66 Vibrolux No sound but reverb squeal

[TerryD]

I have got the pots cleaned out.  The tube sockets cleaned out with DeOxit.  The outside of the amp is clean as a whistle.  The speakers worked with another amp and sound great.  Have newer tubes in the sockets for now.
I get no sound from the amp.  Except when I turn up the reverb pot.  Then it squeals like a stuck (insert whichever political party that would incline you to reply).
I don't know what to do about the reverb squeal. 
From what I've read here, my next step is to change all electrolytic capacitors top and bottom.
Sound like a plan??
 
Thanks, Terry

[eleventeen]

Well, if you have not done that....you probably have an appointment with the cap man. But if the amp is not blowing fuses right now, that move is not called for' except I understand you have a 100% commitment to get the amp going. Otherwise, I would be checking voltages....plates...then cathodes.


What I am saying is, sooner or later, you'll almost certainly want to to change those caps.....but "reverb squeal" is not a pointer to "changing out the e-caps".


I must confess I have somewhat lost track of what's been done and what hasn't on this amp.


Does rattling the amp cause the reverb "crash" sound...?


Are you pretty sure some goofy mod or prior incomplete repair isn't killing your output?

[TerryD]

Yes I get the reverb crash.  I wasn't implying that the reverb squeal would lead to the cap change, In my last post, the advice was to get it running, I should do the least invasive work (cleaning pots and tube sockets) first and then on to caps, if the cleaning didn't work, to get it running.  I'm not sure how or for that matter why to check voltages, plates and cathodes.

[eleventeen]

"I'm not sure how or for that matter why to check voltages, plates and cathodes."

Ummmm.....voltmeter?

You check those because the schematic diagrams available here, in Doug's schematic section, show what those voltages should be and no way can the amp work without those voltage levels being in the ballpark, say +/-15% of spec.

There's only so much you or anyone else can do swapping tubes around.

Your reverb crashes, so you have a reasonable suspicion that the amp works from that point onward...and that's maybe 5/8 of the total amp. You get no (other) sound out of the amp so the fault is likely to reside in the preamp section, and that is logical...if that section that handles the little signal isn't putting out anything, then there's nothing to drive the output section which NEEDS the preamp to amplify the raw guitar signal (millivolts/fractions of 1 volt) into ultimately tens of volts to drive the output tubes. Likewise, that preamp section is needed to drive the reverb circuit whose output works (preliminarily) but isn't getting drive. So nothing gets through. While that could be one of the RCA cables connecting the reverb tank, you'd still get a dry signal even with the reverb tank shorted out or disconnected or anything else. So the finger of doom is pointing to your preamp section.

With all due respect, I'm torn as to whether to advise you further...if you don't know how or why to use a voltmeter then you really and truly ought not be playing around for your early learning experience in something that has 450 volts running around in it....because it could kill you.

[TerryD]

I've checked voltages before with Dan Torres book.  My question is what to check and what does that tell me.
I found something!
  A Spark. It's happening in a part I've never seen before in my Fender past.  I think it might be a tremolo optoisolator by looking at a part in Hoffman's pictures.  That also would explain why I've never seen it before (tremolo).  It has two leads into some rubber casing and two leads out.  It sparks in there when I turn on the stand by switch.  Should that spark?  Could that be the villain?  On my Layout AA964 it's over toward the phase inverter section and has the 10M resister and 100k resister running into this oddball looking thing and then two leads out going to who knows where.

[eleventeen]

No, that's the tremolo "roach" which has a light bulb shining on a photoresistor. It's way of isolating circuits: That which drives the light bulb does not affect, other than the light from the bulb, whatever takes the signal from the photoresistor. Sometimes you can see the bulb flash inside the shrink-tubing. Forget the tremolo for now. It flashes when you come out of standby because that releases the "big power" into the amp. Still, forget it for now.


Edit: Are you saying spark as in open flame type spark? Or the flash of a light bulb? Makes an odor? << If you are getting an open-flame type spark, obviously something is wrong, but even so, my advice would be to cut (or unsolder) the lead going to the light bulb half of the roach and get the preamp working.

