1966 Blackface Vibrochamp rescue

[jeff_free69]

Greetings Gang,

This 1966 Blackface Vibro Champ  followed me home a while back.  Finally found a little time to start the rescue operation, so I opened her up.   

The good news is that it appears to be 100% original and unmolested.

The bad news is that the interior has significant rust on the chassis  (my guess is that before getting kicked to the curb it was relegated to a garage with a leaky roof;  could be that water splashed on top and  infiltrated through the chassis bolts and also down the back etc.)

It seems these modest amps are fairly highly valued, so I think its worth it to invest some time  and resources to restore.   I'm loving how relatively simple the circuit is (Several years ago I restored a 72 SF Twin Reverb and lived to tell the tale, with a lot of handholding from this forum)

For starters I'm planning to do the basics:

• 3 prong cord , death cap  etc
• Replace the Filter cap can
• Replace the 4 onboard Electrolytics

It also seems that these amps were biased very hot, at least for more modern line voltages (mine is a constant 121VAC).    So I'd like to get your opinions on how to handle that.

Otherwise I'll keep it as vintage as possible unless there's some compelling improvements to be recommended.  (For example, are 20/20/20 uF filters caps still considered the best way to go?  I've seen variations that increase some of the values)

More photos with all the gory details to follow...

[Platefire]

If it was mine, I would want to know it the PT and OT lives. Everything else can be cleaned up and fixed

[AlNewman]

Once you complete the things on your list, measure voltages and adjust bias (or B+) to suit.

[Latole]

1- Check if the fuse is the right one.
2- Caps in a can. You can go for JJ brand  20 X 3 at 500 volts . Check for diameter of amp hole to clear the connectors

[Latole]

Can Capacitors:   CP Manufacturing brand are a good choice too . More looking like the original one

https://www.tubesandmore.com/products/capacitors?filters=3109a3124c112a3109

[jeff_free69]

Thanks for the tips. 
I've started the initial cleanup.  What a mess!  Here's some of the before shots.

[Latole]

Thank' for the pictures.

Chassis need a good cleaning
I'll try with electronic cleaner and a brush, more harder than soft

I is a good amp.

[jeff_free69]

more, before...

tha capcan is General Electric 4x20uF

Tubes are RCA;  12AX7's are Mullard

[jeff_free69]

After first round of cleaning
WD40 on the chassis rust;  Isopropyl on the circuit board

[Latole]

more, before...

tha capcan is General Electric 4x20uF
 

Amp use only 3

[mresistor]

Can Capacitors:   CP Manufacturing brand are a good choice too . More looking like the original one

https://www.tubesandmore.com/products/capacitors?filters=3109a3124c112a3109

That's interesting   I don't think I've seen a GE brand cap can in any of my original Champ/VCs. Seen a bunch of Mallorys though.

The CE Manufacturing don't just look like the originals, they are made on the same equipment as the original mallory's. If the amp were mine a CE Man. cap can is all I would use. 

https://www.tubesandmore.com/products/capacitors?filters=3109a3124c112a3109c2341a2356

[acheld]

This amp looks great compared to many.   I don't see mouse droppings . . .

Don't overdo the cleaning.   You want this amp to work well, but have the patina.   Don't replace stuff that doesn't need replacing.  The more original it is, the more it is worth.

Platefire has got the priorities right (as always).

[dude]

As far as biasing, these amps ran hot from the factory, upping the bias 470 ohm resistor will lower the bias but might not be the best option. Hot Blues Plate and many other recommend leaving the 470 ohm and swapping the first two dropping resistors, putting the 10K first to the screen and the 1K to the pre-amp. This change will lower the screen voltage and cool the bias a bit. They say these amps run best at a dissipation of 14/15 watts, hot, if you raise the bias R, the word is it kills the tone.  There's a video on this change that was posted here a short while back, do a search, good info on the change.
You could use a 5K first then 5K to pre-amp, but I would go no lower than 5K first dropping R, a 15K to pre-amp would give the amp more of a brown tone, experiment. These are fantastic amps. IMO, the OT is too small, over-saturates too much. I used an old paper wound SE 10W OT that fit the holes and loved the change, and a 10" alnico helped the tone too. Of course you don't want to butcher the the amp, try an extension speaker. Nice amp.   

