Pro Reverb Cap job and going through.

[proaudioguy]

I have this gem of a 1968 Pro Reverb Amp (black line, drip edge, blue molded caps) on my bench this week.  I’m not a professional tube amp tech, but I have enough experience to get this guy in ship shape.  My main goal is to do as little as possible.  I’m glad I got it instead of that YT dude that cuts all the blue molded caps out and tosses them in teh trash.


I got the softer surface rust off with a soft kids tooth brush and some 99% isopropyl but I hesitate to do much more.  The photo was taken before cleaning it up.  There is still plenty of rust.  Leave it or try and polish it off?  I’m leaning toward leaving it.  Open to any and all advice.  The amp is super clean.  It had been dropped at some point which stripped the baffle screws.  He already had that fixed before I got my hands on it.

[sluckey]

Why is it on your bench?

[WimWalther]

IPA + soft toothbrush isn't a very effective way to remove rust. Use a brass brush & something like WD40 to remove anything loose.

Then degrease it with denatured alcohol or acetone and finally paint on some rust stabilizer / converter to seal up the area.

Or at least that's probably what I'd do.

[Latole]

Vinegar is the best product I found to remove rust.

Put some vinegar in this case ( dog house) and wait. Use some cloth to brush.
Rince with water after.


As sluckey ask ; what wrong with this amp ?

[kagliostro]

Some people don't worry about rust, other like the rat look, I don't like it at all

Put a steel rotary brush on your drill and perform a clean section

When you've finish use a spry zinc can to protect the metal

Franco

[acheld]

I think it totally depends on what you're trying to accomplish.

If it's going to be sold as totally original amp, I'd leave the rust.   It will not have any effect on the function of the amp within your lifetime.  BTW, as most of our forum members will know, that was likely caused by leaking electrolytics . . .

For any other goal, it's dealer's choice.   Personally, I don't like rust, and would wire brush it, clean it soapy water, dry it with denatured alcohol, and coat it with some type of rust convertor.  This would be overkill, my motto.

[proaudioguy]

I think it totally depends on what you're trying to accomplish.

If it's going to be sold as totally original amp, I'd leave the rust.   It will not have any effect on the function of the amp within your lifetime.  BTW, as most of our forum members will know, that was likely caused by leaking electrolytics . . .

For any other goal, it's dealer's choice.   Personally, I don't like rust, and would wire brush it, clean it soapy water, dry it with denatured alcohol, and coat it with some type of rust convertor.  This would be overkill, my motto.

Right on all accounts!


The amp is a recent aquisition of a friend.  He knew not to even plug it in until it was gone through.  I had him order the caps.  He dropped it off the other day with all the parts and a new 3 wire cable.  I have not personally seen one with a 2 wire cable in 20 odd years.  This one is untouched by any tech before.  If it were mine, I’d clean the interior, but its just got sticky dust on it from the few crevices and cracks where it could get in and I hesitate to mess with it much.


If anyone wants photos of the guts of an unmolested 68 Pro, let me know.  It takes a lot of extra work to get them sized for here.  Does Doug have a FB group?

[mresistor]

Photos    a good option is to host them off site.  Photbucket, the cloud, youtube (vids), etc.

[Latole]

Those caps are at the end of their life.
Replace bias filter cap too.

[mresistor]

Those caps are at the end of their life.
Replace bias filter cap too.

I'd surmise that electrolyte rigor mortis set in a couple of decades ago......

[tubeswell]

Running that amp with those filter caps risks destruction of the vintage transformers. If it were mine, I wouldn't leave it - I'd replace all the electrolytics

[proaudioguy]

Thanks for the advice guys. 


I’ll post something else next.  I changed the subject.  You guys can go through this with me.


Yes, new caps.  My friend, the owner, brought them with the amp.

[proaudioguy]

Thank you for going on this journey with me guys.  So I put my glasses on and noticed this burn.  It is all the way through the insulation.  The wire is confirmed to work.  I’ll put heat shrink on it.  This wire is connected to the choke.  I disconnected 1 side to isolate it and it measures 99 Ohms.  Is that the expected resistance of the choke (no shorts in the windings)?


Remember, this is a 1968 black line drip edge PRO REVERB AMP.

