1981 Silverface Twin. No sound from preamp?

[Searing]

I put new tubes and replaced all electrolytics. I can hear hum when turning the output tube matching control. I can hear when I touch pin 1 and 3 with my drain tool. It seems like the output section is working? Just, no sound from the preamp section. Thoughts? Tips on how to diagnose?  Thanks!


Output tube voltages



-056v on pin 1
500v pin 3
500v pin 4
-056v pin 5
500v pin 6
Heaters are all 6.3

[HotBluePlates]

Output tube voltages


-056v on pin 1
500v pin 3
500v pin 4
-056v pin 5
500v pin 6
Heaters are all 6.3

Pins 3, 4 and 6 all being 500v points out that no current is flowing through those output tubes.

If the output tubes are installed, make sure the path from Pin 8 to ground is ~0Ω (or about 1Ω if you have a current-sensing resistor installed).

If you attempted to solder to the chassis, be aware you cannot get a good solder joint unless you have something like a 150w iron (which is flat out huge).

[Searing]

Pin 8 is straight to ground and reads 0ohms on all 4 output tubes. Like you said though, pins 3,4, and 6 all read 500v. How do I proceed? Thanks!

[Esquirefreak]

What are your preamp voltages?

/Max

[Searing]

With meter set to DC


V1


-151.6mv pin 1
-373.3mv pin 2
019.6mv pin 3
05.38v pins 4 and 5 meter set to AC
-002.3mv pin 6
-0.584v pin 7
004.1mv pin 8
2.2v set to AC heater pin 9


V2


-189.2mv pin 1
-356.4mv pin 2
016.9mv pin 3
05.39v pins 4&5 meter set to AC
011.6mv pin 6
-0.582v pin 7
004.2mv pin 8
2.255v AC pin 9


V3 reverb tank not plugged in


054.0mv pin 1
-398.7mv pin 2
000.2mv pin 3
2.357v pins 4&5 AC
047.6mv pin 6
-392.0mv pin 7
000.3mv pin 8
05.38v AC pin 9


V4 reverb tank unplugged


-021.2mv pin 1
-0.420v pin 2
003.3 mv pin 3
05.39v AC pins 4&5
004.6mv pin 6
-0.424v pin 7
003.3mv pin 8
2.421v AC pin 9


V5


034.6mv pin 1
-043.0v pin 2
-000.1mv pin 3
2.426v AC pins 4&5
004.8mv pin 6
-043.0v pin 7
-000.0mv pin 8
05.45v AC pin 9


V6


021.0mv pin 1
-192.0mv pin 2
000.6mv pin 3
05.46v AC pins 4&5
017.6mv pin 6
-254.8mv pin 7
000.5mv pin 8
2.512v AC pin 9

[mresistor]

No plate voltage on V4 pins 1 and 6.   Being a 1981  this should be a Twin 135   and this schematic 
https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_sf_135_schem.pdf


Are you sure you're reading the meter correctly and the meter is working right? Also what is with the - sign in front of numbers? (negative) ?


start with pin # then follow with the reading. 

[Searing]

Yes, negative. Meter was set to DC. I double checked the readings. I have another meter I can try.

[Esquirefreak]

Do you have your meter leads hooked up the right way? Those voltages don't make any sense.

[Searing]

Yes. The orients is correct. I had it set to DC. And, I switched to AC for the heaters.

[Esquirefreak]

I was just asking because if you measure DC and reverse the black and red leads, you'll end up showing negative voltage.

[mresistor]

I think you have some connections messed up with your power supply filter capacitors. There seems to be some plate voltages missing in the preamp. Can you post some photos of the filter capacitors and the guts of the amplifier so we can look at it?

[Searing]

Please ignore J hooks. Lol.

[Searing]

Pre

[Searing]

More pre

[Searing]

Board

[Searing]

Power

[HotBluePlates]

The 2.7kΩ 10w "Cement block" resistor looks toasted, and the voltages after it are essentially "Zero volts."

The discussion around the "positive" or "negative" millivolts in the preamp is missing the fact that these are millivolts, never hundreds of volts.  This is likely just noise being picked up by the meter.  "Millivolts" = "Zero" near enough for our purposes.



