Problem: Weber 5F6A with very low output!

[erndawg]

Post 18 has 750 VAC on the rectifier anodes. Now you have 338 VAC per anode. That is a very significant voltage loss. Did you ever substitute the rectifier? You need to do that and report back.

Your 5 VAC appears to be unchanged from its original 5.2 VAC. But your former 6.5 VAC now seems low at 2.95 VAC to ground. Measure the voltage directly at pins 2 and 7 of the first 6L6 and report back.

Your bias voltage has dropped as well.

What is that current reading across the Standby switch? You have 00.8 A. That is off by ~ a factor of 10 for a nominal current draw. Did you mean 0.08 A? Verify that and report back.

The "before and after standby" voltage checks are often useful in determining if a PT has become lossy or has shorted turns.

If the current draw is indeed 0.08 A (80 mA) at the Standby switch, that rules out any excessive current draw in the circuitry after the Standby switch. Then you have to suspect the PT or rectifier and their associated wiring.

[jibe]

I am on my way to replace the fuse and do a reading across pin 2 & 7 to double check the heater voltage. I'll also do a reading across the pins of the rectifier to see what we have between the reds of the PT. Hopefully a correct current reading is coming and som double checks.



//Jibe

[jibe]

It's so much easier to do the readings with a fuse that actually works

Here we go:

Current across standby 80mA
Filament, pin 2 & 7, 5.8VAC.
Pin 4 & 6 of rectifier, 664VAC.

All on the low side it seems.
I have not replaced the rectifier, I don't have a sub, have to buy one if we think that
could be a source to the problem. I do have the weber solid state rectifier, but that should work shouldn't it?
It doesn't matter that I have the 5VAC secondarys soldered in place does it?



//Jibe

[TD_Madden]

Weber CopperCaps work just fine.

[erndawg]

Current across standby 80mA

That is a good reading. It tells us you have no over-current conditions on your B+.

Filament, pin 2 & 7, 5.8VAC.
Pin 4 & 6 of rectifier, 664VAC.

Not good. As I mentioned a couple posts earlier, you now have to suspect the PT or rectifier and their associated wiring.

I do have the weber solid state rectifier, but that should work shouldn't it?

Yes, that will work fine. Doesn't matter if it is a Copper Cap or just a drop-in solid-state replacement.

It doesn't matter that I have the 5VAC secondarys soldered in place does it?

No, that is not a problem. Drop that Weber right in.


Very carefully check all the wiring around your PT and rectifier.

Check or replace the fuse and make sure the fuse holder is mechanically sound.

Measure the wall voltage and compare to the voltage on the input leads of your PT, and report back.

You said you are doing this work at a camp; is the line voltage low there?

Are you sure your meter is correct? Do you have another one you can use to verify the low 6.3 VAC and low high voltage AC out?

[jibe]

I'm working in a small cabin at our future home, we're about to start building a house there, hopefully with a little place for my "hobbies".
I know the voltage at the input leads is around 225VAC, I'll check the outlet as well. Should be the same. I'll do a reading of what I have at home to get a comparison.

I think my meter is correct, I haven't experienced anything telling me different.
I don't have anything else so......

If I have exactly the same voltage before and after the fuse, would that qualify as a test
of a fuse in good health? I did that the other day, in that case everything is fine.

I'm kinda hoping that I messed up the rectifier and that a switch will solve the problem.
With my luck It'll be a faulty PT and 100$ out of my pocket. Still pretty happy, feels like we been over this thing without finding anything I've done wrong, yet, only a broken RCA jack. It isn't over just yet but so far.....




//Jibe

[MWJB]

Whoa! Filament voltage was 6.5VAC a few days a go, why the drop to 5.8VAC?

If your filaments are 5.8VAC at steady wall AC, then that is too low, try the next lowest voltage PT primary and see what you get, you will need to rebias too.

[jibe]

I tried the solid state rectifier today with no luck, also learned that using shielded microphone cable to make a
amp-speaker cab cable is all wrong. That mistake is now fixed and a proper speaker cable has replaced it. I
didn't know that, this is my first head amp, only had combos before. Maybe this is what made the RCA jack
break, the heat developed by using a cable not suited for high currents. The RCA Jack got pretty hot, doesn't
have to be the Jack itself, maybe the cable is to blame. The OT seems to give me sound anyway so maybe
I didn't break anything.