[AlNewman]

I have a replacement choke for that type of amp, reads 100 ohms.  So, the burn is likely a bad connection or another failed component.
(ie:  your capacitor may have become a low value resistor)

[tdvt]

I disconnected 1 side to isolate it and it measures 99 Ohms.  Is that the expected resistance of the choke?

The Hammond data sheet for a new one specs 108Ω

[proaudioguy]

Thank you!!!!


Here is our progress!


That carbon resister read 6k and should be 4700.  I just happened to have one.


The 2 resisters under the caps are also out of spec but they should be 220k and are about 266k so I figured they were probably OK.  Also, I don’t have those on hand.


I will update as I go through it.

[proaudioguy]

Caps under the doghouse done. 


Anyone ever see an external ground like this before?

[proaudioguy]

Caps are in.  Next up Power cable, then I’ll post voltages.

[proaudioguy]

If anyone knows off hand what the pin by pin voltages ought to be, let me know please.  I’ll start with the rectifier socket.  I have a copper cap my friend brought over, but I also have a tube rectifier in my Princeton I can swap over for testing.

[sluckey]

https://el34world.com/charts/Schematics/files/Fender/Fender_pro_reverb_aa1069.pdf

[proaudioguy]

OK Sluckey, hint taken.  I’m still going to post them.  If everything is perfect, this info is useful to someone later on down the road.

[proaudioguy]

68 pro reverb amp voltages




POWER TRANSFORMER - NO TUBES INSTALLED
Red wires
775vac between them
To neutral 386vac and 387 vac


Green wires
7.17 vac between them
to neutral 3.14 vac and 4 vac


Yellow wires
5.5 vac between them
To neutral 5.9 vac and .5 vac (interesting, the phase must be opposite there to get the 5.5vac result between them)


Rectifier
Measured to neutral
1
2 5.9 vac NO TUBE/ 525vdc w/TUBE
3
4 386 vac NO TUBE
5 387 vac NO TUBE
6
7
8 0.5vac NO TUBE 525vdc w/TUBE




Voltages with tube and without tube.


Tube socket 1
1 312 /516 dc NO TUBE
2
3 2.41
4
5
6 302 /518 dc NO TUBE
7
8 2.57
9


Tube socket 2 NO TUBE
1 303 /518 dc NO TUBE
2
3 2.57
4
5
6 296/518 dc NO TUBE
7
8 2.57
9


Tube socket 3 NO TUBE
1 486/525 dc NO TUBE
2
3
4
5
6 486/525 dc NO TUBE
7
8
9


Tube socket 4 NO TUBE
1 310 /518 dc NO TUBE
2
3 2.28
4
5
6 304 /518 dc NO TUBE
7
8 2.28
9


Tube socket 5 NO TUBE
1 481 /512 dc NO TUBE
2 51
3
4
5
6 432 /463 dc NO TUBE
7 51
8 -0.37
9


Tube 6
1 272 /518 dc NO TUBE
2 62
3 99
4
5
6 271 /518 dc NO TUBE
7 61
8 99
9


Tube socket 7 NO TUBE
1 /-58 dc NO TUBE
2
3 495 /526 dc NO TUBE
4 /525 dc NO TUBE
5 /-58 dc NO TUBE
6 /525dc NO TUBE
7
8 /0 dc


Tube socket 8 NO TUBE
1 /-58 dc NO TUBE
2
3 495 /526 dc NO TUBE
4 /525dc NO TUBE
5 /-58 dc NO TUBE
6 /525 dc NO TUBE
7
8 /0 dc NO TUBE

[proaudioguy]

Reverb is noisy going in to the reverb. A loud reverberated SHHH.  I’ll look deeper at that tomorrow. 
Tremolo is way too fast and not nearly as DEEP as it should be IMO.


I can SEE the lamp sticking out of the end of the heatshink.  Never saw one like that before.  Is that normal?  Should I replace the bug?  Its definitely working.  If its not supposed to be sticking out, maybe I can replace the heatshrink?  Thoughts?


Normal channel switch is noisy.


Some pots are noisy when used.


Random crashing noise comes and goes.  On my Twin I eliminated those by finding the offending resister and swapping it.  Tubes may be a bit microphonic.  Tapping near V1 is very audible.  Moving tubes around gives a bit of a crackle.  I’l work them a bit and se if that cleans them up.  I already tightened the pins.


