Hello again all!! ...determining plate voltage of the RM100

[Pedro]

First off.....hope everyone had a great summer... as I am an avid motorcycle enthusiast, I spent as much time as I could collecting bugs in my teeth through a high speed, direct impact method! Leaves you with a hell of a smile!

...anyhow...back playing and tinkering with the LB again. Has anyone gone through measuring the actual plate voltage of their head? I know the RM100's are around 500 (from what I could find), but I'd like to get the actual numbers to get the BIAS adjusted as accurately as possible. I ordered a new set of tubes and I want to really work on getting the sweetest feel out of this thing.

So...anyone? ...there's also easy\hard ways of doing this...resistors used on pin 3...don't need resistors....need to pull the chassis....don't need to :roll: :lol: ..... and yeah, I know it's dangerous...I have my asbestos moon boots.

Thanks!

P

 

[kc2eeb]

It's between 500 and 510 depending on your line voltage, but, when you use the bias test sockets on the back of the amp you are measuring the actual current draw. You are measuring the voltage drop across a 1 Ohm
resistor. As per Ohm's law Current =Volts divided by Resistance.
Since the resistor is 1 Ohm, the volts(millivolts) convert directly to Amps
(milliamps.)
You're not gaining anything as far as accuracy by opening up the amp and
using the meter in series with the plate especially because, surprise!, most meters, when set to display current, are actually measuring a voltage drop across a calibrated resistance. This is EXACTLY what you're doing when you use the test points supplied on the back of the amp.

 

[Julia]

Randall said it's 530.

 

[kc2eeb]

Pin 3 is the Plate. Pull the tubes and measure it. It will vary under load.
Besides, if you're looking for a quiescent bias reading you're going to get it at the test points. If you then leave the meter connected and play as loud as you're going to get, you can read the total current draw of the tube. Then, ask your ears if it sounds the way you want.

 

[Pedro]

Thanks for the feedback! This makes things very easy.

What I've been getting from the web and on here is between 500 and 530 so it will be interesting to see what it actually does read. After talking with Trace at Voodoo, I want to calculate and test the percentage values he recommended.

Gracias!

Pedro

 

[Julia]

It will also vary per line voltage.

IMO you should bias your amp when your line voltage is reading it's highest.

And for brand new tubes, I'd run them at about 60% for the burn in period.

FYI, i've done the calculations for the RM100 based on the 530V. Valve info from the companies (JJ, SED, New Sensor) Here they are. The low end is 50% and the high end is 70%. I wouldn't go higher than 70% unless you've managed to contract Cesar Diaz to keep your amp running.

EL34/E34L/KT77/6CA7 (these tubes rated at 25W): 23.6 mV to 33.0 mV
6L6/5881 (these tubes rated at 30W): 28.3 mV to 39.6 mV
6550 (these rated at 35W): 33.0 mV to 46.2 mV
KT88 (JJ rated at 42W): 39.6 mV to 55.5 mV

60% seems to be pretty decent. Right now I just want stable.

 

[Pedro]

And for brand new tubes, I'd run them at about 60% for the burn in period.

That's the number Trace said to stay close to...seems to be the favourite of many (60-64 with 62 being the "magic number" for most)

FYI, i've done the calculations for the RM100 based on the 530V. Valve info from the companies (JJ, SED, New Sensor) Here they are. The low end is 50% and the high end is 70%. I wouldn't go higher than 70% unless you've managed to contract Cesar Diaz to keep your amp running.

EL34/E34L/KT77/6CA7 (these tubes rated at 25W): 23.6 mV to 33.0 mV
6L6/5881 (these tubes rated at 30W): 28.3 mV to 39.6 mV
6550 (these rated at 35W): 33.0 mV to 46.2 mV
KT88 (JJ rated at 42W): 39.6 mV to 55.5 mV

60% seems to be pretty decent. Right now I just want stable.

You're the bomb Julia! Thanks for all this info! and thanks to you too kc2!

I do love this site for the wealth of information! :wink:

Pedro

 

[kc2eeb]

You're very welcome! Play on!