Convert Hoffman 68 Custom back to Princeton Reverb

[CordovaNate]

Or...how curiosity killed the amp.

Happy New Year Everyone!

I converted the PR into a 68 Custom a few weeks back.  All went well and I liked it.  But, I had built the cabinet with 12" speaker.  Just to satisfy that %#$@ curiosity, I bought a Jensen P10R-F, built a new baffle board.  Then I thinks to myself, "hummm, I bet that speaker would sound better with the regular PR circuit".  Out with the 68 Custom caps/resistors and back in with the originals.

Now I got...not much.  The sound I get with the volume on 10 is about equal to what I used to get at 1.5 - 2.  I can increase the volume, somewhat, by increasing the Reverb control knob. 

This is what I've done so far to fix it.
Put the .022 PI coupling caps from the 68 CPR back in...no change.
Replaced the tubes in V1, V4, both power 6V6's, and the rectifier...no change.
Measured each heater pin at V1 at 3.35volts
Measured each power tube pin #3 plate voltage at 396 volt, consistent with what I had biased them at.
Using a chop stick while plucking some strings on the guitar, I nudged around those few components I had changed hoping to locate a bad connection...no change.

Any suggestions on what to do next?

[sluckey]

There was more involved than just two .022µF caps. Go back and read your original thread. Then use Hoffman's schematic/layout to put everything back like it was before you got the itch to change things. Should be straightforward. If still no joy start looking for things that may have changed accidentally.

Next time you get the itch, take some pics while the soldering iron heats up. 

[CordovaNate]

I changed everything back to original.  All parts.  The problem surfaced and I noticed the bottom of one of the pi coupling cap was very very close to the top of the turret it was soldered to and I thought maybe it got too hot while I was soldering it and damaged it, so, the 68 caps I just took out...went back in.  I had marked up the layout pdf prior to the change to 68CPR and still had it, along with all the original parts in an small envelope.

Edit: I did not change the NFB resistor back to original value.  I'll keep looking.

[sluckey]

None of those 5 components would cause the loss of volume you describe. I suspect something happened inadvertently while you were swapping those components. Something like a jumper wire came loose or a bad solder joint or another component lead broken, that kind of thing. Just compare your current wiring to the Hoffman layout. Show us some hi rez pics. Measure voltages on all tube pins. All the normal stuff.

[CordovaNate]

I took some measurements. Voltages for pins 1,2,3 on V4 bounced around like crazy, I tried to capture the range.  The included layout markup shows what I was working on.

Edit. The rectifier is a 5Y3WGTB(6087).  It is mis-identified on the voltage spreadsheet.

[sluckey]

V2 pins 2 and 7 should be zero volts. V5 and V6 pins 1 and 8 should be millivolts, not volts. Other than that, I don't see any clues to your low volume problems.

Do you happen to have a SPKR jack and an EXT SPKR jack wired like the original Fender PR? And you mistakenly connected the speaker plug to the EXT SPKR jack?

[CordovaNate]

I do...I didn't.  I wish. that were it.

[CordovaNate]

Well, I think I fixed one problem, and created another.  Now I'm getting zero negative voltage on pin#5 of V5 power tube.  I think the problem is with the coupling cap, but now I'm reluctant to do anything without asking for some more advise.

If you get the time and the inclination, please take a look at these values and tell me what you think.

Thanks

[sluckey]

Pull the output tubes and don't put them back in until you have -33v on V5 pin 5.

Replace R47. Is it fixed?

[CordovaNate]

Power tube removed as soon as I noticed red plating on V5.  Voltage measurements were taken w/o power tubes.

R47 was already replaced once.  Ohms resistance of new R47 prior to placement was around 218, and once soldered onto board, value was 23.8ohms, which is what led me to replace it the first time.  The first R47 also had around that same 23ohm value on board, and I was expecting around 220ohm.

I have re-soldered all the wires that connected to the base of those turrets I provided measurements for.  I'm going to replace the P.I. coupling caps and see what happens, as it seems to me that it's got to be either R47 or the V5 p.i. coupling cap.

[sluckey]

That resistor is 220,000 ohms, not 220 ohms. You don't have to replace the coupling caps. Just lift one lead to eliminate them.

[CordovaNate]

OK...left off the K.

You are right, swapping caps did not fix.  I was at my wits end.  Unfortunately for me, that's usually only a few steps away.

BUT!  It is fixed.  I'll just pretend that it happened in Vegas.

Thank you for sticking with me Slucky.

Nate

[sluckey]

BUT!  It is fixed.  I'll just pretend that it happened in Vegas.

That's great! What was the cure?

[CordovaNate]

Walk of shame time.

I had lifted the board slightly to get a peak underneath.  The wire to pin #5 got pinched under the edge of the standoff when I fastened to board back to the chassis.  The wire is silicone insulated and pretty rubbery.  Even looking down directly over it, that it was pinched was not apparent.  I had wiggled the wire earlier, while tracing it back from pin #5 to the turret on the board, but not "right there".  There are several wires running parallel gathered right there at the standoff, and it wasn't until I decided to completely separate the pin #5 wire from the rest that the issue became apparent.

[sluckey]

The wire to pin #5 got pinched under the edge of the standoff

Haha! No shame. I've done the exact same thing. Same wire. Good snoop job.

[PRR]

> wire to pin #5 got pinched under the edge of the standoff

Thanks for the helpful reminder.

I am actually starting to watch for chit like that. If I'd pinched as many bottoms as I have pinched wires I'd be in real deep trouble. So I tend to try to look-out for pinches. Even try to plan routing to make a pinch harder. Still I managed a good one recently, and on a giant tractor battery.