Bassman 135 question

[bradicusmaximus]

A buddy asked me to look at his 1980 Bassman 135 because of some power dropping issues. Yay me - this was the double-punch of a problem that wasn't readily reproduced and the owner is slightly neurotic. But, that's the fun of this stuff, right?

After going through the amp, it was pretty apparent it had never had a cap job and while there were no gooey messes waiting under the dog house, there were some good bubbles in the old silver mallories. A cap job it is then. Of course, I assume this was done after lunch on a friday because in digging in I find some nasty wiring and other bits of that legendary CBS QC. Rewire the filter board from the eyelets on up. No biggie - it happens.

Do a power on test and check the obvious voltage points. The rectifier shows good voltage and the return from the filters is looking ok, but the preamp has nothing. So, this leads to finding some old ohmite-style resistors that test fine, but decide they don't like passing voltage. To make an already long story a bit shorter, this cascades into rebuilding most of the power section.

Finally, after some parts orders and playing with almost 40 year old solder - not to mention more than one or two bad joints discovered, I power the beast on and start doing tests again. Everything looks pretty good. I even get noise out of the speakers. Woo hoo - given the headache of this old fender, I'm ready to kick back for an attaboy. Note - I love working on old fenders and have never had much issue with them, except some of these late-70's / early-80's models.

Anyway, as I look it all over I've got two things that have me a bit concerned and figured I'd journey to the fount of knowledge to 1) share my pain and 2) see if I'm just being crazy or if there's something I'm missing.

1) I noticed while on, the lamp dims at odd intervals. There is no corresponding voltage dip and I have this through my variac at a consistent 110v ac. I bring this up because my first fear with this amp before the cavalcade of parts replacement was it was an iffy transformer. My assumption though is that the secondaries would indicate a problem if it were something more than a lamp or socket. Thoughts?

2) On metering the amp while operating, it powers up and has a consistent draw of 1.12amps, which seems high to me. It doesn't waver or climb as would be indicative of a tube or something running away. I'm used to seeing that around 30 - 40% lower during operation so again I figured I'd get some thoughts. Though it's possible that again I'm nuts and just too used to working on two-bottle 6V6 and 6L6 amps.

Thanks for getting this far.

[TUBEDUDE]

No voltage dip of the mains i assume. What does the 6.3aV measure, and is it stable? Reflow solder of the lamp contacts if stable. If the voltage is low or erratic you may have transformer issues.

[martin manning]

The incandescent bulb across the heater string is dimming intermittently? If you have monitored the PT voltage and it's steady, then I'd suspect a problem with the lamp socket or solder connections, or perhaps a component in the hum balance network.

[bradicusmaximus]

I have a consistent 2.9 and 3.4 on the two leads direct from the transformer @117vac and pretty much all through the entire set of tubes. Not perfect, but definitely not as imbalanced as I've seen some.

[bradicusmaximus]

The incandescent bulb across the heater string is dimming intermittently? If you have monitored the PT voltage and it's steady, then I'd suspect a problem with the lamp socket or solder connections, or perhaps a component in the hum balance network.

That seems to be where I'm leaning. I was actually less concerned overall with the pilot lamp just, it just seemed annoying and something to get some input on. My more pressing concern was what felt like a higher than normal current draw. Everything seems stable (except for a stupid lightbulb) and the thing is definitely loud, I just didn't want to run into a situation where something that felt off turned out to be a red flag for someone with a fresh brain.

[martin manning]

1.12A and 110VAC is 123W. Not at all unreasonable for a 135W amp.

[martin manning]

I have a consistent 2.9 and 3.4 on the two leads direct from the transformer @117vac and pretty much all through the entire set of tubes. Not perfect, but definitely not as imbalanced as I've seen some.

There's no actual CT on the PT, so all that unbalance is coming from the hum balance nw, and depends on where the hum pot is set (its wiper is the ground reference).

[bradicusmaximus]

I have a consistent 2.9 and 3.4 on the two leads direct from the transformer @117vac and pretty much all through the entire set of tubes. Not perfect, but definitely not as imbalanced as I've seen some.

There's no actual CT on the PT, so all that unbalance is coming from the hum balance nw, and depends on where the hum pot is set (its wiper is the ground reference).

Thanks for that reminder. Appreciate the confirmation on the amperage as well. Happy New Year!

[Stevem]

The hum balance pot only effects 60 HZ hum heard at idle .
These amps most times have more 120 HZ hum then 60.

And even when Fender went up to these big wattage amps they still never addressed the issue they all had of too many preamp gain stages for the amount of filtering stages they had!

[bradicusmaximus]

So I just wanted to bring this to a tidy conclusion in case anyone comes across this in the future and finds use in the info...

This amp is done and happily back to its very neurotic owner and he is happy. It seemed that age took its toll on a lot of components. The heater balance was worked out by adjusting the hum balance pot. The simple act of moving it into a better position and taking the time to balance out the legs made a huge difference in overall stability of both the pilot light as well as ambient noise. The transformer seems to be pretty good and not indicating failures, so I call that a bullet dodged.

All in all, I'm glad it's off the bench, but really reinforces to me that it's so easy to overlook the dumb stuff like old trim pots.

Thanks everyone for the input!

Brad

[martin manning]

Appreciate the follow-up!