Fender Bassman 1968 silverface

[chrisdski]

I just picked up a 1968 bassman silverface amp and cab (2x15). AB165. Previous owner told me it needs some TLC to get it up again. No tubes came with it. I pulled the amp from the chassis and the big black transformer had all it's wires unsoldered from their connections- not sure if it's to power the external power out? All the pots date to 1968, all the wiring and caps look original. I will probably send this off to an amp tech to get it up and running again. Any ideas why the previous owner had the big transformer unhooked (he played guitar)? Any recommendations regarding amp techs, preferably in the northeast US? I will play bass thru this, so I plan on keeping it all stock and prefer not to replace any parts unless absolutely necessary.

[johnallg]

Chris:

http://www.ampwares.com/ffg/schem/bassm ... layout.gif Which tranny?

http://www.ampwares.com/ffg/schem/bassm ... _schem.gif How to hook it back up.

The 7025 tubes are special 12AX7 tubes. Any low noise 12AX7 would do - like JJ or Tesla. 12AT7 tubes are common also.

[chrisdski]

Looks like the power transformer. Is there a way I can easily check the quality of it with an voltage meter?

[chrisdski]

OK, Got a good look at it. Using http://www.ampwares.com/ffg/schem/bassm ... layout.gif the power transformer is at the extreme left center, circle with 9 wires coming out of it. I have traced the layout and see where the wires were unsoldered. The green/yellow wire is cut off close to the transformer exit. All the others are full length and can reach their destination point.

Questions:
1. Is there a way to check this transformer's quality without re-soldering it back in to the circuit and using the schematic tolerances under power up? And does it need to be tubed also to check tolerances for this transformer?
2. The green/yellow wire that was cut goes to ground- this ground should be re-established?
3. I understand the high voltages involved with tubes and that capacitors retain charge even after power off.

[chrisdski]

I emailed the previous owner. He stated he had no idea the power tranformer was unhooked. I am going to resolder it back in. I ordered some tubes from Eurotubes.

[johnallg]

OK, Got a good look at it. Using http://www.ampwares.com/ffg/schem/bassm ... layout.gif the power transformer is at the extreme left center, circle with 9 wires coming out of it. I have traced the layout and see where the wires were unsoldered. The green/yellow wire is cut off close to the transformer exit. All the others are full length and can reach their destination point.

Questions:
1. Is there a way to check this transformer's quality without re-soldering it back in to the circuit and using the schematic tolerances under power up? And does it need to be tubed also to check tolerances for this transformer?
2. The green/yellow wire that was cut goes to ground- this ground should be re-established?
3. I understand the high voltages involved with tubes and that capacitors retain charge even after power off.

1. Only if you have the ohm readings for the windings. You could look for a short, but the winndings for heaters would almost look like a short so that isn't much help without the expected readings.

2. According to the layout drawing, yes. The schematic shows it also going to ground - the heater windings. So yes.

3. YES!! Use a meter or resistor to bleed off the voltages. Especially if you power up without tubes. Remember, tubes ALWAYS need a load on them so hook up the speakers when powering on, unless you have a resistive dummy load.

Also, when poking around a tube amp with a voltmeter, keep one hand in your pocket and one to do the work - that way any shock you get won't go across your heart. ALWAYS USE CAUTION! Don't be afrain, but ALWAYS RESPECT! I've been poked by 800V on a tune GE RF radio and it AIN'T FUN! Hurts for hours, and was only between the fingers on one hand, but the whole arm shook.

[chrisdski]

The plot thickens- going on Ebay and researching bassman amps for sale, you can get some nice pics of the wire layouts. I found a 1967 and a 1969. The 1967 clearly shows the green/yellow wire in question, but the 1969 shows no such wire. A bit more research places my amp in early 1969.

[chrisdski]

looked at the next production line, the AA371- no green/yellow going to ground. I will look a little deeper at the layout to see if thats what i have even though mine has a sticker saying AB165.

[chrisdski]

My bias adjustment pot is wired like the AB165.

[soundmasterg]

I can take a look when I get home at the excellent layouts for Fenders in Dave Funk's tube amp workbook. Hopefully that amp is in there and I can see where the wires should go.

If you can figure out where to hook up all the wires, then you can power the amp up with no tubes in it. If it blows fuses with no tubes in it, then disconnect the secondaries of the power transformer and try again. If it still blows fuses then the power transformer is bad. If it doesn't blow fuses then after you reconnect the secondaries, you can check the voltages at the power tube sockets, pin 3, and it should be in the 450v to 500v or slightly higher range. If those look ok, set the bias pot to max negative voltage, then turn it off. Put the preamp and power tubes in, and then set the bias and play happily. The bias pot could be a bias balance pot, and if it is, then just put the tubes in and set the pot for least amount of hum.

I do work on amps, but I'm in the northwest. If you need a good tech in the northeast, you may talk to Jess oliver, who is the original designer of the Portaflex Ampegs. I'm not sure if he works on Fenders, but if not, he could recomend a tech to you who could.

Greg

[chrisdski]

Thanks Greg. Again, it's the mystery green/yellow wire shown in the AB165 layout from the power transformer to ground. Mine is cut short at the transformer, and the 1969 picture referenced above seems to show an absence of this same wire. Which are the secondary wires out of the transformer, the greens?

[chrisdski]

here is the power transformer in question.
http://www.tubesandmore.com/scripts/fox ... =P-TF22814

[chrisdski]

Well I wired the power transformer back in today. Powered it up and everything looks good. I checked the voltages coming out of it- the red wires were all within specs. The greens were a little weak (3.5 v, should be 6.5 - again, this may involve the green/yellow wire not going to ground?). Checked the voltages going to the tubes, looks good also. The standby switch cut the voltage in and out. I guess at this point I will wait for my tubes and when I get them I will power it up and see how it sounds. Thanks everyone for any help you've given me.

[soundmasterg]

The green wires are for the heater supply, and each leg will be approximately half of the 6.3v. I'm guessing the green/yellow is the center tap for the filament supply, though not for sure on that. I didn't get a chance to look at my info at home yet. You may find that when you get the tubes in and bias it up that it hums too much, in which case the center tap of the filaments needs to be hooked up. They often would also hook a 100 ohm resistor between each green filament wire and ground to create an artificial center tap. You may aleady have these in the amp, or there could be the often used hum balance control which is a similar thing to the resistors but on a pot.

You can check your heater supply by setting your meter to AC voltage, and measure from each half of the heater supply. You should get between 6v and 7v Ac, with 6.3v being the ideal. With today's wall voltages I'm guessing it will be more like 6.5v Ac to 7v AC, which is fine.

Greg

[chrisdski]

OK, I think I've narrowed it down to the bassman AC 568 circuit (or AA 568 depending on different websites)

http://www.schematicheaven.com/fenderam ... _ac568.pdf

Mine has the 150 ohm/ 7 watt resistors coming off pin 8 of the 6L6, and the separate 2000pf capacitors coming off pin 5, both of these go to ground. The AB 165, AA 270, and AA 371 circuits do not have the 150 ohm resistor off pin 8. My ceramic disc capacitor had come loose of ground off pin 5, and really staring at the schematics about this led me to see the differences.

Also, no green/yellow wire off TR1 in the ac 568 which started this whole thread anyways.

I received my tubes today from eurotube- hope to get them in this weekend.