For the 6L6s, you will need tubes in the sockets (or the amp out of Standby, if you did have tubes in the sockets).

[Searing]

Ordered a new 2.7k 10w resistor.

[AlNewman]

Did you test the resistor?

If a 10W resistor fried, and it looks like it did, it would be best practice to test all other paths to ground before replacing.  Otherwise it's likely it could happen again.

[Searing]

How do I test all other paths to ground exactly?

[tubeswell]

How do I test all other paths to ground exactly?

If a power supply resistor is cooked, it means too much current has gone through it. This is usually because a filter cap has shorted and the resistor is dropping all the voltage between the preceding filter cap and the ground*. Look at the filter caps. The way you work this out is if there is 0 volts at a filter cap node (where you would normally expect hundreds of volts).


* if this is the UL twin reverb from 1981, that's 500V^2/2700R = 92.6W


(Moreover, if a filter cap is exposed to VAC it will short. e-caps aren't designed to cope with VAC, they're only designed to cope with  pulses of DC and power supply ripple. The main reason that filter caps to get exposed to high VAC, is if the rectifier is shorted. So you could be looking at a number of problems, i.e., shorted rectifier which then dumps high VAC from the power transformer winding onto a filter cap, causing it to short, and/or a shorted filter cap which causes a supply resistor to cook)

[Searing]

The 2.7k 10w is toast. Reading open. Ordered a few new ones. I tightened all grounds, and resoldered them to the chassis. I will follow up with you all when I replace the resistor. Thanks for your help!

[AlNewman]

You can do some tests with the amp off as well, by testing resistance to ground.

So with the amp off and discharged, with no tubes, and the stand by switch closed, (or standby on), if the 2.7k resistor is open (or disconnected).  From one side of the 2700R resistor you should read somewhere around 30k, and from the other side, you should read around 200k.  It might take a while for the resistance to come up while the caps charge.

I don't see the 2 100k balance resistors in the doghouse, did you replace them when you did the caps?

Is there a possibility a blob of solder made it under the board when you changed the caps?

[Searing]

There were no 100k resistors in parallel with the big filter caps in the doghouse. There are, however, two 100k’s on the far right of the chassis by the power transformer. I was thinking they were an artificial center tap for the heaters, but maybe not?

[mresistor]

There were no 100k resistors in parallel with the big filter caps in the doghouse. There are, however, two 100k’s on the far right of the chassis by the power transformer. I was thinking they were an artificial center tap for the heaters, but maybe not?

Looks to me like the 100K resistors are in parallel with the 220uf filter caps on the schematic.  Artificial center tap resitors would be 100 ohm. So they were missing in your amp?

[mresistor]

the artificial center tap appears to be a 100 ohm pot.

[mresistor]

so these are missing? methinks they are the ones that you mention on the far right of the chassis   


compare your amp to the schematic   

[Searing]

Yea. Following the schematic, those two 100k’s are tied to the filter caps. And, the center tap is the hum balance pot on back.

[AlNewman]

There were no 100k resistors in parallel with the big filter caps in the doghouse. There are, however, two 100k’s on the far right of the chassis by the power transformer. I was thinking they were an artificial center tap for the heaters, but maybe not?

You're right.  the 100k resistors are at that tag strip, and the ground potential of the center tap is lifted by 100k compared to the chassis.

So back to testing, with the amp off, and the 2.7k resistor open and standby ON, if you were to reference your multimeter at the point the center tap connects between the 2 100k resistors, at one side of the 2.7k resistor you should see 100kish, at the other, you should see 130kish.

If you were to reference your multimeter to chassis, at one side you would see 200kish, and the other should see 30kish.

I think that's how that should work.

[Searing]

I have already removed the 2.7k resistor.

[Searing]

Popped in a new 2.7k 10w resistor, and the amp came to life! Thanks all!
Sounds stiff and sterile compared to my ‘63 AB763 I built years ago however. Are these master volume silver faces known for this?

[tubeswell]

Sounds stiff and sterile compared to my ‘63 AB763.

Yeah. I had a 73 quad reverb (same amp circuit with MV) in my stash that I sold off last year because I was lightening the load but also because I have smaller Blackface clones that do amazing cleans but weigh less)