Questions about Rehabilitating a 65 Super Reverb

[Dumbmonkey]

Hi all!
I am currently working on a 1965 Super Reverb that someone I met had sitting in a garage under a blanket.  She tells me that it was previously in a shed, and it sure looks like it!  Covered in dirt and rust. Anyway, she says it belonged to a family member she was close to, and would like to make it usable.  Personally, I think she checked out what the prices are for used ones online, and might be looking to flip it, lol.  I made her an offer on it as is
In any case, I've taken the project on and am in the process of cleaning, replacing and upgrading things.  So far, I've replaced all the filter caps and dropping resistors, the entrire bias circuit, all the cathode bypass caps, screen and screen grid resistors, and the plate resistors.  I've put protection diodes on the rectifier, and I'm definitely replacing the power cord with a grounded one.
My big question is, how far should I go with replacing reisistors?  I'm measuring each one, and if they are way out of spec, I'm replacing with 1 watt carbon film, but is there a point where I shouldn't worry too much?  For example, how important are the 68k resistors on the input jacks? 
At this point, would it just make sense to replace all the resistors on the board?
Thanks in advance for your time!
cc

[tubeswell]

If if were me, after doing the mains wiring safety ground, I’d replace the only the Electrolytic caps (including the bias supply cap) and then make sure none of the speaker coils are open, then check for bias voltage on the 6L6 grid pins, before popping them back in and switching it on with a current limiter. Don’t worry about replacing other stuff until you get it running. The existing resistors have extra mojo, as do the blue astrons (until you find out they are leaky).

[shooter]

WHAT he said^^^^


did a repair on one, pretty sure it was a '65, you might look into "collectability"  this guys amp at the time had a re-sell value in the many 1,000's


a scope n signal generator are great tools after the meter finds all the easy stuff.


the Bias voltage is critical BEFORE you put in the PA tubes with power, it needs to be stable over time AT the tube socket, no major AC riding on the DC


fwiw these are from my initial testing notes for reference.

[Dumbmonkey]

Hey there!
Thank you both for your replies!  I think I got too caught up in the resistors, and forgot that this thing has to fire up first.  I'll get on the mains and check that it powers up and measures and works correctly before diving in too much deeper. 
For the record, this thing is pretty beat up, so I'm not too sure about collectability. 

[stratomaster]

I just recently went through a very similar experience. Mine was an early 67. Silverface cosmetics but completely AB763 inside.  It was a gift, and had been home to many dirt daubers, scorpions, spiders, and a misguided master volume mod before it came into my possession.

Replacing out of spec resistors can very quickly spiral out of control and reduce some of the character that these amps take on as they age.  Far too often people will shotgun replace plate and slope resistors because of Internet stories of noisy plate resistors.  It would be best to know what the resistor is doing at a technical level and making a judgment call as to whether the drift works in your favor or against you.

For example, if a 100k plate resistor reads 112k, but it otherwise functional and not noisy, then that stage will have a slight bit more gain than stock.  You may like this.

Take that same resistor scenario but as a slope resistor, and now you'll have a bit more high end at the expense of some lows and mids.  You may not like that, so replacing the resistor is a good move.

The same applies for cathode resistors. Those drifting high will bias the stage moderately cooler.  It can be a good thing, but in your tremolo oscillator or reverb driver it might result in weaker trem at low frequency or audible distortion in the reverb signal. 

The only resistors I absolutely would change are dropping resistors in the HT, the dropper for the bias supply, and the 6L6 socket mounted resistors. Everything else should be evaluated after you've got the amp running again. 

The other thing to consider on these barn find restorations is conductivity in the fiberboard.  This is a challenge to (temporarily) root out, and is a losing battle in the long run as it will keep happening. Let me know when you get there and I can share some insights from my recent restoration (amongst several others for clients).

