Help: Saw tooth wave on my Plates!?!?!?

[d120f]

Hey Thanks to everyone for reading and hopefully helping me solve this :-)

Been building for a long time. Sometimes you do bone head stuff. I guess this is it for me. I am exhausted and can't seam to find an answer...

I have a 71 twin reverb Non master chassis that I gutted and built a BF twin in. Have the brass plate, Winged-c SED matched quad, JJ + NOS in pre...

New PT (has heater CT so no 100ohm res), Choke, OT (multi tap) and reverb T

I have been inside these amps before...

Grounded as in a real BF except for the 3 pronged chord. so no ground switch

New caps.. Hell new everything all the way round

So the problem:

On the scope the plates show a Saw tooth wave Very defined roughly 20v P-P. o.O IT IS ONLY ON THE PLATES NO WHERE ELSE Cept to say its B+ to the plates so before choke and on secondary of OT..Amp sounds good!?!?!? I will get the frequency of the wave tonight forgot to calculate it from the scope. Will post a picture as well.

It is there but not as pronounced when all the tubes are out. So when the tubes are out the only thing in the system is choke, switches, and OT. If memory serves the Heater frequency was about half of the saw tooth so it is probably in the 120hz range.

The wave is not on B+ when the standby is off second you switch SB on it grows..

I have jumper-ed the filter caps for B+ and the wave form looses about 20% but The ones on the board are still in. I don't think they are bad I guess I could swap them out but the reduction is what I would expect for a parallel move. I will take them out tonight and see if it fixes.. But I am doubtful. You would see it with switch off if it was the caps. (please correct me if I am talking out my ass)

When I say it is grounded .. It is.. I have moved them about and they are all in the right spot I believe cept' for maybe one but I tried that with no change.

1) Pre Ck Rk grounds to plate.
2) PS filter Ground For pre to plate
3) Other PS filter ground to pot side lug of PT
4) Used shielded wire with shrink wrapped business end ground so no loop there
5) Lead dress is tight and twisted on signal, grounds and PS
6) all new sockets tightened pins
7) no continuity between heater pins and Plates..

The amp is pretty quiet.. Slight and I mean slight buzz when the reverb is turned way up but in the #7 range nothing this is with the chassis wide open and un shielded.


Any thoughts or recommends would be Killer.. I never had something like this.. If someone comes back and says that's normal I will shoot my self asn I never had the need to look at this before. Just did it on a whim LOL. It is a saw tooth AC wave.. there shoudl not be any AC on the plates... I have lived by that.. again ( Correct me if I am wrong)

Any quick response or thoughts would be much appreciated..


THANKS IN ADVANCE!

[plexi50]

QUOTE: The wave is not on B+ when the standby is off second you switch SB on it grows..

Change the power tubes and see if it repeats the same wave form

Some power tubes do weird things when near a PT

PT osscillations make some power tubes hum through a speaker with no guitar plugged in. Even with the PI tube pulled

[d120f]

It does this with the tubes out...  so it has to be in the PS/filter I would think

when you take all the tubes out it reduces but is still there just not as big.. It has to be a grounding probelm but I am damned if I can find it.. looking for any ideas thanks for the response!

[Fresh_Start]

Can you put an additional filter cap in parallel with the reservoir cap?  IOW the cap filtering the plate node on the power rail.  Just guessing it's a bad filter cap.  Also, is the PT grounded to the negative end of the first filter cap on the power rail?

Cheers,

Chip

[d120f]

Can you put an additional filter cap in parallel with the reservoir cap?  IOW the cap filtering the plate node on the power rail.  Just guessing it's a bad filter cap.  Also, is the PT grounded to the negative end of the first filter cap on the power rail?

Cheers,

Chip

Thanks for the reply Chip

1) Yup I paralleled each of the series 80/425's it did a 20% cut in it but I just think that that was the added capacitance not because of faulty Filter caps

2)As for the grounding I think I understand what you are asking: Yes The 2 series 80uf filter caps for B+ are grounded to a lug on the PT. But grounding the PT? do you mean CT's, if so yes. The original didn't have a CT for the Heaters. I am thinking of lifting and wrapping it and putting in the 100ohm virtual CT. It just might save the Tranny some day.. It might be a good follow up q.. if there is a CT on the PT for The Heaters should that be grounded with the B+ CT and the B+ filters or else ware. Could this contribute.


