Fender Twin Reverb Troubleshooting

[NoDough]

Hello electron wizards!

Wanted to reach out because I'm having a couple curiosities with a Fender Twin Reverb Ultralinear 135w amp I am working on.

Here is the schematic I am working from:


Amp gut shot:


Curiosity #1: The amp just doesn't want to fire up on the variac. I have tried multiple times to bring this amp up carefully and the pilot lamp refuses to show signs of life when stepping up through the variac. The amp works fine when switching straight on, but I did notice that the on/off switch requires some wiggling to fully engage when it first fires up. The amp also tends to have some trouble coming out of standby as the pilot lamp will dim and sometimes the amp will lose power, but I believe this is a result of the on/off switch needing to be replaced.

Edit: While writing this post I was finally able to get the amp to step up on the variac. Which leads me to believe the on/off switch is in need of replacement.

Curiosity #2: There is 0.097v AC going to one side of the heaters. One side tests fine with 6.3, but the other side tests 0.096v at the pilot lamp.

Curiosity #3: V5 (Tremolo Depth) Tube pulls in 400v at pin 1 when tremolo is off. When the tremolo is engaged pin 1 sees variations in voltage from 150v to 300v. I have checked the tube and it tests as good. I have also tried another tube and have the same result.

The amp works, passes signal, and for the most part sounds fine except for a pronounced 60 Hz hum.

So these are my questions:
1) Is there any reason other than the on/off toggle switch that the amp would not be able to be run off of the variac?
2) Any insight into troubleshooting the 0.097 AC at the filaments?
3) Is V5 voltage variance normal and/or is the 400v too high?

Any help, suggestions are welcome. Thanks!

[AlNewman]

Try replacing the lamp.  You can go to an autosupply store and use a 12v as replacement.  I encountered an anomaly similar to this, and it was a stray strand of wire from the guts of the lamp that was shorting to ground.  Not enough for a direct short, but enough to direct voltage to ground and create poor voltage and hum at the heaters.

[PRR]

> One side tests fine with 6.3, but the other side tests 0.096v at the pilot lamp.

Study the Hum Balance pot. Stray shorts? Turned all the way to one end?  Mark the current setting (it might be right) and center it. Better? Worse?

Center is just a best first bet. They typically give lowest hum when a *little* off center, depending on wiring symmetry.

[sluckey]

So these are my questions:
1) Is there any reason other than the on/off toggle switch that the amp would not be able to be run off of the variac?
2) Any insight into troubleshooting the 0.097 AC at the filaments?
3) Is V5 voltage variance normal and/or is the 400v too high?

1) So put a jumper across the switch to eliminate it as a suspect.
2) Nothing wrong here. Turning the Hum Bal all the way to either end will ground that side of the filament winding.
3) Nothing wrong here either. When the trem is off V5 is in hard cutoff and the plate voltage will rise to 410V according to the schematic.

[tdvt]

You may find this helpful. Much easier on the eyes (& the brain)

[NoDough]

Try replacing the lamp.  You can go to an autosupply store and use a 12v as replacement.  I encountered an anomaly similar to this, and it was a stray strand of wire from the guts of the lamp that was shorting to ground.  Not enough for a direct short, but enough to direct voltage to ground and create poor voltage and hum at the heaters.

Thanks for the suggestion AlNewman! I'll snag some of these. 

[NoDough]

> One side tests fine with 6.3, but the other side tests 0.096v at the pilot lamp.

Study the Hum Balance pot. Stray shorts? Turned all the way to one end?  Mark the current setting (it might be right) and center it. Better? Worse?

Center is just a best first bet. They typically give lowest hum when a *little* off center, depending on wiring symmetry.

Yea the Hum Balance was completely rotated to one end. Understandably as this gave the lowest hum. Will need a proper biasing before the hum balance becomes useful again.

[NoDough]

So these are my questions:
1) Is there any reason other than the on/off toggle switch that the amp would not be able to be run off of the variac?
2) Any insight into troubleshooting the 0.097 AC at the filaments?
3) Is V5 voltage variance normal and/or is the 400v too high?

1) So put a jumper across the switch to eliminate it as a suspect.
2) Nothing wrong here. Turning the Hum Bal all the way to either end will ground that side of the filament winding.
3) Nothing wrong here either. When the trem is off V5 is in hard cutoff and the plate voltage will rise to 410V according to the schematic.

