I have a blackface Super reverb here that I rehabbed. Most of the original board was gone and had been replaced by somebody's idea of a Mesa Boogie a long time ago. I was tasked with putting it back to stock. Everything works perfectly except the Vibrato takes 5 seconds to a minute to kick in. Once it's been going it will turn on immediately the next time you hit the switch. All the components are new since there was nothing to reuse. I originally built it like it would have been with carbon comp resistors and ceramic caps. I have tried numerous known good 12AX7's, both new manufacture and NOS. I completely rebuilt the entire circuit again using poly caps and carbon film resistors as well as replacing the electrolytics on the cathodes. I even replaced the roach. When this happens I've removed the foot switch plug and measured to verify that the tip is connecting to ground. I know usually this is caused by the oscillator not having enough gain from a weak tube or worn cathode bypass cap, but it's all new. I've measured all the resistors for correct value and verified all the wiring and connections. The voltages are very close to the schematic. Any ideas why my wiggle has to wait?
Dave
New does not always equal good. To me it sound like when you have a LCD tv that will only start after multiple presses of the power button. Replacing the power supply caps fixes that. I would check the cap in the vibrato circuit. i would also monitor the voltages in the vibrato circuit during that 5 second interval.Originally Posted by Daver
nosaj
What nosaj said ^^^, also besides possibly a dodgy switch, I'd suspect the caps in the oscillator bridge although they're "new" discs. Whenever I get an old amp that has a weak or non functioning tremolo, I replace the bridge caps with new metallized polypropylene caps (MPP) rated 600 or 630V. Works every time! Panasonic makes 'em, Mouser & DigiKey have 'em, I'm sure others do too, and they're cheap.
I initially had ceramic caps in there. I replaced them with 630V Metallized Polyester caps. The switch "seems" to be OK. As I mentioned, when it didn't work, I'd unplug the footswitch and measure the tip to make sure it's grounded. I've also wiggled the cable and pushed it around and that doesn't change anything. The footswitch is also a new Fender repro. It behaves the same way if I use an alligator clip to make the ground connection. Thanks!
Dave
SO isolate the problem. A trem has two parts, an LFO and some form of interface to the signal path. Scope the trem oscillator tube plate. Does it start oscillating immediately, or is that what fades in?
Education is what you're left with after you have forgotten what you have learned.
Will do. It took me a while to get my scope to display a readable wave form. Now I have to let it sit for a few hours to recreate the problem. I will report back.
Dave
Also have a look at the 2.2meg (and connections) that leads back to the bias circuit.
OK, the 2.2M resistor measures in spec and It has the raw bias voltage on the one end like it should. Everything there looks correct. Since I've had it on the bench again, it hasn't screwed up. Figures. I'm using a jumper plug I made to replicate the footswitch. I'm starting to wonder if I didn't have two problems to start and one was corrected by replacing all the parts in the vibrato again. So I suspect the footswitch, but every time I put a meter on it the resistance drops to 1.7 ohms. Not "ground" enough to work? It doesn't vary at all. I'll report back as things change or don't. Thanks!
Dave
I've had more than one of those switches cause me pain. When you say you are measuring 1.7 ohms, is that across the switch out of circuit, or in circuit from chassis to the junction of 2.2M, and two 1M resistors? Maybe had some corrosion at the RCA jack or plug that has now cleaned up from all the in and outs?
It's weird, because it WAS working fine.....
Randall, I'm measuring it disconnected from the amp. So from the tip of the vibrato plug to the ground on the reverb plug like it would be attached to the amp. The footswitch and vibrato jack on the amp are new. Thanks!
It still wouldn't be a bad idea to measure from chassis to that resistor junction , because that is where it really counts. Plug in the footswitch, wiggle it, see if it fluctuates. I've had problems with more than one new footswitch, sometimes from the insulation being cut too deep at the switch end and cutting into the wires. Also, does it work every time if you use a clip lead from chassis to that resistor junction? Trying to rule out the footswitch, jack and wire from the jack, and solder joint.
It's weird, because it WAS working fine.....
I now measured from the chassis to the resistor junction when the footswitch is engaged and I get 1.2 ohms. Hmmm. I wiggle and twist the plug and cord and it doesn't change. So I go back and measure just the footswitch again and I get about 1 ohm. That's less than yesterday. Weird. So I push that switch a whole bunch of times and the resistance is the same or very close. I try wiggling the plug and cord and bending it and it doesn't change. So I touched up the solder joints inside the footswitch. I'll try it again later to see what happens. Of course, it worked on the bench again. Thanks for the help!
Did you "Zero check" your Ohm meter? I.e. what resistance reading do you get if you touch the test leads together?
