68 SF Bassman volume loss

[MikeP]

Hi guys,

I scored a really great deal on a 1968 Silverface Bassman with the AB165 circuit, knowing that the amp would probably need some TLC. I have always had an interest in tube amps and have always wanted a Bassman, so I figured this amp would be a good place to start. I'm going to outline what the amp is doing and follow up with my observations and what I've tried so far.

Okay, so I power the amp up on standby with a guitar and 4 ohm speaker attached. All tubes light. A couple 12AX7s appear to be *slightly* dim, but it might be my mind playing tricks on me. After the tubes light, everything's nice and quiet, but a very light buzzing can be heard coming from the head. This buzzing stops after about two to five seconds and all is quiet.

Take the amp off standby and turn the volume up to 2. The guitar sounds clear and nice, but the volume suddenly drops off after a bit of playing. If I continue to play, the volume will come back for a while and drop off again randomly. It ALMOST seems like if I strum quite hard, it forces the volume to suddenly come back. Not sure if this is again my mind playing tricks on me or if it's related. I should also note that the dropoff happens at any point on the volume knob on either channel and also that when the volume drops off, it doesn't cut out completely and the guitar can still be heard through the speaker.

Anyway, so here's another issue. If I catch the amp when the volume isn't "cut out" and turn the volume up to 5-7, the amp doesn't get much louder, but does start to distort at that point. I've read that a healthy Bassman should be slicing my head off at that point, volume wise. So I feel like in addition to the volume cutting out every now and then, I also have an overall volume loss issue. Don't get me wrong, it's quite loud at that point, but not unbearable at all. This is through a 1x12.

I have to say, this almost sounds like the whole guitar signal isn't getting through to the amp.

Here are my observations so far:
The cardboard caps have definitely been changed at some point and appear fine visually.
There was one 6L6GC and one 6L6GB when the amp came to me.
Nothing has been modified to my eye. All wiring appears original.
Solder joints appear dark and somewhat corroded.
Pots are not scratchy, but don't turn smoothly at all.

Here is what I've done so far:
Replaced all tubes with known good ones (replaced 6L6GB with a 6L6GC).
Verified the speaker cab and cable are good.
Verified the guitar and cable are good.

Here is what I plan on doing:
Clean all jacks, clean chassis with compressed air/toothbrush, clean all pots, clean all tube sockets, check for loose pots, check for loose jacks, retention tube sockets, tighten down all screws, examine and/or remelt solder connections.

So does it seem like I'm on the right track so far or is there something that I should definitely check out first? Thanks!

[solderstain]

You probably know this already, but just in case... don't assume that a 'visually fine' capacitor (or anything else) implies a 'functionally fine' capacitor. I suspect the caps right away, based on what you're describing.

A good place to start with diagnostics is a full set of voltage readings. Got those?

[billyz]

Sounds like a bad ground to me. Do your amp cleaning and retighten everything first then recheck for the problem. I like to use deoxit behind each jack and pot to the chassis.

Here is an AB165 I just modded into a wreck like tone monster.

[MikeP]

Thanks guys. I was considering changing out all the caps, just because I'm unsure of their age. They might not be stock, but they might also have been changed out 20+ years ago.

I'll order up a new set and pop them in while I'm cleaning everything up and try it out again then.

[billyz]

Thanks guys. I was considering changing out all the caps, just because I'm unsure of their age. They might not be stock, but they might also have been changed out 20+ years ago.

I'll order up a new set and pop them in while I'm cleaning everything up and try it out again then.

It is a good idea to replace the filter caps if they are more than ten years old. The bias cap is sometimes the white molded type ( I think Mallory) they can be still be good and not necessary to change imo. Same is true for the cathode caps on the preamp stages.
The preamp cathodes run at such low voltages compared to the 25V rating that they seldom are leaky or bad.
The AB165 has really good Iron, but you will want to rewire the Output section from the Phase invertor to OT and get rid of the dreaded double feedback loop, the 220K 's around both 6l6's .
after this you will probably need to reverse the Plate leads from the 6l6's to the OT to get rid of the squeal.
Another easy mod is to lift the 470k feedback resistor around the 3rd stage.

