'68 Silverface Bassman AC568

[beakos]

"Oddity when tube 3 pulled? Don't both channels use V3?" IIRC, the bass channel uses 2 12AX7's while the Normal uses one.

I noticed one of the bypass caps had a solder connection that looked kind of black. I'd re-solder that.
The late BF and SF's have a reputation for not so great wiring (as evidenced by the wire wrapped around some of the input wires as shielding). Make sure the wrap is soldered at one end only and try moving them around with a wooden stick and listen for changes in hum or other noises.
Try probing around with the wooden stick and listen for any changes. Tap and press on components / wires / switches etc.
See if any wires carrying a signal are running parallel (and close) to wires carrying power.
Check the grounding buss for any corrosion.
Tighten all jacks, pots etc.
Are the inputs making noise when you touch the 'hot' lead with no plug in the jack. Burnish the contact or re-tension the grounding
Any flurescent(sp) lights or dimmers nearby?
Yes, check the heater wiring to be sure the wires terminate on the same pin of botth output tubes


All I can think of offhand. Keep us posted.

[pchilson]

Ok, I've been through it again.
Changed out all the bypass caps for new.
Heater wires, they were not all connected to the same pins. I corrected that.
Cleaned all the shorting jacks shorting points and tightened all the jacks.
Tightened all the pots, some were not so tight.
Sprayed contact cleaner in the pots.
Chopsticked all solder points, no odd noises or crackling heard at all.
The solder lug for ground wire for the power cord was not very tight on the PT bolt it was mounted to. Cleaned and scratched the surface underneath, tightened it and resoldered it.
The hum is still present and not changed at all however since cleaning the pots I now can kinda hear the wiper in the pot being amplified while I turn it. This is on both volume pots but this effect is not on any of the tone pots. When either of the volume pots reaches full volume the hum turns to a distinct hiss but only at the 9 to 10 position on the pots.

Can this simply be a case of worn out volume pots??? Would that be likely?

I don't posess the knowledge or test equipment to trace down this problem as Enzo suggests. I am kinda at the mercy of everyones experience and a shotgun approach I'm afraid.

Thanks for all your input. Hopefully it will lead to a conclusion soon.

Oh, and just for the record. When the volumes are zeroed the amp is dead quiet.

I'm interested by the fact that you say the noise increases with the volume controls on BOTH channels and that the noise is different on each channel.

What I meant by sounding different is that the hum takes on the characteristic of the channel you are manipulating. ie... bass channel the hum is deeper. Normal channel the hum is brighter. Also when switching the bright switch the hum is brighter.

[Rhodesplyr]

1) Is there any DC voltage on the volume pot terminals? There shouldn't be.
2) How high do you have to turn the volume pots up before the noise becomes objectionable?
3) I wasn't suggesting that 5881s are the source of the problem--just that they aren't the best output tubes for this amp. There's a reason that Fender chose 6L6GCs. I know Enzo hates it when I quote datasheets, but...you might want to check the relevant tube datasheets to see what I mean.

[pchilson]

1) Is there any DC voltage on the volume pot terminals? There shouldn't be.
2) How high do you have to turn the volume pots up before the noise becomes objectionable?
3) I wasn't suggesting that 5881s are the source of the problem--just that they aren't the best output tubes for this amp. There's a reason that Fender chose 6L6GCs. I know Enzo hates it when I quote datasheets, but...you might want to check the relevant tube datasheets to see what I mean.

I will check next time I'm inside it for DC on the pots.
As far as how high turning up the volume before becoming objectionable? At about 2.5 on the dial I can start to hear it. The amp doesn't really start to sound good and full till you hit at least 4 on the dial and at that point it is loud enough to be very clearly heard. You would turn and look if you were across the room. Whatever is creating the hum it is in both channels and is being amplified.

[pchilson]

Not a lick of DC voltage on the volume pots.

