basically a free-for-all and I can move those around with all different kinds for the perfect tone, but the power tubes are a little bit more difficult in that they need to be biased every time they are put in the amp. I have already done a decent amount of reading on other forums about changing tubes, and spent a good amount of time on the Eurotubes website watching their videos.

Here are some pictures of what I have to work with.

New tubes:

A view from above:

A view from below. (I read that you can stick a 1ohm/1watt/1% tolerance resistor between pins 1 and 8 that I can then use to measure with my multimeter. Right now there is just a wire bridging that gap as seen in the pic. I don't NEED one of those bias probes right? I would really rather not have to buy one. I have several digital multi-meters.

Lastly, closeup of the preamp tubes:

and closeup of the power tubes:

It would be very appreciated if somebody could help me through this. I have made at least 50 cables, built my own mic preamps, and tore apart tape machines and other gear many times. I know I can do this, and I'm not afraid to do it.

 

[samth3mancgp]

Okay I just ran through my first time biasing using my OLD tubes just to get my hands dirty. I basically followed the steps from guitargeek.com/chat/amplifiers-9/picture-tutorial-biasing-tube-amp-81368/, except that instead of measuring a single power tube across the 1ohm resistor, I was measuring from the power tube fuse area (combined milliamps from two power tubes). I got the info on how to measure from the fuse area on another forum. I measured 480V from pin 3 with the tubes in, and 76ma from the fuse area. Half of that would be 38ma/tube. So 480v(.038 amps) was 18.24 for my idle wattage?

The last step in the thread I linked to said that you can then mess with the bias trim and run the tubes a little bit hotter or cooler as long as it was "operating within capacity (usually 20 watts, but varies depending on the tube)". I ended up pushing the trim up so I was reading 78ma (39ma/tube). The JJ 6L6GC matched quad that I bought says 19.8 on a little sticker on the side of each one. Is that the maximum idle wattage that they can take? Around where should I adjust the trim for these new tubes to idle at? I don't have a clue how well matched the old 5881s are. Probably not very well since when I first measured the fuse slots one measured 75ma while the other measured 63ma.

Is the reason for measuring BOTH fuse areas simply to make sure that the tubes match? Then I can move them around between their slots to get the closest match in milliamps between the four? As long as they are close to matching, I only use 1 of the measurements to calculate where the bias needs to be?

Hopefully all of these numbers that I am throwing around are not shocking (pun intended ) to you guys and I am doing something right. I haven't jolted myself yet and I don't plan to! I've done it before with other things and I know it sucks! Sorry for the mess of questions guys. I really want to be able to understand and do this myself.

 

[Bart Eeltink]

The first thing I'd like to know: is pin 6 on the power tubes already occupied by something?

If not:
-Solder your precision resistors from pin 1+8 to pin 6.
-Solder the wire connected to pin 1+8 to pin 6.

Steps for biasing:
- Power off
- Connect a speaker
- Set the Master volume to 0
- Insert those new tubes
- Switch the power on, but leave the standby off
- Measure the voltage between pin 5 and the chassis with 1 multimeter and turn the bias pot so that the voltage on pin 5 reads the maximum negative voltage, it could be about -50 Volts.
- Take two multimeters
- Connect Multi 1 between pin 3 of a tube and the chassis
- Connect Multi 2 between the legs of the precision resistor
- Switch standby on
- Multi 1: measure the voltage between pin 3 and the chassis (expect about 400 - 600 Volts)
- Multi 2: measure the voltage drop over the precision resistor, not between pin 1+8 and chassis (expect 20 - 80 mA)

Now some calculations:
You use JJ 6L6GC tubes.
Pmax on 6L6GC tubes: 30 Watt.
Using the cathode resistor measuring method, you will read plate current + 5%, so we're measuring more than we need to.
We need to set the amp to 65% of Pmax, which would be 19.5 Watt.

19.5W/Vplate=Iplate
Iplate* 1.05 * 1 Ohm = VRcathode
I'd use Excel to calculate this.

- Change the pot so your multimeter 2 reads VRcathode
- Check the plate voltage, it may have dropped!
- Do your math again: 19.5/Vplate * 1.05

Repeat these steps about 3 times, that should give you a close value.

There is an other subject on this matter, measuring R on the OT pins.
It's more precise and will also work well.

Measuring the voltage drop between the resistor and chassis may give you completely different values, do not rely on them!

 

[tresmarshallz]

Cool to read this, when I bought my slx a few years ago it had the stock tubes in it from the early 90's they still sounded great.

I think the JJ tube designation of 19.5 is just a rating they use for the tube like groove tubes soft-medium-or hard range, I don't think it has anything to do with wattage.

I use a multimeter and bias socket tool so can't be of any help with the bias method you are attempting, but that is a good idea for you to get your hands dirty with the old tubes first.

I've heard great things about the 6L6 tubes in a Duel Reverb, but never about them in a SLX. Let us know how you think they sound...

Drew

 

[Doug_MN]

The 19.5 box number is a value used by the tube supplier to match with other tubes. It is irrelevant for your purposes.

I just went through biasing my JCM900 and one reccommendation I can offer is to be patient when you measure your bias current. After a 15m warmup I set the bias on my 4100 DR and I found the tubes were showing about 5% bias reading difference. For matched tubes, this was unacceptable and I thought my tube supplier pulled a fast one. After 30m with the amp on, I watched the readings converge to a 1% difference.

Good luck!

Doug

 

[valvefloyd]

Hello,

I have a JCM 900 4100 Marshall header and I changed the power tubes on it, I used EL34 tubes now. I tried to bias the amp without a bias probe, I added an 1 ohm resistor on every tube between pins 1 and 8 and measured the voltage on the legs of it. I measured the same, or almost the same with a small deviation, voltage on the second,third and fourth tubes but different on the first one. For example when I set the bias and measure 30 mV on second,third and fourth tubes, I measure 20 mV on the first one.
Is this something normal? Should it be like this? Do I make something wrong?

Thanks you in advance,