Crate BV120H bias?

[dearlpitts]

ok this sounds good is the video anywhere showing how to do the power chord biasing?

[skot10]

Biasing a Crate BV120H thru the power cord/ac line current draw (rewrite)

ok this sounds good is the video anywhere showing how to do the power chord biasing?

No video but I will write out my simple process:

I use a high quality three prong cord and basically slice the OUTER INSULATION ONLY. When doing this its easy to slice the insulation on the wires inside, so be careful so as not to get zapped or worse.

Usually, the wires inside are green, white, and black. The white is usually it but you may have to try them with your ammeter.

I put the clap-on ammeter on BEFORE I plug in the power chord, if its not the white wire, UNPLUG THE CORD before switching ammeter to a different wire.

Speaker cab connected with correct ohms setting on switch on back of amp. Turn on amp for a minute and then turn on standby (powering on amp) and let amp warm for at least 5 min. This is a no signal procedure so no guitar or chord should be connected to the input.

Turn on ammeter (I use my 2a ac setting) and see what it says. saying 0.00? unplug chord and switch the wire that your ammeter is connected to.

Since the spec is 1.3a@120watts, adjust AP1 (the blue and white adjustable pot that actually says "bias" right on the board) till your ammeter reads as close to 1.3amp as possible. A little cool extends powertube life but can loose dynamics, tone and volume, too hot will way reduce power tube life and can cause onstage failures. I put mine at as close to 1.3 amps as possible but I usually have it at about 1.31 amps.

If your ammeter is cheap you may have to move it so the wire in the clamp is closer to the meter side of the circle to get the correct result but find the spot with the highest reading and bias from there.

If the reading on the ammeter wont stay at one reading, and assuming of course there is not some other problem somewhere, let it warm up a little longer but by 15min it should be fine or as mentioned, you may have some other problem.

Hope this helps, I'm sure it will get some bad comments but on my Crate BV120H, I used this method when replacing power tubes and it sounds great (many of course would argue that this is impossible for a Crate)and has been reliable for a few months no with loud practicing and quite a few live shows.

I ACCEPT NO RESPONSABILITY TO ANY IDIOT WHO SHOCKS AND KILLS THEM SELF USING THIS PROCEDURE.

just saying.

I hope this helps
this post has been edited to correct a stupid misspelling.

Thanks g-one

[g1]

Oh, power CORD, I thought he meant power CHORD (as in biasing while playing power chords). That's why in the other post I told dearlpitts it was a recipe for disaster.
What you are talking about is called AC line current draw biasing.
However, skot10 is talking about a current CLAMP meter, which does not need to be inserted in series. With a standard type ammeter, the circuit must be opened so the meter can be placed in series. Or the fuse can be removed and the meter placed across the fuse holder terminals.

[Jazz P Bass]

Yeah, got me on that one, too.
Wow, Power Chord Biasing.

[dearlpitts]

will this sounds the easyest and cheapest way to go,thanx-now am i brave enough to confidently do this-not sure yet

[Jazz P Bass]

I use a Kill_a_watt (along with an amp meter & a variac)
Kill A Watt P4400 Kilowatt-Hour Monitor with LCD Display

[g1]

Yes, kill-a-watt would be the cheapest and easiest solution. Also the safest and probably cheaper than a clamp type ammeter.

[TomCarlos]

I bought (and assembled) the Bias Scout Kit from Tube Depot. It was a pain- but it is assembled and the pre-check looked good. So I tried it on a BV120H.

Something I learned... for the load- use a dummy load. Trying to take these readings with a speaker connected to the output causes instability. I have some precision power resistors so that makes it clean and steady.

The Probe Kit measures Plate voltage and Cathode current. I installed brand new JJ 6L6GC tubes. According to my reading, I am currently set at -47.4 volts for the Plate voltage. The Cathode current is at 10ma. If I put a AC current clamp meter on the white incoming line, I am reading inconsistent numbers. Depending on the quality of the clamp on, this seems to be a bad way to go. Perhaps it is time to build a simple box - AC in, an inline analog ammeter, and an AC out. This way, I can compare Line current to Cathode current.