[txstrat]

Looking at the back of the amp, right to left.
Preamp tube 1 pulled, nothing on bass channel, normal channel unchanged.
Preamp tube 2 pulled, hum on bass channel, nothing on normal channel.
Preamp tube 3 pulled, hum on bass channel, nothing on normal channel.
PI pulled, nothing on either channel.

Oddity when tube 3 pulled? Don't both channels use V3? Why does the normal channel go silent and the bass not?

V2 is used by both channels. Still odd that bass channel hums when V2 is pulled. Source of hum might be before V2.
Just to make sure, are the heaters of the preamp tubes all on the right pins as well?

Schem: http://www.schematicsunlimited.com/d...sman-ac568.pdf

[pchilson]

Thanks guys. I appreciate all of your help and suggestions. I have found the issue and corrected it.

[tboy]

That's great news; congratulations on your success. Would you care to share what you found, and what you did to correct it?

[beakos]

Yes, please update us - I'm still trying to figure this one out. Was it a bad guitar cord?

[pchilson]

Hi guys. Been out of town for a while. The problem was a broken solder tab # 7 on one of the power tube sockets that the heaters connect to. It never showed up in a chopstick test but started to mainfest itself when the pilot light started to blink out and back on. I started to poke around with the chopstick and sure enough the wire from the tab to the pilot light was just moving around in the hole. I hit it with the solder sucker to clean it up and the whole top of the tab came off... I fashioned a solder lug out of 18 gauge wire, soldered it to the remaining bottom hole on the lug and resoldered the heater wires and viola. The hum was drastically reduced. There still exists a bit but I guess that is normal? Anyway I'm much pleased to have found something wrong to be fixed.


Now I wonder about some tips for this amp as per my initial post. I'm not sure I like the hum balance pot and not having a true bias adjust pot. Easy or hard to change?

I've seen mention of the 470 ohm resistors hanging off of pin 4 on the power tubes should be pulled. Can I do that and why would you do it?

[beakos]

Cool! Glad you found the problem and get 'er fixed.
Might want to do a search online for a Silver to Blackface Conversion. There's a bunch of places that will tell you what to do step by step. I believe Gerald Weber also has instructions in one of his books. Maybe you can borrow from a library? I think the Aspen Pittman / Groove Tubes book does as well. Could also compare your schematic to an earlier model (AA864?)
The conversion isn't too difficult if you go step by step and will make the amp sound more like a BF as well as allowing you to truly bias your amp.

[pchilson]

Cool! Glad you found the problem and get 'er fixed.
Might want to do a search online for a Silver to Blackface Conversion. There's a bunch of places that will tell you what to do step by step. I believe Gerald Weber also has instructions in one of his books. Maybe you can borrow from a library? I think the Aspen Pittman / Groove Tubes book does as well. Could also compare your schematic to an earlier model (AA864?)
The conversion isn't too difficult if you go step by step and will make the amp sound more like a BF as well as allowing you to truly bias your amp.

Thanks for trying to help but I've been searching for the how to's for sometime and only find generalities and not specifics. Thats why I started the thread to see if some of the experts would chime in. Guess not.

[beakos]

I think I can scan some of the stuff if you can hold on for a couple of days. We just moved and I'll have to go searching thru some boxes and I have a funeral to attend tomorrow ...

[pchilson]

Now I wonder about some tips for this amp as per my initial post. I'm not sure I like the hum balance pot and not having a true bias adjust pot. Easy or hard to change?

I've seen mention of the 470 ohm resistors hanging off of pin 4 on the power tubes should be pulled. Can I do that and why would you do it?

No takers? What is the purpose of the 470 ohm resistors? Can they simply be removed and the amp still work or no?

I know it is a dumb question but I've seen posts that say to pull them but have no explanation of why or if there is more to the story.

[pchilson]

Is anyone willing to answer questions? Kinda lonely talking to myself here. Thanks

Did I bring up a taboo subject or something?