diode rectifier in a 5f2-a princeton - did I booboo?

[mort]

Just finished a little 5f2-a using transformers from a modern Champion 600 which has no 5v taps on the PT so I subbed in this rectifier ---



The only other difference from the original fender schematic is that I'm only using one input, so I have the sleeve grounded and the tip is in series with a 68k input resistor.

The problem I'm getting is loud hum that is quiet with no volume and increases with volume. The tone pot seems to affect it too, slightly. The overall guitar sound is great.

I read 3vac on the 6v6 plate so I strapped a 33uf across the first filter and the problem remained and the AC on the plate increased to 6v. I quickly turned it off and I'm starting to double check all my wiring.

Is the mistake probably NOT in the rectifier or filtering??

[Enzo]

Correct, probably not.

Did you make a 5F2 or a 5F2A? The 5F2 has a choke, and takes the B+ for the power tube off the seconds B+ node. The 5F2A saved them some money, by taking B+ from the first node - the reservoir cap - and eliminating the choke for a 10k resistor.

I am kinda hoping you didn't spend extra money to find 8uf caps, but if you did, well enjoy. I would have used more modern 20uf caps.

If the only difference between Fender and you was the B+ supply and the input resistor, then that means you wired your heaters with one side grounded. That is itching to make hum. Recommend wiring the heaters with a twisted pair of wires to the tubes, no ground conection, THEN make a virtual center tap with the pair of 100 ohm resistors to ground that Fender used on a zillion other amps. That will reduce heater related hum. In fact the Champion 600 uses the 100 ohm resistor virtual ground.


Nitpick: If you use only one input jack, and you want it to be just like the fender, you need a 34k resistor, not 68k. 33k is the closes standard value. It isn;t like 68k will screw anything up though.


It can be tricky, but determine if your hum is 60Hz or 120Hz. That will tell us if it is power supply ripple currents or not.

[mort]

I stand corrected .. the heater circuit is also different from the original, as is the switch and fuse being both on the 120vAC line and no cap on the fuse. . I have always done it that way.

this is the drawing I worked from. http://music-electronics-forum.com/a...f2-a_schem.gif

I'll go take a couple photos of the chassis. It's pretty small so it may be difficult to make out what's what...

[mort]

This is the Turntable Tweed (as I like to call it) that is for my 8 year old son.

From the view inside the chassis the volume knob is on the left and has the power switch on it. The tone knob on the right has a switch but it's not wired to anything. The input jack is the red/yellow wisted pair. And the power tube filters are grounded with the rectifier and power cord ground on one end of the chassis and the pots and preamp filter grounds at the other end.

[mort]

Ok so here's something I don't understand.. I was beginning to check voltages and noticed that the heaters have 6.3v when measured from p9 to p4-5 on the 12ax7, but I'm getting 54vac on each heater pin in reference to ground... can't be good, right?

[Enzo]

Don't know yet. You said your amp heaters are wired different from the original, but you did not say exactly how. I seem to see a twisted pair of blue wires, I assume are the heaters, and they seem to join a pair of green wires I assume come from the transformer. Close so far? But did you make a virtual center tap? And if so, what did you ground it to? If not, then there is the reason for the voltage. Your 54v is either coming from circuit connection, or it is induced voltage because the heater wiring is not referenced to ground.

Grounding your heaters through the virtual center tap should reduce your hum. If you want to get fancy, instead of connecting the virtual center tap to ground, connect it to the power tube cathode. That ought to be +20 volts more or less. That has the potential to reduce certain other kinds of hum.

[mort]

Yes the green twisted pair comes from the PT (no CT) and the blue twisted pair simply jumps to the next heater(s). Not grounded.

I have not yet heard of the virtual CT, but I do remember seeing those resistors on the original PCB that was in the 600 that the transformers came from. I will implement the VCT and report back soon. Thanks!

[Enzo]

Even just grounding one side, like the original, is better than no heater grounding. 100 ohms has become the standard, but the values are not critical. 150 or 200 ohms would be fine, or whatever you have in that area that you have two of.

[mort]

I guess I just always took that little heater CT for granted.. I have learned something today.


I temp installed a pair of 100 ohm resistors, I happened to have 5 watters, from the 12ax7 heater to the preamp grounding lug. Cured the problem for sure. There's a tiny bit of hum left which I imagine will go away when I do the permanent fix to the main grounding lug (or I may try the output cathode as suggested).

This thing is a hairy little dog. It will break up at 2! I'm guessing this may be related to the tiny little OT?? Sounds great though, as long as you don't clod hop all over it.

[Enzo]

Even if there were a center tap, it wouldn't have been grounded unless you wired it to be. I bet your 54 volts of AC went away.


Hum is not generic, there are many sources of hum, and each has its own cure. And their cures do not work on each other. In other words, all the filter caps in the world would do nothing to cure your missing heater ground. Likewise, all the grounding in the world will not help a lack of good filter caps. and so on.

COnnecting the heater power to ground cures a lot of hum, but there is also sometimes hum from heater to cathode interaction in the tubes, so elevating the heaters to some DC voltage can help that particular source of hum. That is the reasoning behind connecting your virtual center tap to the 20v of the power tube cathode instead of ground. You may or may not get a benefit from that.

For example: Your amp is single ended, not push-pull. Push pull amps naturally cancel some power supply hum in the power stage. Your single sided amp cannot cancel anything, so any power supply ripple on its plate will be added to the signal. You may be hearing that. You may also have some hum from shared ground current paths. If the cathode ground return of the input stage shares a path with the filter cap ground, then some ripple him may result.


That is why it is important to determine if any hum is 60Hz or 120Hz. They are the same note, but an octave apart. 120Hz is the frequency of your ripple from the power supply. Grounding your heaters solved a 60Hz hum from the AC.

The iron layers in the transformers is another hum opportunity. You need to orient the laminations of the output transformer 90 degrees from the orientation of the power transformer. Other wise they can couple. Simple enough test, pull your power tube and turn on the amp. Is there any hum in the speaker?

I assume you used a three wire power cord? If so, you really don't need that cap the schematic has by the fuse. Did you connect the power cord ground to the same point as any amp circuitry?


Hum is a really big subject. But i don't mean to scare you, none of that may happen.

[mort]

I always observe the 90 degree rule between transformers, and thanks for the coupling test info. I learned something else today -- if I'm reading this correctly you're saying that any hum from the OT and PT coupling each other will be audible without power tube(s) installed?

I've built some amps with transformers in very close proximity to each other and never heard a hum with no power tubes installed... I'm guessing that's something that doesn't come up very often??