[jeff_free69]

Thanks for all the great info, this is a great starting point.  Touching on some comments..

FUSE: confirmed the fuse is is 1A sloblo

~~~~~~~~~~~
TRANSFORMERS
The overall condition may not be  too bad cosmetically , but the evidence is that there was a significant amount of water dripping in.  eg the screws holding down the circuit boards are rusted. That has me wondering about the transformers,  as suggested by Platefire

1) What's the recommended way to investigate this, before investing in a full order  parts?
 Should I just pull the tubes and take basic measurements?  I have a light bulb current limiter.

2) If they aren't good, are there replacements readily available?  (in a quick search I haven't seen anything close )

~~~~~~~~~~
BIAS
Not sure if its the same video referenced by   dude  , but I have seen one  that did the 1k - 10K swap instead of just bumping up the 470R.
Said it had more stable operating voltages and was similar to how it is done on Tweeds.
I will definitely have a look when I get to that point

~~~~~~~~~~~~
FILTER CAP CAN
CE   looks like the way to go ( gotta love good ol Pete for cranking them out). 
Yes it only needs 3, but I like the versatility of 4x20 being able to double up  to reduce hum
UncleDoug put it on the first node (Rectifier), citing the fact that later versions did this
Lyle@Psionic put it on the second (6V6 screen node), to avoid stress on rectifier.
I suppose there are many opinions; its easy enough to experiment once the basics are taken care of

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
SPEAKER
If I do have to replace it, I would not hack in a 10".   I have a few extension cabs I can use to beef up the sound with a bigger spkr.    BTW are the original 8" alnico?

[Latole]

" FILTER CAP CAN
CE   looks like the way to go ( gotta love good ol Pete for cranking them out).
Yes it only needs 3, but I like the versatility of 4x20 being able to double up  to reduce hum. "
_______
I will not reduce hum . Hum is because a bad heater wiring in this amp. See below
It may help to keep sound tight at hi volume, you won't loose bass.

For no more hum ; remove one heater side from chassis ground , add a new wire twisted to the one it had.
Make a an artificial heater center tap with 2 X 100 ohms 1/2 watts each side of the  transformer heater wire. Other side of each resistor must go to chassis ground.

https://www.valvewizard.co.uk/heater.html

[bmccowan]

Cool project - a couple of comments:
I suggest getting the amp working in stock configuration before deciding to modify. Modding first makes it hard to find problems.
If you do not have a variac, I think using the light bulb limiter is the way to go. There is a start-up procedure here on the Forum - good to follow. Transformers are pretty robust, so you are likely to luck out.
In my experience, that speaker will not last if playing at loud volumes. You may want to ensure it works and replace it and save the original. I like the Webers, but WGS speakers have been suggested by many.
You mentioned Lyle@Psionic, his videos on bringing back crudded up Fender amps are great - they make me want to go back and redo some of my projects. I could never do videos as I repaired amps - too much swearing.
Heater wiring - although I think Latole is correct about best practice for these, I did exactly that on my Vibro-Champ and found it made no discernable difference. 
 

[Latole]

Cool project - a couple of comments:

I suggest getting the amp working in stock configuration before deciding to modify. Modding first makes it hard to find problems.

Very wise advice.

[mresistor]

120hz hum can be reduced substantially in these little amps by installing another power supply filter node off the rectifier.  Add a 1k 3w or so off the rectfier and use that 4th cap in the cap can for the first filter, then onward as normal.

[Latole]

I'll start to fix the bad heater's wiring with no ground at PT heater secondary.

Sure you will not ear any hum

[mresistor]

Latole   do you actually know the difference between 60 hz and 120 hz hum? 


And I have several vintage blackface Champs and Vibrochamps that have original unmolested filament wiring and they don't exhibit 60hz hum. One reason is the little 8" speaker. When you connect these little amps up to a 2x12 cab you will hear things (sometimes) that are not really audible through the 8 inch speaker.

