Fender Princeton reverb

[badzuri]

I just finished building a BFPR and somehow I can turn the volume to 10 and it sounds like its on 2-3 at best.Checked circuit over and over and cant find the problem. All brand new premium tubes from Tube Depot. I used blue molded Fender coupling caps I bought from ebay, but I checked range of all caps and they tested good. Any suggestions?

[mhartman]

Bad pot? Wrong resistor value somewhere, maybe ###k for ###R? Are you using 10% audio taper pots (the vintage pots were 30%)? If so, try turning up your bass and treble pots to 7 or 8 instead of 5 (or disconnect the tail resistor from ground on the bass pot to take the tonestck out of the equation while you're testing) Are your voltages looking ok?

[Colossal]

Good ideas from mhartman: possibly a simple mistake with a resistor...that is easy to do when you're tired or working late in the fever to get the amp built...you grab a 470R when you mean 470k!

Also, before you change anything (!), take voltages from the absolute beginning...start at the mains/IEC entry, work your way into the rectifier, through each node in the power supply, to every anode, to every cathode, and every grid. Compare the voltages taken to those expected. My bet is you will find something that doesn't add up.

And as mHartman suggests, look at each resistor to ensure that the value you soldered in is the correct value. Easy to make a mistake there. One time after receiving a large shipment of parts, I loaded some 47R into my 47k drawer, so I automatically assumed that the part was right I always measure every part even as I confirm its written value.

[badzuri]

I checked every value of resistor and everyone of them is correct. The kit came from Mojotone, the only parts I substituted was vintage blue molded Fender caps

[Colossal]

I checked every value of resistor and everyone of them is correct. The kit came from Mojotone, the only parts I substituted was vintage blue molded Fender caps

Ok, if you've ruled that out, then go through and make a voltage table. Something is amiss somewhere. Don't be tempted to change anything until you have carefully eliminated every variable.

I also like to do AC analysis to verify that each gain stage is doing voltage amplification. Start at the input, inject a 1-2V signal (either from a signal generator or a tone output from your computer's 1/8" mini jack). Measure the AC voltage on the V1a grid. Then measure the AC voltage at the anode. You should see the voltage multiply by the gain of the stage. Do that at each tube. If you have say 68VAC on the anode of V1a but it drops at the output of the second gain stage V1b instead of multiplying again, then you *know* you have a problem there. Taking DC voltages at every possible point first and making a table first is imperative though. Don't forget to measure the AC going into the power transformer, the AC coming off the secondaries, and then the rectified DC. Need to rule out the PT as a possible source of error.

When troubleshooting, I do everything I can first without making any physical changes to component values. Often it is very tempting to make changes willy-nilly and this can make a simple problem worse by masking the root cause. You probably know this stuff so I'm not meaning to preach.

[badzuri]

What about the OT, its a brand new Mercury Magnetic but could it be bad, how do you test an OT?

[Colossal]

What about the OT, its a brand new Mercury Magnetic but could it be bad, how do you test an OT?

My bet is against the OT being bad (especially with high QC at Mercury...I use Mercury myself) but certainly concede that everything is suspect, until proven otherwise. You can check the impedance yourself and should get pretty close with the usual means of estimating primary R.

See this procedure:
http://www.marstran.com/Measuring%20Pri ... edance.htm

...and there are certainly other (and possibly better) procedures. This is just the first one I found. This has been discussed here on TAG before, so be sure to do a search and poke around if you want to pursue the test.

If you are handy with a graphics tool, you can take a Princeton schematic and mark up your voltages in red directly at each place that you measured them. Post that and it will be very easy to identify the issue, provided that voltages are causing your problem.

[M Fowler]

badzuri

Keep at it, I also built a BF Princeton Reverb and have the same symptoms, I still haven't found my problem yet. I work on it every now and then.

Mark

[badzuri]

All of the voltages on the Fender schematic are pretty much the same as mine except just slightly higher due to a Mercury PT that puts out 355volts ac on the primaries.

[Colossal]

So I'm guessing you are somewhere around 425VDC on the plates of your power tubes. What is your bias voltage measured at the junction of the grid leaks? Have you done AC analysis to ensure you are passing and amplifying signal from stage to stage? Sure your Volume pot is the correct resistance? Tone stack wired correctly?

You might want to post a detailed gut shot at this point.

[badzuri]

Here are some gut shots

[jjman]

The green safety ground from the power cord should not be attached directly to the cap. I would put it under a transformer screw that has no wire under it currently.

[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

[sonicmojo]

I just finished building a BFPR and somehow I can turn the volume to 10 and it sounds like its on 2-3 at best.Checked circuit over and over and cant find the problem. All brand new premium tubes from Tube Depot. I used blue molded Fender coupling caps I bought from ebay, but I checked range of all caps and they tested good. Any suggestions?

I could be something simple. Poking around with an audio probe might help if it is something with the inputs. It would be a good start to at least eliminate preamp or power amp.

[badzuri]

I have checked the blue caps with my Fluke cap meter and they test right on at .1 and .047, don't know how to do AC test in preamp stages.