Rescue project: 1972 Twin Reverb silverface

[st]

  Or you could change the bias balance control to a bias adjustment control.

I mentioned this to him in reply #11 and one of the best discussions with graphics for changing over to adjustable bias is right here https://el34world.com/charts/bias_conversions.htm


I think its one of those things CBS did to Fenders circuits that doesn't make much sense to keep in todays environment and I'm not a fan of the bias balance circuit. Can be done easily and doesn't take long to do it.

Why's that? Unmatched tubes--especially good NOS tubes--are cheaper than matched tubes, still.

And PRR is right. Twins we're never designed to be biased hot... A balancer pot (biased somewhat cold) is way more useful than an adjustable bias pot.

[2deaf]

  Or you could change the bias balance control to a bias adjustment control.

I mentioned this to him in reply #11 and one of the best discussions with graphics for changing over to adjustable bias is right here https://el34world.com/charts/bias_conversions.htm

I apologize if I stepped on your toes. 

Now that I have looked at Doug's bias mod's, I notice that none of them are the same as the mod's I posted.

[2deaf]

With an almost complete disregard for the Socratic method and critical thinking in general, I submit yet more bias mod's.  Unlike Robrob's and Rob Hull's mod's, the bias level pot may be mounted anywhere.

[shooter]

an almost complete disregard

in this new century, humor and intelligence doesn't compute well with them 

[2deaf]

. . . doesn't compute well with them

Giant ants?

[shooter]

more like pacifist zombies with upgraded tri-corders   

[jeff_free69]

Excellent - thanks for all the tips!

There are lots of things that can be done to increase the dissipation.  You could change the 1K resistor to 1.5K in the bias circuit and see what happens.  Robrob has an adjustable bias mod for silverface Fenders.  Or you could change the bias balance control to a bias adjustment control.

To start, I'll follow the path of least resistance   ( get it, resistance  ?  )

I was wondering which way to adjust that  (up or down)  -
if there's a 50 -50 chance of guessing, then there's a 90% chance I'll get it wrong.   (like when you drop a piece of toast - does it land butter-side up or down?)

Looking  back to the older schematics, they had the 470 ohm there, so I was curious to try that first (easy to clip in another 1k in parallel).    Sure enough, the mV's went the wrong direction:  down.

I don't have a lot of parts in stock, but i did happen to have a 1.5k (planning to replace those Rs cooking on the bottom of the sockets). 
Put that in and all the mV's went up by 1/3 , eg 30 -> 40

So my VI's are now about
20.3W     18.5W            19.8W    18.5W

... just below that magical 70% level.


With these tubes there doesn't seem to be much need to balance.    I think I'll try the level adjust conversion next. 

But I really like the idea of both, because I may eventually change out the tubes.  But I'd have to order another pot etc.


Did a quick test play  on the normal channel (prevol= 8, mastervol = 8) and there might have been a bit more crunch and less brittleness.   Unfortunately i forgot to play right before the change, so I can't be sure.  And you know, there may be some implicit perception bias here (get it, bias ?  awright nevermind).  But with the adjustment mod it will be easy to compare.


More unfortunately, not only does the vibrato channel not vibrato, it no longer even channels any more.  Shame , I was looking forward to hearing that cranked, now its completely dead.    I'll have to look into that too ( could it be some of Them dang ants got in there ??? )

Also there's been a buzz coming from the cabinet on bass notes.  Could be one of those nifty Utah speakers.  Not a big deal I guess, those things don't seem to get much love - do they?  (I've got an old JBL and a recent Greenback I'm interested to compare once things are in order)



 

[jeff_free69]

Decided I'm going to add the extra bias level pot and keep the balance.  Just hate to put in a tiny order when I know there's more coming soon.   Very soon.

I investigated the tremolo issue.  Good news is that the channel works  - when I did a quick check last time I couldn't see the knobs on the other side of the chassis.  So i turned up the volume, but all the tone pots were off.  I didn't realize that, or did I even suspect that would essentially kill the sound so much; so live and learn. 

But then the bugs came out to bite me...

