Bassman 10 75 Model PT Question

[plexi50]

I am having a blank minded day. I have a 1975 Bassman 10 that i have converted to a AA864 Bassman with a Super reverb tonestack.

The power transformer has a different type of rectified circuit and is only gives me a B+ of 280VDC.
I think i am missing something in the way the rectifier is wired. It is currently wired as your typical vintage pre 70's Bassman.
Reference pic below.
 

[sluckey]

How much AC voltage do you measure on the PT red wires (in reference to chassis)? Do you have any filter caps connected to the rectifier output?

[plexi50]

I have 189VAC on each lead in reference to the chassis with filter cap lead removed.

With filter lead attached i read 183VAC & 167VAC.

Below is a pic of the original bridge rectifier board & wiring. I didn't take a very good or complete pic of it either.

The CT lead (red / yellow) on the original rectifier circuit does not appear to be grounded but part of the rectifier circuit.

[sluckey]

The low B+ you are getting is correct for that PT and the rectifier you are using. Using a bridge rectifier (like that PT was designed for) will give you twice the B+ you are currently getting. The CT will connect to the junction of the first two series connected filter caps.

[plexi50]

Ok. So can i connect the CT to the B+ lead at the rectifier anode also going to the first (2) series filter caps? Wouldn't that be the same?

EDIT: Tried that. Fuse blows.
 

[sluckey]

Wire it like this...

http://el34world.com/charts/Schematics/files/fender/fender_bassman70.pdf

[plexi50]

Ok yeah i have been looking at the schematic am doing it now. Back in a bit. Thanks much.

[eleventeen]

I wouldn't do it that way. I'd leave the CT 100% disconnected and connect the B- pole of the rectifier bridge to ground. 



Or you can do it Sluckey's way, probably a bit better. That way (CT to cap junction) serves for the most part just to equalize the voltage across the two caps. it does not change the power supply output voltage. The equalizing resistors do the same thing and act as bleeders.


Now the issue arises whether you have a completely separate bias winding which you "backwards" rectify into a (raw) negative -70 volts or so or whether you have to steal the bias feed from one leg of the HV winding. Either way: You will have potent B+ on the 6L6 output plates, so make sure you have good negative bias of -45 to -55 volts on pin 5 of either 6L6 tube socket before installing any output tubes. Wouldn't want to fry them.


I'm mildly curious, did this amp arrive on your bench modified? Not working? Both? Are you confident that your primary ecaps are good? Because if one were shorted it would cut your B+ in half, roughly, as you originally observed.

[sluckey]

The negative bridge terminal is connected to ground. Connecting the CT to the junction of the two series caps forces the voltage to split equally across each cap. That's a very popular circuit.

[eleventeen]

So...it appears that you have replaced the original 4-diodes-in-a-diamond configuration to a replacement board with the more normal Fender rectifier with two lines of diodes meeting up at the cathode (bar) ends. That ain't gonna work.


However I see no reason why your replacement board can not be *made* to work. The red HVAC leads have to go to *junctions* between diodes, not "ends of strings". The Red wires have to be moved one eyelet to the right. Usually a bridge rect is drawn in that distinct "diamond" way. It can also be drawn the following way, it's exactly the same ckt, but I show this method of drawing to clarify the bridge config as it relates to your particular board. You DO have two strings of 2 diodes in series, yes?





Forget the 12 volts. Now imagine moving the red wires out of the PT one eyelet to the right on your PS/bias board. The B+ is still taken from the junction of the diode cathodes, bar ends, where cathode meets cathode. The ground connection is made by joining the two anodes together. (the a** end as I call it, "A** = Anode) The anode-anode connection is NOT made on your board as is. But the diodes stay exactly where they are, how they are. Also, where and how the bias is produced is a separate issue.


Do you have a separate bias winding?


What you do with the CT is optional, Sluckey's way is probably better.


Be mindful of where you may have connected jumpers on that PS/bias board. Don't leave them behind when you rewire the diodes.

[plexi50]

Ok i have redone the bridge like the schematic and now have 475VDC B+ under load without CT lead connected.
Before i  connect that CT to the first 2 series caps i only have 2 fuses left to play with. I am feeling much better now that the mains fuse is not blowing.


The amp came in as a stock 1975 Bassman 10. He wanted an AA864 Bassman circuit so that conversion is finished. I had sound but the low B+ was way far off.

Here goes/  Edit: CT connected to first series filter caps fuse blows/ Yes the bridge is now as your diagram shows eleventeen.
I do have a seperate bias winding which is going to the 470r 25uf config. Bias is -44 volts.

I had this same issue in about 2005 when Steve helped me sort through it. That amp was a Hammond Frankenstein thing.
I just forget when i run across these particular transformers. 

[eleventeen]

The connection to the cap midpoint should not blow the fuse, but it's very possible you have a leftover ground connection somewhere that is shorting out half your PS. Or, one of the stacked up caps is bad, but then you would not be able to produce your full 475 volts.


WITH the 475 volts going, measure DC across the stacked up caps and be sure the two readings are sort of equal. Don't omit this step before going farther.



So you can leave out the CT connection and rely upon the bal resistors to do the job. But be sure you have those balancing resistors across the stacked-up caps! No higher than 220K, 100K - 150K is fine. Should be 1-2 watt, flameproof preferred.


Strongly believe you have a leftover ground connection you don't want to be there.

[sluckey]

Let's see some hi-rez pics of how you are connecting the CT.

