Bassman questions...

Steve1216

Member
Messages
1,490
just picked up a 66 bassman head. Cleaned out pots, replaced power cord, and removed death cap. It works 'ok'. But had trouble keeping up with band through a 410 the other night. And completely broken up sound by the time I got to 6 on volume. I know it should be louder than that, even accounting for the 8 ohm cab. The tubes, while not original fender tubes, are all mix matched brands from the era. Radio shack, etc. Even the two power tubes are two different brands. Could this account for the weak volume? Which tubes affect volume, mostly?

Also, I'm thinking of what cab I want for it. Want to keep it small. Either 210 or 112. Has anyone done this configuration for a bassman before? I have usually only played them with bigger cabs. 212 and up.
 

Rod

Vibrato & String Bender on Overdrive
Gold Supporting Member
Messages
25,156
You should replace your power tubes with a new matched set and have the amp biased. Have all the tube sockets been retightened and cleaned? Check all your preamp tubes as well...and replace all the large electrolytic capacitors
Bassmans work well with every speaker I've ever tried them with. I particularly like 2/12 cabs
 
Last edited:

MKB

Silver Supporting Member
Messages
9,469
Awhile back I worked on a Blues Deluxe that had an unusually low output, it was loud enough to gig with but not as loud as it should be. Put it on the dummy load and it would only put out around 12 watts before clipping. Turned out the problem was a short that caused one half of the phase inverter to not output signal (took a long time to track that one down, it was a solder sliver on the PCB). Got rid of the short, and the amp put out a little above 40 watts, which is correct.

The point is if the preamp on the Bassman works OK, but one of the power tubes isn't putting out power or the phase inverter isn't putting out the voltage it should, it could make the amp sound like it works but not put out the power it should. It's rare, but one half of the PI tube might be weak or dead for example.
 

swiveltung

Member
Messages
14,483
You need it tuned up. I would suspect power tubes, biasing, and any of those cheap pre tubes. Sometimes sellers put in a bunch of junk tubes and remove their favorites.... at least it appears that way!
A Bassman should pretty darn good with the 8 ohm speaker mismatch. (not all amps do) I have made Bassman open back combos with 12" and 15" speakers. They sound good. You should get at least a 75 watt speaker if you go with one speaker.
 
Messages
6,843
You wouldn't buy a '66 Chevy pickup truck and head off across country without checking the brakes, plugs, points, fluids, belts and hoses would you?? I once witnessed a car that would NOT start until the oil was changed. That is how neglected that car was.....sludge in the pan! Your amp is in the same condition. It needs fresh electrolytics, good general service and good tubes. The preamps, if they are old U.S> or Euro tubes, may be better than what you can buy. They don't wear like power tubes.
In short, you need a good tech to put this amp into proper working order so that then you can enjoy it.
 
  • Like
Reactions: Rod

Robal

Gold Supporting Member
Messages
1,620
Besides the tubes, I would visually check the power supply electrolytic capacitors to see if any of them appear to be bad and/or any of the resistors connecting them appear burnt. (Drain the voltage first so you don't accidentally get shocked.) You can unscrew and remove the metal cover and just look at them. If they are brown paper wrapped and you see any bubbles on the sides, that's a sign of a potentially bad filter cap. If they have not been replaced since 1966, they could well be weak and probably should be replaced as precaution. They are easily replaced by a tech or you can do it yourself if you are careful. I properly working 1966 Bassman is a relatively loud amp. Has it been modified?
 
  • Like
Reactions: Rod

rickc007

Gold Supporting Member
Messages
3,915
I too have a 66, and built a brand new clone, with Mercury Magnetics Fat Stack as the output transformer.

They both break up earlier than other Blackface Fenders, and are known to do so

The Bass Channel, and depending which version you have (aa864, aa165 or ab165) , you may even have 3 gain stages in the preamp section.
The 40/50 watt Blackface Reverb models have use 3 stages, but some of that is to recover from the trem and reverb.
Bassman does not have that, thus, it's a Gainier Amp.

