Biasing with a "BiasProbe" basic

[Skel]

Hi all, I'm sure there are at least a few who are familiar with using a BiasProbe.

According to its instruction manual, the method and formula for setting the bias correctly is:

I(p) = P(D) X 500 / E(p) X Qty.

I(p) = normal bias current, P(D) = rated RMS output power in watts, E(p) = B+ voltage.

I was recently going to bias a 100 watt Marshall head containing only 2 of the power tubes. Also I don't have a load - I realize I need to get one fast - my speaker cab is offsite at my band's rehearsal site. Given this info:

According to my calculations, my I(p) = 50, E(p) = 485V, Qty = 2. Using the bias probe, 50 x 500 = 25,000 / 485 X 2 = 970, or 25,000 / 970 = 25.77 milliamps. So my I(p) should be around 25.77 milliamps. My measurment was about twice that.

My questions are:

I read in Aspen Pittman's book that less negative bias equals more current. Likewise, more negative bias would reduce the current. In my example above it sounds like I need to increase the resistance on my bias pot in order to reduce the bias voltage (?) I assume it's a matter of simply using a screwdriver to turn the bias pot until the current measures what the formula says it should be.

I always hear about bias described using the adjectives of "hot" and "cold". In my situation where the current is twice what it should be, is this considered "hot" or "cold"?

Last, with using only 2 tubes instead of 4, and having no load, is my whole measurement process completely flawed and basically mute?

Thank you.

[PRNDL]

Last, with having no load, is my whole measurement process completely flawed and basically mute?

Usually the output tubes start arcing and will burn out when there's no speaker. Sometimes they'll take out a few resistors, and maybe even an output transformer.

Here's a handy web site
http://www.webervst.com/tubes/calcbias.htm

There's a nice Excel file that shows bias regions in colors, which is handy
http://www.duncanamps.com/software.html

I assume it's a matter of simply using a screwdriver to turn the bias pot until the current measures what the formula says it should be.

Along with checking the voltage, that's pretty much it.
Matching tubes involves comparing the current and swapping tubes until they are less than 5-10 mA apart.

As for your calculation, they look correct, but doing it for both tubes at the same time is confusing, especially if you have a bias probe, which usually measures the current in one tube at a time.

The Duncan bias calculator shows 31-36 mA for an EL34 at 485 volts.

Many older techs state that current bias recommendations are way too hot and are the main reason for short tube life. They'd recommend 36 mA as the maximum, 31 as normal, and cold bias as low as 25.

Colder bias extends tube life at the expense of higher crossover distortion, which is a major issue with HiFi amps, but of little concern with guitar amps.

Anywhere between 25 and 35 would be good (IMHO), although other people may have different recommendations based on personal experience and training.

[Skel]

Thanks for the great info. I would like to just understand (for the record); With no load, are we talking seconds before damage will occur, or is it more like minutes or even hours? Obviously the best way for this not to even be an issue is to have the correct load - I'm just curious. Would an experienced tech even turn on an amp without the correct load?

[PRNDL]

I would like to just understand (for the record); With no load, are we talking seconds before damage will occur, or is it more like minutes or even hours?

If you look, the tubes start arcing immediately. The time for them to burn depends on their condition. Some say that output transformers will arc internally and short pretty quickly too.

Would an experienced tech even turn on an amp without the correct load?

Only by mistake.

My mentor always turns the amp on for a few seconds, then flips the standby switch.

[stokes]

Although it is sound practice not to turn an amp on without a load,if you arent putting a signal thru it it "should" be fine.I say "should" because if you have an oscillation going on that you dont know about,it is like having a signal going thru the amp.

[Amp Kat]

Also depends where the volume pot is set with oscillations. It will most likely fry sooner with the volume cranked over not being cranked but long periods of time at low volumes can do it also.

[Bruce / Mission Amps]

Also depends where the volume pot is set with oscillations. It will most likely fry sooner with the volume cranked over not being cranked but long periods of time at low volumes can do it also.

Also... if the normal output jacks are switching jacks... they will be closed with no speaker cable plugged in.
That means the actual load on the OT is what ever the DC resistance of the wires and OT are.
That will be extremely low and for the short few moments you are checking idle current, that will snub out any damaging problems you might have.
When the switching jacks closed, they are shorted to ground. It is very unlikely that with such a low impedance reflected in the OT there will be any possibility of damage to the OT or power tubes with the volume pots at zero and no signal
applied to the input jacks.

[Skel]

Also... if the normal output jacks are switching jacks... they will be closed with no speaker cable plugged in.
That means the actual load on the OT is what ever the DC resistance of the wires and OT are.
That will be extremely low and for the short few moments you are checking idle current, that will snub out any damaging problems you might have.
When the switching jacks closed, they are shorted to ground. It is very unlikely that with such a low impedance reflected in the OT there will be any possibility of damage to the OT or power tubes with the volume pots at zero and no signal
applied to the input jacks.

Okay - again, thanks for the fantastic direction. This is a late 80's JCM800, with an impedance selector, so I really don't know if it has switching jacks (and don't know what that even means yet, to be honest). I know my DSL 100 has no impededance selector and has separate jacks for the different impedances.

I'm going to swap the 2 Groove Tube EL34's with 4 (supposedly) matched Siemens EL34's I have while I'm at it. It will be nice to not have to worry while I'm doing my first bias setting job. I'm going to follow a previous poster's info and bias it at around 35 milliamps - I've heard that "cold" sounds pretty cruddy, although I don't know the "too cold" sound well enough to be able to imagine it.

Pretty soon I'll be working on my early 80's 50 watt Canadian Marshall JCM800These are a a little different animal but I'll get to find out why it has the circuit board with all the fuses. It runs ok but has low output so I'm actually hoping it's a filter cap so I can learn how to replace one (or really, all of them if a filter cap turns out to be the culprit).

[stokes]

An amp with multiple impedence jacks (not an ext.spkr jack) have no switching jack,your DSL 100 does not have a switching jack.The Canadian and English amps have more fuses because of codes in those countries.