65 or 66 Bassman AB165 setup question

[vargvike]

If this post is incredibly long, i apologize in advance, i just want to provide as many of the variables as possible....

First off, here's what I've got:
50w '65 or '66 Fender Bassman, tube chart says AB165
2x12 50w speaker cabinet (fairly sure it's unoriginal)
Marshall 100w 4x12 1960AX Cab
'82 Les Paul Standard, recently removed the pickup cover from the bridge PU
'96ish ProCo vintage rat pedal

Alright... recently joined a band; practiced for several weeks using only the Bassman head, the 50w 2x12 cab, the Paul (PU cover had been removed for some time now), and the Rat. Things went fine for this duration. Only complaint i had was some muddy tone, but was willing to deal with that.

Enter the Marshall cab. I added this in hopes of pushing more air, having more presence, etc. Now I have switched between using this cab in the 50w speaker jack, and also the Ext. Speaker jack. This has not been consistent, so that's something that warrants further experimenting. I found that after 30-45 minutes of rehearsal with the amp @ about 6 or 7, there appears to be a drastic drop in what seems like volume at first, but is actually tone. The overall sound becomes very hissy, as if it's being strangled or forced through a very tiny hose, if that makes sense.

I have removed the Bassman 50w cab from the equation with similar results. Is this a result of inefficient use of power/speakers, or a symptom of a problem with the amp? Is it possible to use a 100w cab with a 50w head, or should i just give up the ghost?


Thanks,
Ben

[Enzo]

Not sure what you have going on, but always use the main speaker jack first. If you plug into the EXT SPKR jack ONLY, then the main jack shorts across the output.

[vargvike]

yeah, i haven't plugged into the EXT. jack alone. if i'm only using one cab, i'm plugging into the "speaker-50w" jack. i should also mention that running the amp dry at these levels, i'm not experiencing the drop that i was when using effects.

[Tom Phillips]

i should also mention that running the amp dry at these levels, i'm not experiencing the drop that i was when using effects.

Seems like that is a step toward isolating the problem. Next time it happens, plug the guitar straight into the amp. If the problem goes away then look to the effects board for the problem. You may then need to eliminate one FX box at a time.

[Bruce / Mission Amps]

My first guess would be the power tubes are going soft/flat after running for a while with that speaker load.
Very high idle current or soft power tubes going into thermal runaway sound like that.
A bad main filter cap can do all kinds of weird things too.
What do they look like and are the original?

I don't know many Marshall cabs that are 4 ohms, which is what your amp is looking for.
What is the impedance of the cabinets?
Normally a Fendery 2x12 would be 4 ohms but I've seen 8 ohm cabs, 16 ohm cabs and even ran into a miswired one at 32ohm one once!
Aren't most of the Marshall 4x12 cabs 16 ohms?

Anyhow, without knowing the actual condition of the amp previous to this, I'd think it needs a good tune up with new power tubes and filter caps.
Has it been refurbished yet?

[vargvike]

so, last night i checked to see if any of the tubes were microphonic, as a place to start; i used the old rubber pencil eraser method, and i can't really say whether i figured anything out. some tubes made a crackling sound, some made a sound similar to a hum, as in it actually had a tone to it. at least one made a tinkling sound. i assumed that these latter symptoms were microphonic tubes (the hum and the tinkle). what is the crackling sound? by the way, there also was a hum that existed at level 6+ that i seem to have eliminated by shedding the microphonic tubes.

how does resistance work? am i limited to 4ohm cabinets with this particular setup? i only figured i would be using the speakers inefficiently, i didn't think that it would adversely affect the tone...

[12AX7]

I can't tell you what it is exactly without hearing it and reading voltage, but I can tell you what I have seen in the past 30 years that caused similar issues.

First, it happens after the amp has been running for a while, so you should assume it's a thermal issue. I have seen tubes develop internal problems after heating up causing severe distortion, hiss and sometimes even bad squeel. If you have a bad tube, of course all you can do is replace it. I would pull up specs on this model and find out what tubes were originally used and get modern crossovers. The first ones to go will generally be the workhorses at the output stage, but it's also possible that the pre-drive tubes could be worn out. If it has a rectifier valve it could be weak as well, but the output is where to start looking first.

Also, disconnect your AC cable, Pull out the tubes (one at a time, and put them back as you go) and look down into the sockets with a flashlight and see if you spot any corrosion - clean the socket contacts with a tiny jeweler's screwdriver. It will look like white or brown oxidation or rust.

Sometimes you can even wiggle the tubes in the sockets and clear it - don't do it while it's hot or while the juice is on. Other than that, check you're speaker impedances to make sure they match - tube gear is not at all forgiving of a mismatch.

That's all I can suggest unless you know your way around the inside of a tube amp. Please stay out of the chassis my friend - there are voltage levels under that iron that give you a quick trip to the cemetery. I'm not doubting your abilities; I've seen it happen.

[km6xz]

By logic and reason there are some changes that should be looked at first, because they coincide with the observed change in performance, and they may be expected results from that condition.
The most obvious change was adding the 4x12 cab. It has a nominal impedance of 16 ohms. Your amplifier is expecting a lower impedance of 4 ohms. What would be the expected result, does it fit the symptoms? Yes, a 4:1 mismatch in load Z will stress the either the output transformer or power tubes depending on the direction and degree of mismatch.
The tubes might have been damaged, going "soft" which lowers their gain which would change their bias requirement for any expected operating point.
Try running only the 50 watt 4 ohm cab to see it the problem is temporary due to overheating the tubes, or permanent, requiring replacement of the power tubes.
The term "soft" in a tube came from the concept o soft or hard vacuum evacuations and dates far back into history before electronics. An electron tube that has been manufactured well usually has a hard vacuum meaning there is very little in the way of gases left in the tube when it was sealed. A soft vacuum is intentionally introduced into some tubes for special uses. A power tube can generate its own gas by sloughing off metal atoms and impurities that are let in the metals. That can occur with age and by over stressing the anodes, the screen grids or control grid. In this case the anodes. Running a power amp into a large mismatch of loads can start that process, or accelerate it. The transfer function of the tube changes as gases form. Sometime the gases will be reabsorbed by the anode. When run within design spec, a tube should last decades but most guitar amps are not designed with a lot of attention to engineering principles, particularly amps that have been hacked or mod'ed.
You may find that running at 4 ohms your old tubes might be fine. Don't change tubes or anything else until you pinpoint the problem.

The other suggested solutions such as filters etc just do not fit the symptoms, so are very low probability fixes.
As in any field, the highest probability solutions are the very simplest based on "Occum's Razor" a term used for the logic law of succinctness. In this case; you changed something and noticed a change.