Hey J Stokes and other Fender afficienados... Stock zener?

...in BF Bandmaster? OK guys, this is at least the second one I've seen... The PT center tap goes to the zener diode... Right at the spot where there is usually a big blob of solder and the filament CT and HT CT is grounded. I had one in a while back that was the same thing and I dismissed it as a very clean mod...but here is another, and once again, no evidence of there ever being a blob of solder for the chassis ground ??? I pulled off the zener and there is no evidence of solder under it ???http://i10.photobucket.com/albums/a150/timtube/Bandmasterzenerdiode001.jpg

Hi Tim.....

Fender may have had some left over PT that had the 5vac winding for the 5U4 rectifier used on earlier models.
When the power supply was solid stated, the approximately 50 volt drop across the 5U4 was now much less thereby increasing the B+ to the tubes.
In order to get the voltage back down, the Zener was added to the CT because the total load current has to flow through this diode in series with the transformer's center tap to ground. This configuration will result in a
constant 50 or so Volts subtracted from the B+ which, by the way, will be stable regardless of the changes in current drain.

Yet another (or both) consideration is that amps equipped with this Zener could have PT primary windings designed for 115vac mains. The power grid voltage (in the USA) was increased many years ago to increase power company efficiency and at the same time the voltage standard was raised considerably for consumer electronic devices.
Power companys have different standards according to local laws but in general todays mains (USA) voltage is about 118vac to 122vac.
In this case a means of lowering the PT HV is needed and the Zener does work pretty good.

Amazing, never ever seen that before. It's obviously not on any of the factory schemos. and yet you say it looks like it came that way from the factory?

Maybe they got in a batch of PTs that were too hot on the voltage, and instead of sending them back, they took them anyway and threw in the band aid. Probably to meet delivery commitments.

Any way to pull a date code off that diode?

Try this... short the diode and measure the resulting B+ on the 6L6 plates. See if it is unusually high.

I've never seen such a thing, and I doubt that it's original, but if it dates right (wondering if such a device even existed at that time) and the B+ is really high on that transformer, sky high considering some of the voltages they did ship, it may make me a believer.

The solder can be sucked completely clean, leaving only some very minor discoloration on the chassis plating. Sometimes you can smack-em cold and they come off clean. If you look closely at the filament CT, it shows some red on it, indicating the CT leads were probably twisted tighter together previously. Another thing you never see is such a small solder glob at the chassis connection as you see here.

So, I ain't convinced....yet.....

If you look closely at the filament CT, it shows some red on it, indicating the CT leads were probably twisted tighter together previously.

Also the leads look to be exactly the same length, as if to be tied to the same point.

There is absolutely no discoloraton on the chassis. I also pulled the zener and the hole is punched rather than drilled... And if a guy were to add a zener, say to lower the voltage to allow for 6V6s, wouldn't he just leave the filament CT and filters soldered to the chassis, why would you resolder them to the zener washer? After seeing one of these I was not convinced, but after seeing another with the same configuration I am at least toying with the idea that this is indeed another incident of Fender randomness... Sort of like the 3 or 4 December '69 amps I've seen that used 2 paralleled .01 ceramic disc caps in all locations that called for a .02 cap.... See one and call it a mod...see multiples and call it a factory parts shortage.

OK, back to this oddball amp. Shorted zener and had 495V (vs 440V). This is the stock PT, even has the matching date stamp on the underside. I'm now 99%convinced this is stock.

I suspect you are correct. Leo's band-aid to use an out of spec transformer.

check to see if somone swapped the end bells and used a newer souped up PT..

there was a guy in tejas who was doing that and claiming they were rewound..

Are there date codes on the zener that you can see?

as far as Leo making things work...stranger things have happened!
tom

Tom,

The PT looks legit. The fact that there is no discoloration on the chassis where the blob of solder was is somewhat convincing. And most convincing may be the codes on the zener it has the word "SEMCORE" and under that, 5 6429 and then running sideways, 6543. The date code on the amp is 4565. I'm now convinced, in addition, I saw one just like this a while ago and dismissed it as a mod.

So, the transformer is a 125P7D, Tim? That really is way off spec for that part.