Bassman head problem

[fdz24]

Hello from France everybody!

i'm restoring a bassman head, i did a cap job, power filter caps and resistors, all the electrolytics, new tubes, checked all the solders, the amp sounds really good but sometimes when i engage the standby on, the amp "putts' like a motorboat with a lot of distortion and low volume, i have to wait 5-10 minutes before the sound comes back...i checked the voltage on the 6l6's and i have 440v on pin3.
i also checked the voltage of the power supply filter caps and i have only 120-130v on two of them and when the amp warms up everything is ok again with 440v....
if someone can help me it would be nice!!!
thanks a lot
hervé

[pdf64]

Did it do this before it was serviced?
Maybe new part is out of spec?
Maybe there's a broken wire or dry solder joint - could be on the doghouse board, perhaps a link wire underneath.
Gently prod wiring, joints and components with an insulating probe (the classic 'sharpie') to see if you an bring the failure mode on.
Pete

[Jag]

Did you change the cap on the bias supply? If so, make sure you have it oriented properly (+ to ground for the bias supply).

[fdz24]

Hi pete and thanks a lot for your answer!
Yes the amp did it before i changed the parts...the bias supply cap is ok with the + grounded and i have the good negative voltage value (-56v). Also all solder joints are new except under the eyelet board...i think it's a good idea to take a look under the board.
thanks again
hervé

[jmaf]

Isolate the problem. Remove a first preamp tube and see if it still motorboats. Remove the second tube and check. Does the volume control affect it? If not, it's after the first tube....do the tone controls affect it?

Somewhere in there is a path for low frequency oscillation, it's normally in the power supply and power section, but isolating the section is the only way to find out.

[vavlehead]

Hello from France everybody!

i also checked the voltage of the power supply filter caps and i have only 120-130v on two of them and when the amp warms up everything is ok again with 440v....
if someone can help me it would be nice!!!
thanks a lot
hervé

What model/circuit bassman you have??
Ok have you checked the voltage coming off the rectifier diodes. you need rule out the power tranny.
And go down the line... The B+ voltages should be there once you are out of stand by mode.

[Raybob]

Hi pete and thanks a lot for your answer!
Yes the amp did it before i changed the parts...the bias supply cap is ok with the + grounded and i have the good negative voltage value (-56v). Also all solder joints are new except under the eyelet board...i think it's a good idea to take a look under the board.
thanks again
hervé

Yes but did you change that capacitor? If it was time for filter cap change, it was time for bias cap change too. Does that cap test ok?

[fdz24]

Hello Raybob
Yes the bias cap is new too.

[fdz24]

Hello
i have 445 coming off the diodes, when i'm out of stand by mode, i have 445v coming out from the two 70uf caps (wired in serie), but i have only 130-140v out of all the other caps (22uf), and when it warms up (usually 5-10 minutes) everything comes ok...(all the power filter caps and resistors are new).
It's hard for me to give you a good answer about this bassman model, because it's a solidstate rectifier type with a blonde circuit (with the présence pot), strange!!
thanks again.
i put some pictures on my facebook

http://www.facebook.com/media/set/?s...621.1330503895

[52 Bill]

If the voltage after the input caps drop that much, either something is pulling a lot of current or something between the first power supply node is increasing in resistance.

If the current draw was that high, I'd think that you'd see something burning, so I'd check the standby switch for carbon build up and the choke. Read the voltage on each side of each component and see if one or the other is causing the drop in voltage.

If the switch and choke test ok, pull the small tubes one at a time and see if the voltage comes back. This will lead you to the right circuit area to investigate.

[fdz24]

Hey everybody
The problem was the standby switch!! Everything 's ok now.
Thanks a lot for your precious help!
hervé