It turns out the tremolo does work.  The light flashes and it looks like there's a disco party going on inside the roach house. 
And you can hear  it - but only for a small range of notes.  I would say its most pronounced playing the octave on the A string so roughly 220 Hz.   And it gradually diminishes as you go up or down.   On the hi E and B there is no effect.   On the low E , very little.   In way its kinda cool - but how bizarre is that?   
What would cause it?

Also, turning both preamp volumes off elicits a ticking thump of the tremolo.  (not a big deal , but maybe that helps indicate where the bugs are...)

Poking around :

V2 is pretty microphonic.   Haven't tried any jockeying yet, but I'm feeling a new set of preamp tubes coming in the next order. 

I noticed that there's a lot of microphonics and  popping going on when I tap any of the caps, or even the board itself, on the right side (by V3,2,1).   3 screws hold the board down and theres a big warp between the middle screw and the right screw. 
Tapping almost anywhere in between makes a lot of noise (of course everything is dimed for this part).   
But the popping is not 100% consistent.  If I tap a couple of times in a row it pops, but then might not the 3rd time.  Wait a few seconds and that repeats.   
Could this be bad caps (discharging / charging) ? 
Something with the board itself?




Reverb - hooked the tank up and seems pretty much OK.  only issue is that turning the reverb level all the way down still passes a very slight, hi-pass sounding reverb (like a tambourine in an airplane hangar).  Not a big deal, since turning off the switch kills it.    it might actually be useful percussive effect. 
Just wondering if thats normal?

Speaker  Looks like that buzzer went to meet its maker back in Utah.  It actually crapped out during the tap test (told you it was noisy! )    I hadn't paid it much mind, but it turns out there was also a weak solder joint on the other speaker and that one wasn't even playing.   Fixed that and now its the other speaker's turn.   
I am ASSuming that running at 8 ohms with one speaker is acceptable (for the amp - not intending to give 1 speaker the full monty / 100W).
Is that OK ?

 

[shooter]

If I tap a couple of times in a row it pops, but then might not the 3rd time.

pretty common for bad solder joint, tap - conduct - pop

3 screws hold the board down

Old fender boards suck, one of the 1st things I do, remove all the hold-down screws, make sure It's securely "elevated" and see what "effect" that has.

[mresistor]

st says ;
Why's that? Unmatched tubes--especially good NOS tubes--are cheaper than matched tubes, still.

And PRR is right. Twins we're never designed to be biased hot... A balancer pot (biased somewhat cold) is way more useful than an adjustable bias pot.



You and PRR are right about that.. the silverface twins sure weren't designed to be run with a hot bias. I'm sure the bias balancer has usefulness and I can respect that. In my experience I have come to like the simpler adjustable bias of the AB763 - and so have many others. The implementation of both would be ideal then, if you have tubes that need to be balanced in circuit and wanted to be able to adjust the Pdiss where you want it.  Running vintage tubes is a good point.

[Willabe]

3 screws hold the board down

Old fender boards suck, one of the 1st things I do, remove all the hold-down screws, make sure It's securely "elevated" and see what "effect" that has.

Yes, it can sound very different.

[mresistor]

ST  it should still be noted that even with both bias balance and adj bias with a Twin there is still two tubes per side, and each duet should still be fairly matched..

[st]

ST  it should still be noted that even with both bias balance and adj bias with a Twin there is still two tubes per side, and each duet should still be fairly matched..

Agreed, but only because you said "fairly" matched. For all practical purposes, in a guitar amp (!) such as a twin, you can use unmatched tubes without problem unless they are wildly out of spec. Take 4 random tubes, put the two coldest on one side and the two hottest on the other, that way the tubes on each side will be *fairly* matched. Then use the balancer (!) to *fairly* match the two sides.

In my personal view, perfect bias adjustment makes little sense in a tube guitar amp, which is about as crude and lofi a piece of electronic technology you may ever come across... Furthermore, the way bias is set does not influence tone. Much much more importantly, changing the way it is set is almost never the solution to a real problem. In almost all instances I see it discussed, the better advice would to get the amp working properly. (Inspecting the board and its screws themselves is, in that respect, sound advice!)

[shooter]

the way bias is set does not influence tone.

