'68 Bassman, blew Tally light, weird resistors?

halfsmith

TDPRI Member
Joined
Mar 19, 2021
Posts
29
Age
45
Location
california
hey gang,
My '68 Bassman has had a recurring issue where the Tally lamp goes off sometimes- usually after running for >1hr.
The lamp goes off but the amp is still working fine.

I deduced that there would be a joint that loosens when it gets hot.

Upon inspecting around the tally lamp, I did find a wiggle or two that brought the lamp back to life.
I went ahead and reflowed all the joints on the lamp, which included a pair of resistors (100ohm, 1 on each lug to ground).

I checked and reflowed the two greens from the Transformer, two greens off the lamp to the Heaters, and the two resistors to ground from each lug.

When I powered the amp on, the tally worked, then stopped, then with my chopstick wiggle test blew it's filament. Amp works fine still.
I may have pressed something into a short, or some incomplete connection momentarily arced?...

However, in checking continuity around the area, I found that both Green heater wires go to Ground. Is that correct?
How would the tally lamp be lit by two green wires that are both Grounded?
How would the heaters work with both green wires Ground?
Are the Green wires from the lamp to the heaters supposed to be Ground?!

Also, do I want those resistors on the lamp? is that to dim the light? I don't see them on any schematics.

Thanks for any help!
 

Attachments

  • IMG_5271.jpg
    IMG_5271.jpg
    271.2 KB · Views: 54
  • IMG_5272.jpg
    IMG_5272.jpg
    294.8 KB · Views: 52
  • IMG_5269.jpg
    IMG_5269.jpg
    180.3 KB · Views: 51
  • IMG_5270.jpg
    IMG_5270.jpg
    288.1 KB · Views: 54

Ten Over

Tele-Afflicted
Joined
May 13, 2015
Posts
1,247
Location
Central California
Tally light?

You said that it has 100 Ohm resistors to ground. That constitutes continuity, doesn't it? I'll bet the ohmmeter doesn't read zero Ohms to ground.

I see those resistors on the AC568 schematic.
AC568 Bassman JPEG.jpg

The older Bassman's had a center tap on their heater winding. This one uses two 100R resistors to form an artificial center tap. Do not remove them because they reduce hum in your amp.
 
Last edited:

Ten Over

Tele-Afflicted
Joined
May 13, 2015
Posts
1,247
Location
Central California
Now that I have looked at the pictures, I have changed my position. It appears as though this heater winding may have a real center tap connected to ground. If so, go ahead and remove the 100R resistors.

To determine whether the heater winding has a real center tap connected to ground or not, check the resistance between a heater wire and ground. If the reading is really low, it probably has a center tap connected to ground. If the reading is around 50R, then it doesn't have a real center tap.
 

halfsmith

TDPRI Member
Joined
Mar 19, 2021
Posts
29
Age
45
Location
california
Thanks Ten Over!
I see those 2 100ohm resistors now in a lot of amps, the ones in my amp seemed to be recent additions (previous owner did a lot of replacements), so I was suspicious of them.

Still am, one of them has a suspicious burned looking spot, still tests around 100ohms but maybe when it gets hot it slips?
I replaced them both, I only had 120ohm (tested 113) 1W, I figured that'd be ok and I should keep them a matched pair.

Since my bulb burned out, I can't test the repair.

Meanwhile, any ideas as to what else may cause the "PILOT" light to intermittently flick out?
 

halfsmith

TDPRI Member
Joined
Mar 19, 2021
Posts
29
Age
45
Location
california
As is (with the resistors in place) there is 57ohm between Heater wire and GND
-would I have to lift the resistors to do your measurement accurately?
 

Ten Over

Tele-Afflicted
Joined
May 13, 2015
Posts
1,247
Location
Central California
As is (with the resistors in place) there is 57ohm between Heater wire and GND
-would I have to lift the resistors to do your measurement accurately?
Then I would think that the real center tap in the picture is taped-off inside that heat shrink. If you really wanted to be certain, you could lift both resistors and then test the resistance from the heaters to ground.
 

halfsmith

TDPRI Member
Joined
Mar 19, 2021
Posts
29
Age
45
Location
california
I can't be sure which circuit my amp left the factory as, drip-edge transition period. But the PT looks original and it is a 022814 (period correct).
For my circuit there would be CT and no resistors...

-can see on the PT datasheet the GRN/YEL wire (the lifted, shrink-wrapped one in my amp) would be the CT.
 

Attachments

  • pilotlight-CT.GIF
    pilotlight-CT.GIF
    24.3 KB · Views: 13
  • 022814-datasheet.GIF
    022814-datasheet.GIF
    9.6 KB · Views: 8
Last edited:

wangdaning

Tele-Holic
Joined
Feb 3, 2021
Posts
529
Age
38
Location
South of Russia, north of Vietnam
It does seem the yellow green that is wrapped with the HT CT us the heater CT. Maybe they accidentally cut it too short so they decided to go with the artificial. I think it would be better to use the actual CT than the artificial one.
 

