Blackface Vibrolux Reverb needing Attention

[Platefire]

Just got this in for repair this afternoon. The customer said it was a 1965 and he is just wanting to get everything working so he can sell it?? Trying to figure out what schematic would be correct. I downloaded a AA964 but not sure if that's correct? I was thinking an AB763 would be more correct but couldn't find one with verb/trem. How would you determine correct schematic for this amp??

On initial survey it has a three conductor power cord, also noticed the tube sockets had screws securing them instead of brads which make me thing they have been changed. Both speakers are Pyle Drivers. All other exterior stuff looks original.

I fired it up and gave it a test run. Both Normal and Rev/vib channel are strong as far as volume but
in accenting notes or chords getting static and pops. This occurs through both channels! The vibroto doesn't work. I've tried different good preamp tubes and got about the same results. Looks like the problems are internal. Next step is to pull the chassis and check it out. This is my first vintage fender repair job, kind of exited! Platefire

[HotBluePlates]

The circuit should be AA964, or similar. The tube chart oughta tell you, but compare what's in the amp carefully.

... also noticed the tube sockets had screws securing them instead of brads which make me thing they have been changed.

I might be wrong, but I'm about 90% sure all blackface Fender used small (possibly self-tapping) screws to secure the sockets.

The vibroto doesn't work. I've tried different good preamp tubes and got about the same results.

Do you have the footswitch? If not, do you have an RCA shorting plug (one that has a connection soldered from hot to ground)?

The VibroChamp and Princeton Reverb are perhaps the only blackface Fender amps were the trem is "always on" and does not need the footswitch. In these amps, you use the footswitch to kill the trem. In possibly all other blackface amps, the trem is "always off" and the footswitch must be used to turn the oscillator on.

If you look closely at schematics for each, how the footswitch is incorporated in the oscillator is different. In the "always off" amps, the footswitch grounds a 1M resistor, allowing the feedback path to function, turning the oscillator on. In the "always on" amps, that resistor find a ground at the oscillator cathode, and the footswitch point is moved to ground out the feedback signal, turning the oscillator off.

Though footswitches did come with the small amps, Fender probably assumed you'd play them mostly at home, and not need a footswitch like you would for stage amps.

[Platefire]

HotBluePlates

I sure didn't know about the "always off" setup. I do have a footswitch for my Peavey Pacer conversion to a AB763 Deluxe reverb with bias trem I could try.

On the schematic I chose the AA964 because it has GZ34 rectifier and the others are 5U4----so you say that should be close--good! I can tell better when I open it up and compare the board layout.

I noticed it still has a ground switch and the three conductor cord. It will be interesting (when I look inside) to see what they did with the switch--probably abandoned in place??

Thanks so much for your help, always very much appreciated! Platefire 

[Platefire]

Tried the footswitch on the tremolo and it worked beautifully! TVM! Reverb works good too without a switch. In between the static caused by playing a guitar through this amp I can hear wonderful tones and feel some great response.

I tried some different power tubes and still getting static---so I think everything external has been done. I might try a different guitar and cord just to have covered all bases. Platefire

[phsyconoodler]

Plate,
  You need to clean all the tube socket pins and retention them.Then when you get the chassis out replace the electrolytics as required.Check all the basics like grounds and make sure all the pots and jacks are tight.That alone can throw guys for a loop.
   Clean and tight is the order of the day here.

[Fresh_Start]

Hoffman's Vibrolux Reverb schematic is the AA964. 
http://www.el34world.com/charts/Schematics/files/fender/vibroluxreverb_aa964_schem.pdf
Looks a heck of a lot like a Super Reverb AB763 except for the Mid cap in the tone stacks and lower voltages.

While you're checking out that 3-prong cord, you might want to make sure the sequence is:
HOT -> Fuse -> Switch to PT primary
and the neutral wire is connected to the other PT primary

Cheers,

Chip

[eleventeen]

Drool, on that amp. One of the best amps out there, IMO, though I prefer 12" speakers.

I would first check under the cap can to see if the filter caps had been replaced. That might have been done at the same time as the 3-wire cord. This is just to get a fix on whether the thing has been "benched" in recent years. Those caps if original *can* still work, but the odds are somewhat against you. Whether you keep or replace them is a matter of executive decision unless they are bulging and oozing.

But my main point is: Having just light-rebuilt my 1965 Pro Reverb and 1968 AB763 Deluxe Reverb and my insanely modded 1969 Princeton Reverb, it's IMO mandatory to replace the dual 100K plate resistors. You just have to do it. There's no way, IMHO, to chase the fizzyness and "staticyness" and all those miscellaneous ocean-rodent noises out of the amp without replacing those R's. Use Dougs' 3-watt metal film ones. Just replace dead minimum the first half dozen of them.

Hopefully, the caps work and you'll be able to see/hear the difference just replacing those R's makes. If your results are like mine, you'll not let a 60's Fender off your bench without replacing those resistors. And by the way...for all three of my amps it seemed like they needed to cook for about a 15 minutes > half hour before the full amount of noise reduction could be heard. I have no explanation for that, but I can assure you, there's no bigger improvement you'll achieve for $2-3 worth of parts.

