66 Bassman noises/problems?

[Steve B]

I picked up this Bassman head about 4 months ago & finally got around to addressing some of the problems.

There was a noisy "ssssh pop, shhhh" going on some of the time, & the usual remedy was hitting the head to make it stop for a while. I ordered new filter caps & tubes, although I forgot one of the caps.

After changing the tubes, the noise was still there. The whole head is very microphonic, & I found that the bass channel's wiring to be extremely sensitive when tapping with a wooden stick.


The funny thing is, when I turn the bass channel's volume all the way up, the noise goes away.

When plugging into the bass channel, there is heavy distortion happening at 3 & above, totally unlike the normal channel. I realize the 2 channels are voiced differently, but more distortion on the bass channel?

Here is an overall view of the circuit board as well as a detail of the bass channel's controls. If more detailed photos are needed, let me know.



Also, when touching up some solder joints I saw that the resistors would bubble where the wire went into the brown (bakelite, phoenolic?) resistor bodies. Does this mean that they should be replaced?

Thanks,
Steve

[sbluestubes]

Replace all those old carbon comp plate load resistors with new ones.I would also replace all of the e-lytics on the preamp board & the bias filter cap. You need a heavier chisel iron for those ground connections Weller makes a 80 watt chisel iron Marksmen is the name I think any good hardware store has them or Mouser sells them. It is normal for the bass channel to be alot stronger than the normal channel.

[Regis]

That amp looks like somebody else has been inside it since Leo built it, other than you. There was probably those tubular blue caps in it prior to the orange drops, why were those replaced? The blue caps are good. If you still have them I would put them back in. Also, solder drips, new parts, etc.

Unless somebody disagrees, I would go in there with a vaccuum cleaner and clean it out. Scrape all that debris while holding the vaccuum close.

Get a can of contact cleaner or Deoxit 5 and have at it on all the pots. Replace those ceramics with silver micas.

Take bluestubes advice and replace the preamp resistors, that will help. They've probably drifted badly.

Touch a soldering iron to any suspect joint.

Those electrolytics have been replaced but it would only cost a couple of dollars to replace them again and be sure.

Pull all the tubes and retension the sockets, and run some contact cleaner down in them. Wait a couple of hours before putting the tubes back in and firing up though, once I fired an amp up right after I did this and the fumes cought on fire.

Of course make sure the new tubes are biased correctly.

Those are great heads, I have three of them. Good luck.

regis

[Amp Kat]

I'm wondering why they didn't replace the 16uf E-cap under the hood ? All it takes is one 50 year old cap to ruin the whole show ! I 'd get all those paper caps outta there and you may as well leave the orange drops but I also agree with Regis that the old blues are usually still good but I have encountered some leaky ones that seemed to have gotten real warmish. The carbon comps are a question and as stated just depends on how far they have drifted but changing all of them will change the tone and maybe take away some of the old Fender Mojo tone so expect a more tighter sound and maybe less Vintage so to say.If you do all that and it still has problems I'm afraid it could be a cause of the dreaded tweed disease that the boards get from absorbing to much moisture and they get saturated to the point of intermittent problems and the only solution is replacing the whole board. I know it's a lot of work but sometimes the only solution. Good luck man.,

[Regis]

I just noticed the power section, he still has those 220k resistors coming from the power tubes. He needs to get that shit out of there and rewire the whole power section like an AA864: 82k/100k resistors, .1 coupling caps, adjustable bias, etc.

Amp Kat, he said he forgot to order the 16mf cap. About those preamp resistors, i guess maybe he should do some measuring and replace only the ones that are drifted? I dunno, not sure it will make much difference if he replaces them all with more CC. They certainly need to be checked, that might be his noise problem.

[Mick S]

I use a 230 watt soldering "gun" from Radio Shack for chassis soldering. Works very well, heats in 8 seconds, and cools off.

[Steve B]

Thanks, everyone. (I started this reply earlier today, but I wanted to get more readiings before continuing the post. I see some of my upcoming questions were addressed by Regis.)

So far, the only things I've replaced are the filter caps & tubes. The rest is how it was when I got it.

I had already tightened the tube sockets, & also cleaned them & the pots. BTW, that nasty looking pile of solder on the chassis below the bass channel's treble control was already like that when I opened the amp up.

I am thinking that the bass channel's volume pot must be bad. When I plug in with the volume on 0, I still have volume. Plus, when I connect the #1 lug of the pot to ground, things get very quiet. Could this be a temporary fix until I replace the pot?

The biasing has be a little confused. Is this a bias balance instead of an adjustable bias control?


