bassman AA864 filament wiring question

[orrong65]

Hi there,

I have just got hold of a bassman AA864 which has not been touched since 1964. It has untouched solder joints, and a set of original RCA 7025s with NG date codes. It also has leaking electro caps etc which are all to be replaced.

I have read a bit about filament wiring on this forum, which prompts me to ask this question:

I notice that the filament wiring to the 6 tubes is deliberately out of phase -
from pin 2 on power tube 1
to pin 7 on power tube 2,
to pin 9 on the PI
to pin 4-5 on pre-amp 3
to pin 9 on pre-amp2
to pin 4-5 on pre-amp 1

Is the current thinking that this is the best wiring option for lowest noise?

[Structo]

The general consensus is to keep everything in phase for least noise.
Most preamp tubes have humbucking heaters, but every little thing you can do will help in the long run.
I would correct it so both power tubes are in phase as well as all the pre tubes.

[Firestorm]

During the 50s, when Fender typically wired half the filament circuit through the chassis, they were fairly consistent about which pin went to the chassis and which pin got the wire. By the '60s, they pretty much didn't care how they wired the heaters (green wire everywhere). I guess the amount of hum you can inject into the power amp from out of phase filaments is negligible by 1960s standards.

You absolutely do want the output tube filaments in phase, as Structo says. Whether it helps on preamps is an interesting question and the answer is probably not as simple as wiring all the preamps in phase. Sometimes you might want to flip the phase on purpose to cancel hum from an earlier stage. Fender itself was not above correcting a wiring error by making an offsetting error somewhere else.

On most BF Fenders you can leave the preamps wired however you find them. If you rewwire them, you're more likely to break a terminal off the socket than to improve anything. And since the cathode resistors are all bypassed by caps big enough to pass 60Hz, the hum leaks to ground anyway. Different story on unbypassed cathode resistors or only partially bypassed ones like the .68 caps on Marshall lead channels. Some of those hum like crazy.

[Phil_S]

On most BF Fenders you can leave the preamps wired however you find them. If you rewwire them, you're more likely to break a terminal off the socket than to improve anything. And since the cathode resistors are all bypassed by caps big enough to pass 60Hz, the hum leaks to ground anyway...

IMO, there is no harm in following the conventional wisdom regarding the phasing of heater wiring if you are building something new. At the very least it couldn't hurt and might be beneficial. Personally, I am a skeptic on this point. I've built several amps where I paid no attention to phasing of the heater wiring harness and had no issues.

On an old amp like this, I think what Firestorm says is very much on point. Risk probably outweighs reward. You might also diminish monetary value by disturbing the original solder. So, this would only be justified if you are 100% sure it solves a clearly identified problem.

Until you recap the amp (which it probably needs badly), you probably have no clear idea if the heater wiring is in any way problematic. I suggest a minimalist approach on this antique. Only fix something if there is a known problem. Don't try to "improve" it. Just because something seems like a good idea, doesn't mean that it is.

This is a nice looking example. Thanks for the picture.

[Structo]

I agree, recap the amp and then if you are getting some 60Hz noise, correct the heater wiring on the power tubes only and see what that brings.

[orrong65]

Thanks for all the helpful responses!

The amp sort of "works", the normal channel makes a noise but the bass channel does not. I am very conscious of not just tinkering with this because of the fact it is such pristine condition.

I plan to replace components in the following order, checking the amp for performance at the completion of each step.

. cathode bypass elytic caps
. power filter elytic caps and bias elytic cap
. power 1W dropper resistors
. grid resistors
. power tube screen resistors

Also, the third pre-amp stage in the bass channel has an un-bypassed cathode, so I may look closely at that filament for phasing.

Any other comment most welcome.

[Firestorm]

You said the elytics were leaking: If I were you, I'd replace the power filters and the bias supply cap first. A failure there is bad. Then go one step at a time listening as you go. The screen resistors and the grid stoppers on the 6L6s MIGHT be compromised (sitting on top of the tube all those years). It's pretty common for the 100K preamp plate resistors to make Rice Krispy noises (Fender eventually changed them to 1W to combat that). I wouldn't expect filament phasing of a third stage to matter much; first stage, maybe.

