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

[martin manning]

See below...

[sluckey]

OK, we're on the same page now. Sorry bout that. I see the problem. RobRob's drawing is messed up. Simple fix though.

Your yellow wire from the INT pot to the Bias pot should not connect to the wiper. Connect it with the orange wire, either on the board or on the bias pot. Now go check the voltage on pin 5 of the 6V6s. Turn the bias pot to one end and then the other end. Record the minimum and maximum voltages. What are the two numbers?

[martin manning]

Moving the jumper on the pot will fix it.

[sluckey]

Yes, that does the same thing. But the yellow wire is already disconnected from the bias pot. So, IMO it's faster to just put it back on the lug with the orange wire.

[ViperDoc]

HALLELUJAH!!!!!!!!!!!!!!!!

I just flipped the jumper. Now the voltage ranges between -20 and -42 VDC on the bias pot wiper and each power tube socket pin 5. DAMN, you guys are good.

MANY THANKS. MANY.

I'll put that .1 uF blocking cap back and double check it but I think we're goody good to go.

[sluckey]

Maybe you could let Rob know. He's pretty good about quickly fixing little booboos.

[martin manning]

That was a hard to spot error. What tipped me off was when you said the voltage at the diode was varying over the expected range.

[ViperDoc]

I will for sure, he’s responded to me in the past.

I reconnected the tremolo blocking cap and rechecked the bias and all is well. Hot dog.

THANK YOU!!!

[ViperDoc]

Brief tone report: this is the best-sounding Princeton I've ever built. Worlds better than at first with "Zeus bias".

Full disclosure, it's only my second one , but it sounds better than the first now. WOW. Very creamy and smooth until just before full dime.

The log pot on the reverb control is a huge improvement over the linear.

I added a 10K mid pot with a 3.3K floor also, and that sounds great.

The LED switched into the trem circuit slows it down nicely but shelves off some high end, it seems. In general, the tremolo sounds spectacular now whereas my first PR tremolo sounded somewhat weak. I will be pulling and checking that bias!

[Stevem]

So what is your plate voltage now with a -30 on pin 5 ?

One note here.

It's really hard to tell, but in your last photo that blue V+ dropping resistor on the filter can looks like its leads are really close to shorting out on he chassis, so check that.

[ViperDoc]

WELL, there's always something.

So I fired up the amp to check the plate voltages. The bias is set at -40 V. On V6, the plates read at 420 VDC. On V5 there was an oscillating voltage, I couldn't get a solid reading until a "pop" occurred, then all was still on but with no sound coming out of the speaker. Then I could read 420 VDC on both plates. Before the pop, I was getting AC voltage on V5. If I turn the amp off and on again, the output sound returns. ??? Bias is too cold? Why the oscillation on V5? The vibrato is off via the footswitch.

That blue dropping resistor is flying sky high, the picture doesn't show it that well. Thanks!

[ViperDoc]

[redacted]

[redacted again!]

Thank God for naptha.

I found some residue on the 8-ohm connector on my rotary switch, so I cleaned it all off with some naptha on a q-tip. Now I show 404 and 410 VDC on both 6V6 plates.

It's all in details.

[ViperDoc]

So I've been playing the amp here. I set the bias to put -30 VDC on the 6V6 grids.

V5 plate reads 400 VDC.

V6 plate reads .02 VDC.

Is this normal? This does not look normal.

[pompeiisneaks]

That would not be normal no. They should be pretty close to identical. The plates of the power tubes are connected to either side of the OT's leads and other than the slightly different resistance per side which is usually no more than an ohm, you should get identical readings. If you're not, one of a couple of things is wrong:

1. your tube is dead shorted and dropping all of the b+ on that side to ground, but I think you'd see other catastrophic results with that
2. your connection to the pin is bad
3. your ot is bad (extremely unlikely)
4. Measurement error of some kind.

~Phil

[ViperDoc]

I pulled the power tubes and measured the brown-to-red OT primary resistance at 154R, and the blue-to-red OT primary resistance at 165R.