Finally, you'll want to clean the brass ground plate sandwiched between the chassis and pots on the front panel.  Once that is clean and the pots are tightly cinched a lot of the background pops and clicks reminiscent of a record player spooling up will vanish.

Have fun with your project.

[rumpus]

The other thing to consider on these barn find restorations is conductivity in the fiberboard.  This is a challenge to (temporarily) root out, and is a losing battle in the long run as it will keep happening. Let me know when you get there and I can share some insights from my recent restoration (amongst several others for clients).

Someone pointed me to a Lyle Caldwell video on this topic:

i=-Pfe_b5E_wqAw1WR

[stratomaster]

The other thing to consider on these barn find restorations is conductivity in the fiberboard.  This is a challenge to (temporarily) root out, and is a losing battle in the long run as it will keep happening. Let me know when you get there and I can share some insights from my recent restoration (amongst several others for clients).

Someone pointed me to a Lyle Caldwell video on this topic:

i=-Pfe_b5E_wqAw1WR

Thankfully no wax on a 65.

I tend to go a bit further with it. When I take the pots off the front panel to clean the brass plate I lift the board up to access the underside and hit with a flux brush and 99%IPA. The harder to reach areas I spray with alcohol. 

I also take out the insulator board, clean it, and drench it in alcohol which I then dry with a heat gun. I subject it to 3 rounds of this. 

On the top side I clean with a brush and alcohol, then cotton swabs and alcohol.  I then pool alcohol around a target eyelet, use the iron to heat said eyelet and that'll boil off the alcohol, flux, and any crud built up over the years. I'm usually dabbing the area with an IPA soaked swab as this happens to catch the expelled contaminants.

Finally I'll power it up after reassembly and measure the board for DC.  Any areas still leaking a bit get another drenching in alcohol and a targeted heat blast with an SMT hot air rework tool at 450°F.  I'll let it sit on the board for about 10 seconds and make sure that I'm not heating insulation or sensitive components too much with the hot air.

This helps a lot, but doesn't completely eliminate the problem. It's very time consuming, and I go through several fistfuls of cotton swabs.

[Willabe]

Use a light bulb limiter to fire up the amp for the 1st time and after power supply work, B+filter caps, screen grid R's, etc.

No larger than a 100w bulb for that amp, 2 x 6L6.

[rumpus]

Thankfully no wax on a 65.

When did Fender start with the wax? The board in my AA270 Bassman doesn't look waxy - but this is the first one I've looked at so what do I know? It doesn't appear to have been stored anywhere too damp, so maybe I won't have a problem.

I haven't gotten very far on mine - just parts to fix the power supply and other known problems, and some basic cleaning - too many more urgent projects. Once I can power it up, I can start checking for stray voltages. Then I'll know how much more of your excellent advice I need to follow.

[stratomaster]

Not sure when exactly they started with the wax. I'm sure there are others here that know. Without fail the later '70s stuff has it, so my guess is maybe '74?

[Dumbmonkey]

Thanks again for all your replies and advice everyone!
If anyone is curious, I’ve finished what I needed to replace on the board and installed the grounded power cord.
I’m wondering if someone could double check to make sure I didn’t miss anything with the power?
Here’s what I did:
Ground (green): to the chassis.
Hot (black): to hot of courtesy plug, then to center of fuse holder, then from side of fuse holder to one side of power switch.
Other side of power switch to one primary of transformer.
Neutral (white): to neutral of courtesy plug to other primary of transformer.
Was that correct?

[stratomaster]

That'll work. My preferred (but I'm outnumbered by people with valid reasons) method is switch before fuse, but the way you've got it is likely more correct. My reasoning is that I want to have off mean off and eliminate voltage at the fuse.