Ten Over

Tele-Afflicted
Joined
May 13, 2015
Posts
1,247
Location
Central California
There are advocates for the artificial center tap on the grounds that the resistors will burn-up in the event that a power tube shorts its HT to the heater thereby eliminating the path to ground. Others say that eliminating the path to ground in this manner will result in all of the heaters being referenced to HT which could damage the other tubes.
 

Paul G.

Friend of Leo's
Joined
Mar 17, 2003
Posts
4,266
Location
Rhode Island
1. Your heater center tap is not connected, so you need those 100R resistors. They are a false center tap and reference each leg to ground for less noise and more stability.
2. Those lamp sockets are prone to shorting. Sometimes rotating the 2 legs where the heater is connected so they have more separation is the answer. You might want to check your socket with a magnifying glass for any stray wires or solder that is causing a short or near short. As your amp operates, the heat may expand something that then makes contact.
3. If all else fails, a new lamp socket is called for. These are a major pain to remove and install, but, c'est la vie.
 

schmee

Telefied
Silver Supporter
Joined
Jun 2, 2003
Posts
21,212
Location
northwest
Those resistors have nothing to do with a failing light.

Clean inside the socket where the exterior brass of the bulb contacts, clean the tip of the bulb with sand paper or a knife, file .... whatever..... and clean where the tip contacts in at the bottom of the light socket. A Dremel with a brass or steel ball shaped brush works great for all this.
 

peteb

Poster Extraordinaire
Joined
Apr 25, 2003
Posts
5,085
Location
Cascadia
It does seem the yellow green that is wrapped with the HT CT us the heater CT. Maybe they accidentally cut it too short so they decided to go with the artificial. I think it would be better to use the actual CT than the artificial one.

there is precedent that fender installed power transformers that have heater center taps, that are not used and are taped off in the factory.

the blackface champs are this way.
My '68 Bassman has had a recurring issue where the Tally lamp goes off sometimes- usually after running for >1hr.
The lamp goes off but the amp is still working fine.

when I have a pilot light go out, I flick it with my finger.
I feel it is an important thing to do at the time, to establish power on, I guess strumming would be a better test.

the bulb could go out, but the heater current still flows.


usually flicking the bulb lights it up. Sometimes not.


the bulbs themselves have a finite life.

my solution is to keep a pack of pilot lights handy and pop em in as needed.
 

halfsmith

TDPRI Member
Joined
Mar 19, 2021
Posts
29
Age
45
Location
california
Thanks for all the replies!
I have determined the bulb issue was probably caused by either a loose socket/bad connection, or short on the fixture, (ala Paul G), I've rebuilt the whole section and cleaned up the joints, but my bulb blew during my somewhat aggressive chopstick torture tests when said 'short' was activated...

I think I understand the Resistors vs Center Tap thing now, and have more comfort about the Green wires appearing to be continuous to GND (which confounded me), and I suppose the lamp not working is not a big deal as the power to the rest of the amp is unperturbed.
 

Wally

Telefied
Ad Free Member
Joined
Mar 17, 2003
Posts
42,178
Location
Lubbock, TX
I can't be sure which circuit my amp left the factory as, drip-edge transition period.

The chassis stamp inside shows a date ’0368’….3rd week of 1968. The circuit change is dated to May, 1968…AC568….for the Bassman. Your amp had the AB165 circuit when it left the factory, I would think. I see no hints if the mixed Biasing scheme of the AC568.
as for the pilot light, the bayonet fitting probably needs to be ‘adjusted’ so as to make good contact with the bulb.
 

halfsmith

TDPRI Member
Joined
Mar 19, 2021
Posts
29
Age
45
Location
california
The chassis stamp inside shows a date ’0368’….3rd week of 1968. The circuit change is dated to May, 1968…AC568….for the Bassman. Your amp had the AB165 circuit when it left the factory, I would think. I see no hints if the mixed Biasing scheme of the AC568.
as for the pilot light, the bayonet fitting probably needs to be ‘adjusted’ so as to make good contact with the bulb.
The newest (original) component I can date is from later '68. So AB165 or AC568 are both possible, the AC568 has the virtual ground so that could have been how it was originally built. It appears to have been mostly BFd, and when I took it to Skip Simmons for a look over he moved a wire that was apparently not in the correct place.
 

Wally

Telefied
Ad Free Member
Joined
Mar 17, 2003
Posts
42,178
Location
Lubbock, TX
The newest (original) component I can date is from later '68. So AB165 or AC568 are both possible, the AC568 has the virtual ground so that could have been how it was originally built. It appears to have been mostly BFd, and when I took it to Skip Simmons for a look over he moved a wire that was apparently not in the correct place.

out of curiosity, what component and date code is that? IF that code is later than the tube chart stamp, that would be odd, ime.
That virtual CT on the heater winding is not original…or at least the resistors are not original.
 

halfsmith

TDPRI Member
Joined
Mar 19, 2021
Posts
29
Age
45
Location
california
Wally, I'm not sure, it was years ago I did my best to date it, I think there was a pot from August '68 - this amp had the tub chart torn out when I got it. I agree the resistors do not look original.
 

zook

Friend of Leo's
Joined
Aug 6, 2003
Posts
2,946
Location
Cochise, AZ
Put a drop of solder on the bulb tip so that it makes a firmer contact with the hot terminal inside the Socket.
 




New Posts

Top