[plexi50]

Got sidetracked with house work. This is the best pic i can get from my camera at this time of the original bridge rectifier caps i had removed earlier.
I connected the CT to the B+ first rail series caps lead and that's when the fuse blew. Edit: (I wired it wrong to the B+ recto)

The amp is working fine but would like to sort the CT issue out if there is any issue left.
 

[sluckey]

I connected the CT to the B+ first rail series caps lead and that's when the fuse blew. The amp is working fine but would like to sort the CT issue out if there is any issue left.

You cannot connect the CT to the diode board. It has to connect to the JUNCTION of the two series caps that make up the first stage filter. They are located under the doghouse. LOOK AT THE SCHEMATIC!

[plexi50]

Looking again at schematic.

[plexi50]

Ok got it Steve. I wont forget this next time i run into this type of PT. Fired up,smooth and quiet. B+ 485VDC and solid.
Your the man! Thanks much all!

[plexi50]

The negative bridge terminal is connected to ground. Connecting the CT to the junction of the two series caps forces the voltage to split equally across each cap. That's a very popular circuit.

Not to be forgotten. 

[sluckey]

I knew you would get it.

EDIT... In one of your pics it looks like the CT may have originally connected to an eyelet on the rectifier board. That eyelet would not have connected to any components on that board but would have been used only as a tie point for the red wire that connected to the junction of the series caps. Your splice looks very good.

[plexi50]

I knew you would get it.

If had payed attention to the actual PT i was working with and stopped thinking of the usual Fender PT senario it would have been less stressy.
I have this block in my mind that is sorta stuck in all things Fender of the 50's & 60's. Thanks again!

[sluckey]

Easy to think like that. Not many Fender amps use a bridge rectifier.

[plexi50]

Whatever happened to plexi anyway? I have grown to really love vintage Fender amps overall.

[sluckey]

Ha! There was a time that Bassman would have made a nice plexi.   

[PRR]

> I wont forget this next time i run into this type of PT.

TIP: when you scavenge a PT, grab the rectifier also, or at least a precise copy/sketch.

There's several ways to do windings and rectifiers. If you mix & match, you often end up at half or TWICE the designed voltage, and way-off from what that PT can handle with any likely tube and OT set.

Yes, pre-1965 "all" amps used bottle rectifiers. Cathodes are expensive. The bottle was a dual common-cathode rectifier. This forced the PT to be 50 cents more expensive (more turns, 3rd wire), but far cheaper than adding cathodes and heaters.

By 1965 Silicon diodes were practical and affordable but Fender was not re-speccing their PT orders just for that. (Anyway the early Si were not outright cheap, so a 2-diode plan was still sweet.)

Also in that period: high-Voltage diodes cost more. There is a Doubler with one winding, 2 diodes, and 2 main caps. Diode and cap voltage ratings are less than full voltage, so the total cost was sometimes favorable. This plan also gives a half-voltage output, useful in some huge amps. Bogen did a few. One of the SilverTones stacked several Doublers (musta had a crate of 100V diodes and caps).

There is much drag on g-amp design and even new designs lean on the old CT and 2-diode plan.

When diodes are cheap, one winding and 4 diodes is less overall money. A true clean-sheet design would explore that first.

The 4-diode bridge *with* a CT again gives a half-voltage tap, handy in big amps and (in Fender) a 1/4-power mode.

With so many options, differing very little in chassis but a LOT in final output, you really need to steal the rectifier along with the PT. Since you really took the full PT and OT set (and presumably similar final tubes), whatever worked in the Bassman 10 should be fine for the AA864 (+/- a few Watts).

[plexi50]

There is 65VDC plate difference between the Bassman 10 (75 watter) @ 485VDC and the Bassman AA864 @ 420VDC.
But the amp doesn't mind. I did have and was going to use the original bridge rectifier but my thinker wasn't thinking.
In the end i had to go back and rebuild the bridge rectifier as it was in the amp originally with that PT. Lesson learned.

It sounds big and beautiful. Oh, and yeah the atrocites i have committed with some of my first Bassman amplifiers.
Sham on me. 

[plexi50]

Fuse blew this morning after 2 minutes. It blew while i was turning up the volume to 10.
My current limiter light did not show any dimming. Other than that the amp sounded great before the fuse went south.
 

[sluckey]

Bassman 10 75 uses a 2.5A slow blow fuse. What size are you using? I'd probably use 3A Slo.

[plexi50]

I am using a 3amp slo. No voltage leakage across the ceramic bridge caps. I am using Dougs 90ma DC - 4  Henry choke

A Note: I am not using the ultra linear OT screen voltage leads. They are heat shrinked off. 

[plexi50]

I have decided to use a new 330-0-330 PT. I really do not want 485VDC B+ on the plates.

[sluckey]

I have decided to use a new 330-0-330 PT. I really do not want 485VDC B+ on the plates.

Do not use a bridge rectifier with that PT unless you want to see some nice fireworks!

[plexi50]

Oh no. That bridge board will be stripped and rebuilt as the standard vintage SS fender bridge.
  Yeah that would be fireworks.

[PRR]

> use a bridge rectifier with that PT unless you want to see some nice fireworks!

Been there. Done that. Didn't get the T-shirt. Or the video.

Actually I was watching. I expected ~~~400V. I saw it rising 500, 600, and got it shut-down before the real fun started.

That habit has saved a few builds. Like the time I got a 120V:24V *backward* and was trying to pump 50V caps to 840V. (I saw the lights dim before the meter hit the stops.)

[plexi50]

I used oven mittons on a Sound City 200 Plus with 749 volts. Talk about flirting with the scary unknown/
Schematic said 620 volts but i found 749VDC.