[Latole]

Latole   do you actually know the difference between 60 hz and 120 hz hum? 


And I have several vintage blackface Champs and Vibrochamps that have original unmolested filament wiring and they don't exhibit 60hz hum. One reason is the little 8" speaker. When you connect these little amps up to a 2x12 cab you will hear things (sometimes) that are not really audible through the 8 inch speaker.

To be honest, not really.
But I do know how to get rid of all that noise.

[Latole]

I will add a grid stopper resistor to 6V6 pin 4.

[AlNewman]

I don't think it runs as hot as it may look.
Granted, there's 21V over the cathode resistor, .0446A.
Screens will drop that to around .04A
(342-21)*0.04= 12.84W
Even at modern voltages you're likely still safe.

If anything, I'd personally add a dropping resistor off the rectifier just to get it to the specs on the schematic, and then the rest of the amp falls where it should.  If you have an extra cap in your can, you could use that to make another filter there if you want.

A screen resistor may be a good idea.

[jeff_free69]

Thanks - so much great info to digest.
Of course, i will be starting with the baby steps and see where things go.

Re:  Heater wire  twisted pair / artificial center tap...
After Uncle Doug doubled the filter on the rectifier node, he then did the twisted pair conversion.    He discovered that the transformer had an unused centertap tucked away and used that.  His was an early 66, mine is a later 66 , and i think I have the same unused wire. ( In each case he demonstrated  a reduction in hum with a close-miked external 12" spkr .)
But this is above my paygrade at the moment.

I think I'm just about set do the basic 3 prong conversion and to start the power up.

Couple of more shots :
The pilot light, showing more corrosion  (a little blurry , but there's some rust on the base) 
My fear is that the pots have the same damage internally.  BTW there's something odd about the Volume and Bass pots.  Any ideas about what's going on here?

[tubeswell]

I’d pull the tubes and test the transformers and speaker first. If those work, then maybe replace the cap can and see what voltages you get. Avoid changing anything else if it works (except to ensure it has good tubes). Don’t bother cleaning away the patina. The more stock it is, the more $ it will fetch

[bmccowan]

Don’t bother cleaning away the patina. The more stock it is, the more $ it will fetch

I agree on the stock components. But I do not think cleaning the amp hurts the value of the amp at all, unless you take abrasives to it. If you want to play it - and who does not want to play a Blackface Fender, cleaning the circuit board may be necessary to get it sounding good. Stray voltage tends to leak across a cruddy eyelet board.
Your best approach depends on your intentions - fix and sell or fix, play and maybe sell later. If the later, I'd avoid destructive mods and save any parts that were changed. I'm always in the play-it camp - that's what it was made for. Of course I have a vintage Alfa Romeo that I drive. Am I hurting the value - probably. Do I care? Not a bit - guitars, amps, and cars are to be enjoyed. And now a little voice is telling me to step off the soapbox.

[Latole]

Tubes socket 's contacts could be rusty too.
Follow last Tubeswell advice ; check PT

[mresistor]

.  BTW there's something odd about the Volume and Bass pots.  Any ideas about what's going on here?

Are you talking about the plastic center shafts protruding from the backs?  Any markings on the backs of those pots? I haven't seem many like that before.
Or is it something else?
Pots should be cleaned and lubed with some de-oxit spray and worked back and forth a bit.  The brass plate looks really clean.

[jeff_free69]

.  BTW there's something odd about the Volume and Bass pots.  Any ideas about what's going on here?

Are you talking about the plastic center shafts protruding from the backs?  Any markings on the backs of those pots? I haven't seem many like that before.
Or is it something else?
Pots should be cleaned and lubed with some de-oxit spray and worked back and forth a bit.  The brass plate looks really clean.