Your focus should be on the first two little tubes in the amp. The plates of those tubes are pins 1 and 6. Those pins should all have right about 200 volts on them, measured relative to ground. That could be maybe 175-220 volts with normal slop. The pin 1 terminals are the first halves of the tube. The pin 6's are the second halves of the tube. The one in the corner of the chassis is the first tube in the "normal" channel, the next one over is the first tube in the reverb channel. Now you have to figure out if one or both is working which requires that they get their volts.


The cathodes are pins 3 and 8. The cathodes of V1 and V2 should all have about 1.5 volts relative to ground. I said the plate (pins 1 and 6) could vary quite a bit and still be "OK". The cathode volts can't. They can't be zero, they have to be pretty close to that 1.5 volt number.


You have the output section apparently capable of making sound because the reverb crash comes through. But the preamp signal isn't getting through or isn't being created in the first place. If that is true, then it's likely that neither tube is getting power. That would be a tad rare but in a 50 year old amp, anything is possible.


If those voltages are good, then you move farther down the line, checking plate voltages in the same way. Take note of the reverb driver tube, that has big volts (425, not 200) on its plate. Your job is to chase down the problem like cornering a rat and then you have to squash it.


Somewhere, your signal is disappearing or not being made in the first place. A broken wire or bad solder joint can break the connection between successive stages. A bad solder joint can do the same.


But for the most part, the initial stages of troubleshooting, and by that I mean the point where you take out your meter and start poking, usually starts with checking plate voltages.

[TerryD]

Thanks a million.  I'll work through this and see if I come up with anything.
probably just the light in the roach...it's quite a flash though.

[TerryD]

Eleventeen, hope your still out there in cyberspace.  I have the voltages off of my preamp tubes.  I'm now putting them up so that hopefully someone can tell me what to do with this information.  Hopefully what parts I need to change.
I took these numbers at two separate times so I'm quite sure they are accurate.  The strangest thing happened with the cathodes.  I got readings when the amp was off on some on some of them. Also I've had high input voltages in my house before, but it seems strange to me that I get really high input voltages only on V1 & 2.   
I hope my format of communicating these below is intelligible.  If not, let me know.
 
Tube & pins                           layout/schematic volt number                                          numbers I actually got
V1
pin 1                                       170                                                                                       200
pin2                                        180                                                                                       292 (wow!)
pin 3                                       1.2                                                                                        1.41
pin 8                                        1.3                                                                                       .78 amp off/amp on    10.  blank
 
V2
pin 1                                      160                                                                                         189
pin 2                                      180                                                                                          292 (ouch)
pin 3                                      1.2                                                                                            1.54
pin 8                                       1.3                                                                                          .079 amp off/ amp on 1. Blank!
 
V3
pin 1                                     400                                                                                              453
pin 3 & 8 connected             8.2                                                                                              .07 amp off/amp on 10.blank!
 
V4                 
pin 1                                     175                                                                                                211
pin 6                                     170                                                                                                 210
pin 3 & 8 connected             1.3                                                                                                  1.61
 
V5
pin 1                                     350                                                                                                  443
pin 6                                      405                                                                                                 391
pin 3                                      2.0                                                                                                  0
pin 6                                      6.5                                                                                                  .01
 
V6
pin 1                                    160                                                                                                  180
pin 6                                    160                                                                                                   197
pin 3&8 connected               7.2                                                                                                  10. Blank!

[HotBluePlates]

...  The strangest thing happened with the cathodes.  I got readings when the amp was off on some on some of them. Also I've had high input voltages in my house before, but it seems strange to me that I get really high input voltages only on V1 & 2.   
 
Tube & pins                           layout/schematic volt number                                          numbers I actually got
V1
pin2                                        180                                                                                       292 (wow!)
pin 8                                        1.3                                                                                       .78 amp off/amp on    10.  blank
 
V2
pin 2                                      180                                                                                          292 (ouch)
pin 8                                       1.3                                                                                          .079 amp off/ amp on 1. Blank!
 
V3
pin 1                                     400                                                                                              453
pin 3 & 8 connected             8.2                                                                                              .07 amp off/amp on 10.blank!

I think where you have "Pin 2" written above that you really meant "Pin 6"? Pin 2 is a grid for all 12A_7 types, and should be zero.


Also, you should measure d.c. voltage with the amp on but the Volume control on 0. No benefit is gained by turning the volume up (and it may confuse other readings).