There is a tonal change, enough to bother you, subjective.
the last amp I set bias, you could definitely "hear" the amp come alive going from fridged to warm bias

[mresistor]

. Much much more importantly, changing the way it is set is almost never the solution to a real problem. In almost all instances I see it discussed, the better advice would to get the amp working properly. (Inspecting the board and its screws themselves is, in that respect, sound advice!)

Strange you would say this.  Who said converting bias adjustment circuits from one type to another in a SF Twin was a prelude/substitute to troubleshooting or problem discovery? It was a simple  recommendation for when the amp is up and running, not a suggestion to help find something wrong.  And as shooter elucidates above... being able to take a Twin to that sweet spot in bias - in consideration of the output tubes characteristics is in many ways a good thing.  Tell me sir.. why would Doug Hoffman discuss changing bias circuits to AB763 from SF bias balance if it wasn't considered a good thing to do? And why would Shooter and myself find it better to be able to have that adjustment capability based on what we have heard out of the Twin amplifier. I have had 0 complaints.

[st]

I didn't mean to suggest converting the SF to the BF bias arrangement is a bad idea; I think instead it depends on personal preference and your tubes (matched or not). The truth is, the thread starter, who is a self-declared newbie (all due), asked for advice on restoring this amp, indicating he wanted to stick to the original schematic, and immediately someone mentions changing the bias arrangement. Same thing happened in a different recent thread (which I now can't seem to find). Anyway, don't take it the wrong way. I don't think it's bad advice, it's just that I prefer a "first things first" approach and I don't consider the bias arrangement a "first thing" (unless of course there is no neg bias voltage).

And apologies to the thread starter as well. I didn't mean to hijack the thread.

[jeff_free69]

That's alright, I enjoy the banter and appreciate the info, even when it spins into a world of its own. Sometimes those are the most entertaining parts!  Fortunately I work so slowly it fills my dead-air nicely  :)

To me adding the bias level adjust just increases the versatility without committing to any SF vs BF conversions.   
Plus, I don't see any harm as if it were  defacing a vintage museum-quality collectors item.   
I see some people like to "bias by ear", so I'm interested to hear the difference for myself, once I know what the reasonable limits are. 


Only a bit  progress this week.  Traveling for the job and had a high school band reunion this weekend.  Aint saying what year, but there was plenty of Led Zep, Bowie, Allman Bros and Cocaine (the song) on the set list. 

Took a look at that buzzing speaker.  wow - never seen anything self destruct so completely (pretty much started the reconing process for me but i don't think I'll be going there).   I tacked on 50W worth of 8 ohm resistor in parallel to keep the amp happy and make sure the other speaker doesn't follow suit just yet.

Took another look at the board and tried removing the screws. 
Didn't really reveal anything new ; if anything that made it worse.  Ch1 ( all the way by  the right screw) even got a bit microphonic  when tapped (but no popping like the other channel).  Seems worst at the high point of that bulge (kinda wish I could simply screw it down there).

Also noted on Ch2:
Sticking the chopstick in between the terminals on the Volume pot or bright switch induces a slight hum (doesn't happen on any other pots)

Swapped V1 and V2 and the tube in the v2 socket was still more microphonic than the tube in v1 socket.

Tapping the wire from input jack to pin 7 also produces the popping noise

The terminals on socket 2 are "wobbly", from the underside.

Not sure if this explains all the noises, but that socket is trouble.  I'd swear there was a contact mic on that board - it comes from anywhere.

[st]

That's alright, I enjoy the banter and appreciate the info, even when it spins into a world of its own. Sometimes those are the most entertaining parts!  Fortunately I work so slowly it fills my dead-air nicely  :)

To me adding the bias level adjust just increases the versatility without committing to any SF vs BF conversions.   
Plus, I don't see any harm as if it were  defacing a vintage museum-quality collectors item.   
I see some people like to "bias by ear", so I'm interested to hear the difference for myself, once I know what the reasonable limits are. 


Only a bit  progress this week.  Traveling for the job and had a high school band reunion this weekend.  Aint saying what year, but there was plenty of Led Zep, Bowie, Allman Bros and Cocaine (the song) on the set list. 

