Princeton Reverb screen/grid resistors

[POWDOG]

The amp evolves! I fixed the lead dress around the reverb driver (was the wire between pin 6 and 1). 12AT7 is back in and quiet! New development...I lost my tremolo. Rechecked voltages and connections. With 29.3 volts from 6V6 pin 8 and ground (remember...this one is cathode biased), and 404 VDC on the plates and a 400 ohm cathode resistor, I'm now dissipating 13.6 watts per tube. A little higher than I want to be. Could the hot bias be the cause of the lost tremolo? Everything looks pretty good, not sure where to look. Suggestions? Thanks guys.

[martin manning]

Can you tell if the oscillator is working? Measure the voltage at the intensity pot wiper and see if it is fluctuating.

[POWDOG]

Yes, it is fluctuating. Not a huge swing, but something.

[martin manning]

In fixed bias amps with bias-varying trem, the p-p signal at the junction of the bias feed resistors should be about the same as the bias voltage. Given that the frequency is ~10 Hz at most, it might be hard to measure with a DMM. In any case it will give you RMS (approximately), or ~70% of peak so turn the speed all the way up and look for VAC at about 1/3 of cathode voltage or better.

[POWDOG]

This is true with fixed bias amps. Is is the same with cathode biased amps? This is the first time I've built a PR with cathode bias, so I'm kinda working with a new set of problems.

[martin manning]

I'd think the trem signal would want to be at least in the same ballpark.

[POWDOG]

Okay. Should be 1/3 VAC of the cathode of V4 or the 6V6's?

[martin manning]

The trem signal VAC should be ~1/3 of the DC voltage on the cathodes of the 6V6's.

[POWDOG]

Good morning. Well, I went back to the 330 ohm cathode bias resistor on the 6V6's. Measured 26.8 volts from pin 8 to ground, 392 on the plates, and hooked up my bias meter which read 40 mA for each power tube. I believe the range for cathode biased amps is 35-45 mA, which puts me right in the middle. When I put my MM at the junction of the two 220K feed resistors, I get 0 vac and .04 vdc. I've triple checked all my solder joints, making sure there's continuity at the right points and no ground issues. Not sure where to go from here. It's strange that the tremolo DID work, and then suddenly dropped out. Help.

[martin manning]

What is the resistance to ground from the junction if the 220k's?

[POWDOG]

With the Intensity set on 1 I have .03 ohms at the junction of the feeders. With Intensity on 10 I have 212K ohms.

[martin manning]

That sounds good... So with the intensity turned up how much LFO signal do you have there?

[POWDOG]

It swings from about .037 to 0 vdc (270K - 230K ohms).

[martin manning]

If the oscillator is working, you should be able to work back towards the oscillator from the top of the intensity pot and see if the signal reappears. If the oscillator is not working or not producing much signal I would suspect a failure of one of the three phase shift caps.

[POWDOG]

I was trying to put off replacing those. I overbuilt this amp, all 1 watt carbon film resistors and 600 v orange drops. But there must be a failure, especially since it worked fine at start up and then just quit. For the build, in order to slow down the tremolo, I started with .02 uF where it called for .01 and .03 where it called for .02. Oh well, I guess that's where I go next.