For all pin 8 connections which read low (essentially zero), you should check that the tube
1. Has the heater lighting up
2. Has good firm contact in the socket (not so loose the tubes want to fall out)
3. Has continuity from the tube socket pin to the cathode resistor and from resistor to ground; Verify by measuring esistance with the amp off from the tube socket pin to ground (it should match the value of the cathode resistor).


When I was in the Navy, they called the condition I think you're seeing "Balloon/Rock". You have a high supply voltage from the B+, which connects through a plate load resistor to the tube plate. The tube cathode has a resistor which connects it to ground. If the tube draws current, there is a voltage drop created across each of these resistors.


On the cathode side, you read that voltage as a voltage at the cathode which is increased above 0v (ground voltage) by the voltage across the cathode resistor. In many Fender amp stages, this will be something like 1-2vdc. On the plate side, the voltage drop across the plate load resistor is apparent because the voltage on the plate pin itself is lower than the supply voltage by the amount of the voltage across the resistor.


If the tube is not passing current, the voltage drops across the resistors don't happen (Ohm's Law: Voltage = Current x Resistance; no current, no voltage across the resistor). The cathode then drops, like a rock, to ground voltage (0v) while the plate elevates like a balloon to the supply voltage.


So where you had odd readings, it looks like your tube isn't passing current. The connection to the tube plate looks okay (because there is voltage there), so now you need to check that the cathode has good connection to ground through the cathode resistors.


The outlier possibility is that the heaters aren't lighting up (the tube won't pass curent if the heaters don't work) due to a wiring problem or an actual open heater in the tube.

[6G6]

V1A, V2A and V4 seem to be OK.
Check for broken solder at pin 8 on the ones not reading 1.5-2.

[TerryD]

You are right I meant pin 1.  Thanks for the info.  Guys let me work on this info and I'll get back with any new info...I'll have to check with this new info.  Thanks again.  This means a lot.  Terry

[eleventeen]

You've gotten a "two sides of the same coin" kind of explanation of what (it seems) like you have going on.


Preamp tubes. We seem to be zooming in on those as our culprits.


Theoretical: A tube is working when it is passing current. When it is passing current, there will be a voltage drop -across- both the plate resistor and the cathode resistor. Conversely, when it is NOT working, there will be no or almost no voltage drop across said resistors.


So in your preamp(s) [there's two channels, right?] you have these two tubes, both are dual triodes, that's like having four separate tubes. ALL of them get their plate volts from the same place [node "D" on your schemo] and ALL of them have those 100K plate resistors, physically arranged in that unmistakable "V" shape on your parts board. All those tube sections work on about the same volts. The "hinge" part of the "vee" should be about 300 volts, and the two legs (of both) should be about 200 volts. Each one of those 100K resistors should have about 100 volts across them.....leaving about 200 volts for the tube plates. All the cathode resistors have in the neighborhood of 1 volt across them. Any big variation on that theme is nonworkingness.


[by the way this is a straight ahead illustration of ohm's law: The 100K plate resistor is about 100 times as large as the cathode resistors of 1500 or technically, 820 ohms, and when the tube is working, 100 times as many volts drop across the plate resistors as across the cathode resistors. 100 volts across the plate R's, 1 volt across the cathode R's. Yes?] 


Physical: The "hinge" or common point of those "Vees" should all measure the same volts, about 300.  Looks like they do, or will. The "legs" of those "vees" are...V1 plate A, V1 plate B, V2 plate A, V2 plate B. Forget for the moment measuring volts at the tube socket. Measure right on the "leg" side, the far side of those 100K resistors. Preliminarily, any of them that measure 200 volts shows a working, conducting tube. Any of those that show 300 volts shows a nonworking tube. Period!


It would not surprise me terribly to find an open or impossibly flaky 100K plate resistor in that 50-year old collection, but it could also be a bad tube pin contact, a broken wire, a hinky solder joint or a bad tube.


But you absolutely know: Where you have 300 volts on your V1 or V2 plates (and remember any voltage measurement can *easily* be +/- 10%) you have a nonworking tube.