[Dumbmonkey]

Hi again!
I have the amp up and running.  Voltages look good so far, and I have the amp making sound.  I did have to replace a section of questionable heater wiring, so now all the preamp tubes are lighting up correctly.
Quick question for anyone, I have channel one (Normal) working perfectly, but Channel 2 (Vibrato) has really low output.  Does the AB763 circuit require that the reverb tank be connected for that channel to work fully, or should it work fine with or without the reverb tank?  I can connect the reverb tank if so.
If something needs to be connected, can I jumper the reverb in and out with an RCA cable for testing and such?
Thanks again for everything!

[stratomaster]

Last question first: No, you don't need the tank connected to get full volume.  But it's detrimental to the reverb transformer's health to run it without a load for extended periods of time.  Do not use an RCA cable to bridge the two jacks.  This will give a boost effect that will sound unpleasant at best.  If you're just taking measurements and such on the bench, then running without the tank connected is ok.  But once you start running significant signal at higher volumes it's best to connect the tank.

As for why you have low signal, it really could be anything.  Common causes are a busted solder connection at the grounds to the brass plate, broken connection between the input jacks, a failing component, a bad connection somewhere else in the signal path, loose pots on the front panel, etc.

You'll need to troubleshoot to find it. 

[pdf64]

If it's not convenient to connect the reverb tank whilst testing the dry signal path / power amp etc, it's simple to just pull the valve in V3. That avoids stressing the reverb transformer if the secondary is left open circuit, or bothering with a dummy load for it etc

[tubeswell]

Channel 2 (Vibrato) has really low output. 

After you’ve tried a preamp tube swap for channel 2, test the coupling caps in channel 2 for DC leakage. But don’t just do a shotgun change out of all the coupling caps- test each one properly and only change it if it is actually leaking DC. (If DC gets onto the next stage’s grid, it will cause bias to change enough to reduce gain drastically).

[Dumbmonkey]

Hi all (again),
It looks like I have some troubleshooting ahead of me.  Oh boy!
Tubeswell, can I check for DC leakage with just a multimeter, and pardoning my lack of experience, how should I go about doing that?
Thanks again for everything!
cc

[Dumbmonkey]

Hi
I checked for dc using my multimeter, and I found some at the .047 coupling cap that comes off of pin 7 of V1.  Could that be causing an issue?  All the tone stack caps look fine, but there is dc on the caps in the phase inverter section, should I be concerned about that too?
Thank again!
cc

[stratomaster]

How much DC? 

10mV, not a big problem
100mV, noticable and could cause issues
1V+, big problem

[tubeswell]

Hi all (again),
It looks like I have some troubleshooting ahead of me.  Oh boy!
Tubeswell, can I check for DC leakage with just a multimeter, and pardoning my lack of experience, how should I go about doing that?
Thanks again for everything!
cc

You need to unsolder the ‘ac lead’* and lift it out of the circuit, then clip a V-meter onto that (now floating) lead, switch the amp back on and watch the meter. If the cap is normal, you’ll see some changing DC voltage on the meter as the HT voltage comes up to normal power-on voltage, but then it should show 0VDC after everything has stabilised. (Remember that changing voltage will be seen across any coupling cap - but constant DC voltage should not be present). If there is any DC voltage present after the HT has stabilised, the cap will be bad.


*i.e., the other (dc) lead needs to stay connected to the plate of the preceding stage for this test to work.

[Dumbmonkey]

Hi again all,
I apologize for all my inexperience in this, and want to say thank you for all your time and patience with me. 
In any case, I'm still plugging away at this vibrato channel issue.  I've tried replacing a couple capacitors that I thought might be an issue, but apparently, they are just fine, so I put the original ones back in. 
I'm at a loss as to what this issue could be with the lack of volume and response from this channel.
I went and measured voltages at the preamp tubes, and was hoping that if I posted them, someone might have some possible insights.
Here's the measured voltages followed by the voltage listed on the schematic, there's definitely something odd going on at V5...
V1
1:280  (270)
3:1.922  (2.1)
6: 284.5  (270)
8:  2.154  (2.1)

