bias probes educate me please!!

martinw

Budwiser said:

Here's a little maths to work out bias voltage on the marshall dsl 2000 ..50 watt ..

Ht is 460 vdc cathode resistor is 1 ohm on pin 8..marshall say 45mv per tube..way hot and it's near max wattage of the tube so I bias them at 38mv that works out about 17.48w per tube.. so 460 times by 0,038mv =17.48w or near to 17.5 watt.

Except that when you alter the bias like that, the HT will no longer be 460V, so your maths will be out a little. You're broadly right though, and in the case of the DSL the difference will be negligible, or at least safe.

That's not true in every case though, so it does need pointing out.

ThePrettyDamned

martinw;135166" said:

Budwiser said:

Here's a little maths to work out bias voltage on the marshall dsl 2000 ..50 watt ..

Ht is 460 vdc cathode resistor is 1 ohm on pin 8..marshall say 45mv per tube..way hot and it's near max wattage of the tube so I bias them at 38mv that works out about 17.48w per tube.. so 460 times by 0,038mv =17.48w or near to 17.5 watt.










Except that when you alter the bias like that, the HT will no longer be 460V, so your maths will be out a little. You're broadly right though, and in the case of the DSL the difference will be negligible, or at least safe.That's not true in every case though, so it does need pointing out.

I understand none of this.

Which is why I'd always take an amp to a tech. It's probably simple enough, and I'm quite handy when it comes to learning stuff, but I just don't fancy it. Even if the amp had an external bias pot and stuff, as some do, I wouldn't use it.

I'm a scaredy cat.

ICBM

ThePrettyDamned said:

I understand none of this.

Which is why I'd always take an amp to a tech. It's probably simple enough, and I'm quite handy when it comes to learning stuff, but I just don't fancy it. Even if the amp had an external bias pot and stuff, as some do, I wouldn't use it.

I'm a scaredy cat.

External bias test points and trimmers are the way to go, and it really puzzles me that so few manufacturers fit them. Fender did as long ago as the 'The Twin' in 1988. If they're designed well you shouldn't be able to set the amp too far outside the safe range, and there's no risk to the user at all - that's the whole point. Anyone can bias an amp like this with a multimeter.

jpfamps

ICBM said:

ThePrettyDamned said:

I understand none of this.

Which is why I'd always take an amp to a tech. It's probably simple enough, and I'm quite handy when it comes to learning stuff, but I just don't fancy it. Even if the amp had an external bias pot and stuff, as some do, I wouldn't use it.

I'm a scaredy cat.

External bias test points and trimmers are the way to go, and it really puzzles me that so few manufacturers fit them. Fender did as long ago as the 'The Twin' in 1988. If they're designed well you shouldn't be able to set the amp too far outside the safe range, and there's no risk to the user at all - that's the whole point. Anyone can bias an amp like this with a multimeter.

The external bias test points may fall foul of the regs in certain territories as the HT could be exposed in the event of a valve short.

Regardless, it makes life easier for everyone, so I fit them on our amps.

ICBM

jpfamps said:

The external bias test points may fall foul of the regs in certain territories as the HT could be exposed in the event of a valve short.

Only if you don't take proper precautions. A 1W 1-ohm resistor in parallel with a 1N5401 or similar to limit the maximum cathode voltage to 0.7V and a 1A HT fuse should make it foolproof.

martinw

They'll just invent a better fool.

ThePrettyDamned

martinw;135617" said:

They'll just invent a better fool.

Yeah, that's... Er...

That's me.

ICBM

martinw said:

They'll just invent a better fool.

This is true, and a really world-class fool can circumvent almost all safety mechanisms. Over-engineered voltage limiting should stop mere jobbing fools though .

ecc83

Great (or even  much at all! ) precision  is  pointless IMO for  guitar amps because...

1) There is no need to bias "hot". 25mA per  25W pentode seems to work ok. If of course you insist on running the valves at 90% of their Pa limit, you might have a problem.

2) 5 minutes (or 5 secs!) after your oh so precise setting the mains will shift up or down. Now bias SHOULD track mains V, a bit and it does but not perfectly.

3) Immediately after a good full power thrash, bias with be all over the shop and is unlikely to return to within 5% of original setting.

4) If biasing after fitting new valves (main raison detre!) the grid current of those valves will slowly decrease by up to 400% over an hour or two and once again, bias setting will wander.

Dave.

