Fender 1962 Concert Brown face 6g12a re-form modern caps?

[Derrick111]

I re-caped a 1962 Fender Concert Brown face 6g12a with Sprague Astrons a few years back, then through unexpected events, I had to store the amp for many years. I know the proper way to reform electrolytic caps is using a current limiting resistor, but it can be a little tricky to figure out how to do this for different amps. I must say, I am more familiar with Marshalls than Fenders. Anyhow, does anyone know the proper way to do this in the 6g12a Brown face Concert?

Thanks!!

[Jon Snell]

Remove the output valves.
Leave in standby and use your variac to slowly increase the mains voltage fromabout 25volts up to normal mains voltage over about 5 minutes whilst looking and listening for hissing or bulging/heating of the two tank capacitors before the standby switch.
Remove the supply and discharge the two tank capacitors.
Place a 100k 2W resistor across the standby contacts, keep in standby mode.
Repeat the process with the valves removed, looking and listening.
Any faulty ones will be obvious.
When satisfied with the result, power off, fit the output valves and run as normal.

If you do not have a variac, insert the 100k 2W resistor between the centre tap of HT winding and ground, test as above.
The 100k resistor will fail instead of your capacitors if they are faulty so wear eye protection and be careful as there are lots of angry pixies in there and they like to bite!!!

[Derrick111]

Thanks for the reply! Sorry, I used Sprague Atoms throughout the amp, not Astron caps... I posted very late ;-) I don't see a way to edit this. OK, two questions... what are you referring to by the two "tank capacitors"? Also, what if I put the 100k 2w resistor in line with the red wire coming directly off the rectifier? Wouldn't that form all the B+ caps at once instead of the tank caps being powered without the limiting resistor (though still slowly with variac)?

[Jazz P Bass]

What capacitors did you change?
If the filter capacitors are relatively new,then I really, I don't see why the need to 'reform' them.

[Helmholtz]

If the filter capacitors are relatively new,then I really, I don't see why the need to 'reform' them.

As the amp hasn't been used for "many years" with the Sprague replacement ecaps, re-formimg makes sense.
Re-forming can cure excessive leakage caused by over-storage/exceeded shelf life.

[Jon Snell]

Thanks for the reply! Sorry, I used Sprague Atoms throughout the amp, not Astron caps... I posted very late ;-) I don't see a way to edit this. OK, two questions... what are you referring to by the two "tank capacitors"? Also, what if I put the 100k 2w resistor in line with the red wire coming directly off the rectifier? Wouldn't that form all the B+ caps at once instead of the tank caps being powered without the limiting resistor (though still slowly with variac)?

(
The tank capacitors are the two capacitors the bridge rectifier charges before the standby switch. Powering up slowly will make sure they do not suffer full power at once and the 100k will limit any damage, (if there are faulty capacitors).

[Derrick111]

Thanks everyone.. I completed the cap reform and it worked great!! I ended up unsoldering the red wire from the diode board to the main board and putting a 100K resistor in line, then powered up slowly with a variac and no tubes. The measured voltage across the 100K resistor started at around 100v and I let it sit there reforming till the voltage got down to around 5v. Then I removed the resistors, soldered the red wire back in place, installed tubes and powered back up, but again, slowly with the variac in case of trouble looking for smoke and listening for bad noises. Once I reached full voltage on the variac, I tested the bias voltage and shut things down. I was getting -53v which is quite high. Will have to sort that out. The 6g12a schematic shows a 15K resistor between the bias supply caps, but mine is a 10K and looks stock. What bias resistor do you all use?

[Leo_Gnardo]

I tested the bias voltage and shut things down. I was getting -53v which is quite high. Will have to sort that out. The 6g12a schematic shows a 15K resistor between the bias supply caps, but mine is a 10K and looks stock. What bias resistor do you all use?

Before you start into fiddling with the bias note the resistor to be selected isn't the 10K or 15K, but the ground leg of the bias voltage divider, showing 56K on the schemo.

-53V may seem "out of line" until you consider the plus/minus 20% advice on most Fender schematics. What we do is measure bias on the output tubes operating in the amp, and adjust bias voltage accordingly. Good to have a 'scope and load resistors, then you can find the bias setting where crossover distortion occurs and set your bias just a tad below that so the crossover notch is gone. You can install a trim pot in series with a fixed resistor so you can scan a variety of voltages, also there's no harm in paralleling resistors to select your sweet spot.

Also keep in mind, once you have set bias for the pair of tubes in your output department, should you change tubes you'll have to repeat the process for them. Tubes vary, all over the place these days especially. I've seen bias voltage in Concerts and similar amps range from -42 to -55 V, with bias current in the 30 to 35 milliamp range.

[g1]

I tested the bias voltage and shut things down. I was getting -53v which is quite high. Will have to sort that out.

Not sure how you are meaning 'high'. In case you are not familiar, more negative bias voltage means lower idle current. From a safety and reliability perspective, you are better to err toward lower idle current. So the -53V is not a concern as far as that goes.