V2
1:255.2  (270)
3:  2.047  (2.1)
6:  274.5  (270)
8:  2.18  (2.1)

V3
1:  459.2  (450)
3:  4.56  (8.4)
8:  4.56  (8.4)

V4
1:  281  (270)
3:  2.18  (2.0)
6:  287.8  (270)
8: 2.18  (2.0)

V5
1:  452  (280)
3: 0       (2.5)
6:  403.2  (390)
8:  0       (170)

V6
1:  298.4  (280)
3:  67.1  (106)
6:  305.1  (230)
8:  67.1   (106)

[stratomaster]

V1
1:280  (270)
3:1.922  (2.1)
6: 284.5  (270)
8:  2.154  (2.1)

V2
1:255.2  (270)
3:  2.047  (2.1)
6:  274.5  (270)
8:  2.18  (2.1)

V3
1:  459.2  (450)
3:  4.56  (8.4)
8:  4.56  (8.4)

V4
1:  281  (270)
3:  2.18  (2.0)
6:  287.8  (270)
8: 2.18  (2.0)

V5
1:  452  (280)
3: 0       (2.5)
6:  403.2  (390)
8:  0       (170)

V6
1:  298.4  (280)
3:  67.1  (106)
6:  305.1  (230)
8:  67.1   (106)

I have a feeling if you hit the button on your vibrato footswitch your voltages on V5 will come into alignment. That tube is the tremolo oscillator and driver for the bulb. Unless your issue gets better/worse with the Intensity knob, then I suspect it's not the issue.

Nothing in your DC voltages are pointing to an obvious problem with your vibrato channel.

I think your issue is in the signal path between the triodes. If you don't have a scope I'd recommend getting the Fender Super Reverb reissue schematic. It has AC voltages at each stage. You can make a reasonable attempt at replicating those measurements with a DMM capable of VAC RMS measurements.  You'll need a signal generator. I use a phone app and an 1/8" to 1/4" cable.  You can even use alligator cables in a pinch.

This will allow you to track the change in signal amplitude as it travels through the amp.  If it drops where it shouldn't--or doesn't rise as much as it should, then we focus on the associated circuitry.

[tubeswell]

^What Stratocaster said^


Also

...test the coupling caps in channel 2 for DC leakage. But don’t just do a shotgun change out of all the coupling caps- test each one properly and only change it if it is actually leaking DC. (If DC gets onto the next stage’s grid, it will cause bias to change enough to reduce gain drastically).

[Dumbmonkey]

Thank you both Stratomaster and Tubeswell!
I printed out a 65 reissue schematic in a nice 11x17" size (hooray for free school copiers!) and I've marked off all the test points for the vibrato channel preamp signal.  (unless you think I should check out the phase inverter section as well?) I have a signal generator on my phone, so the 1k sine wave shouldn't be any issue. 
Thanks for your help, I'll keep y'all posted.
cc

[stratomaster]

Cool. Regarding the PI, we'll burn that bridge when we get to it.  The fact that your issue is isolated to a single channel rules the PI out. That doesn't mean it's 100% healthy--just that it's not the immediate concern.

[pdf64]

... I have a signal generator on my phone, so the 1k sine wave shouldn't be any issue. 

A potential divider between the phone output and amp input might be helpful in achieving the specified 20mV signal voltage at the amp input.
Be aware your meter might not be good enough to accurately measure up at 1kHz, especially at only 20mV.
But the signal voltages noted can be used as a guideline to estimate the difference in signal levels at the different test points at lower frequencies.
200Hz is often within the capabilities of most DMMs. But I haven't checked this kinda thing for a while, so the spec of ghe typical DMM might have got better.