Yes, those.  I'll see if can get a close up next time.   Haven't Deoxited yet, but they are super stiff.  And the power switch is completely shot.  So i suspect that, like the pilot light, there may be internal rust on these components

[HotBluePlates]

" FILTER CAP CAN
CE   looks like the way to go ( gotta love good ol Pete for cranking them out).
Yes it only needs 3, but I like the versatility of 4x20 being able to double up  to reduce hum. "
_______
I will not reduce hum . Hum is because a bad heater wiring in this amp.

Fender made a step-backward in engineering in the late-50s 5F1 Champ, and all Champ/Vibro Champ amps afterward.  They connected the single-ended output transformer to the main reservoir cap at the rectifier output.  There is significant ripple at this cap, that ripple gets impressed across the OT primary, and results in hum at the speaker.

The 5E1 Champ (and every earlier Champ amp) fed the 6V6 screen and output transformer from the same cap:  the 2nd cap of a C-R-C or C-L-C filter.  The result is substantially reduced ripple at the 2nd cap, less ripple impressed across the OT primary, and less hum in the speaker.

"Going backward" to the good 5E1 power supply (even if using a 470Ω resistor instead of a choke) has been standard practice on this forum for a good 15 years or so.

[dude]

Schematic of added for Vibro champ with 4 E cap can.  But I have a question with about the screen supply, why is the screen voltage X, taken from the pre-amp supply (X & A) and not at point A..? It's the same voltage at both points, why isn't the (X & A) near the power supply? Is there a reason for getting voltage down the board.

[jeff_free69]

Sweet! will definitely put this on the top of the list once the basics are taken care of.   (plus your copy of the layout is so much clearer than the one i have - I can actually read the values.  Mine must be a 100th generation copy from  a mimeograph machine)

I think I get the idea - very clever.   looks like the tricky part will be fishing out the wire from the rectifier to the gopher hole on the board, so it can be moved to the extra cap element.   


  got a little vacation time coming (gotta make sure i don't miss all the sweltering heat).  So in a few days I should be able get moving ...

[HotBluePlates]

Schematic of added for Vibro champ with 4 E cap can.  ...

I drew that schematic & posted it on another forum.  It is merely Fender's original Vibro Champ schematic with an additional filter-section and an added resistor.  These "added components" are placed before the existing caps/connections.

... But I have a question with about the screen supply, why is the screen voltage X, taken from ...

Look again:  "X" is at Pin 5 which is the control grid (G1), not the screen grid (G2) which is at Pin 4.

   - "X" connects to a 0.02µF cap and 220kΩ resistor.  There is no power supply connection here.

   - "A" connects from an eyelet in the power supply, then passes through a hole (that "X" also passes through), and connects to the eyelet that the 100kΩ plate resistors share.  This IS a power supply connection, from the last filter cap to the 12AX7 plate loads.

[jeff_free69]

I drew that schematic & posted it on another forum.  It is merely Fender's original Vibro Champ schematic with an additional filter-section and an added resistor.  These "added components" are placed before the existing caps/connections.

Great!  I believe it was mentioned  that the resistor value between the 20uF elements should be 470 ohms, correct?

[mresistor]

470 ohm or 1k ohm are acceptable   HBP did mention 470 ohm above.

[jeff_free69]

I did the powerup and basic measurements, and I'm quite relieved to report there was no smoke or flames.

The power switch is shot, so I had to modify my light current limiter rig to add one there, and build a little test jig.
Turns out the 1A SB fuse was no good either (guess what I didn't have and had to buy.)

Here's what i did / measured.  All are done with the current limiter and 4 ohm dummy load
Some measured voltages were way higher than spec  and some way lower   (I assume the limiter has an effect )


1)  Removed all tubes
PT secondary:   366 VAC
Lamp/heaters:  7.2 VAC
~~~~~~~~~~~~~~~~~~~~~~

