Fender Twin Reverb 1976 Silverface "Restoration in progress”

[Avraxas]

Ok we have news!

I replace all the components. Test it with current limiter all ok! Plug it to wall and works like a charm!

Sweet sound! Reverb works, vibrato works! Everything works!

Just I here this sound, this noice:

https://www.dropbox.com/scl/fi/k7k10s86ex87tgfadnhc2/Video-6-8-25-20-41-47.mov?rlkey=fbezvzvtad02z3oop0o2vzaed&st=s5b8rqey&dl=0

Any suggestions to reduce it?

Thank you!

And something else. Any tests I should do in voltages or anything to be sure that all is as should?

[stratomaster]

Time to start troubleshooting.

Pull the PI tube. If the noise stops it's in the preamp. Then pull tubes in the preamp starting with the input end to determine where it's happening. Once you find the stage responsible then it's time to ground grids, bypass plates, etc, until you find a few components that are suspect.

Divide and conquer

[Avraxas]

Thank you all!

The problem with sound it was just a micro metric adjustment in Hum pot. Now the sound is crystal clear!

Seems like everything is ready with the amp. Tubes are ok, all pots are working etc.

I want to ask if there are a pilot measurements that I have to do with my multimeter before i close it.

For example the bias adjustment, or anything else that is good to check.

Thank you

[pdf64]

Have 1R cathode current sensing resistors been fitted to the 6L6 sockets?
I had a quick review of the thread and unfortunately didn't notice a suggestion to do so.
They're very useful to check the anode / cathode current each valve is drawing at idle.
An alternative is the measure the resistance of each OT primary leg (anode to centre tap), then measure the idle V DC across each of those leg resistances.

[Avraxas]

Can you drive me to do this please?

[pdf64]

Can you drive me to do this please?

Fitting the resistors, or measuring across the OT?

Either of these are the easiest / simplest / least risky methods of assessing whether the bias setting is appropriate for the valves.

The benefit of the 1R cathode resistors is that they are specific to each valve. So it's easy to identify if 1 valve if ghe wuad is weak / different / not working for some reason.
Whereas the OT method assumes that the 2 6L6 on each leg of the OT are drawing the same current.

[Avraxas]

So as I read else where I need 1 ohm resistor with 1% tolerance on every 6L6 tube from pin 8 to ground, yes? Need to be 1W or better 2W? Any type?

Then I will able to put my voltmeter across each resistor and measure the mV. 1 mV across the resistor = 1 mA of current.

That’s it?

And what mA should I get? And I adjust mA through bias pot yes?

Need to do this mod too? Or it’s good to do it?

https://robrobinette.com/Silverface_Amp_Mods.htm#Convert_Bias_Balance

Thank you

[Avraxas]

Thinking the following:

If I put 1ohm resistor I will have to deal with 25 to 40 mV measurements yes? To be able to measure that with accuracy I need a multimeter that cost 500€.

Maybe it’s better to put 10 ohm so my measurement scale opens a bit?

[stratomaster]

You don't need an expensive meter to read millivolts.

What gave you that idea?  The free ones that Harbor Freight used to give away are capable of it with accuracy enough for tube amp work.

Also, we get away with 1 ohm resistors in fixed bias because of how close to a wire they are.  The more resistance you add to the cathode connection, the more you're shifting into mixed cathode/fixed bias.

If I were king for a day I'd mandate 0.1 ohm current sense resistors be used in place of the 1 ohms we all use. Then I'd raid the treasury and vanish without a trace.

[pdf64]

1R 1W 1% is a good spec.
They'll pass 1A happily.
If a 6L6 shorts, it's possible that more current than that will pass. To protect things from that, fit HT fusing, and connect a diode, eg 1N4001 or better, across the resistor, banded end to chassis common, as per many modern Fenders https://el34world.com/charts/Schematics/files/Fender/Fender_65_super_reverb.pdf

[Avraxas]

You don't need an expensive meter to read millivolts.

What gave you that idea?  The free ones that Harbor Freight used to give away are capable of it with accuracy enough for tube amp work.

Also, we get away with 1 ohm resistors in fixed bias because of how close to a wire they are.  The more resistance you add to the cathode connection, the more you're shifting into mixed cathode/fixed bias.

If I were king for a day I'd mandate 0.1 ohm current sense resistors be used in place of the 1 ohms we all use. Then I'd raid the treasury and vanish without a trace.

That’s the point. That cheap multimeters don’t have accuracy for this measurement. Even worse accuracy with the 0.1 ohm that you say.

Maybe I am wrong. But think about it

[stratomaster]

You're wrong.

A middle of the road Extech 300 series (at a cost of 10% what you've claimed you need to spend) claims accuracy of 0.5%. 

https://manualzz.com/doc/1526348/extech-instruments-ex330-12-function-mini-multimeter---no...

If that holds, then the resistor tolerance and the screen current effects in the measurement swamp the meter accuracy as sources of measurement error.

But if you prefer though, you can leave the resistors out and learn to do the output transformer voltage drop method--referencing the center tap and each leg of the primary to ground and subtracting for reduction of shock risk. I think the current shunt method has fallen out of favor for safety reasons.

[glass54]

My standard now is 1R 1W 1% MF (Cheap in lots of 100 from many suppliers). Fitted to Cathode of course and solid Ground.
...and Fluke 79 (very accurate) at mV DC. No probing around High voltages and Transformer. Very important for our OCD friends.
(inspired decades ago by a very talented "Mr Sluckey" What else can I say but "Thanks Steve" )
Regards
Mirek

[Avraxas]

Thank you all for the explanation