Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Big chief on February 09, 2016, 10:38:16 am

Title: Fender 140 amp
Post by: Big chief on February 09, 2016, 10:38:16 am

Purchased fender 140 amp,
Installed new tubes turned on and Blew on of the power Tubes
Can you tell me what is wrong
Thanks

Title: Re: Fender 140 amp
Post by: sluckey on February 09, 2016, 11:25:10 am

My crystal ball says... Not much to go on. Could just be a faulty tube.

What voltages do you measure on pins 3, 4, and 5 of all 6L6s?

Title: Re: Fender 140 amp
Post by: Big chief on February 09, 2016, 01:21:17 pm

Voltage on V 6 pin 3. 558 4 120 5 560
                  7 pin 3 560 4 558 5 558
                  8 pin 3 560 4 0   5 560
                  9 pin 3 558. 4 0  5 558
Have replaced electrilyte caps checked the 470 resisters they are in line
This amp as I'm sure you know has low power and high power switch all test were done on low power as I have blown 3 6l6 tubes today.

Thanks for all you help

Title: Re: Fender 140 amp
Post by: jojokeo on February 09, 2016, 01:40:41 pm

I'm not familiar with this amp but check / test your coupling caps after the phase inverter to be leaking DC? Especially the side that your tube is blowing. Did you see major red plating before one of the tubes blew? Also we're assuming you are checking bias conditions as well as your OT and ground connections on pins 1&8? Your voltages are out of whack. Double check them. V6 pin 4 is low - should be similar to V7's. You shouldn't have high voltage on pin 5's which are your grids...please check again. If amp is fixed bias should show a negative voltage on all pin 5's.

*Suggest you use a current (light bulb) limiter while you troubleshoot to keep from any further damages until you figure things out.

Title: Re: Fender 140 amp
Post by: sluckey on February 09, 2016, 02:04:22 pm

Quote

Voltage on V 6 pin 3. 558 4 120 5 560
                  7 pin 3 560 4 558 5 558
                  8 pin 3 560 4 0   5 560
                  9 pin 3 558. 4 0  5 558

I don't believe those voltage readings for pins 5. Those should be about -50vdc in high power or -25vdc in low power. V6 pin 4 doesn't look right either. Pull all 6L6s and recheck all those voltages.

Title: Re: Fender 140 amp
Post by: Big chief on February 09, 2016, 08:19:43 pm

Tubes pulled
Pin 3 263 pin4 -02 pin 5 256 v6
Pin 3 263 pin4 -20 pin5 260 v 7
Pin 3 263 pin4 -17 pin 5 256 v8
Pin 3 263 pin4 -17 pin 5 256 v9

Should I replace the 1500 resistors and the 470 resistor on tube sockets
Thanks

Title: Re: Fender 140 amp
Post by: PRR on February 09, 2016, 08:42:51 pm

Are you counting pin numbers right way round?

There are sometimes very teeny numbers on the socket. Usually on terminal side, but sometimes on top.

The diagram in the data sheet is as-seen from the *bottom*.

Title: Re: Fender 140 amp
Post by: Big chief on February 09, 2016, 09:47:55 pm

Correction
V6 pin3 264 pin4 264 pin 5 3.3
V7 pin3 267 pin4 267 pin 5 20.9
V8 pin3 267 pin4 267 pin 5 18.5
V9 pin3 268 pin4 268 pin 5 18.5

Thanks for all your help

Title: Re: Fender 140 amp
Post by: PRR on February 09, 2016, 09:58:54 pm

At the lo-power setting 265V, negative 20V on pin 5 is reasonable, negative 3V is surely bad.

But this amp drives the pin-5s in pairs, so you would expect two to be the same?

Look REAL good for broken connection or bad solder around V6 and whatever V6 pin 5 connects to.

Title: Re: Fender 140 amp
Post by: Big chief on February 10, 2016, 01:36:33 pm

Thanks for all your help
Repaired loose connections on V6 and replaced resistor
All is well now
Again thanks for all the help

Title: Re: Fender 140 amp
Post by: jojokeo on February 10, 2016, 02:37:42 pm

Good to hear you fixed it but too bad it cost all those tubes.