65 Blackface Bassman

[Manlizard]

So I just traded a Fender Pro junior straight up for a "broke" blackface bassman. When I plugged it end everything lit up fine, but when I played it it sounded almost like a ring modulator. There was also a nasty 60 cycle hum.

I've already ordered new power caps as well as bypass caps. I've already made my cap discharger thingy. It's a lead jumper wire with insulated clips and a 10k 2 watt resistor soldered in the wire.

Is there anything else I need to know before digging in?

[Gibsonman63]

Sounds like you are on the right track. Just the usual.

Convert it to a grounded power cable while you are in there.
Check all the tube sockets and tighten them up so they are snug
Clean the pots
Check the input jacks. They are grounding jacks, so make sure the contacts close well when the instrument cable is not plugged in. If you have a burnishing tool, run it between the contacts to clean them.
Check the fuse. You would be suprised how many fuses get changed out with 20amp fuses for the sake of getting through a gig. Make sure that it is the correct rating and it is a slo-blo
Don't forget to change the bias capacitor. You need the 50Vdc cap for that one, not the 25Vdc like the cathode bypass caps.

Put some decent tubes in there, bias it up and you are ready to rock.

[Manlizard]

I did forget the bias cap!!! Thanks for the heads up.....

As far as adding the three prong cord, do I just bypass the ground switch, 600Vdc cap and ac receptacle? Then ground the ground wire to the chasis?

[Gibsonman63]

Clip out the death cap. Bypass your ground switch completely in case you want to add presence control there later. I usually wire up the receptical so the original wires will reach. Hot goes to the backmost part of the fuse holder. The other side goes to your transformer. Nuetral can go the the receptical and the other wire from the transformer goes there as well. Ground goes to the chassis. You can ring lug it and put it under one of the transformer nuts or get a big ass soldering iron and solder it to a chassis ground.

You may want to check the heater wires as well. For your two power tube, each heater wire should land on the same pin on both power tube sockets. This makes them hum cancelling. Fender did not pay attention to it and lots of amps have crossed heater wiring. It is not dramatic, but every little bit helps.

[Manlizard]

I know I'm reviving an old thread here,

I've replaced all electrolytic caps, changed to a three prong plug, modded the "normal" channel by taking out a gain stage making it more AA874 flavor (it's an AB165), fixed the wacky bias circuit, removed the local feedback resistors, and changed the phase inverter(negative feedback resistor and coupling cap). The amp sounds great, well just the normal channel. I do have some questions/concerns....

When biasing, I measured 495v plate/cathode. Is this too high? I haven't measured just plate voltage, should that measurement be lower than the plate/cathode measurement?

When plugged in and playing the normal channel, it's sounds great and is nice and quiet, until I start messing with the bass channel volume. When I turn the bass channel up I get a nasty static and a slight volume drop. Keep in mind a don't have the channels jumpered or anything. COuld this be a bad ground somewhere? Or maybe a bad input jack?

[tboy]

...changed the phase inverter(negative feedback resistor and coupling cap).

Can you tell us more about exactly what you did here? Did you change the negative feedback arrangement from AB165 style (feedback to grid circuit) to AA864 (feedback to cathode circuit)? If so, you also need to reverse the polarity of the output leads to the speaker jack or you'll be getting positive feedback instead of negative.

[Manlizard]

Can you tell us more about exactly what you did here? Did you change the negative feedback arrangement from AB165 style (feedback to grid circuit) to AA864 (feedback to cathode circuit)? If so, you also need to reverse the polarity of the output leads to the speaker jack or you'll be getting positive feedback instead of negative.

That's exactly what I did, and yes, I did reverse the ouput leads....