[Platefire]

Found tubes in wrong locations. Reverb input driver has a 12AX7, tremolo tube had a 12AT7 and PI was a 12AX7----so I put the 12AX7 from the reverb driver in V1 location, installed one of my used 12AT7's in Verb driver location and swapped the trem and PI tube around--now tube set up is back to stock lineup.

Opened up the chassis this morning. First took the cover off the doghouse for the filter caps. New Sprague Atoms in there. Even though the soldering and lead dress was terrible sloppy, all connections had continuity. Two 10K CC power rail resistors looked like they could be new--still reading in resonable range of 10K. Everything here seemed to be OK, so I moved on.

Next gut layout. The three conductor cord-terrible again. Improper strain relief installed to hold cord in chassis so they put some kind of clear calk around cord on inside of chassis to hold it though--still lose!
To make it worst, they still had the AC power tied in with the ground switch    

Looked at the board--mostly still original with the blue caps. All the cathode bypass caps has been changed and some CC resistors with the same sloppy lead dress and soldering--though still obtaining continuity! The board has waves in it but still working OK.

Continued on the next post

[Platefire]

I then checked ground connections, pot tightness, cleaned pots, moved some wiring lead dress to get more seperation in wiring. I Checked some voltages as follows:

To OT=450V
6L6 Plates=455V
Bias=-50.8
PI Plates=198/195
Normal/Vib Channel Pre's=186/189

Power section voltages seems to be about 30V higher than schematic shown voltages? Maybe regulation of the bias down to -42 would correct this? Will the bias pot accomplish this?

BTW-I found the problem with static/poping with each note played ---I found a bad solder connection with the 100 Ohm resistor to ground at the Intersection of the .1 cap at front of PI resistor network. Where the 100 ohm connected to the .1 cap/22K R/820 Ohm R was loose/or un-soldered. I de-soldered the joint, re-attached resistor lead and re-soldered it and amp seems to be working fine now---so I think I got it?!?! It's late now so I got to go to bed. Platefire

[stratele52]

Higher voltage is normal , these amps are made to work at 110 volts AC and today we have more voltage . Check also the 470 ohms 2 watts resistor on 6L6 tubes sockets. Sometimes one is burn.

[Fresh_Start]

Personally, if I had the amp opened up anyway, I'd de-solder and re-solder any of the messy, new solder joints.  I learned the hard way the continuity doesn't always equal good connection.  Try chasing gremlins in a Blues Jr. if you want to know how I figured that out!  However, your customer might not want to pay you for the necessary time.

When I had a mid-60s Princeton Reverb to fix up, I blue printed the value of every component.  Overkill for your situation, but there are a couple of components I'd pay special attention to:  the 3.3meg reverb mix resistor (I found over 4.5 meg there), slope resistor and tones stack caps, coupling caps (big value drift might hint at leakage), and plate resistors for the PI.  I'd also check for DC voltage on preamp grids, pots, etc. to track down any leaky caps in the tone stack etc.  Yes, I understand that value drift might be the key to an old amp's "mojo", so just because it's off-spec by more than 20% doesn't mean replace it automatically.

You haven't mentioned the bias supply.  Does that electrolytic cap look like a replacement?  That's one I'd replace if it looked cross-eyed at me.  Honestly, the two 10uf cap pi filter in Doug's library produces a much cleaner bias supply voltage, but putting it on the old board would be a pain.  I've got a nice layout for a replacement bias board I can share if you want.

Hope that helps and doesn't cover too many things you already know.

Respectfully,

Chip

[Platefire]

Thanks Guys for all your support and suggestions on this! I really do appreciate it and it was a great  help.  

The customer just wanted the minimum to correct the problem so he could sell it. The main problem was the cold solder joint and it is corrected. There is a lot more I could do to bring the amp up to speed but I want do it unless he request it. I did do some corrections on minor things I found along the way to finding the main problem. He mentioned an amp show this month he would like to make with the amp I think to sell it there, so I'm trying to get it out fast.

It's been a blast fooling around with this old amp and the customer said he has a friend that has a Marshall that needs attention and he also has a Marshall he wants me to worked on---so I'm hoping and praying this will lead to more. I'm attaching some pixs of the Vibrolux--sorry I put it back together before I got a gut shot

Thanks, Platefire

BTW-The amp sounds great---reverb and tremolo is working fine--nice and quiet operation with plenty of punch!!

[Fresh_Start]

What, no gut shots?

Where is the love?

Chip

[HotBluePlates]

... He mentioned an amp show this month he would like to make with the amp I think to sell it there, so I'm trying to get it out fast. ...

If it weren't for the speakers, it would be a fast, easy sell, given that you got it going again!

[Platefire]

Yep! On my first vintage amp repair I should have took more pictures and I usually do on all my amp projects. I'll try to better next time.

I told the customer the more original the amp the better the sell would be. He mentioned that he had some vintage correct speakers he could install. What would an all original Vibrolux sell for these days?? I would guess off the top of my head---two to three Grand? This amp in it's present condition-maybe one grand? I really haven't researched what current price would be. Platefire