I am very interested to hear the outcome of this. My 1971 Super Reverb behaved exactly the same. Only after a dead cold start up it would take up to a minute sometimes to engage vibrato.
Sometimes it would take 20 sec, but always a delay. Once it did engage and the amp was left on, no problem. No delay all night at a gig for example.
I rebuilt the vibrato circuit too. I scratched my head for some time, going over everything and eventually gave up. Someone mentioned that it was most likely
do to a conductive board. I was not prepared to replace the board to fix it.
Daver did you replace the entire board? Or using the same stock one. Curious. Thx. Keith
If the board is conductive, it need not usually be replaced. I have had success with a heat gun baking the conductiveness off the surface.
Education is what you're left with after you have forgotten what you have learned.
The zero, lead only resistance is .4 ohms. The meter is a Fluke. I'm not at home now so I don't have the model number. The board is brand new. I started from scratch.
And now a word from the theoretical section.
The oscillator in the amp is a phase-shift oscillator. Phase shift oscillators oscillate at the frequency where the phase shifts around the gain loop (that's those caps and resistors feeding back to the grid of the oscillator tube) equal 180 degrees, and where the overall gain from the grid of the oscillator tube to its plate, and back through the lossy capacitor/resistor network is equal to or greater than one.
However, the startup time, how long oscillations take to build up, is determined by that loop gain. If the loop gain is ***exactly*** unity, it will sustain an oscillation, perhaps started by noise, but never build up to full size. If the loop gain is less than unity, it will ring like a bell, but never sustain an oscillation. If it's greater than one, the oscillation will build up at a rate determined by the "excess gain" over unity. With high excess gains, startup is almost instantaneous, but there will be distortion.
So if the oscillator takes too long to start up, either (1) the tube doesn't have enough gain, perhaps because of a faulty cathode cap, high drifted cathode resistor, etc., or (2) the feedback loop from plate to grid is attenuating the signal too much, causing too much of the signal to be lost before getting back to the grid.
That's why the values of resistance, footswitch cabling, contacts, and so on affect the start up time. There is another issue, and that is that the feedback network has a variable loss at different frequencies, so the LFO will oscillate better at some speed settings than others, generally at the higher ones. That might be a good thing to check.
Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!
Oh, wait! That sounds familiar, somehow.
Thanks R.G. I have tried numerous new stock and NOS tubes with no variation in how it acts. All the resistors and caps are new, measure in spec and then were all replaced again. How would the feedback loop be attenuating the signal too much? Are you thinking like drifted resistors and caps or something else? When you're talking about checking it at higher speeds, do you mean to start it with the speed turned up to start?
I tried it again today. When I hit the footswitch, it didn't start. After about 5 seconds I shut it off before it started. I then removed the footswitch plug from the amp and stuck in my jumper plug. It started right up. It keeps looking like the footswitch even though it seems to be grounding when I measure it. Thanks everyone.
Well I wired up a bare footswitch to my jumper plug and used that instead of the regular footswitch. The amp is sitting loose in the cabinet at this point. So I verify I have good continuity to ground when the switch is engaged, turn it off, switch on the amp and play for 30 seconds or so. Then I hit the switch and what did I get? Ticking. No vibrato just ticking for 20 seconds and then the vibrato started. Aaahhh.... Another new wrinkle. And, yes, I did rebuild the opto side of the vibrato circuit when I did the oscillator. That has to be a clue. Why would it tick with no vibrato? Thanks.
Does the neon lamp flash when it does this?
I don't know. It's completely sealed. I'll have to put a cut in the shrink tubing to see it. If the bulb were not coming on, would it make the ticking noise? Thanks.
OK it works now. I'm not 100% sure why. I cut open the bulb/ldr shrink tube to see if the bulb was blinking when I heard just ticking. The vibrato worked. I moved the bulb a bit in relation to the ldr because it didn't seem to be aligned well. I then put some silicone glue on it to hold it in place and glued down the shrink tube I cut back. It has since continued to work flawlessly for days in its own cabinet using the footswitch. I can't believe I had two bad roaches from two different places. Further, I wouldn't expect that to cause a delay in the vibrato. I would think it would just not work or be weak. Go figure. I can let it sit for 24 hours and it will fire right up. Thanks for all the help.
Dave
Could be that the legs in the bug were touching, and by cutting it open you disturbed them enough to solve the issue. I recently had a (I forget) that had a nasty crackle at volume. When chopsticking, I found it to be the bug and opened it - the legs were apparently so close that they would touch when there was enough vibration - I separated them and no more problem.
Could be. It's just odd that two different ones did the same thing. It's been flawless for a week now.
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