Took a look at that buzzing speaker.  wow - never seen anything self destruct so completely (pretty much started the reconing process for me but i don't think I'll be going there).   I tacked on 50W worth of 8 ohm resistor in parallel to keep the amp happy and make sure the other speaker doesn't follow suit just yet.

Took another look at the board and tried removing the screws. 
Didn't really reveal anything new ; if anything that made it worse.  Ch1 ( all the way by  the right screw) even got a bit microphonic  when tapped (but no popping like the other channel).  Seems worst at the high point of that bulge (kinda wish I could simply screw it down there).

Also noted on Ch2:
Sticking the chopstick in between the terminals on the Volume pot or bright switch induces a slight hum (doesn't happen on any other pots)

Swapped V1 and V2 and the tube in the v2 socket was still more microphonic than the tube in v1 socket.

Tapping the wire from input jack to pin 7 also produces the popping noise

The terminals on socket 2 are "wobbly", from the underside.

Not sure if this explains all the noises, but that socket is trouble.  I'd swear there was a contact mic on that board - it comes from anywhere.

Both the input wire and the volume/bright switch are sensitieve areas. There is no reason to worry if they only cause noise when agitated. If they also pick up noise from vibration, you'd have to do something about it. Is the input wire shielded?

Re: the tube socket, you could try tightening the pin sockets... Sometimes replacing the socket is the best thing to do, but keep in mind the socket itself is not microphonic. It may however, introduce vibration to something on it that may be microphonic (like a tube or a ceramic Cap)

[jeff_free69]

Both the input wire and the volume/bright switch are sensitieve areas. There is no reason to worry if they only cause noise when agitated. If they also pick up noise from vibration, you'd have to do something about it. Is the input wire shielded?

Re: the tube socket, you could try tightening the pin sockets... Sometimes replacing the socket is the best thing to do, but keep in mind the socket itself is not microphonic. It may however, introduce vibration to something on it that may be microphonic (like a tube or a ceramic Cap)

Yes - its that shielded wire.   
The induced hum when prodding the pot terminals was pretty low compared to the other racket, so what you say about being expected makes sense.   

I pulled the tubes, and using a jewelers screwdriver, squished the clips inside each socket hole to get a tighter grip on the pins.  Also re-deoxied.   

That seems to have helped a bit as far as the popping, for the most part.  One area (eyelet that connects to V2 pin 1?)  can still do it , but at least its not sounding like the 4th of july already.
The terminals still wiggle within their holes, so I'm thinking of  replacing the whole socket anyway (the clips in V3 socket wiggle too)

But  the board is still microphonic as heck, so maybe we're talking mainly about caps.  I can play that first .1uF  all the way to the right on chan 1 like a bongo.  It sits well above the board and sounds like its  twanging away.

While I put together this next order, I'm also looking at the tubes:
So far I have a pretty good feeling about the Ruby power tubes.  Swapped a couple around so the match within each side is better, then rebalanced so they are now all biased to within ~ 1 mv of each other.

But the pre-tubes are a bit of a mystery, at least as to what they are.
Here's what they should be  / and what I have
V1     7025   /   RCA 12X7 
V2     7025   /   ?  (markings obliterated - looks very similar to V1 )
V3     12AT7 /   RCA  12AT7
V4     7025   /   RCA  7025
V5     12AX7 /   RCA  12AX7
V6     12AT7 /   ? ?      (markings obliterated, does not look like any other tube)


I gather 7025 is an obsolete reference to a "better"  12AX7 (please feel free to correct). 
Note that I have (1) 7025 in V4,   but V1 (and probably V2) are 12AX7.
I have no idea what the V6 PI is - doesn't resemble any others.  (maybe they were getting fancy and popped in a new one).

Any recommendations here?
 

[st]

For all practical purposes: 7025 = 12ax7. If you have to replace with new tubes, go with 12ax7 (there are no new production 7025s).

A little pin wiggle in the tube socket is normal as long no tube is installed. The socket pin should not wiggle on the tube pin when installed.

