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

[ViperDoc]

There may be some operator error, but there's more to it.

So with tubes in, power on, I get 400 VDC on the V6 plate, then some noise and eventually a 400 VDC when probing the V5 plate, but the idle sound out of the speaker changes, gets more quiet, even absent. Then I plug a guitar in, play with no reverb or vibe, all sounds good and then "click"---NO SOUND out of the speaker. Fuse is good. Lamp is on. No sound. I turn the amp off, turn it back on and I can play it again. ???

Buy stock in sledgehammers now, boys.

[pompeiisneaks]

What voltage readings do you get with the tubes pulled?

~Phil

[sluckey]

Weird things can happen when you poke the output tube plate with a meter. Even on an amp that's working just fine. If your amp works fine, let it be.

[ViperDoc]

I am a firm believer in the K.I.S.S. rule, although you'd have no reason to believe it. Poking the PT plate is a Pandora's box? No problem. But now this thing will shut off in the middle of playing. I cobbled together some voltage readings with the 6V6 tubes out, foot switch and reverb tank removed:

[ViperDoc]

Still cutting out. I put the tubes in to rule out my probe, this amp cuts out when I turn it up. In the middle of playing it cuts out, then it came back for a bit, then shut off again.

[sluckey]

Those voltages would be a lot more meaningful and helpful if they were put on a SCHEMATIC and the output tubes were plugged in.

[pompeiisneaks]

I was just suggesting pulling the tubes to see if the voltage on both tube pins was the same, or if you still were seeing the low voltage. i.e. the two anode pins.

~Phil

[ViperDoc]

"Hey guys, what do you make of this Schemanicotti?"



I will make more sense of all of that, good call.

[Stevem]

I think the extreme current you have been pulling thru the amps OT due to the condition it was run in has broken down its primary winding insulation and the cutting out and popping your are hearing is that winding arcing over to the secondary winding!

[pdf64]

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

[ViperDoc]

...here’s a schematic courtesy of Sharpie 1.0:

Here's a better bias circuit in Sharpie 2.0 (chisel tip). First resistor is 68k, then 10k after the diode, 10k bias pot (standard Fender with screwdriver slot), and 15k to ground. The caps are 10u and 22u. Your 100u will be very slow to charge given the 100k in front of it. The extra cap on the output will keep the ripple low. The bias pot is wired in a fail-safe way, too.

Is this accurate? (Lights are back on.)

PR ADJ BIAS MMANNING.png

[ViperDoc]

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?

[pdf64]

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.

[martin manning]

...here’s a schematic courtesy of Sharpie 1.0:

Here's a better bias circuit in Sharpie 2.0 (chisel tip). First resistor is 68k, then 10k after the diode, 10k bias pot (standard Fender with screwdriver slot), and 15k to ground. The caps are 10u and 22u. Your 100u will be very slow to charge given the 100k in front of it. The extra cap on the output will keep the ripple low. The bias pot is wired in a fail-safe way, too.

Is this accurate? (Lights are back on.)

Yes, that's it. 1W rating for the 68k, 100V for the 10u, 50v for the 22u. You should be able to reuse your existing bias supply board.

[ViperDoc]

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