Most techs (myself included) would heartily recommend you replace in a 50 year old V-lux ALL those 100K plate resistors with the 3 watt metal film ones Doug sells, or equivalent. You could also wait until the amp is working with the present resistors, THEN replace those 100K's....it's usually stunning how much hiss and whoosh and crap noise will go away: It is IMO by far the cheapest thing bearing the greatest potential improvement you can do to an old Fender. I strongly reco you buy those next parts buy. (there are a few more than just these four, do 'em all for 30 cents each)

EL34 edit:
Guys, please learn how to crop your images to make them smaller
There's no need to have that much wasted space
This is what the image above looks like when it was cropped

[eleventeen]

Incidentally, you should take a small screwdriver blade and push against the sides of those 100K resistors lightly. Power off, this is strictly a mechanical item although of course it will affect amp performance. Occasionally those resistors can crack in half, completely invisibly, and they look fine. But the cracked internal carbon element is making only the slightest contact with its other half. This will really fool ya...there's enough conduction across the cracked resistor to drive your voltmeter, but not enough to let the preamp ckt work. Sometimes the resistor is cracked and you'll just push the halves apart when you nudge it. 


On future Fender repairs where you suspect any preamp goofyness, you should go to those "V" resistor formations right away, measure the hinge and measure the legs, ~~300-200-200 volts wham bam.


 

[HotBluePlates]

Incidentally, you should take a small screwdriver blade and push against the sides of those 100K resistors lightly. Power off, this is strictly a mechanical item although of course it will affect amp performance. Occasionally those resistors can crack in half, completely invisibly, and they look fine. But the cracked internal carbon element is making only the slightest contact with its other half.

He's got voltage at the plate pins of the sockets, so 99.9% chance the plate load resistors are working fine. But he almost certainly has no plate current; cathode connections, grounds or unlit heaters seem the most likely fault.

[TerryD]

I'll be working on this info.  Could anyone tell me how to check the heaters.  How to physically do that?  I know they are the green twisted wires to all tubes.

[Willabe]

Measure them for voltage from green to green and not 1 green wire to ground. Heater voltages can be +/- 10% and will be fine.


            Brad     

[TerryD]

Okay!  I think I'm getting....somewhere?  My heater DC volts are better than good.   V3/reverb cathode 3 &8 to resister is good but resister to ground is 1.  Also V5 pin 3 to resister is good but to ground is 1. pin 8 to resister is good but resister to ground is 1.
I'm hoping and hoping these are the culprits and only culprits.  So go a head and replace these resister to see what gives?

[eleventeen]

"Could anyone tell me how to check the heaters."

First, check to see they are lit up!  So far, in my recos as to how to check plate voltages and what those might mean, I have definitely assumed you have observed the heaters lit up! If they're not lit up, there ain't no [working] tube there.

In terms of voltmeter, (my practice is to) start with the farthest tube from the power transformer, V1. Whatever heater volts are getting (or not) to that tube have to travel the greatest amount of wire and the largest number of solder joints.  Green to green should be a tad over 6 volts, the spec being 6.3 volts. 4.5 or even 5 volts is too low even if the heaters are lit up, something's not right. Every tube should read the same amount of volts, green-green wires, running up in the air over the tube sockets.

[eleventeen]

" V3/reverb cathode 3 &8 to resister is good but resister to ground is 1.  Also V5 pin 3 to resister is good but to ground is 1. pin 8 to resister is good but resister to ground is 1."

No! >>There is no pin on ANY little tube that connects to ground! There are pins that may show "zero volts" but NONE of them connect to "ground".

The little tubes...all of them...should have 6.3 volts (+/- not very much) >>>AC<<< (remember all your prior measurements have been DC! The heaters run on AC, right out of the transformer, so you have to change the range of your meter) between pin 9 and the joined pins 4 & 5. All the same, all should have 4 & 5 joined together (that's where one green wire goes---to BOTH of those pins, soldered together) and the other green wire goes to pin 9. This is true for all the little tubes. Just follow the green wires and measure those pins where those greens connect.  Most likely, if the first small tube has good heater volts, you are OK.

[TerryD]

Crap!  I decided to change three cathode "bypass?" resisters because I wasn't getting a reading.  I was getting all the volt readings just fine but on these resisters sometimes I get readings and sometimes not.  It seems like the resisters were fine..I think.  What should I do next?