It can get loud, but it's not a headroom amp.

Unspoken Gem though ... it is rawkus when it breaks up, great rock tone.


.
 

Steve1216

Member
Messages
1,490
Besides the tubes, I would visually check the power supply electrolytic capacitors to see if any of them appear to be bad and/or any of the resistors connecting them appear burnt. (Drain the voltage first so you don't accidentally get shocked.) You can unscrew and remove the metal cover and just look at them. If they are brown paper wrapped and you see any bubbles on the sides, that's a sign of a potentially bad filter cap. If they have not been replaced since 1966, they could well be weak and probably should be replaced as precaution. They are easily replaced by a tech or you can do it yourself if you are careful. I properly working 1966 Bassman is a relatively loud amp. Has it been modified?

I did look for bubbles on the caps. I didn't see any. It has not been modified except for, as I say, me replacing the two prong cable with a grounded one.
 

Tony-Cliffton

Silver Supporting Member
Messages
1,251
replace the power tubes first, you need new matched tubes anyway. If it fixes it, great. Then pack it up and bring it in for a tuneup.
 

Vanyu

Member
Messages
738
If the filter caps are original, I'd replace them regardless of how they looked. A 50 year old electrolytic is a 50 year old electrolytic. I'd also change the bias caps while you're in there.

Give the tubes a physical once over, make sure the plates are not warped on the sides, and check the condition of the micas and make sure they're not separating. If they look okay, check the idle current and make sure they're in the same ballpark. If you have the stock AB165 bias supply, then get them in the same ballpark if they aren't already. With the AB165 supply, you're able to use mismatched tubes since your bias control is actually a bias balance control, and this could be why your amp has two different power tubes in it. Just adjust the pot until both tubes read the same at idle. If you're amp's bias supply has been converted to AA165 spec and your tubes are too far apart, it's time for new tubes. If you've made it this far, then check the tubes for microphonics by giving it a gentle tap with a pencil or chopstick. If they're in need of changing, you'll hear the chopstick through the speaker.

Once you ensure that tubes are all running as they should, and you've got fresh filter caps in there, you should be good to go for a little while longer.
 

toddincharlotte

Silver Supporting Member
Messages
1,875
4 Ohm 2x12 would be louder than the 4x12 8 Ohm. Having said that something is up. I use a 1x12 8 Ohm cab and worry about standing next to it at around 4-5.

As others have said, these amps are definitely "rockers". They are considered blackface amps but shouldn't be compared to amps with the AB763 circuit [Twin Reverbs etc] that are known for their scooped cleans.
Think edge of breakup, hit it with a boost for crunch, use an OD for big drives.

These amps are rated at 50 watts but I know a tech who bench tested one at original spec and the was around 38 watts.

I would take to a tech for a bill of health. Could be power tubes, caps etc. Put $200 into it man....it's a classic rocker and worth that!

Here's a demo I did a couple of years ago, what to expect....

Cleans: 0:55 - 2:15 Light Drives (JHS Morning Glory): 2:30 - 3:00 Moderate Gain (Timmy): 3:10 - 3:40 Red Barchetta [exceprt]: 4:10 - 5:25 Ten Years Gone [exceprt] 6:00 - 7:35
 

rickc007

Gold Supporting Member
Messages
3,915
ESPECIALLY the bias caps! These $1 items can take out your power tubes and, if you're REALLY unlucky, your output transformer when they fail. High priority on these.

Out of curiosity, how so ?
Cuz they fail open, or short ?

.
 

Blue Strat

Member
Messages
30,723
Out of curiosity, how so ?
Cuz they fail open, or short ?

.

Short or partial short ("leaky"). That pulls the bias voltage toward zero causing the power tubes to draw higher and higher current. I've seen the power tubes pulling 200mA (thought it was 20 when I first looked) and happily doing so. If they do this for long enough, they'll take out the primary of the output transformer.
 




Trending Topics

Top Bottom