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Post subject: Bassman 100 SF 70's problem
Posted: Thu Jan 31, 2013 12:11 pm
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Hey Folks,
Have 70's SF bassman 100 with mstr vol knob.
The owner bought this from a guy who attempted to install a grounded cord. Really did it wrong and ended up wrapping the fuse with TIN FOIL and powered it on and literally smoked the power tranny.
Owner bought a new Weber transformer https://taweber.powweb.com/store/022756sch.jpg.
This is the one I have installed in the amp.
I used the black and brown for the primary. The greens for filaments, 2 reds for diode bridge, blue/w red stripe for the other connection on the diode board, green-yellow and
red with yellow to ground. Tapped off the 2 red w/whites and solid blue wires.
First off could anyone verify the way I have wired it? Weber didn't know for sure.
With all the tubes in place and power switch only on and I flip on the standby it blows the fuse. Without the 4 6L6's you can turn on both the power switch and the standby and it doesn't blow the fuse.
Any ideas frorm you folks?
Thanks
Tube Dude


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Post subject: Re: Bassman 100 SF 70's problem
Posted: Thu Jan 31, 2013 4:08 pm
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tube dude, Sounds like the bias is maybe out of spec (too hot). Put in a new proper amp fuse. Put in a BIAS PROBE or similar adapter to read the idle bias. If it's way above 40mV read (or 40mA current draw) per tube, turn amp off and inspect the BIAS POWER SUPPLY.

This amp has NO BIAS POT but a BALANCE POT. So, what you need to do is read the voltage after the 3.3k-ohm dropping resistor in the Bias Supply circuit. Should be -57VDC (+/- 5VDC). If not, you need to look into that circuit before proceeding.

REMEMBER, you'll be dealing with a HOT amp and many hundred of volts DC.


If all this sounds Greek to you, a good amp tech maybe in order.


http://www.webphix.com/schematic%20heav ... _schem.pdf
Good luck!


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Post subject: Re: Bassman 100 SF 70's problem
Posted: Sun Feb 03, 2013 9:01 am
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BMW2002TI,
Well, after really going over this schematic and finally finding a very good and accurate layout, this amp didn't have the 3.3k resistor at all. I have been going over this part by part and realized the guy that attempted to install the grounded plug really screwed the pooch.
I installed the 3.3k from the fourth leg of the balance post to the single diode connected to the 470 ohm resistor on the diode bridge board and now the amp works although I can't yet get rid of the hum with the balance pot. I can set the proper voltage but there is a loud hum. May be the 80uf at 50v or one of the tubes.
Thanks for your help.
Tube Dude


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Post subject: Re: Bassman 100 SF 70's problem
Posted: Sun Feb 03, 2013 9:33 am
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tube dude, Check ALL grounding points in that BIAS POWER SUPPLY. Do you get similar idle bias readings with all 6L6GCs?

Also, a bit of contact cleaner in the sockets of those 6L6GCs may also help.

Good luck!


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Post subject: Re: Bassman 100 SF 70's problem
Posted: Fri Feb 08, 2013 7:07 am
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BMW2002Ti,
Grounding was not the problem...
Here is something else someone had done to this amp. On pins 1 and 6, the anodes, of the 12AT7, someone installed 2 120k resistors. It calls for 47K. I replaced them and this amp has come sweetly and extremely quiet. The customer says it takes all his pedals with no noise whatsoever.
I had also replaced all the PS caps re- tensioned all the tube sockets. Man it sounds and performs great.
Take care!
Tube Dude


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Post subject: Re: Bassman 100 SF 70's problem
Posted: Fri Feb 08, 2013 7:11 am
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Good news! You didn't lose too many units of blood over this amp.

And owner is satisfied! Time to kick and and have a brewsky.

:mrgreen:


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