[eleventeen]

"I decided to change three cathode "bypass?" resisters because I wasn't getting a reading."

Pay closer attention...those resistors pass ~1 milliamp at 1.5 volts and thus dissipate .0015 watts which is practically zero. The chances of them being bad is also practically zero. (The plate resistors are quite the different story: There we have 100 volts of drop and as per prior posts, 100 times as much power or .1 watt or maybe .2 watts on a bad day. Now that's well under half a watt [their power rating] but long experience has shown that the thermal noise carbon resistors create is ugly.

Now the CAPS placed in parallel with those cathode resistors...those DO go bad in a 50 y/o amp. On an electrolytic capacitor shotgun run through an old amp, many people change all of those out.

What is far more important is: Why do you think you are sometimes getting readings and sometimes not? Presumably you are measuring either on tube pins 3 or 8 of any of the little tubes OR on the parts board where those "duos" sit. Are the solder joints corroded and your meter probe isn't making good contact?
If your meter is >>NOT<< giving you good readings, you're going to mislead yourself very badly. Very.

One of the elements here is certainty. I know you are newish to this and so it seems maybe out of reach. You're going to have to manage how hard you poke at stuff with your meter probes and get a better idea of what your "eyes" are telling you. If I've expressed that clumsily, I apologize.

[HotBluePlates]

I was getting all the volt readings just fine but on these resisters sometimes I get readings and sometimes not.

You need to ask a question only you can answer: Did you get readings only sometimes because there was an intermittent connection, or did you only get readings sometimes because you didn't have good contact with the resistor from your meter leads?


I'd probably recommend getting the measurement right at the tube socket pin or with the meter lead against the leg of the resistor. You'd be surprised how sometimes your meter won't want to register a reading when you poke the probe tip against a solder joint.

[TerryD]

Okay  Please hang in there.  I checked the voltage on the plate resisters and found two that were nasty on the tube end.  Both read 300.  I changed those with new 100ks for now just to get things happening and I get the same 300 reading and of coarse no sound.  Thanks, Terry

[Willabe]

Well 11teen (maybe HBP too?) explained this to you. If you have no dc voltage drop across the plate R then that tube is not drawing any current.

Could be the heaters are not lit or it could be the cathode R is not soldered up correctly, either at the tube pins (for 12 _ _ 7, pins 3/8) or it's ground connection.

You can try this, set your meter for resistance and with the amp off and unplugged from the wall and the B+ filter caps drained, clip 1 lead to pin 3 the other lead to chassis ground, it should read very close to the value of the cathode (K) R. Then try checking pin 8 the same way.

What do you get? 


                Brad     

[Willabe]

You never did say that your getting 6.3 AC on the heaters. (you were measuring for DC?)


              Brad   

[HotBluePlates]

You never did say that your getting 6.3 AC on the heaters. (you were measuring for DC?)

Do not measure voltage.

Turn the amp on. Turn off the lights. Look at the tube envelope. Is there stuff glowing orange?


There is an outside chance of measuring heater volts, but the heater in the tube not lighting up (and several ways for that to happen). Make life easy and just look to see the tube is lighting.

[TerryD]

I seem to get some glow from them all, more or less, but nothing from V5. 

[HotBluePlates]

You can pull out V5 and measure ohms from pin 9 to pin 4, and from pin 9 to pin 5. Infinity ohms would be bad (tube has internal open heater).


I'd bet money the socket itself is wired correctly (unless someone tinkered). If tube measures okay, but tube doesn't light, try a different tube (to rule out odd structure where you can't see heater readily, or odd-diameter pins). Glow? Maybe you can use the old tube in a different amp or different socket. No Glow? Maybe the socket pins are dirty or loose.


A break in the green heater wires seems impossible because V1-V4 sockets get fed from V5.


If readings don't start making sense (no way to get cathode volts), then I'd have to see the amp in front of me... Does your meter auto-range voltage? If not, be sure you have your meter set to the lowest range above the expected voltage. That might be a 2v or 4v range on your meter (maybe I should ask what meter you are using).

