Princeton Reverb preamp coupling cap leakage

[justsoldern]

I have a Princeton reverb AA1164 sounding kind weak and farty. Going through it I notice I'm getting some residual DC voltage after the preamp coupling caps. My question is how much is too much? After V1A the Tone caps seem ok. I'm measuring 4 mV DC after the 0.1 mf tone cap. Only 0.3 mv after the 0.47 bass cap and 6 mv after the 250 pf cap. That seems reasonable to me. My concern is coming out of V1B. After the 0.02 cap where it splits to the reverb driver I'm getting 50 mv DC (using a Fluke 115 meter) Is that too high?? It's also resulting in 30mv DC at pin 2 of the AT7 reverb driver and 10 mv DC on pin 7 of V3A. I've swapped tubes around with no changes. Does that 0.02 need to go? what's an acceptable level to be seeing there?

[The Dude]

My first thought when I hear "weak and farty" as the symptom is bad/old/worn filter caps.

[Helmholtz]

Going through it I notice I'm getting some residual DC voltage after the preamp coupling caps. My question is how much is too much? V3A. I've swapped tubes around

Anything below 100mVDC won't have a noticeable effect. Some negative 10mVDC at tube grids are normal grid leak voltage.

[J M Fahey]

Weak and farty?
Start by injecting clean 100mV sinewave, 400Hz to 1 kHz, your choice, , set all tone controls to 6 or 7 and start rising volume until clipping.

How much do you get clean signal?

Do you see anything weird , such as very unsymmetrical clipping, visible hum (it appears as sawtooth peak modulation), oscillation, etc.?

Besides worn dry old caps, as suggested by the dude, IŽd expect weak tired old power tubes.

[justsoldern]

Thanks all, You guys are great. I haven't dug out the scope yet. but plan to in the the next few days. What I have done is replace that 0.02 cap coming out of V1B and it brought the DC after it down to about 5 mv down from 50 mv (measured in circuit). The reverb pot was also very intermittent so I replaced it. cleaned all the tube sockets and other pots with deoxit. I have traced through the entire circuit for DC readings. all look ok. I also took AC readings with a 30 mv sine wave input. The schematic from fender for the princeton reverb reissue has expected AC readings in it, so I went by those. Everything seemed reasonable.. I have a scope but haven't used it in quite a while. I'll hook it up. The tubes are only a few years old. Also the cap can and all electrolytics were replaced at that time, so I don't expect them to be bad. I'm not seeing unusual AC ripple on the B+ so I'm thinking the power supply caps are still OK. At this point I'm also suspecting the speaker is tired and may need replacing. It's sounding much better now if I use a different speaker, so maybe it was a combination of things? I will check with the scope and report back in a few days though

[justsoldern]

OK hooked up a scope. then sent in a 100 mv 1 K sine wave. Very nice clean symmetrical waveform until volume on 4. On 5 the tops of the wave have flats but bottom of the wave still good. On 6 the tops are flat sloping slightly down and the bottoms are also getting flat slightly sloping up. the length of the flats are longer on top than on the bottom. 7 to 10 on the volume not much difference, from 6 but a bit closer to a square wave. Don't see anything too crazylike hum or oscillation. Does this mean the tubes aren't well matched or one tube is not performing as well as the other? With all my recent maintenance and a known good Weber speaker it is sounding much better. Scope pics are volume 4 and 7

[Leo_Gnardo]

OK hooked up a scope. then sent in a 100 mv 1 K sine wave. Very nice clean symmetrical waveform until volume on 4. On 5 the tops of the wave have flats but bottom of the wave still good. On 6 the tops are flat sloping slightly down and the bottoms are also getting flat slightly sloping up. the length of the flats are longer on top than on the bottom. 7 to 10 on the volume not much difference, from 6 but a bit closer to a square wave. Don't see anything too crazylike hum or oscillation. Does this mean the tubes aren't well matched or one tube is not performing as well as the other?

The kind of asymmetrical clipping you see is normal for a Princeton. You can get some relief - that is a more symmetrical looking waveform as you go into clipping - by using a stronger output drive tube (12AT7) and/or increasing the bias current in the output tubes. Your vibrato won't be as deep with a 12AT7, which can be overcome by decreasing the value of the normally 1 Meg resistor* that feeds vibrato signal thru a DC blocking cap to the depth control. And bias much beyond 20 mA for 6V6 isn't advisable. 25 mA tops. So, you can leave your prize "as is" or try either of these solutions. What's to be gained? A couple watts of clean power. Will it sound different/better ? Not much.

* I ordinarily parallel the 1 Meg resistor with a 330K more or less when using a 12AT7 drive tube. This restores vibrato to about the same depth it had with a 12AX7 drive tube. Don't forget to reverse the process should you decide the 12AX7 is the drive tube for you.