Gonna make a note to make sure that heater circuit still has the right voltage Pole to Pole . Maybe one leg was shorted and I am throwing AC to the chassis. and make sure the power chord is grounded to the right lug.

[sluckey]

That sawtooth is normal on the output tube plates. It's also normal on the first filter cap. The sawtooth should be 120Hz. It's the charge/discharge waveform of the 1st filter cap trying to smooth out the rectified halfwave pulses.

Not to worry. The push/pull output circuit will cancel that waveform if the tubes are decently matched.

[d120f]

That sawtooth is normal on the output tube plates. It's also normal on the first filter cap. The sawtooth should be 120Hz. It's the charge/discharge waveform of the 1st filter cap trying to smooth out the rectified half wave pulses.

Not to worry. The push/pull output circuit will cancel that waveform if the tubes are decently matched.

You da man.. I had a feeling that that was what was going to come back.. you don't here it at all. Its silent. that's why I said tell me I'm wrong. See that Never to old to learn. On the way to work I said to myself that when I get home put each leg of the Primary of the OT on a scope and add teh channels to see if it cancels as I cant hear it through the speakers..

Thanks so much!

[sluckey]

put each leg of the Primary of the OT on a scope and add teh channels to see if it cancels

They won't cancel on the scope unless you invert one channel. The reason they cancel in the amp is because of the common mode rejection properties of the PP output stage. IOW, that ripple voltage is the same on each plate. When the same ripple current (or voltage) is applied to each primary end of a pp transformer, the net or combined current will be zero (common mode rejection), so no ripple signal is coupled to the secondary and your speaker. The better your circuit (tubes) are balanced, the more hum rejection the amp will have.

Now if you were to unbalance the pp circuit, maybe by pulling the tubes on one side of the OT, you should be able to hear that sawtooth, or see it with your scope connected to the speaker.

[d120f]

So the Wave inserted into the OT through the center tap comes out either side of the primary in phase.... I guess that would stand to reason... If not it would be added in the PP circuit.

[d120f]

After all the work to figure this out I cleaned up the wiring last night. It was originally a chassis with board and some wires no 9pin sockets. So when I built it I used the original heater wire on the Power tubes... F'ng CBS they had alternated phase on the heater wires.. There had been a slight buzz on the reverb return (phase issue) after 7 on the control. the thing is dead and I mean dead silent now with every control dimed on the amp..

I put in RCA grey plates in V2 and V4... The thing is the creamiest Twin I ever heard. I am contemplating busting out for a quad of sylvaina str's....

I built this to go with the copy of Jerry Garcia's Alligator strat.. Here is a picture of Jeff Matson Playing it.




It has all hand made bras parts I carved from blocks of brass. It has all the grounding and electronic mods Alembic did. You can take your hands off the guitar with the amp dimed on an even switch poss and it is dead silent... No ground to the bridge. Has a period correct fet buffer in it Duncan antiquities. Shallers, abr-1... Its a fun guitar. It is a copy of the Europe '72 version. It sounds exactly like the record. Its a tone that dead heads consider his best and no one has duplicated it... Until now ;-)

Thanks again all!

[plexi50]

If you had a heater CT it would solder straight to chassis ground at the PT end of course.
If you dont have a heater filiment CT then it is a good thing to add two 100 ohm artificial resistors that would go from the pilot light or power tube heater pins straight to chassis ground

[d120f]

If you had a heater CT it would solder straight to chassis ground at the PT end of course.
If you dont have a heater filiment CT then it is a good thing to add two 100 ohm artificial resistors that would go from the pilot light or power tube heater pins straight to chassis ground

Thanks.. I am all over that. my only issue was  seeing the rise and fall time of the cap charging. Never looked at that. LOL the Heater thing I just mentioned was me trusting Fender to wire the heaters correctly . Shouldn't be surprised when they didn't. Funny. My grounds are spot on. I bet that is why the chassis was parted out as no on ever would have wanted it. with the hum in the reverb circ.. LOL