Any more advice?

[sluckey]

OK Sluckey, hint taken.  I’m still going to post them.  If everything is perfect, this info is useful to someone later on down the road.

You asked for voltages. Every voltage you need in on that schematic.

It's OK to be able to see the end of the neon bulb in the roach.

[proaudioguy]

Voltages are high but the schematic has a 5U4GB and the amp has a 5AR4.

[Latole]

Caps under the doghouse done. 


Anyone ever see an external ground like this before?

Yes.

[Latole]

Don't forget to check amp fuse, too many owners put a too powerful fuse over years unable to find the right one.
Power transformer is not protected

[WimWalther]

On first glance, that burned spot looks like the wire arced over to the cap lead.

Cloth insulation becomes comprimised by picking up moisture & pollution from the air.

[Latole]

Reverb is noisy going in to the reverb. A loud reverberated SHHH.  I’ll look deeper at that tomorrow. 


Normal channel switch is noisy.

Some pots are noisy when used.

Any more advice?

Easy to clean with good contact cleaner
Spray in access hole from inside the amp , not from the shaft as I read too often
Turn knob at least 20 times

Channel switch ? you mean bright / normal slide switch ?
Contact cleaner and slide 20 times and more

[stratomaster]

Reverb is noisy going in to the reverb. A loud reverberated SHHH.  I’ll look deeper at that tomorrow. 
Tremolo is way too fast and not nearly as DEEP as it should be IMO.


I can SEE the lamp sticking out of the end of the heatshink.  Never saw one like that before.  Is that normal?  Should I replace the bug?  Its definitely working.  If its not supposed to be sticking out, maybe I can replace the heatshrink?  Thoughts?


Normal channel switch is noisy.


Some pots are noisy when used.


Random crashing noise comes and goes.  On my Twin I eliminated those by finding the offending resister and swapping it.  Tubes may be a bit microphonic.  Tapping near V1 is very audible.  Moving tubes around gives a bit of a crackle.  I’l work them a bit and se if that cleans them up.  I already tightened the pins.


Any more advice?

Check the small metal barbs that hold the switch body to the wafer with the solder lugs.  Sometimes these come loose and the switch becomes noisy and intermittent.  Sometimes you can cinch these down to restore function. Other times they break and the only option is replacement. 

For the Tremolo depth, cover the top of the amp with something and test again.  Sometimes light bleeds into the ldr roach assembly and affects the depth and performance of the effect. 

[proaudioguy]

One of the speakers is rubbing or has a delaminated VC.  Its a UTAH.  I do not FEEL a rub but its playing a rattle.  I replaced it at least temporarily with an Early 1980s Pyle.  Sounds pretty much exactly the same.  Kind of blows my mind.  I hated that Pyle in the Deluxe Reverb II, but it sounds good in this amp.  If he doesn’t want it, I’ll try it in the Princeton I built in 2014.  Then I could put that JBL back in my twin.  LOL.

Pots are clean!


Amp sounds STELLAR.  There are a few noises I can attribute to carbon comp resisters but they are only really noticable with the reverb return all the way up.  Reverb is SHORT.  I like them LONG.  I’m not saying the “model” is short as I haven’t taken the tank out to look at the part number (and I don’t plan to), only that I prefer them a bit longer.  It is a great reverb though.


Tremolo is really weak.  I have all the parts on hand to rebuild it completely if necessary. 


What is this amp worth?  He offerred to sell it to me.  I will have a solid full day of work in, plus maybe $55 in parts.  My day job pays $95/hr, but there is no way he could ever afford to pay me that much.


Other than 1 broken tube, it has 2 of the original 7025, original 12AT7.  A different 12AT7 that is just about as old, a pair of RCA blackplates, an old GE 5AR4 (untested) and a WZ34 Copper Cap, as well as a set of old Sovtech 6L6 which he just bought assuming the RCAs were bad (they aren’t).  Given the amount of work I have in, what should I pay?  Its a player’s amp, but its also a 1968 black line with snowman 8s, and drip edge.  I’ll post photos when its back together.

[proaudioguy]

Had to replace the bug.  All good now.

[proaudioguy]

1968 Pro Reverb
Drip edge, black line, snowman 8s.