The "microphonic board" sounds worrying. Are you sure you can't narrow it down to a specific component (ceramic cap*)? Sure it's actually microphonics and not intermittent connections/shorts to ground? Have you tried lifting the board as shooter has suggested?

[jeff_free69]

The worst offender that  I noted  a while back (and  still is, by far),  was the  little .003 ceramic cap connected to V4  pin1
 > if you gently  rub it, it sounds like the chopstick is on your eardrum

(Edit:) turning down the reverb level eliminates this completely.
And with the reverb off
 ... that big  blue .1uF on ch1 still makes a nice bongo sound, maybe more like a conga drum.. , the blue .047 uF next to it is the bongo 

I don't have many stock of parts on hand.   So I don't see any reason not to order the all the parts at once and  methodically replace them all.



TUBES:
Also thinking of trying some different AT  and AX tubes to see what differences there might be. (nothing crazy exotic, but for  under $20, why not)
The RCA's I have could be original (tired?)  and  there's at least one (the unmarked PI) "who knows?" (wish I paid more attention before i cleaned them, not realizing the paint might come off).  It strikes me that the PI needs to be in tiptop shape.

TubeDepot has a tube comparison chart for many different brands (brightness, gain, noise, etc)
Whats in your PI slot (or would be if you had a 72 SF Twin)?
Favorite 12 AX7's here?

[st]

Personally, i always track down the problem first, before doing something as drastic as rebuilding a whole board. To me, that's the fun part--and you learn a lot from it. But I can see why you'd prefer to do it all at once...

Anywho, the microphonic ceramic cap has to go. It will produce noise under vibration, which you always have in a combo amp. Replacing with a new ceramic will do, so will any other type of cap.

Trying some new tubes is also a good idea. Any new production preamp tube will work, which one is up to personal preference. I have a weak spot for the sovtek 12ax7wps, but many would disagree. The pi takes a 12at7.

[jeff_free69]

I hear ya, but buying parts one at a time is too inefficient ( and you're talking to the king of inefficiency! 
Gonna be some good practice soldering in tight spaces.

Aside from that one bad ceramic, those 2 blue blobs have some microphonics going on too.

And 2 other ceramics  (feeding pin1 of each channel) should be 250p according to schematic, but  are measuring 320 and 350.  unless Fender built it like that  on purpose I'd think they should go too.

And lets not forget the Tremolo issue   - it only works on a range of notes.  I haven't heard any theories on what might cause that weirdness, so what else can I do except change out those caps for a couple of bucks?

BTW I take it silver mica are acceptable instead of the ceramics here  (upgrade or snake oil?)


Uploading some shots of the tubes. 
V3 is AT
V6 (far right) is supposed to be also, but doesn't look anything like V3 or any other tube.  Of course it could be  something new n' fancy, but who knows...

[2deaf]

V6 (far right) is supposed to be also, but doesn't look anything like V3 or any other tube.  Of course it could be  something new n' fancy, but who knows...

Chinese.  Better than Russian, usually.

[jeff_free69]

Chinese.  Better than Russian, usually.

Looks like  it could be a Ruby (like the power tubes) - it has those little horizontal metal sections that distinguish it from other AT7's.
How do you mean better?   build quality ?  sonically?

Not having had the chance to compare yet, I think I'd might like something on the warm (yet crunchy) side, and quiet (for recording)
What's the general rep on Ruby - your basic Chinese tube?

I have a weak spot for the sovtek 12ax7wps, but many would disagree. .

I'm not finding that one (wps)- is it an older version?     finding LPS, WA,B,C  etc

[2deaf]

Looks like  it could be a Ruby (like the power tubes) - it has those little horizontal metal sections that distinguish it from other AT7's.
How do you mean better?   build quality ?  sonically?

The transconductance and amplification factor were more consistent for the two triodes in Chinese tubes than for Russian tubes for the ones that I tested.  I've never had either one fail in an amplifier, so I have no reason to think that one has a better build quality than the other.  There is no way I could tell any kind of subtle sonic difference between the two having been diagnosed with "profound hearing loss" clear back in the '80s.   

[st]

Sorry. Typo. Should be lps...