In any event, I am looking up some reference material and I guess different resources will give you different settings. One book recommends -52 volts on the Plate but does not give a recommendation for the Cathode current. I can't seem to adjust the voltage below -47 volts.

Any ideas on an average Cathode current setting for this amp?

[km6xz]

Current read at the cathode will depend on the tubes so the exact value desired depends on the set of tubes you are using. The AC current reading should be pretty steady unless the clamp is not fully closed. Viewing the signal at the onset of clipping with a scope will tell you more about how it is going to sound, by adjusting so cross-over notch just disappears and the top and bottom peaks just begin to change from a sine wave to a flatten of the peak, is going to more predictably give consistent sound than going by some recommended cathode current. Cathode current should be checked however, to verify that the intended anode dissipation is not exceeded. Tubes vary by brand and design in all parameters, and even between any two coming off the same production line. A plate dissipation that is fine for one brands/model might not be fine for another.
Record the mains current for future reference. It makes a good quick check of the condition of the tubes. If, in 6 months the reading it the same, there is no need to check using other methods. If it is off, it is time to see what changed.
I log every amp's line current at idle when it is on my workbench and file that away for general averages for model and series. It take 10 seconds to get an overall impression of electronic performance when an amp comes to be checked out. With only AC power, output connections and a calibrated signal generator input, noting idle current from the wall(ammeter and volt meter are both permanently connected to the variac) and scope, load, spectrum analyzer, and true RMS ac meter are all connected though a patchbay to the output terminals of the amp. Literally, 10 seconds is all that is needed to tell how it is going to behave, sound, and function just from those visual indicators of power in, power out, distortion, noise, harmonic distribution and efficiency. Usually, depending on the tubes, the optimum settings of bias will be different than the internet rumors specify.

[TomCarlos]

Thanks for the reply.

I monitored the Cathode current and saw it change at some point. I readjusted the trim pot and it changed later again. The trim pots on these amps are very flimsy. I'm tempted to install either a mini 10 turn 1/2 watt or drill a hole on the back side of the chasis and install a "real" bias trim pot. I hate working on this thing upside down!!!

Yes, I will hook up a scope and follow the Scope Bias Method... but only after I swap out the trim pot and make sure I have some stability first!!

[Jazz P Bass]

Just a hint: the -47Vdc is the BIAS pin voltage, not the plate voltage.

It will not go any lower than that because at that point you are beating the piss out of the valves.
At idle!

Get a Kill_a_Watt meter.

Set the bias that it draws 100-120 watts from the mains at idle.

[TomCarlos]

JPB - Thank you. I stand corrected. Pin 3 is the Plate. Pin 5 is the Grid (Control).

I spent the day reading several sources on Biasing and the different techniques. And here is what I did.

(1) Using a clean signal generator fed into the clean channel, I used a scope and adjusted the Bias control to minimize any notches that appeared on the rising and falling edges of the sine wave (remove the crossover distortion) that appeared in the output. By the way... output was a precision 8 ohm resistor. You increase the gain just before clipping and recheck.

(2) DC voltage measurement at Pin 5: -45.5 volts (for the heck of it)

(3) DC voltage measurement at Pins 3 and 4, 456 Volts

(3) Use the Bias Probe kit to measure the current coming out of the Cathode: 45ma

(4) Calculate Power Dissipation of 6L6GC tube: 456vdc x .045a = 20.52 watts

Max Power Dissipation for the 6L6GC is 30 watts - I am good.

Kill-A-Watt Meter reads 1.32 amps

The Clean channel sounds nice. The Dirty channel? Well, that is another problem. Something is wrong there. But that is for another night and another thread.

Thanks for the education and pushing me to learn this stuff!!!!

[Jazz P Bass]

They are your tubes

I still think that is set way too hot (for longevity)

Set the K_a_W to read watts.

My guess is you are pully 150 some watts at idle.

Again, thai's fine for EL34 tubes.

Not fine for 6L6 tubes.

[g1]

JPB, you posted the factory bias spec. in post #5. It is .8A mains current. For crossover method they say reduce till just visible, not till it disappears.
If you can cool it down without the tone suffering a lot, you will get longer tube life.

[Jazz P Bass]

100 watts mains draw at idle.
I could go with that.