[Dumbmonkey]

Hi again!
I'm sorry, but I'm going to off on a tangent here.  I haven't gotten around to checking with the signal generator, but as I was trying the channel out again with a guitar last night, I noted that as I increased the level on the Reverb control (no tank connected yet), the volume of the channel increased significantly.  Is this normal, or could it be the indication of something I should be checking out?  Maybe something that narrows my search for what might be the issue in the channel?
Thanks in advance!
On a side note, how crucial is a 12AT7 in the Reverb circuit?  Will subbing a 12AX7A cause any problems?

[stratomaster]

When troubleshooting it is best to eliminate variables. Do not deviate from the print, especially if you're unable to evaluate the impact of those deviations for yourself.  If you absolutely must substitute a tube, then put at 12AX7 in for the PI and use a 12AT7 in the reverb.

Good observation with the reverb, but please do not try to short cut the troubleshooting strategy.  Systematically and methodically evaluating your amp vs the print is a skill unto itself, eliminates variables, and builds familiarity with the circuit.

There's a good chance the reverb circuit was modified to operate as a lead boost at some point in this amp's existence. Take a look at the RCA jacks and measure resistance tip to tip with the tank disconnected.  It should be open circuit. If it's low 100s ohms then someone likely modified the stage to act as a boost of 330Hz+

[Dumbmonkey]

Wow, this thread is really getting ridiculously long and I'm so sorry that my inexperience is taking up so much space. 
Thank you all for putting up with me!
It seems the sine wave plan is going to be a bust as I don't have a the right means to measure the testing points, I only have standard multimeters.  If anyone can recommend one I should have, let me know and I'll order one.  I probably ought to have an oscilliscope, but I wouldn't even know where to start from there, lol.
Anyway, I'm still at a loss, but now I'm noticing that the phase inverter section is noisy and when I chopstick it it makes all sorts of banging noises, which seem to go away as the amp warms up.  I've tried a couple different tubes in there but it still happens.  Any thoughts on that, or as mentioned before, is that a separate issue.
Also, does anyone think there might me something with the pots on this channel?  They all seem to work, but the bright switch doesn't although both the switch and the bright cap measure fine.  Actually, pretty much all the components measure within specs.
I'm getting frustrated with my lack of knowledge and experience, and I'm almost thinking about shotgunning the parts to find the culprit (don't like that idea tho...)
Any advice is welcome.
cc

[stratomaster]

What meter do you have?  Most are capable of mV AC measurements. You don't necessarily need an oscope if you're not chasing a waveform issue. A DMM will be just fine for amplitude tracking.

[tubeswell]

Hi again!
I'm sorry, but I'm going to off on a tangent here.  I haven't gotten around to checking with the signal generator, but as I was trying the channel out again with a guitar last night, I noted that as I increased the level on the Reverb control (no tank connected yet), the volume of the channel increased significantly.  Is this normal, or could it be the indication of something I should be checking out?  Maybe something that narrows my search for what might be the issue in the channel?
Thanks in advance!
On a side note, how crucial is a 12AT7 in the Reverb circuit?  Will subbing a 12AX7A cause any problems?

You should not be getting this. Check the resistances in and around the reverb level pot against the schematic. Include checking the 3M3/470k divider in the dry signal path and the 220k from the reverb pot wiper to ground.

[Dumbmonkey]

Hi again Stratomaster!
I have regular multimeters, so I'll try those again.  Sadly, I don't have a dummy load, so it got pretty loud down there...

[Dumbmonkey]

Hi Tubeswell,
I had check that section earlier and replaced the 470k and 220k.  The 3.3meg seemed within spec.  I did notice that when I measure the 220k in circuit, it measures in the 400K+ range.  Does that seem right to you?
cc

[stratomaster]

Hi again Stratomaster!
I have regular multimeters, so I'll try those again.  Sadly, I don't have a dummy load, so it got pretty loud down there...

Pull the power tubes or temporarily short across the power tube grids. You're just signal tracing right now.  The later will be equivalent to turning a cross-line master volume down to zero. It will mess with your PI voltages, but you'll be able to signal trace in silence.

