TomH
Joined: Mar 15, 2006
Posts: 766
Oberlin, Ohio, USA
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Posted on Apr 19 2016 12:31 PM
On Saturday, I bought a '68 Pro Reverb. It's a drip edge from early '68 and it has the blackface circuit, which wasn't changed until June '68. In the used guitar store where I bought it, I turned it up to 4 and it sounded great. I didn't turn it up any louder. Got it home and discovered that it gets this horrible buzz-saw sound at 4 1/2.
It's only the Vibrato channel that's doing it. The Normal channel is OK. It gets traditional breakup starting around 5 on the Normal channel, but it's not this awful buzz-saw distortion that I'm getting on the Vibrato channel.
I recorded my Mosrite playing through the Pro Reverb at 3, 4, 4 1/2 and 5 1/2.
https://soundcloud.com/tsunami-tom/zoom0001
Note how the distortion starts at 4 1/2. Again, it's only the Vibrato channel that's doing this. The Normal channel is OK.
If you can take a listen and let me know what could possible be wrong, that will help a lot. I love this amp, and I want to fix it, but I'm at a loss. Any incite will be appreciated!
Tom
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caddady
Joined: Feb 14, 2010
Posts: 802
N.E. Ohio
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Posted on Apr 19 2016 12:45 PM
did you try different speakers? run the amp int a diff cab. or swaping around preamp tubes 12ax7 i think they is.
cadman
— http://www.reverbnation.com/thegreasemonkeyz
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caddady
Joined: Feb 14, 2010
Posts: 802
N.E. Ohio
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Posted on Apr 19 2016 12:51 PM
12at7 tubes
Circuit:
AB668, AA1069
Config:
Combo
Control Panel:
Silver, forward facing w/ blue labels
Front Control Layout:
68-76: Normal: In, In, Bright Sw, Vol, Treb, Bass
– Vibrato: In, In, Bright Sw, Vol, Treb, Bass, Rev, Speed, Intensity – Pilot Lamp76-82: Normal: In, In, Bright Sw, Vol, Treb, Mid, Bass
– Vibrato: In, In, Bright Sw, Vol, Treb, Mid, Bass, Rev, Speed, Intensity
– Master Vol/Boost – Pilot Lamp
Rear Conrol Layout:
68-76: AC Outlet, Ground Sw, Fuse (2A), Power Sw,
Standby Sw, Speaker Jack, Ex. Speaker Jack, Vibrato Jack, Reverb Jack, Reverb Out, Reverb In
76-82: AC Outlet, Ground Sw, Fuse (2½A), Power Sw,
Standby Sw, Speaker Jack, Ex. Speaker Jack, Line Out, Tube Matching Adj,
Vibrato Jack, Reverb Jack, Reverb Out, Reverb In, Hum Balance
Knobs:
Black skirted w/ chrome center, numbered 1 – 10
Cabinet:
68-69: 19½” x 26½” x 10″ (49.5 x 67.3 x 25.4 cm)
70-82: 19½” x 26″ x 10½” (49.5 x 66 x 25.4 cm)
Cab Covering:
Black Tolex
Cab Hardware:
Black strap handle, 5½” Chassis straps, 16″ tilt-back legs, corner protectors, glides (68-71) or casters (72-82)
Grille:
Blue sparkle grille cloth w/ aluminum frame (68-69) or w/o aluminum frame (70-80)
Logo:
Grille mounted, raised, chrome & black script “Fender” w/ tail(68-74) or “Fender®” w/o tail (74-82)
Weight:
58 lbs. (26.3 Kg)
Speakers/Load:
2 x 12″/4 ohms (8 ohms each in parallel)
Speaker Model:
Oxford 12L6, Oxford 12T6, Utah 12″, Rola 12″ ceramic
Effects:
Reverb, Tremolo
Output:
40 or 70 (late models) Watts
Preamp:
Normal: 7025
Vibrato: 7025, ½ 7025
Power:
2 x 6L6GC
Bias:
Fixed, adjustment pot (AA668?) or balance pot (AA1069)
Rectifier:
5U4GB
Phase Inverter:
12AT7 (long tailed)
Other:
Reverb Driver: 12AT7
Reverb Recovery: ½ 7025
Tremolo: 12AX7(photoresistor)
Comments:
The AA1069 schematic shows a Mid control but it did not get installed on producion models.
Fender switched back to the blackface era cosmetics some time in 1980. Pro Reverbs produced between late 1980 and 1982 have a black control panel and silver sparkle grille cloth.
A Boost switch, master volume, and hum balance control was added before Pro Reverb was discontinued.
https://jazzatelier.files.wordpress.com/2008/08/pro_reverb_aa1069_schem.gif
— http://www.reverbnation.com/thegreasemonkeyz
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TomH
Joined: Mar 15, 2006
Posts: 766
Oberlin, Ohio, USA
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Posted on Apr 19 2016 12:53 PM
Thanks. Since it's fine at louder volumes on the Normal channel, I'm figuring that it's not related to the speakers. I haven't looked at the tubes, yet. That's usually the first thing I try, but wanted to get some input before I do. Could be coupling capacitor, plate resistor, cathode bypass cap, cathode resistor, grid resistor -- probably a lot of other things, in addition to a tube.
