From Blues Jr to AA1164 Princeton Reverb

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PCB gets me in a bad mood. Kind of like cars at some point in the 80s went from serviceable in the driveway to "we'd rather you not work on your own stuff."

People hear you work on amps and the next thing you know it's oh here's this radio, record player, microwave lol not really. But the newer amp conversation I have can be along the lines of "some of the guts in this amp are borderline disposable in terms of quality, the hassle to even get to the point of a 'simple repair' can easily lead me to the desire of throwing this out the window." Ok so I'm not gunning to be the neighborhood repair guy.

The Blues Jr is a popular amp but decisions like mounting tube sockets holding hot biased tubes directly to PCB has been haunting surviving examples, not to mention the jacks So I had enough of all of that and got full of myself and told a guy I'd rip out that stuff and put in handwired components for what that amp cost originally. I was just barely ready for the learning curve that ensued but today I corrected my last dressing blunders and she came to life.

The operation is everything I had hoped for all the way up to the onset of distortion which can get this ratty edge to it on the mid and high end. I'm eager to track this down and after replacing tubes and trying a couple speakers, reflowing connections, it's still there. I realized the only transformer I kept was the OT which is intended for 6BQ5s and now running 6V6 (also tried 5881s with a bias adjust), but am I wrong to assume the 8000 Ohms primary impedance works for either 6V6 or 6BQ5 pairs in push-pull?

The owner will mostly be after clean tones but when everything is perfect up to the point of breakup and then the breakup has an inharmonic edge to it, I'm wondering how to best approach this. All voltages are within 5-10% of the published schematic and the 6V6 tubes are right at 28-30ma and 400VDC on the plates. The only mod I added to the circuit was a bias pot and SS rectifier followed by a 5-watt 100 Ohm dropping resistor connected to the first reservoir cap.

As always any help appreciated

cv

[AlNewman]

Do you have an as built schematic and pictures?

[SEL49]

Disconnect the NFB wire. Any better?

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Do you have an as built schematic and pictures?

I can take some pics - as for schematic I tried to stay as close as possible to the original with the exception of the rectifier. Let me take some detailed shots and get them up here.

Ad for the negative feedback that's a good call. Folks here are turning in for the evening so I'll be back on this in  the morning,

regards

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I think I found the issue. The last PT had no heater CT so I had 2x100ohm resistors to ground off V1 so there was a low voltage hi amperage AC loop through the chassis. You can see that is lifted in the background of the attached shot. The "mother ground" hight tension CT is only secured to a PT nut at the moment, I will get the 80watt canon after that and bring it to ground with or close to reservoir cap #1 ground point.

Getting there and thanks again - I will experiment with the neg feedback which I believe is at 2.2K now. Also the amp is high gain so I dropped V1 to an AY7 which helps.

[SEL49]

I think I found the issue. The last PT had no heater CT so I had 2x100ohm resistors to ground off V1 so there was a low voltage hi amperage AC loop through the chassis.

There is ZERO current flowing through those 100Ω resistors into the chassis. There is 31.5mA flowing through those resistors but it simply flows from one side of the filament winding to the other side of the filament winding. That ground connection simply references the filament circuit to ground to lower hum that may be introduced between the filament and cathode inside the tubes.

NFB may or may not be a factor with your amp. Reverse phase does not always result in a scream/howl sound. Sometimes it's just a subtle harsh sound. I've seen people go for years with the phase being wrong, thinking that's just the way the amp should sound. My point is that with any new build you should always prove that the NFB phase is correct. There's a 50/50 chance that it's wrong. Very easy to do. Just disconnect the NFB wire at the speaker jack or the board, whichever is more convenient. Then you will know for sure.

[dude]

I was never a fan of the Blues Jr, beside the design faults, hot bias, sockets mounted to PCB Bd, the distortion was just not pleasing to my ears. Changing to 6V6’s was a good move, but your client may be trying to get a sound the circuit is not going to give. A Fender Deluxe Reverb circuit sounds closer to what your client wants. Have you searched schematics here, Slucky Deluxe lite is one of best sounding amp, IMO that gets a nice clean to a great distortion, loves pedals and many mods can be added. Just my 2cents.

Hope you get to where you want with the Blues Jr.
Sorry, l must be getting old, no more a Blues Jr but a Princeton, my bad.


Note: l always thought you'd get that loud squeak if the NF was out of phase, once l had an amp l built that had some crazy noise when the distortion came on, l was losing it, trying everyhing to no avail. Using Google a cause could be NF out of phase, but never had a squeal when l first turned on the amp..? I ignore out of phase as the issue, but reading more not only could you get the squeal but a bad distortion, l didn't disconnect the NFB but switched the OT leads to the power tube plates, noise gone...