67 Bassman Hum

[Valvehead]

Im betting most of you have been thru this one......

Working on a 67 Bassman --loud hum - Volumes on 0 . power tubes in but no phase inverter tube in . The back story= owner was playing and they blew a breaker during practice. Reset and turned on this amp--it went up in smoke.
I found a shorted 6L6 and burnt screen grid 470 . I replaced the 2 470 ohm sc. grids and the 1.5k's at the sockets. new 6L6 tubes now.

Chasing the hum still , I decided to get a re-cap kit for it (electrolytics) . So all those are new---everything in the doghouse is new. Voltages look fine everywhere. Bias voltage looks good . The hum still persists.. This is with Volumes on 0 and NO phase inv tube installed .. just the 6L6 tubes

Checked all grounds . It does have a 3 wire ac cable. Bias cap and resistor are new also. Everything from the Hum pot to the tubes is good.

Im starting to suspect one of the 6 rectifiers (or more) . Checked everything in bias supply ( has the original diode still ) I cant find any resistors way out of spec .

Last thing i tried is clipping in 2 , 100 ohm resistors from pilot light ac to ground = no difference .

Any ideas ???

video---

https://youtube.com/shorts/hBA4EB5BXgI

https://www.thetubestore.com/lib/the...-Schematic.pdf

[Helmholtz]

What are the voltage drops across either side of the OT primary, i.e. measured from CT (B+) to each plate?

[Greg_L]

Im betting most of you have been thru this one......

Working on a 67 Bassman --loud hum - Volumes on 0 . power tubes in but no phase inverter tube in . The back story= owner was playing and they blew a breaker during practice. Reset and turned on this amp--it went up in smoke.
I found a shorted 6L6 and burnt screen grid 470 . I replaced the 2 470 ohm sc. grids and the 1.5k's at the sockets. new 6L6 tubes now.

Chasing the hum still , I decided to get a re-cap kit for it (electrolytics) . So all those are new---everything in the doghouse is new. Voltages look fine everywhere. Bias voltage looks good . The hum still persists.. This is with Volumes on 0 and NO phase inv tube installed .. just the 6L6 tubes

Checked all grounds . It does have a 3 wire ac cable. Bias cap and resistor are new also. Everything from the Hum pot to the tubes is good.

Im starting to suspect one of the 6 rectifiers (or more) . Checked everything in bias supply ( has the original diode still ) I cant find any resistors way out of spec .

Last thing i tried is clipping in 2 , 100 ohm resistors from pilot light ac to ground = no difference .

Any ideas ???

video---

https://youtube.com/shorts/hBA4EB5BXgI

https://www.thetubestore.com/lib/the...-Schematic.pdf

Did you lift the heater CT when you tried the 100 ohms off the pilot light?

[g1]

60Hz or 120Hz? Did you replace the bias filter caps?

[Valvehead]

Did you lift the heater CT when you tried the 100 ohms off the pilot light?

no

[Valvehead]

60Hz or 120Hz? Did you replace the bias filter caps?

120 i think---check the video . bias cap new , but not the diode

[Valvehead]

What are the voltage drops across either side of the OT primary, i.e. measured from CT (B+) to each plate?

2vdc / 11mVdc

[Helmholtz]

2vdc / 11mVdc

The side with the 11mV probably has a winding short. This would explain a 120Hz output ripple/hum.
Test the OT out of the amp.

[Valvehead]

Heres the secondary =

[Helmholtz]

Heres the secondary =

I'd say this is the primary.
As resistance values look good, this means that one of your power tubes is not conducting.
You wrote that voltages look "fine everywhere", but please measure again the screen pin voltages.

[The Dude]

Do you have a bias probe so that you could check each tube current separately?

[Valvehead]

Do you have a bias probe so that you could check each tube current separately?

i do not . Ill check the current tonight though

[Helmholtz]

Do you have a bias probe so that you could check each tube current separately?

Not necessary as we have the individual voltage drops across the primary halves.

[g1]

2vdc / 11mVdc

Swap the tubes around and see if the low value stays with the socket, or with the tube.

[Valvehead]

Ok, found something ....voltages checked again at 6l6 sockets and bias current is 27ma/18ma . I swapped many 6l6 in and out trying to get that closer .

Everything looks ok except pin 3 on one of the 6l6 sockets . When i touch the probe there my meter starts beeping like crazy and the screen goes wild ! Over voltage ?? Also , the hum gets Way louder when i touch that pin. This is with or without a tube installed .



EDIT------- This might be a clue -- it only does that with the PI tube in . If i remove the PI the voltage is normal at 463vdc . Tried different 12 at7's