Princeton Reverb Extremely High Voltages without Tubes

[HumanJHawkins]

Hi all,

I've just built a Princeton Reverb and think I have a power supply section problem. It's really odd actually... All voltages from the PT and GZ34 rectifier are good as follows:

With power tubes installed, but no preamp or reverb tube:
   Wall voltage: 117.5v
   Heaters and pilot light: 6.44v
   PT Secondary: 325vAC
   B+ from GZ34: 450vDC

Here are my voltages in red next to Fender's:



I've put a choke in, instead of the 1K resistor, so expect the plate voltage to be a bit higher. But the two 18K resistors test to 18.5K each and are intact. It's like voltage is being fed around them somehow.

Anyone have any advice where to start?
Thanks much.

[TIMBO]

WELCOME Human, NANU NANU. 

Try putting the preamp/reverb tubes in.

[PRR]

The 300 and 250 feeds are sagged-down by preamp tube load. Without the little bottles it *will* un-sag to essentially the full source voltage.

Your other numbers are correct if the PT was wound for 110V and you give it 117V. I suspect nearly all old Fender designs show higher B+ today than they did in 1953.

[HumanJHawkins]

So, I put the tubes in and re-measured voltages. I fixed my bias issue (had a wrong resistor value), and discovered that I was being confused by a modification in a Mojotone layout I was using. They feed V4 pin 6 from the filter stage most people refer to as "C" instead of "D" where Fender put it. I reverted this to the Fender AA1164 design.


In short, all but one voltage reading are now spot on (10% over anyway, but within tolerance.). But one severe issue remains. V4 pin 1 is at 435 volts (vs. about 280 expected based on my other voltages.) I inadvertently had the cathodes of the power tubes lifted when I first measured this. I could feel the buzz of the current, possibly from the 12AX7 on the other side of the socket.


I fixed the power tube cathodes, and now get a crazy buzz... Not just from the speaker I think. I think aside from the speaker buzz/wail, the whole chassis is vibrating at an audible buzz frequency. Needless to say, I shut it off quick. I've checked the routing and the resistor values. Nothing discovered yet. Will give it fresh eyes and re-read the schematic and layouts tomorrow.


Thanks all.

[sluckey]

V4 pin 1 is at 435 volts (vs. about 280 expected based on my other voltages.)

This is the tremolo tube. Sounds like it's not conducting. What is the voltage on pin 3? With power off what resistance do you measure from pin 3 to ground?

I fixed the power tube cathodes, and now get a crazy buzz... Not just from the speaker I think. I think aside from the speaker buzz/wail, the whole chassis is vibrating at an audible buzz frequency.

Disconnect the NFB wire from the speaker jack. If that fixes the problem swap the OT primary plate leads that connect to pin 3 of each 6V6.

[HumanJHawkins]

Ahhhh... It was the OT primary leads. Disconnecting NFB causes it to work with that rough no NFB sound as one would expect. On V4 pin 1, it dropped to about 400v with NFB disconnected. I am guessing that when NFB is properly hooked up it should come down further. Will see soon.


Curious thing... I also measured V4 pin 3 to ground and find it is non-conducting. Should that have a measurable resistance, or is that what we were hoping for?


Woot. Thanks much!

[sluckey]

Curious thing... I also measured V4 pin 3 to ground and find it is non-conducting. Should that have a measurable resistance, or is that what we were hoping for?

Should be 3300Ω.

[HumanJHawkins]

Curious thing... I also measured V4 pin 3 to ground and find it is non-conducting. Should that have a measurable resistance, or is that what we were hoping for?

Should be 3300Ω.

Ah. Just via that 3300Ω to ground. Makes sense and that resistor was the issue. Apparently tremolo doesn't work so well when you put a 3.3M resistor there instead of a 3.3.K resistor. O_o

This amp is cooked. In a good way. Sounds fantastic. I added a mid pot and a variable NFB on the front, so need to make labels for it still. But I can hear a bit o' Keith Richards in there... Just need to learn how to play to let that sound out properly. Here it is:






Thanks much all.

[eleventeen]

Beautiful job! Very nice.


Tiniest, most subatomic nit imaginable: On the on-off switch, if you install a nut and hopefully a lockwasher of some type on the inside of the chassis, less of the threaded bushing will be exposed. (at some point, it will interfere with the 6V6 socket though) It's actually important to have a nut on the inside of the chassis because unless that switch is a real, USA-made Cutler Hammer or other high-end switch, overtightening the nut will pull the guts out of the switch or distort it in some way so that it breaks.   


Your work is outstanding!

[HumanJHawkins]

Beautiful job! Very nice.

Tiniest, most subatomic nit imaginable: On the on-off switch, if you install a nut and hopefully a lockwasher of some type on the inside of the chassis, less of the threaded bushing will be exposed. (at some point, it will interfere with the 6V6 socket though) It's actually important to have a nut on the inside of the chassis because unless that switch is a real, USA-made Cutler Hammer or other high-end switch, overtightening the nut will pull the guts out of the switch or distort it in some way so that it breaks.   

Your work is outstanding!

Thanks. I didn't understand at first, but I get it now... Keep the nut behind the lockwasher to avoid stress on the switch housing, right? Cool. I'll keep that in mind.

Thankfully, plenty of room on the switch for that. I wish the average pot had about an extra mm of length to it's bushing. This chassis was aluminum and slightly thicker than a typical steel chassis. I wanted to do a custom plexiglass faceplate, but there is no way the bushing on the pots would have been long enough. I barely managed to get some of them installed with the thinnest of lockwasher or other locking mechanisms and the thin standard faceplate.

Man, this is a good one. Wish I could play well enough (and record well enough) to really show it off.