[solderstain]

With all due respect, you guys... you're nuts. AA864 Bassmans are RARE RARE RARE. DO NOT unsolder anything you don't have to. AB165 Bassmans are a dime-a-dozen - mod to your heart's content. But if that AA864 is truly untouched... LEAVE IT THAT WAY. Filter cap changes are acceptable, but leave the rest alone! Don't alter the heater string, even if it is out of phase. If it bothers you enough, re-sell it to someone who will leave that rare bird stock, and find an AB165 Bassman to tinker with if you absolutely feel you need to play with the soldering iron.

I'm all for tinkering and such - I'm probably the worst 'violator' on this site... but there are some things NOT to tinker with - AA864 Bassmans are an example.

My $.02...

[M Fowler]

That's why I buy mojo chassis and use Magnetic Components transformers and not destroy old Bassman's.

But the real reason is the old Fender amps cost too much

[Firestorm]

With all due respect, you guys... you're nuts. AA864 Bassmans are RARE RARE RARE. DO NOT unsolder anything you don't have to. AB165 Bassmans are a dime-a-dozen - mod to your heart's content. But if that AA864 is truly untouched... LEAVE IT THAT WAY. Filter cap changes are acceptable, but leave the rest alone! Don't alter the heater string, even if it is out of phase. If it bothers you enough, re-sell it to someone who will leave that rare bird stock, and find an AB165 Bassman to tinker with if you absolutely feel you need to play with the soldering iron.

I'm all for tinkering and such - I'm probably the worst 'violator' on this site... but there are some things NOT to tinker with - AA864 Bassmans are an example.

My $.02...

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.

[orrong65]

Thanks to all the responders, - its good to hear a diversity of views on old amps like this.

My first priority is to get the amp working fully and quietly, and just having cleaned up two blackface Vibro Champs and a blackface Princeton I know how far you have to go with old components. I am definitely not going to change parts for the hell of it, or just to get out small amounts of noise.

That said, it will be electrolytics first, and then, depending on the results, the 1W resistors (replaced with new 1W cc's). Only then will I look at the plate resistors (replace with cc's).

A lot of the tone of these amps comes from the ceramic disc caps, and replacing these with silver micas often changes the tone for the worst - makes it too sterile or brittle.

I have found that a lot of the ceramic caps in these era amps are dead or shorting, and so I have a stock of 60s ceramics on hand just for this application.

Carefully as we go.

[billyz]

I am with most everyone on this one . Change the Power supply and bias caps, I like the F&T brand. It is ok to uprate the bias cap a little. You might have a hard time finding the exact value. I would measure the resistors , if they are open or more than 20% off, then change them with CC of the same rating. Much depends on your soldering skills . If you are neat and careful, few would know you have even been inside . As for plate load resistors, if they are not noisey and you like the way the amp sounds, do not change them. Yes , they drift, the value is one aspect of what gives the amp its mojo tone.

I have found putting the Heaters in phase to help sometimes. I had a similar Bassman in last year for an annoying hum. 2 other (well known) techs had worked on it an could not get it quiet. Well, after much head scratching I discovered a few of the heaters were out of phase. I rewired them, you would never know. Hum went away .
But, I would not bother if after replacing the caps it is not an issue.

And if you really think this the holy grail of collectible fenders then keep the original power cord, Most collectors I know prefer the old 2 prong cord intact , yes I know.

[martin manning]

I have found putting the Heaters in phase to help sometimes. I had a similar Bassman in last year for an annoying hum. ... I discovered a few of the heaters were out of phase. I rewired them, you would never know. Hum went away.

Do you recall if the out-of-phase heaters were on power or preamp tubes?

[billyz]

I have found putting the Heaters in phase to help sometimes. I had a similar Bassman in last year for an annoying hum. ... I discovered a few of the heaters were out of phase. I rewired them, you would never know. Hum went away.

Do you recall if the out-of-phase heaters were on power or preamp tubes?

To be honest , I don't remember exactly. It was more than one.

[martin manning]

Some thoughts on this for further consideration...

Assuming that some 60 Hz hum is injected at the cathode by the filament, it seems that power tubes should be wired in phase to take advantage of common mode cancellation. A LTP PI should be taken care of by this as well, regardless of which way the filaments are wired, while a cathodyne PI would not. Dual triode pre-amp tubes have the two filaments in each envelope wired in phase. If the two triodes in a single envelope are used in adjacent inverting gain stages, there might be some cancellation possible there. These may be some reasons why out-of-phase pre-amp tube wiring doesn't matter (or does), since it would depend upon the particular topology.

MPM