New Princeton Reverb build--Low B+ post -GZ34

[pompeiisneaks]

Monitoring the power tube cathode current in the 'normal working' and 'cut out' modes may help to indicate where the problem lies.

Thanks for the suggestion. Do I need a 1R resistor off the PT cathodes to ground to measure that?

Yes, or bias probe etc.
Dunno why people don’t default fit current sensing resistors to power tube cathodes.

cuz it's super expensive, like 30cents each and takes an extra like 10 minutes of hard work that's why duh!



~Phil

[ViperDoc]

Seems like it's worth the effort.

[martin manning]

Thanks. Is that a circuit you use only on PR builds?

The same basic circuit is fine for almost anything. You have to change the first resistor to get the voltage range centered, and if you are working from a 60VAC bias tap, then I would make the caps larger (47u and 22u) to keep the ripple low.

[ViperDoc]

Thanks.

Before I fire this thing up and measure it again, I want to put the 1R resistors on the PT cathodes to ground. I've got a small bag of 1 watt carbon film 1R resistors. They all test around 1.4 ohms, assuming my DMM is calibrated. My assumption is that with a perfect 1R resistor, you can simply measure cathode voltage and that will numerically equal cathode current via Ohm's Law. It's simple enough to factor in the tested values, but do you recommend shooting for a 1% resistor value tolerance or less, or is 1.4 R close enough for bias purposes? It would seem to me it would be 40% off!

[martin manning]

Ignore your measurement and assume the resistor is 1Ω within its marked tolerance. Your meter is likely not capable of accurately measuring resistances that low.

[sonofmickel]

1 ohm is pretty small to be measuring.
With my meter I put a 100k in series with the 1 ohm and see if they are actually closer to spec?

Whoops, Martin beat me to it.

[ViperDoc]

Thanks, guys.

[ViperDoc]

OK, I did my current best to update my schematic to the build and placed voltage readings as just measured. The amp still cuts out after a bit, and I can make it come back if I probe the PT sockets, but it cuts out again. I installed the 1R resistors to ground off of pin 8 of each 6V6 cathode and I read 42mVDC on each cathode.

[pdf64]

Why bias so hot?
Maybe because the bias trimmer doesn’t have much effect (if arranged as per the schematic)?

[ViperDoc]

Indeed, a very good question. I re-biased for 27 mA cathode current on the power tubes. The amp is still playable for several seconds and then there is a fizz, pop and no sound. It will come back again, but then cut out when playing again.

[sluckey]

It's possible that the 6V6s and/or your OT were damaged while you were barbequing due to the defective bias circuit. So, monitor your cathode current. Does it change when the amp cuts out?

[ViperDoc]

I need to make myself some clip leads for my DMM. It's hard to tell if any wavering cathode voltage is due to my hands. But it appears that one side goes down, then up and the sound comes back. Accompanied by the fizz pop.

Also guilty of barbecuing. I'll find some new tubes and take some new voltages. If my new tubes behave similarly, I assume I'm getting a new OT for Christmas?

Awesome.

[ViperDoc]

So I put some known good 6V6S power tubes into the amp, biased them at 27 mA on the cathodes and the amp takes a ceremonious sh!t as before. So unless you chaps have any other recommendations, Ima buy me a new OT.

Now let me admit, I decided to try a multi-tapped OT for this build in the off-chance I wanted to run another 8R speaker cab in the aux. output with the option of selecting the 4R tap. Worth it/not worth it in your experience?

[Stevem]

27 ma is very idle current for 6V6 tubes run at your amps plate voltage!

I never bias Deluxe reverbs at more then 18ma especially if you you do not want the amp to get too dirty too soon!

[pdf64]

I need to make myself some clip leads for my DMM. It's hard to tell if any wavering cathode voltage is due to my hands. But it appears that one side goes down, then up and the sound comes back. Accompanied by the fizz pop.

Also guilty of barbecuing. I'll find some new tubes and take some new voltages. If my new tubes behave similarly, I assume I'm getting a new OT for Christmas?

We're some way off being sure that the OT is causing the problem yet. eg losing one side of a p-p pair can't, per se, cause the sound to cut out.
27mA still seems high.