f^*%ing Blues Jr

[sbluestubes]

I have a cream board Blues Jr on my bench that has been a nightmare it had a few half assed repairs that caused problems from filter cap replacements where the leads of the old caps were used as binding post for the new caps and the heat made weak joints.Now it has developed a problem with the phase inverter socket or the ribbon cable from the mother board cleaned up all the solder joints on the socket and re-soldered and the same for the ribbon cable connections but whatever tube I put in the socket goes to humming loudly when vibration hits it and if I push on the tube it will go away but return when I remove pressure from it. should I replace the socket and use wire to replace the ribbon connector ? Any help would be great !

[acorkos]

i'm not sure if they use the same sockets, but i had a hot rod deville with a broken pin on one of the preamp tube sockets. not a difficult replacement as long as you get the correct socket.

[Mark Black]

Is it a hum/buzz or low-frequency feedback howl?

When you press on the tube you also put pressure on the PCB which may be damping vibrations in some other component or one of the other tubes. Can you try pushing on the tubes and/or other components & PCB with something like a chopstick to localize the problem?

Also the Blues Jr's have issues with oscillation, especially when working with the back cover off. It has to do with the output transformer primary leads being in close proximity to the phase inverter wiring & components, and can be aggravated or made better sometimes by just moving your hand or a test probe anywhere near the area (like when you grab the driver tube). An oscillation may be massively loud at a frequency above our hearing and can so load the power supply as to introduce excessive ripple with resultant hum. A scope on the output would reveal that in short order if you have access to one.

Sounds like a real fun one...

[Enzo]

The amp may be oscillating at very high frequencies - above audio. But when the amp is cranking out RF as hard as it can, in the audio band it makes some hum and also usually a sort of washed out quality to the sound. REadily visible on a scope.

If the output transformer wire crosses the ribbon cable near the power tubes, make sure it is as close to a right angle as possible.

[LBrewski]

I worked on a Blues Jr that had a buzz problem when the back was off; sounded like a bee swarm when played through. I thought it was a microphonic tube but it went away when I put the back on.

So the back has a shield that provides some capacitance for signals on the ribbon cable, without it the phase inverter can break into oscillation. Assuming that you are experiencing the same thing, you may be able to stop it from happening by re-dressing the ribbon cable. Take a look at the page below for a better explanation.


Billm Audio Cream Board Phase Inverter Oscillation