Also measured the wall outlet, 227VAC same as I have on the input to the amp.

The last thing I was about to do was to try the 220 primary instead of 240 but I didn't have time to, my ride
home arrived. Will try to go back, fix that and report back here later on tonight.
I'm not sure when to use 220 and 240, think I used the 240 because someone told me that was OK.




//Jibe

[MWJB]

Rule of thumb is to use the next PT primary tap up from what your actual wall voltage is, so technically 240VAC would be OK - unless you had a 230VAC tap? If you have under voltage heaters & low B+ it's worth trying the next tap down.

You don't want to exceed 7VAC on the heaters (anywhere from 6.2-7VAC will be OK), or have much over 500vdc on the power tube plates with a reasonable current load.

[jibe]

Here we go:



Switched to 220V Primary, rebiased to 35mV on one tube, 37mV on the other, (how much can it differ and be OK?).
469V B+, Heaters 6.4VAC, -49V bias.

Had to do this in a hurry, gotta bake a cake, my son turns 6 today
Didn't have time do a total checkup today but from what I can see the puzzle should be solved, or?
Unless heaters to the rectifier went up higher than it should. Soundwise everything is ok, powerful clean sound that starts
to break up at around 7-8 on the volume knob. Also a bit more output than before.



Just in case we're finshed here I'd like to give thanks to you all for helping me solve this problem.
I still have changes I'd like to do but hopefully I can start a new thread to discuss caps and tubes and such.
After all this I'm not sure I dare to change things

Thank you very much!!!!





//Jibe

[MWJB]

Looks good!

Tubes can be +/- 5mA an still be considered a matched pair, 2mA difference is unnoticable (to most folks...there's always one...) at reasonable current.

[jibe]

Ok, then we can consider this thread closed I guess, about time
Thanks a million, again!






//Jibe

[mazzas]

I am very happy for Jibe's success but was hoping his problem was more like mine. I just stumbled across this thread while trying to troubleshoot similar symptoms on my own amp build (AA864 Bassman). I get *very* low output levels. I tried the treble pot wiper test suggested by erndawg and heard almost nothing on the output. So... seems like a problem in the power section???

The amp is built based on these:
http://www.fenderholic.com/schem/bas...a864_schem.gif
http://www.fenderholic.com/schem/bas...864_layout.gif

I can supply a list of parts and pictures.

One obvious deviation from the schematic is the substitution of 100uF/350 caps for the 70-350 caps in the power section. Problem?

Also, the PT that I sourced does not have a center tap for the 6.3V heaters so I wired up a "virtual" center tap. Wiring 1/2 of the winding to ground (the way it's shown) produces no change. I've tried it both ways to no effect and so have left my vct in place for now.

I *think* everything else is to spec.

Here are some readings to get started:

Line voltage: 120.2V
PT Heaters: 6.49V
PT Bias tap: 51.2V
PT Power: 361V

Bias pot: -61.3vdc (biased cold)

B+: 480vdc

Filter caps (left to right on the schematic):
430vdc (high by about 50v)
464vdc (high by about 54v)
476vdc (high by about 56v)

6L6
pin 3/4: 476 (high by about 56v)
pin 5: -59.5

12ax7
pin 6: 285vdc (high by about 45v)
*I hear "scratching" when I touch these pins
pin 8: 2.1vdc (high by about 0.3v)

12at7
pin 1: 200.4vdc (low by about 20v)
pin 2: 109.2vdc *I hear "scratching" when I touch this pin
pin 3: 116.2vdc (high by about 20v)
pin 6: 285vdc (high by about 80v!!!) <==== PROBLEM???
pin7: 74.3vdc

[MWJB]

12AT7 pins 1 & 6 should be within a few volts (+/- 10-15 max) or so of each other, at the moment your PI is unbalanced and will have poor fidelity & little clan tone.

Double check plate resistor values.

[mazzas]

Thanks, MWJB. I'm going to repost my message over on the troubleshooting forum, as it's a more appropriate place for this. I have located some resistor values that are a little out of spec and am going to address those and take new readings.

I sincerely appreciate your assistance.