distortion on leading edge of note

[pula58]

My two cents.

It looks to me like your phase inverter is driving the grids of the power tubes into saturation. That "nipple" is a tell-tale sign of that (cold showers will do it too). I suggest reducing the gain of the PI so that the phase inverter will start to clip before its peak voltage swing exceeds the bias voltage. That way, the grids of the power tubes will never be driven positive (when they are driven positive, they start to conduct current which the phase inverter cannot supply--hence the nipple).

You might try increasing the size of the dropping resistor in the B+ string (the one between the choke and the PI node).

I kind of agree (re: fwd biasing the grids), yet, I have used this same PI (Fender style) and output stage before without this problem, and, it appears in countless Fender AB763's..So, I can't figure-out why this particular amp is misbehaving this way.

power supply voltages are:

center tap of OT: 452V
screen grid supply: 450V
phase inv supply: 429V
preamp supply: 385V

WHy should fwd biasing the grids cause a low frequency (almost sounds like 60Hz) artifact?

[pula58]

output of both sides of phase inverter. This is "before" the grid stoppers. i.e., this is output at PI, not at the grids of the output tubes.

[Jana]

The AB 763 also has about 100 less volts on the PI node.

[pula58]

The AB 763 also has about 100 less volts on the PI node.

Being a big Fender Fan I have seen most of the AB763 schematics and, if I recall (correct me if I'm wrong here), the power supply voltage (usually marked "C" in the Fender schematics) for the PI plate loads falls-in at about 425-450V for the Pro reverb, bassman, super reverb and twin reverb. So, my voltages seems quite close to those voltages. I have another amp I built, very similar to this one, that does not have the same problem.
So, I am puzzled.....

[Jana]

I gave my .02 worth. Try it or not--it's your choice.

[pula58]

I gave my .02 worth. Try it or not--it's your choice.

Thanks Jana, I appreciate your input, just trying to understand why my other amp, with nearly equal voltages and parts, behaves so well, whilest this one does not! Same OT, same circuit, same choke, only thing that's different is the power supply tranny. I was using a Hammond 291EX. now I am using a custom Heyboer part (with very similar specs as the Hammond)


Interestingly, I just looked at the FFT of the amps output. As I turn up the output amplitude (as measured at the speaker output jack (with 8 ohm 200W speaker connected), I can see some 60Hz and 120Hz components in the FFT start to increase right at the point where the output tubes grids go into fwd conduction. This is what I have been hearing.

So, there is something going on that creates 60Hz and 120Hz frequencies in the output, and it happens when the output tube grids are forward-biased.

[Firestorm]

This has to be a lead dress issue. There are "identical" AB763 amps where one behaves and one doesn't. Most of the ones I have corrected had issues with the tone stack leads, but the other source of problems is the grid leads to the output tubes. Sometimes 1 inch too long is too much.

Jana: I loved the "cold shower" comment. You crack me up.

[LeftyStrat]

This has to be a lead dress issue. There are "identical" AB763 amps where one behaves and one doesn't. Most of the ones I have corrected had issues with the tone stack leads, but the other source of problems is the grid leads to the output tubes. Sometimes 1 inch too long is too much.

Jana: I loved the "cold shower" comment. You crack me up.

I agree with Firestorm.

Only other difference is "I was using a Hammond 291EX. now I am using a custom Heyboer part (with very similar specs as the Hammond) "

Perhaps coupling between the PT and OT? I might try removing the mounting bolts for the OT and twist it slightly to see if it made a difference.

[Structo]

Just curious, are you using a 12AT7 for the PI?

What kind of plate voltage do you have on the PI?

I found this "No Normal" channel layout in my files.

[pula58]

Just curious, are you using a 12AT7 for the PI?

What kind of plate voltage do you have on the PI?

I found this "No Normal" channel layout in my files.

Yes, 12AT7 for the PI

The "C" power supply voltage is about 413V according to measurement made this afternoon. Plate V's of the PI plates are 214V (105K measured from plate to trim pot wiper) and 224V (92K measured from plate to trim pot wiper).

The grounding scheme in the "no normal" AB763 layout: proven to work?

