Fender Princeton reverb

[cbass]

Check for DC on the end of the caps where there shouldn't be any.Even a couple of volts can really scerew things up.
Even if a cap measures good.It can still leak at high voltages

[sliberty]

When I have low output, or no output, I find a simple Weber style audio probe to be the quickest way to find the part of the amp that is causing the issue. It allows you to hear each stage, so you know immediately when the level drops down instead of boosting up.this is conceptually similar to the AC analysis mentioned earlier in the thread.mi would probe at the input, and then at the plate of each stage. While dng so, it would turn the pots to ensure that they are behaving as expected as well (when probing the plate immediately after those pots of course. This test never fails to zero in on the part of the circuit that is dropping signal where it shouldn't, and it can be performed in 2 minutes.

[badzuri]

Good call! i'll try it tonight

[rdjones]

Just an observation, I don't like the area around the filter cap's terminals and cutout hole.
It looks to close for comfort, but if there were a short I'd expect that dropping string resistor or something to smoke.
So it's probably not a problem (yet).

The original Princeton layout could benefit from some modernizing, especially the power cord inlet and grounding.
Modern safety code calls for the green AC earth ground to have it's own dedicated mounting point near the inlet and not shared with any other hardware or electrical connections.
Even though it's very common practice to use a transformer nut that practice is discouraged by (some country's) safety code.

rd

[rdjones]

Nice build, man. That thing is pretty authentic looking, save for the orange drops. I've also been thinking about buying some NOS blues on fleabay, but I've been scared of them being leaky, though I hear they are quite resilient in this regard. Have you measured to see if any of yours are leaking?

David

I've got a boatload of blue molded pulls from organ chassis' and have yet to find a bad one, but I've not tested them all yet.
A few values off somewhat but still quite usable.

As with any cap of this construction type watch for cracks around the lead entry area.

reddog

[cpollack]

I had a similar experience back in the ’80 when I was starting out wrenching on amps. A friend had his dad’s ca. ’68 SF Vibrolux Reverb which had been in storage for at least ten years in an attic. It had the same condition; it just wouldn’t give up the goods or get as loud as other 2x6L6 amps on 3 or 4. He remembered it being plenty loud when he was a kid watching dad play at local bars and such. I went in and checked everything. Nothing was visibly burned and no cold solder joints were found. The tube rectifier was good and voltages were OK. The speakers weren’t great but were loud enough when fed from another amp. We decided to blame the SF phase inverter and PS dropping resistor values so I blackfaced it. Still not much luck. He assured me that the old tubes were fine, but when I dropped in a pair of cheap Soviet 5881s it roared back. The old RCA 6L6s were simply worn out. So, even though your tubes are new, do you know for certain that they are good? Do you have any that you know work from another amp which you can sub in? A working amp makes the best tube tester sometimes. Check the small tubes, too.

[mhartman]

How well do you know the speaker that you used? I had a Pro Reverb that came with Celestian V30s in it. It was loud enough to blow down the house. I changed the speakers over to original Jensens and it was much, much quieter (like 1/2 as loud).

[badzuri]

Brand new Weber speaker pretty much made for Princeton Reverb, as far as tubes I don't have any others to try but these are match pair made by Tung-Sol. I know that doesn't matter but going to try the audio probe tonight and see if I can find anything wrong.

[Blackburn]

Nice build, man. That thing is pretty authentic looking, save for the orange drops. I've also been thinking about buying some NOS blues on fleabay, but I've been scared of them being leaky, though I hear they are quite resilient in this regard. Have you measured to see if any of yours are leaking?

David

I've got a boatload of blue molded pulls from organ chassis' and have yet to find a bad one, but I've not tested them all yet.
A few values off somewhat but still quite usable.

As with any cap of this construction type watch for cracks around the lead entry area.

reddog

Cool, man. I really want to use some in a build. Think you might want to sell me a few?

[badzuri]

Can someone show me the signal path to test with my new audio probe please, and including the reverb

[JazzGuitarGimp]

Vdo you have the vias voltage set correctly?

[badzuri]

Yes bias set with bias calculator and Hoffman bias probe

[Colossal]

Can someone show me the signal path to test with my new audio probe please, and including the reverb

To do basic AC analysis set your DMM to AC mode. Attach your black probe to ground. With the amp on and standby off (be sure you have a load attached to the speaker jack) inject a 1V AC signal into the amp's input high gain jack. Then probe Pin 2 (the grid) with your red probe and record the value. Then probe Pin 1 (the anode/plate) and record its value. You should see something like 1V(ish) on the grid and if the amp is amplifying correctly, Pin 6 should be something like 59VAC or so (maybe more or less) I think the typical 7ender gain stage is something like gain of 59 with 100k/1k5/22uF. The tone stack immediately follows the first gain stage and will "load" the first gain stage, reducing the gain of the signal slightly. The 100k "slope" resistor minimizes this loading so the output impedance is kept manageable, minimizing the shift to the DC load line. Then do the same for Pin 7 (grid 2) and Pin 6 (anode 2) of V1b. The point is just to ensure that when you put a small signal in, that you are getting a big signal out of each gain stage.

mhartman brought up another great point about apparent volume and the speaker. A client came to me with a Headstrong Lil King S that he wasn't happy with the boomy tone. It is basically a Princeton Reverb with 6L6s in the power section and a slightly modded cathodyne. It had an Eminence Legend speaker stock and I found it to not be very loud. Cranked it was pretty loud and you get that hard, blatty 7ender tone but it wasn't all that loud, even dimed. So apparent volume is relative to the listener. I plugged the amp into a 1x12 with an EV and it got a lot louder and a helluva lot better sounding, for my tastes anyway (still boomy before modding). The Legend was not bad with this amp, in fact it had a very sweet top end with nice chime.

Yes bias set with bias calculator and Hoffman bias probe

What is the (negative) voltage at the junction of the 220k/220k grid leak resistors? This is very important. I asked this earlier.

[badzuri]

O.K going bad solder joint using audio probe fixed that and volume is blasting loud, and the term is heavenly. Reverb on the other hand is not present, just get buzz when reverb knob is turned up. Can I trouble shoot that with the audio probe, and how Please almost there. Thanx

[rdjones]

O.K going bad solder joint using audio probe fixed that and volume is blasting loud, and the term is heavenly. Reverb on the other hand is not present, just get buzz when reverb knob is turned up. Can I trouble shoot that with the audio probe, and how Please almost there. Thanx

The send can be checked by hooking up a speaker to the send RCA jack, the reverb driver transformer can easily power a speaker (don't forget to have a speaker/load connected to the speaker jack at all times).
You can test the return by sending an audio signal into the return RCA jack.

Does the reverb pan have the usual isolated input, grounded output jack arrangement ?

rd