[Guest]

As a die-hard (but untrained) DIYer I earlier found info on the 'net on how to bias the EL-34 power tubes in my Rivera M60 amp, which has bias pots. The link has disappeared meanwwhile, and I moved house and misplaced the info. In addition to the usual warnings about lethal voltages, it consisted of where to connect the test leads and roughly what voltage to aim for. Could anyone tell me this?

 

[Wild Bill]

"As always, it ain't that simple..."

As a die-hard (but untrained) DIYer I earlier found info on the 'net on how to bias the EL-34 power tubes in my Rivera M60 amp, which has bias pots. The link has disappeared meanwwhile, and I moved house and misplaced the info. In addition to the usual warnings about lethal voltages, it consisted of where to connect the test leads and roughly what voltage to aim for. Could anyone tell me this?

Couldn't find a schematic for the M60 with a quick google. Saw some circuits for older Rivera amps that show no easy provisions for quick biasing. Maybe your amp is different.

First off, you have to measure the plate voltage on the output tubes. Stating a specific bias reading without knowing the plate voltage is like describing the length of a fish with only one hand - it's meaningless! Even different amps of the same model might show a bit of a plate voltage difference and this affects the bias "sweet spot".

http://www.aikenamps.com has some great stuff about biasing and also you can google up Randy's "Tone Lizard" site which also has some great info.

You don't likely have a bias test point built in so you'll either have to add a couple of resistors to the circuitry or use what's called the shunt method, where you measure the DC resistance of each side of the output trannie winding with the amp turned OFF and then measure the voltage drop across each winding with the amp idling with no signal. Ohm's Law will give you the plate current. You can then adjust the bias control if you need to change it.

It's a little more ticklish with the high voltages involved but the nice thing about the shunt method is that you don't need to install bias test points.

You absolutely should read Mr. Aiken's FAQ.

 

[Guest]

Thanks for the input, Bill. BTW I was recently given one of your cards at a store in Burlington ON with a recommendation when I inquired about an amp tech. The biasing info I found previously was provided by a more knowledgable (than me) Rivera user, and involved connecting a test lead to a specific pin on one of the power tubes, and to another point (ground?). As you probably can tell I have no idea what I'm doing technically, but managed to follow the instructions without zapping myself, with satisfactory results. It's now time to change the power tubes again and I was hoping to do it myself again, but my random filing system let me down.

 

[Wild Bill]

Thanks for the input, Bill. BTW I was recently given one of your cards at a store in Burlington ON with a recommendation when I inquired about an amp tech. The biasing info I found previously was provided by a more knowledgable (than me) Rivera user, and involved connecting a test lead to a specific pin on one of the power tubes, and to another point (ground?). As you probably can tell I have no idea what I'm doing technically, but managed to follow the instructions without zapping myself, with satisfactory results. It's now time to change the power tubes again and I was hoping to do it myself again, but my random filing system let me down.

Glad to help, but you've now confused me with how you checked your bias!

Most of the time there is NO specific pin on a power tube that lets you check the idling current! The cathodes are grounded so you can't measure anything there. Perhaps the Rivera already has put 1ohm resistors to the cathodes that will THEN give you a bias test point. You still have to know the plate voltage and do a little bit of math.

You can put your meter on the main signal grid pin and you'll see 30-50 negative volts of dc bias voltage. This is the adjustable voltage that is used to change the idle current and set it to the right amount. Some old amp schematics (like Ampeg) would tell you to set this voltage to a particular point but with modern tubes that vary much more than in the old days this is generally too loose to be useful.

With a cathode biased amp you will measure a voltage at the cathode and you can then use Ohm's Law together with the value of the cathode resistor and figure out the bias current but those type of amps are pretty well self-biasing and no adjustment is provided or needed.

Whatever, hope it works out! As I keep saying, as long as disco doesn't make a comeback life won't be too bad...