Thanks sluckey for this insight. I always appreciate your wealth of knowledge, and now see how the circuit work with the hum balance. Also, I jumpered across the switch and was able to get the amp to work perfectly through the variac. So the power switch will absolutely be removed and replaced. 

[NoDough]

So I'm diving further in to this amp, but there are a couple more details I want to clarify before I begin changing out parts. It seems there have been 2 mods done to the amp (master volume deleted & blackface style biasing).

Does anyone have some insight into the biasing mod? The closest resource I can find is this premier guitar article:
https://www.premierguitar.com/gear/ask-amp-man-better-biasing-for-silverface-twin-reverbs

Here are gut shots of the mod:

The 220k resistors which feed to the V6 coupling capacitors. I will probably replace these with metal oxide resistors, but feel free to advise if I should go a different route.


The 10k adjustment pot. Lug 1 goes to ground via a 3.3k resistor. Lug 2 goes to the 220k resistors. Lug 3 goes to the Trem Jump Start and a filter cap (100uf 100v) in the doghouse.

From what I can tell this mod has installed a traditional bias knob. Which makes biasing simple. All I would need to do is make small adjustments while continuing to measure as I make the turns until I get 70% dissipation. Feel free to let me know if that is incorrect or if I am totally off-base in terms of the mod they have done here.

For the second mod (deleting of the master volume) I just want to make sure that I didn't miss any of the components that have been taken out.


Those are the parts that I couldn't find when going through the amp. Let me know if I am potentially mistaken on that.

Thanks!

[sluckey]

The 220k resistors which feed to the V6 coupling capacitors. I will probably replace these with metal oxide resistors

The 10k adjustment pot. Lug 1 goes to ground via a 3.3k resistor. Lug 2 goes to the 220k resistors. Lug 3 goes to the Trem Jump Start and a filter cap (100uf 100v) in the doghouse.

Sounds right. Don't mess with the 220Ks. They will last forever. No need for metal oxide.

For the second mod (deleting of the master volume) I just want to make sure that I didn't miss any of the components that have been taken out.

Those are the parts that I couldn't find when going through the amp. Let me know if I am potentially mistaken on that.

The 820Ω and .01µF you circled are part of the NFB circuit. They have nothing to do with removing the MV. The amp will have a less refined sound without those NFB components. You may like the sound. I can't imagine playing a TR without NFB. The blackface TR has an 820Ω but not the .01µF.

Look at the reverb transformer. There's another part of the MVS connected to the RT green wire. The other side of the MVS connects to a 1K and 470K. All that can be removed.

[NoDough]

Sounds right. Don't mess with the 220Ks. They will last forever. No need for metal oxide.

Would it be a total waste of time to change these to bumble bee carbon comp to at least match the rest of the amps CC's?

The 820Ω and .01µF you circled are part of the NFB circuit. They have nothing to do with removing the MV. The amp will have a less refined sound without those NFB components. You may like the sound. I can't imagine playing a TR without NFB. The blackface TR has an 820Ω but not the .01µF.

Look at the reverb transformer. There's another part of the MVS connected to the RT green wire. The other side of the MVS connects to a 1K and 470K. All that can be removed.

Can't thank you enough for this. Both the 820Ω and .01µF were hiding in plain sight.

[NoDough]

So now that I know what absolutely needs to be replaced, the on/off switch, I'm headed down the rabbit hole of components that should be, can be, or would be unnecessary to be replaced.

1) Death Cap
My understanding is that I can just solder this off of the middle lug of the ground switch and call it a day.
But is there any reason to replace this with a .047 µF Class Y Safety Capacitor?



2) Navy Blue Blob .047 Coupling Capacitors & Beyond
So I have 2 Blue Blob coupling capacitors that I will definitely replace due to age, and 3 Cathode Bypass Electrolytics I will replace due to poor previous repair work. Outside of those replacements I am debating going full on cap job from the orange drops throughout the amp over to SoZo blue molded.
I have these in some other Fender clone builds and really like them, but is there a reason I should be convinced not to do this?



3) Replace 470Ω CC Screen Resistors with 3W MO.
The previous owner decided to replace only half of the screen resistors on the power tubes.
Would there be a reason not to make the switch on the other two?
Also noted here that only one power tube grid stopper made the grade for a 3W grid stopper.
Any potential reason for that?



