5E3 rebuild

[koen]

This weekend I spent a lot of time on taking apart my 1 year old Mission 5E3. I've always ghad some issues with hissing, and I thought that checking and reflowing all the joints, etc could improve that. And it did, the amp is now very quiet, so time well spent.

Before closing it up I measured the voltages, with the listed range between brackets:

V1 #1: 121V (105-120)
V1 #6: 115V (105-120)
V2 #1: 145V (145-160)
V2 #6: 178V (185-205)
V3 #3: 366V (345-365)
V3 #4: 320V (305-320)
B+ 377V (355-370)
before V1 plate resistors: 232V (215-235)

Except for the B+ which is a bit too high, and V2#6 which is a bit too low, they all are fine. I guess that depends on the tubes as well, so I am not worrying too much about it.

One other thing I noticed is that the resistance measured over each 100 Ohm cathode bias resistor on the V3 and V4 sockets measures 50 Ohm. I looked at the schematics, and they are in series, so I don't quite understand that.

Also, the resistance between the two filament leads is 0 (zero) Ohm, I'm not sure why that is. Although now that I think of it, the filaments are a closed circuit in the tube, so that makes sense - correct?

Soooo, anyway, my 5E3 is back together. We'll see how it behaves at practice later this week.

[MWJB]

377v isn't bad for B+, 366v at the 6V6 plate (pin 3) is fine.

"One other thing I noticed is that the resistance measured over each 100 Ohm cathode bias resistor on the V3 and V4 sockets measures 50 Ohm. I looked at the schematics, and they are in series, so I don't quite understand that." The cathode bias resistor for the power tubes is the large 250ohm resistor on the board that connects to both power tubes pin 8s. If these 100ohm resistors connect to pin 2 or 7, then they are the heater virtual centre tap. The junction of the 100ohms may be grounded, or connect to the hot side of the aforementioned 250ohm resistor (6V6 pin 8). Either way 50ohms seems fairly reasonable as a reading.

The resistance between the 2 filament leads will read "0ohms", or lowest default setting on the meter because of the very low resistance of the heater winding of the PT.

[koen]

Thanks for the clarification on the 100 Ohm resistors and filament resistance.

[koen]

So I just saw this thread: Stock 5E3 B+ Voltage

The second poster states that a B+ over 370V could cause distortion at all volumes. I am actually experiencing that, could this be related? When I just got my 5E3 a year ago I never had distortion at lower volumes. Is a B+ shift something that can happen over time?

If so, is there anything I can do about it?

[MWJB]

"The second poster states that a B+ over 370V could cause distortion at all volumes." Absolute PIFFLE! There are plenty of 5E3 made by Fender that sound fine & run over 370v on the B+.

Your distortion is not related directly to your B+, 366v at the plate is perfect, you do not need to "fix" this.

Post some pics of the build, check integrity of grounds & remake any grounded component eyelets that read higher than default ohms to the chassis with an ohmeter, check routing of grid wires, keep them short & away from plate & heater wires.

[koen]

Ok, thanks for the clarification - and I learned a new word

Was fighting mostly a crackling input last night - turned out there was a drop of solder on the inside of the input jack that got slightly touched by the cable plug. Was kinda difficult to see. Practice is tonight so I can test it with the full band.