bassman AA864 filament wiring question

[solderstain]

Also with all due respect, I don't believe anybody suggested modding the AA864. Putting one more or one less twist in output tube filament wires is not a mod; replacing dead caps or bad resistors is not a mod. I've had to restore enough hacked amps over the years to be sensitive to the issue, but unless an amp is a museum piece (like still in its original shipping box), simple maintenance is usually appropriate.

To each his own, as they say.

I suppose it all comes down to what any one individual considers a 'mod'. For an otherwise unmodified, relatively-rare mid-'60s Bassman (I don't consider electrolytic replacement to be a 'mod'), changing the wiring of the heater string IS a mod (to me). I DON'T consider that to be in the 'simple maintenance' category. That's not how it left the factory. To SOME people, that makes a difference.

I used to own a '67 Bassman - AB165, and other than the power cable and the power switch, it was TRULY untouched inside. Even the original electrolytics everywhere. I wanted to do the hybrid circuit on it, but once I pulled the chassis and saw that there were NO redone solder joints outside of the power cable/switch replacement, I took pictures to document it, and put it back together. I sold it instead, and got a pretty penny for it from someone for whom 'unaltered' was important. It's obviously not important for everyone. I found a '66 that had been 'improved' and went on with my project.

Like I said - to each his own... just my own opinion.

[Structo]

I didn't notice it was a AA864 at first glance.
From what I have read they do seem rather scarce.

Just for grins and giggles you might list it on eBay just to see what kind of price it generates.
If it goes for enough you could simply use that money to finance a build.

Or if you really dig the bassman circuit, do the maintenance and keep it.

There are some crafty guys that can put new E caps inside the old wrappers so appearance wise it looks vintage.
Not sure what that does to collectibility but just another idea.

[orrong65]

OK,
thanks for all the comments, they are very helpful. Here is another question related to the Bassman clean-up:

There seems to be a variety of 6.3V filament circuits in Blackface amps.

On the Princeton Reverb AA1164 the filament is center-tapped to earth, and it has two 100 ohm resistors to earth as well. Most blackface AA763 and AB763 circuits just have the earthed centre tap. The Deluxe Reverb AB763 just has the centre tap, but the AB868 circuit has the earthed centre tap and the two resistors to earth.

So, some do and some dont, and it seems that the later circuits have both the earthed centre tap and two 100 ohm resistors to earth.

I notice the Weber schematics say that you should have either an earthed centre tap, or two 100 ohm resistors to earth, but both are not necessary to lower noise as far as possible from the filaments.

Whats the thinking on this - were the 100 ohm resistors added to later revisions to further quieten the noise??

[Structo]

The early tweed amps used a grounded heater scheme that was later to be decided to be faulty and unsafe.

The later brown and blackface amps either used a center tap heater supply that was grounded or if the PT was lacking the center tap, an artificial center tap is created by installing a 100R resistor off of each leg to ground.
Many feel the 100R tap to be superior because you can use higher precision resistors and have the heater supply better balanced.
The center tap on heater windings is frequently not balanced very well and can cause some hum problems.
This is why Fender used the heater hum balance pot on many amps.
Another trick that can be done is to elevate the heater supply by grounding the winding center tap or 100R resistors to the cathode of a cathode biased amp, this raises the positive voltage floor but does not increase the actual voltage to the heaters and reduces hum.

[Firestorm]

OK,
thanks for all the comments, they are very helpful. Here is another question related to the Bassman clean-up:

There seems to be a variety of 6.3V filament circuits in Blackface amps.

On the Princeton Reverb AA1164 the filament is center-tapped to earth, and it has two 100 ohm resistors to earth as well. Most blackface AA763 and AB763 circuits just have the earthed centre tap. The Deluxe Reverb AB763 just has the centre tap, but the AB868 circuit has the earthed centre tap and the two resistors to earth.

So, some do and some dont, and it seems that the later circuits have both the earthed centre tap and two 100 ohm resistors to earth.

I notice the Weber schematics say that you should have either an earthed centre tap, or two 100 ohm resistors to earth, but both are not necessary to lower noise as far as possible from the filaments.

Whats the thinking on this - were the 100 ohm resistors added to later revisions to further quieten the noise??

The Fenders have one or the other, not both. Post Tweed and pre Silverface, it's the center tap version (unless a replacement transformer has been installed). At some point in the Silverface evolution, the center tap went away and the resistors were added. Probably the transformers were cheaper that way. If the amp came stock with the resistors, the heater winding is not center-tapped; the ground connections you see will be the center tap of the HT winding and (if there is another lead) a transformer shield.