My layout is a bit different > I have three grounds (you can see them all in the photo of thr chassis I posted - I added labels to them i nthe photo)

1) near the input jack, with buss wire extending from there to last pot on the front panel. Any pots that need ground get it from that buss. Also, any points in the preamp part of the circuit board get their ground there too. The tail of the PI is grounded to that buss wire too.
2) the bias pot has its own ground, the upper right hand mounting bolt of the power tranny.
3) a star ground, with the following connections
A) ground lead of 1st power supply cap
B) ground lead of center tap of PT HT secondary
C) ground side of bias circuit capacitor
D) center tap of PT heater secondary
E) output tube cathodes
F) AC power cord chassis ground connection
G) PT hum shield connection

The black lead of the OT goes straight to the outer ring of the speaker output jack. The jack is mounted using an insulating washer. But, this point is also connected back to the star ground

[Firestorm]

Once more, it's got to be leads, not grounds. Fender grounded near randomly and got away with it. You have to look at your signal leads and see what is in-phase and what is out-of-phase. In-phase leads of the same magnitude "talk" to each other: that is the problem with the standard Fender tone stack wiring.

If you put a higher signal lead anywhere close a lower signal lead of the same phase, God help you.

Phase is not absolute; a cap and resistor shift it a certain number of degrees (which you can calculate). You've got to be looking for a positive-phase reinforcement type coupling that is creating a parasitic anomaly.

[jelle]

Disagree, fender had a clever and highly functional grounding system and touching it will change things.

[Tillydog]

Yep, PI looks nicely balanced!

... my initial bias board for this amp had a bias adj as well as bias balance adj. So, I could exactly match their bias, and still had this problem).

The OT is rated at 40W, but, I had this very same problem when I used a 55W outpu ttranny with this amp.

Do you still have independent bias to each side?

One thing that happens when the grids are driven into conduction is that you get current flow in the grid leak resistors. If you have independent bias supplies they need better filtering to avoid bias voltage fluctuations if/when the grids start to conduct (IMHO). Even with a common bias voltage, grid conduction can push the bias voltage around if the bias supply has too high an impedance. Might be worth scope-ing it out.

If the effect is always exactly 60/120Hz, then it could be insufficient power supply filtering or an under rated transformer. You could look at how the ripple voltage varies with load.

If you have an under-rated OT, it will cause more current to be drawn as it becomes saturated, so loading up your power supply more than you think (and maybe explaining the odd (IMHO) way that the peaks are clipped in your scope shots).

Is the effect still there with the NFB disconnected? It could also be instability in the NFB loop triggered when the loop becomes non-linear due to grid conduction. Lowering the PI headroom by reducing its B+ (as suggested by others) would probably help.

Andy

[pula58]

I eliminated the two spearate bias networks a few days ago, now there is just one. I havea 10uF 100V cap at the bias node to gnd. So, that node is pretty quiet (I have looked at it on the scope)

The effect showed up even with a much larger OT

I have 47uF as 1st power supply cap
then, 4H 90mA 108ohm choke,
and 47uF at the screens

Bear in mind that even if I drive the PI input directly (i.e., bypass the preamp)I see the problem. But, beside the point, I have wired the tone stack a little differently than Fender. The mid and bass tone-stack caps are soldered directly to the mid and bass pots. So, there is much less inches of wiring.

I tried disconnecting the feedback around the output stage, OT and PI - it made no change in this behavior.

I'll have to check to see if the 60Hz and 120Hz frequencies are independent of the signal frequency. And, re-check grid wiring.

[Firestorm]

Disagree, fender had a clever and highly functional grounding system and touching it will change things.

Point taken. Interestingly, if you ever completely disassemble a Fender BF/SF, you find small "insulators" between the ground bus and each pot, as if they wanted to control the number of actual ground connections in the preamp. I don't agree that Fender grounds were "clever," but they were functional. Leo was a radioman and if you know how to get a radio under control, a guitar amp is a no-brainer.

I still think the OP's issues will turn out to be signal lead routing, but technically, grounds are signal leads too, so you may totally correct.