66 fender bassman blackface head

[cockandswan]

hey guys,
i don't know where else to turn.

i have what appears to be a modded AB165. some parts of it now look like the AA864 and AA165, but some of it is completely off the charts. i can post a picture soon (incase anyone wants to try whatever this mod is out) but i had a little problem with a repair.

i was replacing all the resistors (because i was getting some static noise discharge) and for some reason my "Normal" channel is now MUCH quieter than the bass channel.

where should i look for bad components? a tube? a resistor? a cap? biasing?

thanks,
johnny

[52 Bill]

where should i look for bad components? a tube? a resistor? a cap? biasing?

Welcome to the board!

If it happened after you started replacing resistors, I'd double check everything that you did, making sure that you didn't mis-wire or mis-value something. Also check to see if anything has come loose from moving other stuff around as you worked on the amp.

If only the normal channel is reduced in volume, then the problem must be in the pre-amp section. Yes, a bad tube or resistor or cap could cause the reduction in volume. Start by checking the voltages on the tube in each section of the pre-amp. Compare the voltage to those found on the schematic. They don't need to match exactly, but they will give you a guide to what range of voltages should be at each terminal of the pre-amp tube.

You said that the amp has been modified. Do you know what the mods were supposed to do? Do you want the amp to do whatever it was, or do you want to put it back to original condition?

[cockandswan]

this looks different from any of the mods i've seen online, but i'm not too good at reading schematics. i'm gonna post some pictures as soon as a can. i'm trying to restore it to the modification, since that's the sound i know and like.

i'm just trying to get a better understanding of how this amp works. where is the preamp section? i'll check all the tubes. one is definitely more microphonic than the rest, but the tech who heard me tap it said it was fine.

-johnny

[BCRGreg]

Make sure that the pots are all original. There was a guy that had a mod going around that bumped the gain ridiculously so he went to a 2 meg vol pot to calm it down(not the best Idea). We had a couple come in with that stupid mod and undid them while laughing.

I have the schem around here, I'll see if I can find it. It also involved buggering the B+ to give it a more "metal" sound.

EEDIOTS!

[cockandswan]

yeah, i'm not sure what most of the mods are suppose to do. i'm using it for a 3/4 scale musicmaster bass and some analog synths. it sounds really good for those.

i DID turn it on with the diode in backwards. it was a diode with a yellow band on it that was connected to a big 470 ohm resistor that attaches to the red/blue stripped connection to the transformer. it blew the fuse. could that have also damaged something in the normal channel?

thanks for the help,
johnny

[BCRGreg]

yeah, i'm not sure what most of the mods are suppose to do. i'm using it for a 3/4 scale musicmaster bass and some analog synths. it sounds really good for those.

i DID turn it on with the diode in backwards. it was a diode with a yellow band on it that was connected to a big 470 ohm resistor that attaches to the red/blue stripped connection to the transformer. it blew the fuse. could that have also damaged something in the normal channel?

thanks for the help,
johnny

Perhaps. Replace the diode and turn on the power and plug a dummy plug into the input. Flip the standby on. No blow, then go!

As a rule, always check the grounds along the strip, flow the solder joints(a solid LOOKING joint may not pass current very well).

[cockandswan]

i'd love to replace the diode, but i don't know how to identify it. it has a yellow band, and it's black and it looks like it has two other indentations on the body of the diode, but not actual colored rings. also there are no numbers on it. i don't think the schematic says what the diode is either.

any ideas on what type and vaule diode i should replace it with?

[52 Bill]

any ideas on what type and vaule diode i should replace it with?

That is the bias supply diode. You can use almost any silicon diode rated at least 100v @ 1 amp, to replace it. I'd recommend a 1N4004 or a 1N4007. It should be wired so that the banded end is towards the 470 ohm resistor.

If there was no bias to the power amp you may have damaged more than something in the pre-amp.

[cockandswan]

i'll replace that diode right away. there is no signs of damage to any resistors, but i'm guessing it wont be that easy.

what else do you recommend i look for as far as damaged components?

the amp functions normally with the exception of the "normal" channel being so quiet.

[52 Bill]

the amp functions normally with the exception of the "normal" channel being so quiet.

I can't see how the amp would work without a working bias supply.

The bias supply controls the power output tubes, without bias they will draw too much current and overheat, possibly damaging the output tubes and related resistors, etc.

[Rick Erickson]

How did you determine that the diode was in backwards? Not by comparing it to the others in the amp I hope. Reverse that diode and you can kiss your Bias Filter cap goodbye too. No, it won't do anything to your normal channel, unless your modification relys on the bias supply for some reason. Sounds to me like you may be in over your head...

RE

[cockandswan]

well, i had pulled the diode because it was unmarked and looked a little blackened on the leads. i put it back in backwards and turned it on. that blew the fuse, so i'm hoping that prevented too much damage.

i checked the schematic to find the correct direction. i guess it wouldn't hurt to replace the bias filter cap as well.

as far as being in over my head, i am an electronics student so there's a good chance that'll happen frequently... besides i'm willing to pay to repair my mistakes

[cockandswan]

yeah, the amp works great on the bass channel, so maybe the bias supply is okay?

[BCRGreg]

There are a million sites with all of the parts labeled on the schems, find one that matches your amp before anything else blows up.

[cockandswan]

i can't find an exact match, so i'm using the AB165 schematic.

i measured the voltages on the 12ax7s and found that the two middle tubes aren't getting any voltage. replacement time right?

now how to decided! groove tubes, mesa or chinese?