4) Rectifier diode replacements
So these are clearly original diodes on the bias board. As I have seen Sluckey advise not to bother with replacing these until they fail, I am leaning away from bothering with them, but I'm curious if there would be a reason to replace otherwise. One of the other threads dealing with these 135 ultralinear amps discussed replacing the diodes with ultra fast versions.
Any reason to bother with replacing these?



Here is the other thread for reference: https://el34world.com/Forum/index.php?topic=23749.0

Thanks for the continued help 

[shooter]

If it ain't broke don't fix it, applies to many things in life, in this case, make it work, play til the fingers bleed, then you'll have a good idea IF... "the treble's a lil ice-picky"...  "how do you get more..."

[sluckey]

I don't see a reason to replace anything you just mentioned.

[mresistor]

The older CC 470 screen Rs are not flameproof and if they burn up they can make a real mess inside. IMO I'd replace them with metal oxide/film resistors like the others and it would look nicer, not that anyone's inside the amp but the owner or techs.

[NoDough]

If it ain't broke don't fix it, applies to many things in life, in this case, make it work, play til the fingers bleed, then you'll have a good idea IF... "the treble's a lil ice-picky"...  "how do you get more..."

I like this line of thinking. .......buuuut..... What would you did if it works, but you want to take the amp from "working" to "as if it just came off the factory floor"? What would you do to get it back to the quietest most reliable amp you own?

Just playing a little bit of devil's advocate to pick your brain 

[NoDough]

If it ain't broke don't fix it, applies to many things in life, in this case, make it work, play til the fingers bleed, then you'll have a good idea IF... "the treble's a lil ice-picky"...  "how do you get more..."

I like this line of thinking. .......buuuut..... What would you did if it works, but you want to take the amp from "working" to "as if it just came off the factory floor"? What would you do to get it back to the quietest, most reliable amp you own?

Just playing a little bit of devil's advocate to pick your brain 

[shooter]

I got paid to fix broke things, the 1st thing you learn  (See above post)


if it's working, since what, the 70's, it's reliable.


FIND what's not to spec, ohm_age, voltage, scope_age, note the variation from spec, decide if it has ANY effect on original tone, if you n the amp go back that far.
by doing that you're "spec'n the amp", once you know the spec's, you know everything there is to know electronically.  Now turn off all the test equipment, play like a Rockstar for a week, swap in 2-4 cabs with different speakers, Now you know everything you need to know musically.


now you are equipped to modify the amp to your specific musical talents 

[NoDough]

I don't see a reason to replace anything you just mentioned.

I follow you on this except for when it comes to the "death cap". From what I can tell that is a metallized film cap (https://www.tedss.com/2020069705) which in the case of failure would run the risk of shock. The advantage of keeping the capacitor over just simply removing it altogether would be that if the amp is used in a venue with improper wiring the ground switch could still be used to reduce hum.

Am I correct in this line of thinking? Feel free to let me know if I am off base here. 

[NoDough]

I got paid to fix broke things, the 1st thing you learn  (See above post)

I'm new to all of this fun stuff, so I have yet to learn this valuable lesson. 

FIND what's not to spec, ohm_age, voltage, scope_age, note the variation from spec, decide if it has ANY effect on original tone, if you n the amp go back that far.
by doing that you're "spec'n the amp", once you know the spec's, you know everything there is to know electronically.  Now turn off all the test equipment, play like a Rockstar for a week, swap in 2-4 cabs with different speakers, Now you know everything you need to know musically.

My plan, if I do go with replacing things, would be to note signal path capacitor values. This would allow me to keep any tone that time has changed. Any other tidbits I might need to think through if I dive down the rabbit hole of replacing things?

[mresistor]

I don't see a reason to replace anything you just mentioned.

I follow you on this except for when it comes to the "death cap". From what I can tell that is a metallized film cap (https://www.tedss.com/2020069705) which in the case of failure would run the risk of shock. The advantage of keeping the capacitor over just simply removing it altogether would be that if the amp is used in a venue with improper wiring the ground switch could still be used to reduce hum.

Am I correct in this line of thinking? Feel free to let me know if I am off base here. 

What I do in older amps that are going to be used by performing artist in who knows what venue or building; I remove the present death cap and replace it with an Y2 safety cap. Simple and not expensive and the ground switch is still functioning.  I previously said X1/Y2 but looked and they are just class Y caps. So yeah I agree with that thinking.