New PT , how to implement it?

[2deaf]

The only time the reading across the 1R resistor is relevant is at idle with no AC signal applied.  Readings under other conditions should be done for recreational use only. 

[uki]

It seems like a pretty big swing from 30mV to 70mV, almost 140% more,  To go from 30% to over maximum for the tube. But if it is only momentary...??

21k bias resistor in place, the idle current is 18-19mV  and that jumps to 70mV while playing really hard but only momentary, it did hit that much 2 or 3 times in about 5 minutes.

With the bias resistor, as you increase its resistance you are supplying less and less -Vdc to the tube. Less -Vdc = more current. The max safe amount for that resistor is something that gives you the correct bias. Unsafe would mean you have too much current for the 6L6 and it will red plate and/or not last as long as you might want it. 

Tube data says:
Ia   = 72 mA(se)  134 mA(pp)
Ig2 =   5 mA           11 mA
which one to look at ?

I think this is the same question as above but a in different way, what is the safe current ?

[EKDENTON]

Tube data says:
Ia   = 72 mA(se)  134 mA(pp)
Ig2 =   5 mA           11 mA
which one to look at ?

I think this is the same question as above but a in different way, what is the safe current ?





Ia is anode current
Ig2 is a grid current


I think your okay on your bias, any spikes would be just monetary, your average is within limits. You like the sound and you wont be hammering the guitar 100% of the time ;-)

[2deaf]

Tube data says:
Ia   = 72 mA(se)  134 mA(pp)
Ig2 =   5 mA           11 mA
which one to look at ?

Neither.  Those are typical characteristics for Class A₁ at operating points that aren't even close to yours.  They have nothing to do with the maximum current that your tubes can draw.  The only number on there that is of any use to you is the 30W rating under limiting values.  The only good that number is going to do for you is to determine a bias point that you desire. 

The bias point has nothing to do with the maximum current your tubes are going to draw at full tilt.  There is no way you are going to measure this current with a 1R resistor and a $6 meter.

[pompeiisneaks]

Tube data says:
Ia   = 72 mA(se)  134 mA(pp)
Ig2 =   5 mA           11 mA
which one to look at ?

Neither.  Those are typical characteristics for Class A₁ at operating points that aren't even close to yours.  They have nothing to do with the maximum current that your tubes can draw.  The only number on there that is of any use to you is the 30W rating under limiting values.  The only good that number is going to do for you is to determine a bias point that you desire. 

The bias point has nothing to do with the maximum current your tubes are going to draw at full tilt.  There is no way you are going to measure this current with a 1R resistor and a $6 meter.

I don't know that I'd 100% say "no way" it's going to be wildly inaccurate, and at best you could massively go below expected 70% dissipation rating, say 60% just to be sure the 'errors' in that cheap of a meter aren't going to cook tubes ;).  but I do agree it's not super smart.  If you want to be doing this, you should at least try to buy one of th 50$ or so meters out there on amazon or the like that have a moderate rating etc.  Or if you don't even want to go full 50$ range, this meter has decent ratings for the price: https://smile.amazon.com/Mastech-MS8268-MS8261-Digital-Multimeter/dp/B000JQ4O2U/ref=sr_1_5?ie=UTF8&qid=1488991635&sr=8-5&keywords=multimeter+autoranging and is only 24$.

That's loads better than a harbor freight 6$ one.  (I tried using one at my father in laws because I was out of state and didn't have any of mine, and MAN that thing sucked!)

~Phil

[uki]

That's loads better than a harbor freight 6$ one.  (I tried using one at my father in laws because I was out of state and didn't have any of mine, and MAN that thing sucked!)

~Phil

   

Man thanks for the hint !  In the city downtown there a neighborhood that have several stores that sells electronic stuff, best prices there too, been asking in one of the big stores, the price range for a good meter, starts at around R$100(U$32) and goes up to R$1000(U$320) and beyond !  Check out: http://loja.multcomercial.com.br/catalogsearch/result/index/?p=1&q=multimetro

About the bias circuit, I found a 25.4k resistor and I did like it a lot how the amp is sounding now, it doesn't go too hot like with the 38k one and even the 28.5k doesn't sound as good. Looks like the balance between the tubes aren't too bad.
V7    24.8mV
V8    24.6mV
V9    26.2mV
V10  25.8mV

Gonna leave this way for now.....   gig next saturday !!!!

[2deaf]

I don't know that I'd 100% say "no way" it's going to be wildly inaccurate, and at best you could massively go below expected 70% dissipation rating, say 60% just to be sure the 'errors' in that cheap of a meter aren't going to cook tubes ;).

The current that I was referring to was the maximum current that an output tube draws, not the DC current at idle.  It sounded like they were trying to determine maximum current by playing a 6-string chord and measuring DC volts across a 1R resistor.   

[pompeiisneaks]

Oh gotcha, you meant specifically that's the 'wrong way' to measure it period.  Or do you mean the crap one couldn't measure the mV across a 1ohm on the cathodes? 

~Phil

[PRR]

> the price range for a good meter

The R$ 31 meter (US$ 10) looks to my eyes like a $10 Chinese meter. These are often fine meters.

I am not sure what you get "more" for the R$ 60 (US$20) meter.

Both are safety "CAT1". This means you can use them inside appliances (including guitar amplifiers), but maybe not in a home fuse-box and certainly not at your electric meter (which you should never do-- your electric supply system has enough problems without you messing with their stuff).

[2deaf]

Oh gotcha, you meant specifically that's the 'wrong way' to measure it period.  Or do you mean the crap one couldn't measure the mV across a 1ohm on the cathodes?

I think that a $6 meter can measure the relatively flat DC voltage at idle across a 1R resistor.   But when you get that tube going and the current is wildly fluctuating at a wide array of frequencies, who knows how that meter (or any meter) will interpret it.  So I am questioning the methodology.  Who cares about the maximum current draw, anyways?  It is a function of just about everything EXCEPT the bias point. 

[2deaf]

. . . the price range for a good meter, . . .

I like the Multimetro Osciloscopio  at R$ 2,540.00 with three easy payments of $265.14 USD (cheaper than yesterday).  Just kidding, don't buy it.

With very little objective reason, I like the Hikari HM-2010.

[PRR]

> With very little objective reason, I like the Hikari HM-2010.

Because it is yellow. But it is not a Fluke.

It does however have " Impedância de Entrada: 10MOhms", a useful spec not given on the ten-buck meter?

[2deaf]

The majority of meters seem to be yellow these days, probably to imitate Fluke and increase their sales to confused dopers.

This is one of the few that lists some specifications, even more than Hikari lists on their website.  It has an input impedance of 10M for DC and 1M for AC, but it doesn't list the input impedance for resistance readings. 

You could also print out a sheet with all of the choices on it, throw a dart at it and select the one closest to the dart penetration point.   

[PRR]

> it doesn't list the input impedance for resistance readings. 

Probably meaningless as stated. If the meter has resistance it just confuses things.

DMMs usually have a constant-current source ("infinite" resistance) and a volt-meter (not necessarily with the 10Meg used for volt-only measurement). So they try to be "infinite". Which can't happen. But if the ohmeter Z is much-more than the highest resistance it will measure, that is OK.

Yes, the highest (and lowest!) resistance the DMM will read correctly is sometimes important. But >10Meg gets into leakage of cheap plastics, and sub-Ohm requires special technique to control contact errors.