Help with a Rivera Sedona

[5thumbs]

I have a friend that brought his Sedona to me for biasing. He had changed all the tubes in the amp, all the preamp tubes (The Tube Store 7025s) and the 2-EL-34's (Winged C). He had played the amp (I think mainly electric) for a few months and then started getting some noise. I pulled the chassis and tried to bias the tubes, but I am not getting any reading in mA's on pin 8 of the EL-84. I followed the instructions from the manual…moved the speaker load to output #2, leaving output #1 empty, no input and no reading on pin 8. I am getting ~450VDC on the plate (3), ~450VDC on the grid (pin 4), ~ -33VDC on pin 5 (control grid), But no milliAmp (DC) on pin 8, although it does give me ~54VDC reading. Now when I plug the speaker back into output one I get runaway oscillations (loud squeal) with no input signal present. What is going on?!?! Everything seems fine, I re-seated the tubes, tried a pair of spare EL-34's I had, re-seated V1 and switched preamp tube positions but still can't stop the squeal. I am not an electrical engineer, but I grew up in the '60's helping my dad in his TV/Radio repair shop and I have built and repaired many tube amps over the years, so I know enough to be dangerous ;-) Help!

[pdf64]

There's something very strange if the cathode is 54V; is there a fuse between the cathode and ground? Need a schematic really, can you attach it from the manual?
Pete

[Jazz P Bass]

I would tend to look beyond bias if there is a noise problem with the amp.
At any rate, which Sedona do you have?
I believe these are cathode biased amps.
The bias procedure is a bit vague on what is being measured.
The opened jack may allow a series reading from the cathode resistor to ground.
In that case your meter should be set to read DC current. (if it was not already )

[5thumbs]

Hi Pete,
I wish I had a schematic! I can't find one anywhere online and I sent Rivera an email last week, but it has not been returned. I can't see any fusing between the cathode and ground but the tube circuit board is half hidden under the output jack circuit board and I was hoping not to have to disassemble 25 things just to set the bias! ha! It's never easy!

Hi Jazz P Bass,
Obviously the squeal is not bias related, but that didn't start until I pulled the chassis and tried to set the bias. The amp says Sedona and it has 2-EL34's and 5-12ax7's a 12" woofer and a tweeter. I used the same print out that you posted and followed their procedure exactly, but I did not get any DC mA reading on the cathode. In fact I got no reading with a multimeter or with a Weber Bias Rite plugged in between the sockets and the tubes. I'm a little lost without some kind of schematic or layout that helps to trace the circuit on these printed circuit panels.

Thanks for the input, I really appreciate the help!
Tim

[52 Bill]

In fact I got no reading with a multimeter or with a Weber Bias Rite plugged in between the sockets and the tubes.

When you used the socket adapters which speaker jack did you use?

As best as i can figure, the output tube cathodes are grounded through the #1 speaker jack, kinda like a battery cut off on an FX pedal. No speaker in #1 no power through the output tubes. This means that if you place your ammeter from the tube cathodes to ground you will read the idle current directly in mA.

Have you checked your meter for current reading? On one of my meters, there is a fuse that only protects the current meter function so all of the other functions will work fine, but it is an open circuit when trying to read current.

And if you want to try the socket adapters plug the speaker into jack #1.

[5thumbs]

Thanks 52 Bill,
I had the speaker in #2 as the biasing instructions stated when I tried the Bias Rite. I will try it in 1 and see if I get a reading. You also have a good point about the meter fuse, I haven't checked that meter in a while as to wether the amp settings are working correctly! Good info, thanks, I will post back when I can get back home to the amp.

Tim