So I have been given free reign to gut a Blues Jr and do whatever I want with it

[glebert]

It does have a nice thick piece of corrugated cardboard between the tank and cab floor. And as for the reverb level, the effect at 10 sounds about like the average reverb level at about 3. Was actually considering increasing the reverb level(scroll up in the thread) for those that like it at 4 or 5 so now I have two objectives that are competing with each other lol.

Just a thought, maybe the objectives aren't competing but actually completely aligned. If you have to turn the reverb up to 10 to get "normal" reverb levels then presumably that is where people are using it. Maybe it starts feeding back at band volumes because you are trying to amplify too small a signal from the tank and have essentially a high gain path that is underdriven by the intentional signal and then gets hijacked by the feedback signal.

[mort]

^^^^ interesting. I'll keep that thought in mind.

[dchang0]

Ted Weber came up with an interesting way to vibration-isolate the tank while bolting it to the bottom of his Princeton Reverb (6A14) kit: he uses two short steel springs, one drywall bolt stud, and a long wood screw for each of the four holes in the tank. The two springs are essentially used to suspend the tank and dampen vibrations in both directions, much more so than the included white translucent grommets that Leo mentions.

Still, the tank is open to the air if suspended in this manner. It would probably benefit from the other damping treatments that Leo mentions.

Definitely consider trying other input cap values than 500pF. glebert may be right--you might simply be sending too little signal to the reverb. When I changed my 500pF, I got a ton more signal and midrange into the reverb, resulting in overdriving the tank (but no feedback). Essentially, it meant that to keep the cap change, I'd have to reduce the gain on the driver side. I was too lazy to do so and just put the 500pF cap back.

[ric]

You said this is a back line amp.

No doubt you tried switching the pan end for end b4 mounting it to keep the most sensitive end away from the PT.

I'm wondering if it's sitting next to another amp on stage that has it's PT near that sensitive pan end.

Would explain why it's fine on your bench, but has trouble on stage.

Just a thought.

[Enzo]

The reverb pan is NOT to be screwed down tight, screws only in enough to let it float. That is the point of the spongey grommets.

I also wondered if the weak reverb might be from the cables being reversed.

My list:
bag
cardboard cover
check all four corner springs for being connected.
Wiring
Strip of foam weatherstripping down the middle of the flat top surface to reduce resonance.

[mort]

Doing some play testing at higher volume than I could last night. A couple of things...

Note: the amp had a 12at7 for the reverb, NOT a 12ax7 like the last drawing shows. I did sub in a 12ax7 and also tried out a 250pF and 100pF with both reverb tube selections. The 12ax7 helped considerably with the reverb level and the pF cap swaps may have increased the reverb effect just a smidge, but for now I just went back to the 500pF.

I can get the amp to feed back but it's independent of where the reverb level is set. Either volume control can be cranked and it's fine so long as the other volume isn't up too much. If the master is dimed, it plays fine until the preamp volume gets up to about noon or so. If the preamp volume is dimed it will start to feed back when the master gets up to about 10 o'clock.

Just from general observation it seems like the preamp gain is just too strong. What's the most efficient way to calm down the EF86 gain?

[mort]

Ok I experimented with the plate and screen resistor values for the ef86. The amp has calmed down a bit but could still stand to be calmed down a little more. Should I raise the 33k input resistor value a little for a little less input signal or raise the plate resistor value? To be honest I don't fully understand the theory behind what I'm doing, just doing some semi-educated experimentation...

This drawing has all voltages measured and edited today.

[Dave H]

Just from general observation it seems like the preamp gain is just too strong. What's the most efficient way to calm down the EF86 gain?

You could try removing the EF86 cathode resistor bypass capacitor.

[ChopSauce]

I'd like to gut a Blues Junior, too. Not sure the outcome would be that great, though

What's the most efficient way to calm down the EF86 gain?

I've been recommended the read of The Valve Wizard -Small Signal Pentode about this subject & sorry: I am not clever enough to explain it myself.