I'm just hoping somebody recognizes this sound and tells me what it is. Maybe that's hoping for too much, because it probably could be a variety of things. I don't know if Youngstown is anything like Cleveland, but taking it to a guitar repair shop in Cleveland is guaranteeing that you won't see it for a couple months. I'm hoping to avoid that!
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TomH
Joined: Mar 15, 2006
Posts: 766
Oberlin, Ohio, USA
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Posted on Apr 21 2016 10:09 AM
Last night, I did tube swaps for all the tubes. Unfotunately, that
didn't help. Just want to post my issue again, in case somebody
missed it. I bought a '68 Pro Reverb and the issue is that it gets
this horrible buzz-saw sound at 4 1/2 and above. It's like a bad
fuzz tone sound.
It's only the Vibrato channel that's doing it. The Normal channel
is OK. So, I recorded my Mosrite playing through the Pro Reverb at
3, 4, 4 1/2 and 5 1/2:
https://soundcloud.com/tsunami-tom/zoom0001
Note how the distortion starts at 4 1/2. If anybody recognizes this
and can tell me what needs to be replaced, I'll appreciate it very
much.
Tom
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Gilette
Joined: May 04, 2014
Posts: 734
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Posted on Apr 21 2016 11:53 AM
Sounds bad! Guitar sounds fine btw. 
It's narrowed down to the vibrato channel, which is good. Now all you have to do is to check every part in the signal path from the input of tube 2 all the way up to the end of tube 5. Look for evidence of arcing in tube sockets, check the integrity of pots, loose or cold solder joints, broken wires, etc. Also check the output wires of the reverb pan. These wires break sometimes and they're possibly resonating in and out of contact at certain volumes.
I once re-soldered every connection in an amp to get rid of a similar problem, and I have a tweed Deluxe with the same issues on my workbench. Good luck!
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plexi69
Joined: Oct 25, 2014
Posts: 66
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Posted on Apr 21 2016 01:36 PM
When you swapped tubes did you notice if the sockets for the vibrato tubes were loose, where the tube pins go in? I have seen where loose sockets caused tubes to vibrate inside them. Pro Reverbs are nice amps that have tube rectifiers, very underated but can play any music style. I have also seen filter caps with bad connections do the same. Good luck.
— HEY THERE, HI THERE, HO THERE !!!!
Last edited: Apr 21, 2016 13:38:15
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TomH
Joined: Mar 15, 2006
Posts: 766
Oberlin, Ohio, USA
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Posted on Apr 21 2016 03:12 PM
Hey thanks for the advice! Yes, a couple of the tube sockets were loose. That will be the first thing I address. I can also test the filter caps. I'm not sure if I'm knowledgeable enough to actually trace the signal path, but I might know somebody who can help me with that.
Yes, the amp sounds really good at 4 or less. The reverb is exceptionally strong. It'll be a great amp, once I get this one issue fixed.
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plexi69
Joined: Oct 25, 2014
Posts: 66
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Posted on Apr 21 2016 03:26 PM
I have taken a toothpick and poked it in each pin socket and the pried it to tighten. Just go slowly and make sure your unplugged. Good luck.
— HEY THERE, HI THERE, HO THERE !!!!
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Badger
Joined: Nov 16, 2013
Posts: 4536
Wisconsin
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Posted on Apr 22 2016 06:49 AM
TomH wrote:
Hey thanks for the advice! Yes, a couple of the tube sockets were loose. That will be the first thing I address. I can also test the filter caps. I'm not sure if I'm knowledgeable enough to actually trace the signal path, but I might know somebody who can help me with that.
If your normal channel is fine it's not likely going to be your filter caps; their role is something common to the whole amp. Does the symptom exist using either input of the Vibrato channel?
— Wes
SoCal ex-pat with a snow shovel
DISCLAIMER: The above is opinion/suggestion only & should not be used for mission planning/navigation, tweaking of instruments, beverage selection, or wardrobe choices.
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Gilette
Joined: May 04, 2014
Posts: 734
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Posted on Apr 22 2016 08:04 AM
Maybe this narrows it down for you.
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TomH
Joined: Mar 15, 2006
Posts: 766
Oberlin, Ohio, USA
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Posted on Apr 22 2016 11:00 AM
Thanks Frank! That's really helpful. I'll save that image and take it with me next weekend.
I found somebody more knowledgeable to help me with this. He's actually Cambeezy's brother's friend and he sent me this email message:
"I listened to the audio clip and its possibly a leaky bypass capacitor or capacitors passing DC causing the distortion. As long as the Clean channel can be turned up loud with no distortion the speaker and power amplifier circuits are probably all ok. If it were the large power supply filter caps, the amp would be humming all the time clean and reverb."
