Help with using Weber Biasrite

[jackboy8]

Hi there- please can someone help me make sure I am reading my Weber biasrite when using with my 1987 reissue with metro ptp?

I have the V option 2 head biasrite and it reads 454 when set to v that I believe is the max plate dissipation? I have put in Svet pair then Eh EL34's and both times got this reading !

Then when I set the biasrite to the I option I get a reading of 51.2 and tube A and 49.7 on tube B with the Svets . I assume this is miilamps?

I am concerned that this is high- the amp was retubed by a tech a month ago- However maybe I have missed something,for example am I meant to calculate 70% of the I readings ( this being max recommended idling setting) to get a more sensible approx 35 ma bias reading?

I am of coures taking care not to get a shocK!!

I would appreciate a nice simple gude

thanks

Nick

[myker]

i bought mine about 6 months ago, it has three wires coming out of it, i dont know how to use mine either. lol.
454 is your plate voltage, the smaller numbers are your dissipation i would assume.
mike

[Dai H.]

Hi there- please can someone help me make sure I am reading my Weber biasrite when using with my 1987 reissue with metro ptp?

I have the V option 2 head biasrite and it reads 454 when set to v that I believe is the max plate dissipation? I have put in Svet pair then Eh EL34's and both times got this reading !

Then when I set the biasrite to the I option I get a reading of 51.2 and tube A and 49.7 on tube B with the Svets . I assume this is miilamps?

I am concerned that this is high- the amp was retubed by a tech a month ago- However maybe I have missed something,for example am I meant to calculate 70% of the I readings ( this being max recommended idling setting) to get a more sensible approx 35 ma bias reading?

I am of coures taking care not to get a shocK!!

I would appreciate a nice simple gude

thanks

Nick

454 should be the plate voltage on pin 3. "51.2" and "49.7" should be the current draw in milliamps. Have a look at this:

http://www.webervst.com/tubes/calcbias.htm

remember lower is better for reliability esp. if you crank it all the time when you use the amp. So it's encouraged to go lower than 70% dissipation if you use it on 10 all the time. Also the hotter you run it and the more the tube is used, the faster it wears out.

[Guest]

Thanks guys-

Dai- can I take it then that the ma reading on the Biasrite is taken as read when using the Weber tables and not adjusted to 70% of value on bias rite?

For example on my other 1987 Marshall it is reading V 458 with Tube A 65 and B 71 - is this this is very high ? Amp sounds nice and crunchy

thanks again

Nick

[Dai H.]

the figure you see (ex."51.2" and "49.7") should be the current that tube is drawing in mA. The user (you) has to make the adjustment to (in this case class AB) 70% (or less) using the bias adjustment inside the amp. And yes, the numbers you gave do seem to indicate that both amps are running hot. I ran the tubes with the plates cherry red once when I was a total beginner and trying to install a post-phase inverter master volume (and did it wrong...). The amp sounded fantastic, nice and thick, beefy tone. However when I looked in the back and saw the EL34 plates burning red, I nearly $**T myself, lol... Sometimes people run a bit hotter on purpose because it sounds better and accept shorter tube life and higher unreliability. I think running hotter might work in a number of ways. It could a) might no difference, b) sound better, or c) possibly sound worse. Now that you have a biasing tool, you can try biasing hotter and cooler so you can decide yourself what you like and want (guess that would be the beauty of it). Those types of biasing tools I'm guessing typically use a 1ohm resistor inserted between cathode and ground to sense the voltage drop over it, then convert to a mA reading. Since that would include the screen current draw, the reading should be off by a little bit, maybe 5%, so maybe that is something to keep in mind. Also, it's probably safer to try pushing things in a Marshall when the plate voltage is lower (since that usually means the screen voltage is lower--the plate is the big metal piece, the screen grid is a coil of wire) and you aren't pushing the ratings of the tube as much. Personally I'd rather go easier on the tube since some of the tubes I have are almost extinct old tubes, but as I mentioned I think there is some leeway for individual choice. Sry for rambling on, lol...

[jackboy8]

many thanks Dai- not rambling but educating me!

Last question ( for now???)- I have metroamp type2 ppimv on both the amps so when testing bias and adjusting should that knob be fully clockwise ,ie no attenuation but full volume?

regards

Nick

[Dai H.]

no problem! It's a static setting meaning with no signal, so set it with all controls down. If you tried to do it with a signal going, the numbers would jump up and down, so you wouldn't be able to do it. You can try googling for sites to explain biasing. I suppose it can be a pain to read through tech stuff (esp.math--I hate math... ), but even just a little of the basics can go a long way if you're getting into the DIY thing.

p.s. one more thing you might want to consider (esp. since it's summer!), is the ambient temperature. Hotter (like the sun beating down) should be more unreliable, so if you find yourself in a situation like that, maybe some cooling like a fan would be good. Help dissipate the heat better.

[jackboy8]

Thanks again Dai !

Just to conclude this thread I have heard back from Ted Weber who confirms that I take the readings from the biasrite fors "I " setting being the milliamps on the Tubes as read.

He also confirmed my readings were high and therefore I needed to adjust bias to get readings in the 33ma to 39 range

Best wishes

Nick

[jackboy8]

Thanks again Dai !

Just to conclude this thread I have heard back from Ted Weber who confirms that I take the readings from the biasrite fors "I " setting being the milliamps on the Tubes as read.

He also confirmed my readings were high and therefore I needed to adjust bias to get readings in the 33ma to 39 range

Best wishes

Nick

[McGoogle McDougal]

Just remember that when you ever change tubes you may get a different measurement on the plate voltage. The plate voltage will also change as you adjust the voltage. The 35ma recommendation for EL34's is based on what is the typical plate voltage on a 100 watt plexi. Here's how to figure out where the 70% figure comes from:

1. Maximum dissipation for EL34's is about 25 watts
2. 70% of 25 is 17.5
3. 17.5 / 454 = .038 (this equates to 38ma)

So setting the bias at 38ma, given 454 volts palte voltage, results in 70% dissipation. 65% disspation (16.25 / 454 = .035) puts your target closer to 35ma. Just remember that as you adjust the bias pot your plate voltage will change a little, so always use the formula above to know for sure where you're at. You can decide by experimenting if you like it a little hotter or colder. Just make sure that the plates of the tubes turn red you have to go colder! As others have said, tube life will be longer when they are biased colder.

One other thing - the talk of "watts disspation" above has no relation to how many watts the amp is putting out. This is strictly related to the bias.

Eamon

[rjgtr]

The nice thing about the Weber Bias tools is that you can get them with the plate voltage reading as well as just the current draw. That makes zeroing in a lot easier. Most of the time people just assume the plate voltage is a certain value, which might not be correct.