2) Installed Rectifier
PT secondary:   347 VAC
Rectifier p8:      458  VDC
Filter caps:        460/468/470  VDC
Lamp/heaters:  6.6  VAC
~~~~~~~~~~~~~~~~~~~~~~~~~


3) Install ALL tubes
Rectifier p8:      210  VDC
Filter caps:        183/194/205   VDC
Lamp/heaters:   4.9  VAC

6V6
p3     188V
p4     195V
p8     0V

12AX7 Vibrato (on)
p6   195V
p1,2,7,8  varying

12AX7 preamp
p1   131V
p3   1.0V
p6   129V
p8   0.9

See anything alarming, before I go ahead and retest without the current limiter?  (I'm hoping to see voltages closer to the spec).
Or should i just go ahead and order the   electrolytics etc now ?

Please let me know your thoughts and any other suggestions for next steps...

[bmccowan]

Assuming the limiter behaved as expected, yes I would test without it and record voltages.
I also suggest that you go ahead and order and replace the electrolytics.
And if you don't already have them - two varieties of Deoxit will help those pots - D5 for heavy duty cleaning and F5 to lubricate them after cleaning. A lot of folks just use the D5, but I am a sucker for emptying my wallet. Actually I think the F5 helps the pots stay smooth longer.

[Calboy]

Post #35 shows pin8 cathode voltage at 0v with the 6v6 plugged in with the current limiter.
Did the current limiter show any shorts or did it pass initial start up?
I would check the pin8 connections.
Could be bad 6v6 tube.
Even with current limiter I would expect some pin8 voltage unless the bulb wattage is really high.

[jeff_free69]

Post #35 shows pin8 cathode voltage at 0v with the 6v6 plugged in with the current limiter.
Did the current limiter show any shorts or did it pass initial start up?
Could be bad 6v6 tube  Even with current limiter I would expect some pin8 voltage unless the bulb wattage is really high.

OK  - back to work

To get restarted I double checked my measurements.  The bulb was glowing dimly; (I'm not experienced enough to know for sure how much is acceptable.  The bulb   is 100W.     I measured the voltage at the fuse at 86 VAC.

I don't have a big stock pile of spare tubes, but i did happen to have an old Fender 6V6 GTA, of unknown provenance.  It also showed 0V on p8, and no significant change on any of the other pins.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
I then retested without the Current Limiter.   While the voltage coming out of  the transformer  looks reasonable, everything else still looks way too low ( based on the AA764 schematic)
         
                 Expected  /   Measured (no  current limiter)

5Y3  p4,6 red   315 AC  /   338 AC
       p2,8 yel    355 DC  /   305 DC


FILTER CAPS       
                      320  /   253
                 340  /   282
                 355  /   305

6V6     
     p3        342  /   266
   p4          340  /   281
   p8       21  /   0      

12AX7  vibrato (on)
   p1,7      170 varies
   p2       1.6  varies
   p6       340  /     281
   p8       175  varies

12AX7  preamp
   p1      205  /   170
        p3      1.6  /   1.6
   p6      200  /   79
   p8      1.5  /   .9
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
FWIW  I powered down, discharged and took some measurements on the components -  all in circuit
The can filters were all about 22 uF

The cathode bypass electrolytic caps were all in the general neighborhood except the 25 uF Bias cap on the 6V6, where there's 0V on p8. 
It measured off the scale on the highest range  (400uF). 
It also appeared to be a dead short (assuming the 470 ohm resistor was not).   
I'm not sure how measuring like this affects the results,  but wondering if this is might go away when i replace the caps?   


What might be the cause of the generally low voltages
- bad Rectifier? I don't have any spares to test)
- Bad filter can caps? 
- Do you think the PT is healthy?

 

[AlNewman]

What might be the cause of the generally low voltages
- bad Rectifier? I don't have any spares to test)
- Bad filter can caps? 
- Do you think the PT is healthy?

 

Low voltages=something is drawing too much current.
Bad filter can caps=definitely possible
Healthy PT=yes....so far

Have you tried testing resistances on your caps?  They could read perfect on your meter and still be basically a resistor to ground.
It almost seems like the bypass cap on your 6v6 is a direct short, which would definitely explain low voltages.  This isn't an amp you are just going to throw away or give up on, you want to repair it, so a good plan of action may be to start repairing before you break something that isn't already broken.

Replace the electrolytics and check values of all resistors, make sure your transformer windings aren't open or shorted, clean the pots and switches, and then start checking voltages.  Seems like you're putting the cart before the horse, at least to me.

[mresistor]

I would replace the 6V6 cathode capacitor. I'd use 47Uf 100v electrolytic there.  Save the old one. 

[Calboy]

Definitely replace the 6v6 cathode capacitor.  Measure the 470ohm resistor and measure the 6v6 pin8 to ground which should be ~470ohm.
Do this/these first and test as opposed to doing a bunch of stuff at one time and then testing.

[AlNewman]

I don't really understand the mojo of a 60 year old part that is waiting to fail.  Sure, save the caps in a bag, cut them open and put new caps inside so it looks vintage.  Make it so you don't have to wince every time you turn it on.

[mresistor]

I've still got a couple of bags of old Fender mojo hanging around    -   don't really know why.  Most of the blue ones are the dreaded death caps.

[bmccowan]

I've still got a couple of bags of old Fender mojo hanging around    -   don't really know why.  Most of the blue ones are the dreaded death caps.

I have same. I went to the Crossroads in Clarksdale, Mississippi and sprinkled them around in a circle. I sat in the center for an entire night. Nothing.

[mresistor]

 

[jeff_free69]

..Healthy PT=yes....so far
 Have you tried testing resistances on your caps? check values of all resistors, make sure your transformer windings aren't open or shorted, 

1) Transformer:  removed tubes and took a bunch of measurements for shorts etc   around the transformer:
Primary: 
Black1- Black2   5 ohms

Shorts? (X = no)
Black – Gnd       X
Black  - Red p6   X
Black  - Red p4   X
Black  - Yel  p8   X
Black  - Yel  p2   X
Black  -  Green   X

Red p4 - Red p6  380 ohm
Red p4 -  Gnd    195  ohm
Red p6 -  Gnd    185 ohm

Yel p2 – Yel p8    .3 ohm
Yel p2 – Gnd      X 
Yel p8 – Gnd      X 

Green - Gnd   .5 ohm
~~~~~~~~~~~~~~~~~~