[2deaf]

And lets not forget the Tremolo issue   - it only works on a range of notes.  I haven't heard any theories on what might cause that weirdness, so what else can I do except change out those caps for a couple of bucks?

I don't think there is anything wrong with the oscillator capacitors since you can see the bulb flashing and you can hear the classic Fender tremolo thump.  It seems more likely to me that the opto-isolator is malfunctioning.

[jeff_free69]

And lets not forget the Tremolo issue   - it only works on a range of notes.  I haven't heard any theories on what might cause that weirdness, so what else can I do except change out those caps for a couple of bucks?

I don't think there is anything wrong with the oscillator capacitors since you can see the bulb flashing and you can hear the classic Fender tremolo thump.  It seems more likely to me that the opto-isolator is malfunctioning.

Yeah been thinking about that - I haven't spent a lot of time studying how the vibrato circuit works, so i will have to review.

I was originally thinking that  caps in general are frequency dependent, and in this case dirt cheap to replace.     

Here's were things are still not clear for me yet (feel free to , you know..) :
But now that I think about it, it doesn't seem to me the 3 caps  that modulate  the rising and falling voltage that ultimately controls the photo- part of the roach, are in the signal path at all.   So then  it would not help...

OTOH  the -resistor part of the roach is a variable resistor, which in general I would not think to be frequency dependent.   
And that its job is to bleed off signal going into the PI:
 Light on = low resistance = more signal going to ground (and not into PI, so reduced volume)
 So is that considered  "in the signal path" (and could be frequency dependent) ?

Maybe i just should've ordered a new roach too, but cest la vie, for now.
Some different colored caps and a couple of tubes are on their way....

Also, don't think I mentioned it, but this channel is highly hissy.  No wonder- it's pretty complicated!

[terminalgs]

I debated starting a new thread but this one has a lot of good 72-75 Twin/Vibrosonic information and I feel my comments might fit better here instead of a new thread. I recently repaired a 74 Vibrosonic (Twin w/ one 15") and this thread was useful!


The amp had lots of noise problems and under the hood it was obvious that it had been worked on a good bit.  all caps had been changed, and someone spent some money on Jupiter caps and nice Sprague 22/25Vs,  but workmanship was shoddy for a few repairs. 


Especially under the dog house:

• left most: spliced axial lead.  I suspect tech was in hurry and put it in backwards and later fixed the mistake.
• resistor values changed and completely wrong.  maybe misguided plans to revert to pre-BF?? dunno.
• and the decision to abandon Leo's grounding scheme for ???

Fixing these things plus re-flowing all the ground solder joints on this board and the primary board resolved most of the noise issues.


I did have another noise find & fix and I thought it might help others:


with the amp on I went along each lead wire from the primary board to the sockets with (insulated) needle nose pliers and gave each wire a jiggle,  when I got to the wire that goes from the reverb driver grids (V3) to the eyelet of the 500pf/1M union.  amp noise dramatically increased as I neared it with the pliers.  this is a high impedance wire and it is unshielded. the reverb knob was turned down, so I think the noise was possibly going the other way: from wire back through 500pf to V4B.  It seemed like it was acting as an antenna!   I replaced the wire with some shielded RG174  (from hoffman's store),  grounding at the axial lead of nearby cathode bypass cap,  and overall level of noise was reduced (and not just with proximity of needle noses...)   here's a picture:











Last here's a nuance of the Vibrosonic.  the Twin schematic of this era shows a 100R under the Long tail pair PI which serves as the NFB circuit along with the 820R.  This one has 2 100's in parallel and both look original to the board.  One of  Robinette's  pages mentions this ( https://robrobinette.com/AB763_Modifications.htm ) as 100ohm for two speaker amps and a 47ohm for 1 speaker amps.  It seems that applies to this Vibrosonic.  here are the two paralleled 100 ohm resistors at the top of the board:

[jeff_free69]

Nice!
Although I've not updated this thread in the past year, I have not forgotten it.  The 72 Twin is doing very nicely, using the clean channel and some new speakers as a pedal platform.   Resolution for 2021  is a return to normalcy and to complete the other channel rehab.  And then I  also have a old VibroChamp sitting there waiting for some attention and TLC.
Happy Holidays!