loud hum appeared in initially-working princeton reverb new build

[bdale]

I recently completed a Princeton Reverb build using Hoffman parts.  Initial turn-on and test went great!  My son plugged in his Strat and we played around for quite a while with the amp chassis perched on a plastic utility tub under my bench and a random 8 ohm speaker attached.  Super-quiet, everything worked perfectly...

Then I bought a Weber 10F150T driver and built a cabinet for the whole setup.  The first time we fired things up, with all controls set to 1 and nothing attached to the input, when power was applied we had the usual couple seconds of silence followed by a few seconds of the expected *very* soft hum through the speaker.  Then all of a sudden, the hum went to very high volume.  Kind of like hum at full-power max volume?  Moving the volume control made no difference.  Plugging in an instrument made no difference. Reseating all the tubes didn't change anything.  Tapping on tubes with a fingernail gave no new data.

I pulled the chassis out of the cabinet to inspect, and can find nothing changed/wrong that's immediately obvious. 

Running the chassis outside the cabinet makes no difference.  Pulling the shield cans off the 9-pin tubes made no difference (they were not installed during the initial successful testing).

Pulling V1 makes no change.  Pulling V1 and V2 makes no change.  Pulling V1, V2, and V3 caused the problem to go away.  Swapping the 12AX7 tubes around doesn't seem to change the symptoms.  Having the reverb tank attached or detached doesn't seem to matter.  Ditto the foot pedal switches for reverb and tremolo. 

It's Sunday afternoon, I don't have any spare tubes in the house, and after all the work I put into the cabinet having the amp stop working like this is really frustrating.  So I'm going to take a break and get back to this tomorrow.  In the meantime, are these symptoms a "smoking gun" for those more experienced with diagnosing tube amps?  In any case, suggestions about what to look at next would be greatly appreciated.

[tubenit]

It is difficult to tell by your description but if a hum at a very loud volume means howling .....................

Then you may need to change the OT wires?   Easy way to check is to disconnect the NFB.  Does the howl go away?  IF so, swap the wires on the OT.

You can do a search for OT wires & oscillation.

With respect, Tubenit

[shooter]

+1

"smoking gun"

Then I bought a Weber 10F150T driver

If your original speaker and new speaker were - are wired dif, could effect nfb.
you coulda wiggled loose an iffy wire, sounds like you have it narrowed to maybe possibly V3 which in the Hoffman version is where nfb connects

[eleventeen]

The greatest possibly IMO is the failure of an electrolytic cap or, one of your tubes developed a heater-to-cathode short. Both of those are pretty rare, but not impossibly rare, and definitely not impossible.


You say the amp was working OK, then all you did was to change speakers, in other words, you touched nothing inside the chassis, moved no wires (although the insertion of a 1/4" plug into the speaker makes a tiny movement of "things") and made no ckt changes.


Yes, it frustrating to try to diagnose things with no test gear nor spare parts. It's voodoo.


Here are some ideas to try.


1: You have tried pulling tubes starting from the preamp end of the amp. That is somewhat logical, but it's not the case that the input guitar signal goes thru V1, then V2, then V3, then V4. You say when V1 and V2 gone = no change. But pulling V1,2 AND V3 made the hum go away. Now, I find that telling, because V3 is the reverb driver. Only a *part* of the full signal goes thru it. The amp should still work (without reverb of course) lacking a V3. Does it? IOW, you should be able to pull V3 and be able to play a non-reverb amp using either Ch1 or ch 2 with no hum. A good thing to test. >>Pull only V3<< and try the amp.


2: The immediate suspect has to be the 12AT7 in V3. This tube is fed very high volts and is driven hard in any reverb-Fender and if you know your Fender blackface amps, you know this tube gets really hot, much hotter than the others. I am thus pointed to a H-K short in V3. Maybe this tube suffered an early death. The amp does not NEED V3 to work. But V3 can certainly induce hum if it has an internal H-K short. If the hum goes away with that tube and only that tube pulled, suspicion grows > H-K short V3.


3: *IF* you can pull V3 and have a working amp (with no reverb) get the reverb tank cabled in. Power up the amp and shake the reverb can a little. The reverb control has to be off zero, set it to 3-4-5. Do you hear crashing when you move the reverb can?


4: You say you have swapped V1 and V2. These are ch1 and ch 2 preamp tubes. The next thing to do is to pull V2 and place it into V3 socket. (The amp now has all tubes installed except for V2, and V3 is replaced by a 12AX7 instead of the normal 12AT7) Now, (a) see if hum goes away, and (b) see if you have reverb. The 12AX7 should work fine as the reverb driver (you may get a lot more 'verb than normal, we would expect that from the higher gain 12AX7 in place of the mid-gain 12AT7)


I again lean toward a H-K short in V3. Bad tube. 

[sluckey]

Now, I find that telling, because V3 is the reverb driver. Only a *part* of the full signal goes thru it. The amp should still work (without reverb of course) lacking a V3. Does it? IOW, you should be able to pull V3 and be able to play a non-reverb amp using either Ch1 or ch 2 with no hum. A good thing to test. >>Pull only V3<< and try the amp.

V3 is not the reverb driver. Half of V3 is the reverb recovery and the other half is the amp stage just prior to the PI. When you pull V3 you will lose all guitar signal. Take a look at the schematic.

[eleventeen]

Sorry, I misread Deluxe Reverb.

[bdale]

Quick update.  On a lark, I re-attached the "random" speaker and everything went back to working fine.  A quick check with the meter that was within reach shows that the "random" speaker has a DC coil resistance of 4.3 ohms and the nice Weber is about 6.1 ohms.  So, clearly, the new driver is just loading the output tubes differently (less?), triggering an oscillation.  I'll try poking with the negative feedback circuit next.

I now believe I can solve this, but I've promised the family we'll do other things tonight and I have to leave town on business for a few days tomorrow, so might be next weekend before I get time to work on the amp again.

Thanks for the replies, though, you guys are great and I'll definitely report back when I get this figured out and/or get stumped!

[HotBluePlates]

...  On a lark, I re-attached the "random" speaker and everything went back to working fine.  A quick check with the meter that was within reach shows that the "random" speaker has a DC coil resistance of 4.3 ohms and the nice Weber is about 6.1 ohms.  So, clearly, the new driver is just loading the output tubes differently (less?), triggering an oscillation.  ...

Bottom-line: do as Tubenit suggested and unsolder the feedback wire from the speaker jack to the board. Try the Weber again.


Assuming both speakers are marked 8Ω, you're measured d.c. resistance just says there is a different balance of resistance to reactance to wind up at 8Ω nominal for each speaker. That may be enough to alter what frequency an oscillation due to positive feedback manifests itself. Unhooking the feedback wire will tell you for sure if you need to swap output transformer wires.

[bdale]

And it's all good now.  Thanks tubenit, et al!

Pulled one end of the negative feedback 2.7k resistor and the issue went away.  Flipped OT primary leads and re-attached the resistor, and my son is now happily playing through his rep on his home-built Tele clone... ;-)

I'll make time sometime during the holidays to get photos and my build notes up on the web.  But thanks again, gang, you and the amp are awesome!

[HotBluePlates]

Thanks for the follow-up, and congrats on the fixed amp!!