GilgaFrank

ICBM;127316" said:

Any bias probe that uses a 1-ohm resistor is as good as any other really. Just avoid any where you read *current* directly and not *voltage*

All bias meters or multimeters measure voltage! The 1 ohm resistor is there to create a small voltage drop and the meter measures the voltage across the resistor. Ohm's Law (V=IR), if the voltage drop is 0.042 Volts then the current through the resistor is 42mA. This is the principle for any multimeter reading current, they just switch in a known resistance and measure the voltage drop and use that to calculate the current flowing.

martinw

I'm sure @ICBM will be grateful for that lesson!

ICBM

GilgaFrank said:

ICBM;127316" said:

Any bias probe that uses a 1-ohm resistor is as good as any other really. Just avoid any where you read *current* directly and not *voltage*

All bias meters or multimeters measure voltage!

No they don't. Do some research on this.

At least one model simply breaks the cathode circuit and requires you to measure *current* directly. This is dangerous for the exact reason martinw said.

(And yes, I do know that you're still actually measuring voltage inside the multimeter .)

Try this one, for example…

http://www.el34world.com/charts/BiasChecker3.htm

If you're unfortunate enough to use that on a valve which has a short, you'll have the full HT present on the banana plugs.

bandmaster188

I just bought this, am I going to die :-O I know it's an overpriced box of junk, but I think it's gonna serve my amateur needs just fine! http://www.hotroxuk.com/tad-biasmaster-2-octal-probes-bm2.html

ICBM

bandmaster188 said:

I just bought this, am I going to die :-O I know it's an overpriced box of junk, but I think it's gonna serve my amateur needs just fine! http://www.hotroxuk.com/tad-biasmaster-2-octal-probes-bm2.html

For some reason the link in your post doesn't show and I thought for a minute you'd bought one of the Hoffman ones...

I can't tell whether the TAD does it right or not - there's no indication in the documentation.

It does seem rather a lot of money for a couple of adaptors and a meter that looks like (but is much less useful than) a cheap multimeter, although it does have the slight advantage that you can check both valves at the same time.

bandmaster188

Well according to the paperwork, it's a highly accurate piece of test equipment. )

GilgaFrank

ICBM;136111" said:

GilgaFrank said:

ICBM;127316" said:Any bias probe that uses a 1-ohm resistor is as good as any other really. Just avoid any where you read *current* directly and not *voltage*



All bias meters or multimeters measure voltage!





No they don't. Do some research on this.

At least one model simply breaks the cathode circuit and requires you to measure *current* directly. This is dangerous for the exact reason martinw said.

(And yes, I do know that you're still actually measuring voltage inside the multimeter .)



Try this one, for example…

http://www.el34world.com/charts/BiasChecker3.htm



If you're unfortunate enough to use that on a valve which has a short, you'll have the full HT present on the banana plugs.

Perhaps you could do my research for me and tell me how to measure current without measuring the voltage drop across a known resistor? Thanks in advance.

ICBM

GilgaFrank said:

Perhaps you could do my research for me and tell me how to measure current without measuring the voltage drop across a known resistor? Thanks in advance.

You're trying to be pedantic about something which is of no importance.

Yes, we all know that the actual measurement inside the meter is done by passing the current through a resistor and measuring the voltage across it. (Although in fact, in an analogue meter that voltage is then used to drive a small current through the movement coil - the lowest current range *is* usually just the coil itself and so is a direct current measurement.)

That's completely irrelevant here. What matters is whether the bias probe uses a resistor to generate a measurable voltage (the right way) which is sent to the meter, or no resistor and relies on passing the current out of the amp to be measured by the meter (the wrong way, because it's potentially dangerous).

Worse, you're in danger of confusing people who don't know or need to know why one matters and the other doesn't. If you can't see why it's an important difference without resorting to trivia about how the meter works internally... perhaps you make or sell one of the dangerous type of bias probes?

ecc83

I agree with ICBM. We are in danger of losing the plot here. But!

Talk of volts, amps and Rs takes me back to first year at tech'! Voltage was also known as "Electro Motive Force and it was the paradox of measurement that you could not have a current without an EMF* to "push" it and you could not have an EMF without a current to cause a voltage drop!

Back in that day, much cleverness was needed to avoid loading effects from screwing voltage measurements, no such thing as "infinite" impedance DVMs (for the equivalent of 20 fags and a pint today!)

*Then came Superconductors of course!

 

Dave.