[TerryD]

I swapped tubes and now I see a glow in all the tubes.  Maybe I should just change all those cathode caps and filter caps I'll need to do that anyways and maybe that would do the trick.  I feel kind of lost now.  I will reread all the info I got here so far.  Maybe I don't have all my ducks in a row quite yet. I'm sure my meter is ok as I read all the cathodes at the same time.

[HotBluePlates]

Caps won't make a difference.


Try re-measuring all the d.c. voltages. Maybe you just didn't get good contact before.

[Willabe]

Turn the amp on. Turn off the lights. Look at the tube envelope. Is there stuff glowing orange?

Make life easy and just look to see the tube is lighting.

Doh! Your right why complicate a simple thing.


             Brad   

[6G6]

Try this; on each cathode resistor set your meter on resistance and check from the end away from the tube to the chassis.
If it does not read zero, you don't have a ground connection.
If it does read zero, then maybe the resistor is cracked or there is a bad solder joint.
You can try reflowing them if there is a question about the solder joint.
That won't take long and will cost nothing.
 

[plexi50]

I cant really interpit your voltages from the chart text you posted. Pull the power and preamp tubes. Turn on amp and keep standby off. Check the voltages off each of the power transformers red leads going to your rectifier tube in the A/C setting on your DVM. What  are the voltages of each lead?


More after you reply/

[TerryD]

I can't believe it!  I got it running.   Thanks for all the help and patience.  I read and reread all the posts till I had a little more "certainty" as to what the heck I'm doing.  6G6's comment at the end put the nail in the coffin.  All it was was a ground wire that looked connected.  I never trouble shot before.  That was fun.  Thanks again. I used it all.

Now, I still have the reverb squeal.  How do I deal with that?

Also, How big of an iron do you need to sodder those ground connections to the chassis.  I had to makeshift the ground to a nearby ground wire because my 30 watt would not put a dent in the chassis sodder joint.

[HotBluePlates]

Also, How big of an iron do you need to sodder those ground connections to the chassis.  I had to makeshift the ground to a nearby ground wire because my 30 watt would not put a dent in the chassis sodder joint.

100w+ solder gun. The bigger, the better. A smaller iron may even melt solder and make a connection that looks good, but if you hit the joint with a screwdriver (used like a chisel) it will pop right off the chassis. I know because I've done it before. 


If the makeshift ground is working, then just use that.

Now, I still have the reverb squeal.  How do I deal with that?

Unplug the send wire from the chassis to the input of the reverb tank (marked "Reverb Out" on the chassis). Play the amp at good volume with the output of the tank still connected back to the chassis. Do you still get squeal?

[TerryD]

Actually I didn't even have the reverb tank connected because I have my chassis on a little work stand with an extension wire to the speakers.  I thought it would be just "not in service" when not connected.  I plugged the reverb tank in and it works perfectly fine...no squeal at all.  That make sense?

Man this is great.
 This thing sounds so good, I could die.

Finally:

I think I should clean up the sodder joints.  A few I messed with had big globs of sodder that just went down into the board.  The sodder joints should be freshened up...right?  Whats the best way to do that?

Replacing the cardboard Mallory electrolytic caps on the cathods and the filter caps and the resisters to the plates would be minimalist and still a good thing to do..right?         
- If the amp was used very little could the caps still be good??

Thanks,
Terry

[HotBluePlates]

Actually I didn't even have the reverb tank connected because I have my chassis on a little work stand with an extension wire to the speakers.  I thought it would be just "not in service" when not connected.  I plugged the reverb tank in and it works perfectly fine...no squeal at all.  That make sense?

Cool. Then don't play with the reverb unplugged. 

I think I should clean up the sodder joints.  A few I messed with had big globs of sodder that just went down into the board.

Don't worry about it. There is an insulator board under the main board, so they're not hurting anything.

Replacing the cardboard Mallory electrolytic caps on the cathods and the filter caps and the resisters to the plates would be minimalist and still a good thing to do..right? 

If you don't have hum and the filter caps haven't burst, I'd leave them alone. Replacing the filter caps and bias cap would probably be fine if you wanted to do preventive maintenance.

The cathode bypass caps have probably dried up some and reduced in value. That would shave bass a little, which interestingly most people do when they build a similar circuit for high-volume playing. So if the amp doesn't sound unbearably thin or weak-sounding, I'd leave it be.