At least he obviously knows more than I do. Cambeezy recommends him and that means a lot. I also appreciate that he took the time to listen to the sound file. Anyway, he's going to look at it next weekend. I'll let you all know what it is that fixes the problem! Thanks again for the input!
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Badger
Joined: Nov 16, 2013
Posts: 4536
Wisconsin
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Posted on Apr 22 2016 12:21 PM
That's interesting, thanks for the update. I had listened to the sound clip also and it'll be interesting to correlate that with whatever the diagnosis ends up being. Hope to hear back when it gets fixed.
— Wes
SoCal ex-pat with a snow shovel
DISCLAIMER: The above is opinion/suggestion only & should not be used for mission planning/navigation, tweaking of instruments, beverage selection, or wardrobe choices.
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duncanjames
Joined: Dec 20, 2011
Posts: 156
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Posted on Apr 23 2016 05:06 AM
I once had a sound somewhat similar to this on a Gomez G Reverb chassis I had bought. It turns out it had something to do with how I had mounted the reverb pan. Coincidentally it also started coming in around 4 1/2. When I disconnected the pan, it was greatly diminished if not completely gone. I haven't had the issue since mounting everything in a proper combo cabinet.
I'd suggest you try disconnecting the Reverb In and Out cables from the back of the amp. If that seems to help, check the mounting on the pan, maybe try a different pan and see if it still does it. Good luck!
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benadrian
Joined: Jan 16, 2013
Posts: 15
Echo Park, Los Angeles
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Posted on May 03 2016 04:15 PM
Hello,
I know you said you did a tube swap. Did you also swap the reverb recovery tube? 1/2 of that tube carries the full vibrato channel signal.
My thoughts, though, make me think this.
-
bad solder joint. Some component in the vibrato channel is loose, and the vibrations cause it to make and break contact. You can try giving the amp a physical thud and listen for popping sounds. Make sure the reverb is turned down to avoid the surf crash. If you hear noise when you gently beat on the amp, then it's something mechanical.
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bad coupling cap. Coupling caps allow the AC audio signal to pass through, but block the DC of the power supply which powers the individual tube stages. Sometimes these go bad and can let through "bursts" of DC.
Luckily, tube amps are pretty straight forward designs. This should be a relatively easy fix for a technician.
Cheers!
Ben
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TomH
Joined: Mar 15, 2006
Posts: 766
Oberlin, Ohio, USA
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Posted on May 04 2016 10:19 AM
Hey thanks, Ben!
On Saturday, I took it to a guy, Al, who mostly builds ham radios,
but he's extremely knowledgeable about circuits. Even though I had
swapped the Groove Tube 12ax7 and 12at7 tubes with new Groove Tubes,
and had swapped the JJ 6L6 tubes with new similar JJ 6L6 tubes, and
the 5AR4/GZ34 rectifier tube with a Weber copper cap (didn't have a tube
replacement), the first thing that Al did was to replace all of the
12ax7 and 12at7 tubes with used tubes that he had. He had a tube
checker and first checked all of the original tubes that I had back in
it and they were OK, but he
wanted to replace the 12ax7 and 12at7 tubes, because he said the Groove
Tubes were higher gain (more powerful) and he thought the Pro Reverb
would like the RCA/Sylvania tubes better. Then, when we were taking out
the amp, we discovered that the nuts on one of the speakers were all
loose. So, we tightened those. On the amp, all of the filter caps had
been replaced, but none of the other caps or resistors had been
replaced. Except for the filter caps, everything is original. I
suspected that would be the problem, but Al checked each capacitor and
resistor and they checked out fine. Then, he took some of the
capacitors out and checked for leakage. That was fine too.
So, not sure if anything had been the issue, we put it all back together
and - viola - the problem was gone.
Neither of us know for sure what the problem was, but Al gave me the
tubes he had put in as a gift and just charged me a minimum for the few
hours he spent checking out the amp.
Like I said, this issue ceased; however, the amp still breaks up around
"6" or "7", which is normal, but I don't really like how the break up
sounds. That's OK, because I'm not a blues player! I had planned on
selling my Showman and using this Pro Reverb for all gigs; however, I
changed my mind and decided to keep the Showman. So, I'll be using the
Showman at our outdoor summer gigs and I'll be using the Pro Reverb
for indoor gigs. It gets awfully loud before there's any breakup. So,
it's more than enough for indoor gigs, but it sounds great at lower
volume, too.
I'm attaching a picture of my two amps. I'm in love with both of them.
They sound so good. I'm glad that I decided to keep the Showman. It's a
classic!
Just picture a sunburst Mosrite and a sunburst Jazzmaster between the
two and that'll be my gear! Thanks to everybody who helped out with
this. I wish that I could tell you exactly what the issue was, but at
least it's not an issue anymore!
Last edited: May 04, 2016 10:28:42
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