2) Lifted leg of bias resistor and cap

470R =  650 ohm
Capacitor :  capacitance - no reading ;  1ohm


~~~~~~~~~~~~
It doesn't look to me that there are any shorts on these measurements, but i am perplexed by the HT secondary reading (red wires) being so high = 380 ohm
Is that normal??  With the Rectifier tube out of the circuit, shouldn't the impedance through the windings be pretty low? 

The "470" bias resistor  =  650 ohm seems reasonable,  at least for now ( I've seen many people increase it to this or higher to cool the bias)
The cap is shot, for sure...


Anything else i should look at ?  Do i need to unsolder the PT connections to make these measurements?

[mresistor]

That cathode resistor is way out of tolerance. Id replace the 470R with a new 1 watt metal film flameproof resistor probably around 600-750R. It depends on what the power dissipation of the 6V6 is when you check it. Might still be ok with 470R. Some people replace the KR with up to 5 watt resistors, but I should think 1-2 watt would be ok. Isn't that a 1 watt in there stock?

[mresistor]

Your HV winding on the PT is fine  195+185=380 IMO

[jeff_free69]

Thanks for confirming.   
I noticed the math was fine, but the high value had me concerned. 
Pure ignorance on my part and  not terribly important,  but if the measurement is just through the windings, i wondered why not something lower,  like  the Primary side  (Black1-Black2 =  5 ohms)

Anyway as long as nothing looks horrendous here I will order some parts...