[Dumbmonkey]

Pulling power tubes will be ok?  I'll give that a whirl.
cc

[stratomaster]

Yes. Your DC and filament voltages across the board will go up. So it's not the best thing to do long term. But will be fine for troubleshooting purposes.

[Dumbmonkey]

Cool, thanks!  I'll report back.

[Dumbmonkey]

Well that was interesting.
Pulled the power tubes and ran the test tone in. I didn’t get very far with my measurements as the 1st testing point (supposed to be 20mv) showed no voltage. I tried a couple other points and got either 0 mv, or really low readings.
I’m thinking that maybe the input packs need to be replaced or rewired?

[stratomaster]

Check the solder connection between the two jacks. It tends to crack and causes the jack to lose ground reference. You'll often hear it snap into place if you reflow that connection. Tighten the jacks to the chassis with a socket wrench before reflowing.

You'll also need to make sure you're putting the 20mV signal into the amp. Confirm that your signal generator is outputting that amplitude. You can do this by measuring tip to ground outside of the amp before connecting to the input jack.

[tubeswell]

Hi Tubeswell,
I had check that section earlier and replaced the 470k and 220k.  The 3.3meg seemed within spec.  I did notice that when I measure the 220k in circuit, it measures in the 400K+ range.  Does that seem right to you?
cc

The 220k is wired between the pot wiper and ground so it will not measure as 220k in circuit - rather it will measure as being in parallel with other resistance, which will vary depending on the pot rotation. At which points is the R-meter clipped when you measure 400k-ish? That doesn’t seem right.

[Dumbmonkey]

Hi Tubeswell,
I had check that section earlier and replaced the 470k and 220k.  The 3.3meg seemed within spec.  I did notice that when I measure the 220k in circuit, it measures in the 400K+ range.  Does that seem right to you?
cc

Hi,
I was measuring the 220K right at each end of the resistor.  I'm not 100% sure about the measurement I was getting, but I'll check it when I get home and report back.
Thanks for your time!


The 220k is wired between the pot wiper and ground so it will not measure as 220k in circuit - rather it will measure as being in parallel with other resistance, which will vary depending on the pot rotation. At which points is the R-meter clipped when you measure 400k-ish? That doesn’t seem right.

[Dumbmonkey]

Hi Tubeswell,
I measured that 220k resistor with the reverb level all the way down across the actual resistor and got 481k.
Man, I wish I could figure out what the heck is going on here...

[tubeswell]

You should be measuring 470k||220k when the reverb level pot is cut. If something is funny with one of the coupling caps in the reverb driver circuit or the vibrato channel, you might get a funny resistance reading.

[Dumbmonkey]

Tubeswell,
So 480k with the reverb cut is ok then? If so, then that section seems good.
Any other troubleshooting ideas?
I’m starting to think I’ve gotten too far over my head with this…

[tubeswell]

No it should be 470k||220k with the pot cut (as this puts the resistors in parallel). However, if there is a leak in say the coupling cap after the second stage of the Vibrato channel, this could put other resistors (the 100k plate resistor on the 2nd stage in the vibrato channel and the 3m3) into the equation. Or if the 500pF cap going to the reverb driver was leaking DC, this could put the 3m3 and the 1M in the equation.

[SEL49]

Maybe it's as simple as the 220K is open or has a bad connection. I would disconnect one end of the 220K to measure its resistance.

[Dumbmonkey]

Hi,
The 220k in question is one that I replaced.  I did remove it and double check it and it does measure correctly out of circuit.  I'm assuming that something else is amiss there. That right in an area where I replaced cathode bypass caps and resistors, so I'm going to remove those and clean and re install those to make sure they are correct as well.

[tubeswell]

Test the 20nF coupling cap in the vibrato channel for DC leakage.

[tubeswell]

See Fig 4-4 on P57

[Dumbmonkey]

Tubeswell,
Thank you so very much!!! That will be so very helpful!