[xtian]

Replaced all tubes with known good ones (replaced 6L6GB with a 6L6GC).

Did you really replace BOTH power tubes? Just want to make sure you do try a known good PAIR before you run down other paths.

[MikeP]

Yes, I replaced both power tubes with known good ones. It was not a matched pair, but they are both good.

Actually, I have a weekend update to this one. I took a magnifying glass to the power caps and spotted a dark "burnt" spot on the bottom of two 20uf caps, plus the same two caps have very small blisters. So I ordered up some new ones and see if that helps.

[solderstain]

At the risk of sounding repetitive, have you taken a full set of voltage readings yet? Voltage readings are a REALLY basic, informative diagnostic tool, and can be more revealing than your eyes can. Without voltage readings, you're on an Easter egg hunt.

Take a set of readings when you first fire the amp up, then go play for a while or whatever you do to get the voltage to drop, and take them again.

[MikeP]

solderstain, sorry about that I missed that question earlier. No, I don't have voltage readings yet. Me being quite new to this, I don't exactly know where to take them from. My dad would, and I'm sure he will when I bring the amp to him. Only thing is he's away on business for a while and I don't really want to go prodding around with the amp powered on without him. I was planning on taking the amp to him along with some new caps when he gets back.

However, if measuring voltages is something easy enough for a new guy like me to do, by all means let me know where to take them from. I just don't like the idea of sticking my hand(s) in a powered up amp. I know how to discharge the caps and everything, so I'm perfectly comfortable poking in the amp *without* power.

[Gibsonman63]

If you look at the Fender layout drawing, the voltages are there. Use a clip on the ground lead on your meter and clip it to the chassis. Print an extra copy of the layout and mark your voltages on the layout next to thier voltages as you go. Only probe with one hand and keep your other hand in your pocket. You'll need to plug the amp to a speaker or dummy load to protect the output transformer. If you are still uncomfortable, then wait for help.

Wall voltage is a few percent higher than it was in the 60's, so your voltages may be a few percent higher.

If your filter capacitors are bubbly, I would definitely replace them before I did much else.

[crbowman]

Mike,
Be CAREFUL!
If you are not familiar with taking voltages, by all means wait for your Dad.
There are voltages present that can kill you.

[solderstain]

Cool, Mike - I wasn't trying to force your hand in any way; it's more trying to have your trouble-shooting be based on some hard facts about what your amp is doing.

I get it about your newness to this - being 'new' doesn't mean 'don't proceed'. There are some good tips here already - get a clip-on ground lead, and when taking the other readings, have one hand in your pocket and handle the meter probe with your other hand. Slow and steady. No super-double-Dutch-chocolate-mocha-espresso beforehand, either. No liquids nearby to accidentally knock over. Good, thoughtful work practices apply for everyone, regardless of how many times some of us have done it.

It was also a good tip to make a copy of the schemo and write down your actual voltage readings next to the factory-recommended readings. You'll see where there are big variances (if there actually are any), and then you can concentrate your attention in that area of the amp.

[MikeP]

Thanks, yeah I won't be messing with anything in there until my dad is available to either watch what I'm doing or do it himself.

Sometimes I get just enough knowledge to get into trouble, and that's when I really have to be careful, heheh.

On a sidenote, would a 20uf@500v cap be okay to replace the 25uf@50v bias board cap? I know it's okay to go up in voltage, but would a 5uf drop be too much?

[Gibsonman63]

That 500V cap will be huge compared to a 50V cap. You'll have a hard time fitting it on the little bias board. The 25/50V caps are available for $2-3. There is another recent thread with lots of suppliers if you have trouble finding it.

[MikeP]

Ahh, good point. I didn't even think about that. Actually, upon closer inspection (my eyesight definitely isn't any good), I've found that it's a 50uf@50v cap on the bias board. So 20uf would definitely be way too much of a drop. I have a 50uf@50v on the way, so we're good there.