Now, after reading Regis' later reply it does make sense. I kept wondering why adjusting the trim pot only did very little, but did mess with the relationship between the 2 tubes. It's currently 46 ma @ 447V.

The amp really sounded good before doing the work, I just wanted to quiet it down a bit, & make sure everything was running right. I still don't think it was too bad of a deal for $150.

Once again, thanks for all the help,
Steve

[Amp Kat]

Not a bad deal at all Steve and it sure sounds like that pot is your main problem and you really need to get it outta there as it can cause all of the symtoms you told us about. Sometimes you think you have a bad tube or coupling cap and the pot is leaking DC onto your signal wreaking havoc with the whole scenario. It can pop and fizz but it most likely lost it's ground which is why it quiets down when you grounded it. Regis changing those plate resistors to Metal film 2 watters will reduce noise in the form of hiss a tad but unless they are really drifted way far up (mainly plate resistors) the noise is really an effect of increasing gain and noise and gain are proportional but in a well designed amp if you changed from carbon comp to metal film I'm not sure you could hear a difference. Everyone tells me they are quieter but unfortunately when I need them too they don't . I guess it can't hurt though.

[Steve B]

I found the cause of the microphonic noise...

The bass channel volume control was wired incorrectly.

The wire I'm pointing to should be going to the middle lug of the pot, & the other wire to the first lug.


Steve

[Regis]

I've been looking at these pics, and thinking, so stand back.

Steve, you say this is a 1966 Bassman, how did you determine that? I usually use pot codes, but I couldn't read yours from the pics. Is it Blackface?

The reason I ask is due to some peculiarities in the pic of the right side: The preamp section is wired like a Silverface AA371 model with the 1.5k/1mf cathode setup and pin 6 goes to the 220k resistor instead of the 100k/.1 cap. It has the 220k resisters in the power section like an AB165. On the other hand it has cloth wiring like a Blackface AA864.

I know Fender was screwy during their transition between Leo and CBS, but this has got me going

In any event I still think you should rewire the power amp to AA864 specs, it will make a huge difference in tone.

[Steve B]

I'm glad you questioned, because, today, I have been wondering the same thing myself, especially considering how the bias pot is set up.

I trust the guy who sold it to me, & if he said it's a 66, either it is, or he thinks it is. I don't know the history of it prior to him owning it.

This is what it looks like.


The tag date code "PE" could possibly mean May of 1966, but who knows if it was stuck on from another amp, & who knows if the cabinet goes with the chassis.


I'm not sure what numbers you need from the pots & trannys, but here are some photos. If this doesn't help, let me know & I'll take more.



Steve

[Bruce / Mission Amps]

The filter cap on the main B+ filter board is 49th week of 1965 and the preamp cathode bypass cap is 9th week of 1966.

Also, get rid of the yellow cathode bypass caps replacing them all with anything made in the last couple years.
Yes the volume control is wired wrong so turn the two wires on the lugs around so the wiper of the treble goes to the top of the volume pot.
Undo all the eyelet board mounting screws and shoot a little deoxit in between the eyelet board and insulation board, then blow the space out between the two boards with compressed air... and I mean use something with some air pressure punch behind it.
Leave the bias supply alone and replace the 15K resistor that is grounded to the bias balance pot's body with a 6K8 resistor in series with a 22K trim pot wired as a variable resistor.
Now you have a combination balance and current level setting bias supply, that mod is a winner.

[Tubebass]

Great tips in this thread guys, thanks! I'm about to start work on an AA 3-71 and all this info will come in real handy!

[Steve B]

Well, I didn't get to do all of the recommendations, but after adjusting the bias (hum) balance, the old Bassman sounds great! Dead quiet when I'm not playing. No telling what the current readings are on each power tube, but I turned the trimpot to the position where the hum was the lowest, like the schematic suggests. No unusual plate glowing either.

I still plan on replacing the old caps that I hadn't done, & add in a real bias adjustment, & some other things suggested. Quite a few of the resistors are off as far as their values, & I'll probably address that, too.

Thanks to all who helped!
Steve

[Sweetfinger]

People,....Ya gotta love 'em!

I like how the previous owner replaced just about every cap in the thing, but NOT THE ELECTROLYTICS! I'm pretty sure that the LAST thing that was bad in that amp were the old blue Cornells.
Looks like the power tranny is a bit rusty. You might hose it with a little rust stop spray paint.
IMO the blackface and early silver Bassman heads are just about the best sounding stage amps ever for getting a good solid rock guitar sound.